---
title: "Volcanoes"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 19
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/19-volcanoes
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 19 — Volcanoes

On 17 May 1980, a geologist stood on a ridge ten kilometres from Mount St. Helens, in the north-west of the United States, and took the photograph on the left below: a smooth, snowy cone, one of the most graceful [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of the country. For two months the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) had been trembling, and its north side had been swelling outwards like a balloon. The next morning, at 8:32, a small [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) shook it. The whole swollen side slid away, and the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) exploded sideways. When the same ridge could be photographed again, in September, the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) looked like the photograph on the right.

![Mount St. Helens seen from Johnston Ridge, before and after the eruption of 18 May 1980. The summit and the whole north side are gone; a crater open to the north has taken their place. Photographs: Harry Glicken, U.S. Geological Survey (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-89c59de71c41.jpg)

![Mount St. Helens seen from Johnston Ridge, before and after the eruption of 18 May 1980. The summit and the whole north side are gone; a crater open to the north has taken their place. Photographs: Harry Glicken, U.S. Geological Survey (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-eed67db223dd.jpg)

*17 May 1980, the day before.*

*10 September 1980.*

*Mount St. Helens seen from Johnston Ridge, before and after the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) of 18 May 1980. The summit and the whole north side are gone; a [crater](#def-g7-volcanoes-vent) open to the north has taken their place. Photographs: Harry Glicken, U.S. Geological Survey (public domain, via Wikimedia Commons).*

In this chapter we open a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) up, to see what drives an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption); we find out why some [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) pour out quiet [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) while others blow themselves apart; and we look at the [world map](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-world-map) again, to see where the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) are and why.

**You already know.**

A *[volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano)* is a place where molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) from deep inside the Earth comes out at the surface, and the [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) or [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) that it builds there. During an *[eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption)* it throws out [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption), ash and gas ([Chapter 8](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#ch-g4-volcanoes-earthquakes-first-look)). Below the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) is made of hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) which is solid, although it can creep very slowly ([Chapter 16](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#ch-g6-inside-the-earth)). Basalt and granite are [igneous rocks](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-families): they form when molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) cools and hardens ([Chapter 15](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#ch-g6-rocks-and-minerals)). The outer shell of the Earth is broken into [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) which move a few centimetres a year; their edges are the [plate boundaries](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) ([Chapter 18](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#ch-g7-plate-tectonics)).

## 19.1 Inside a volcano

The [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) is solid. Yet in a few places, deep under the surface, a small part of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) melts. The molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is lighter than the solid [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) around it, so it rises slowly, squeezing up through cracks, like a drop of oil rising through water.

**Definition 19.1 (Magma).**

*Magma* is molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) underground. It is a hot liquid which may also carry crystals and, dissolved in it, gases such as water vapour and carbon dioxide. Magma that comes out at the surface is called [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption); as it comes out, it loses most of its gas.

Rising [magma](#def-g7-volcanoes-magma) often stops on its way, a few kilometres below the surface, where it gathers in a large pocket. There it waits, sometimes for centuries, before an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) empties part of it.

**Definition 19.2 (Magma chamber).**

A *magma chamber* is a large space in the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) under a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), a few kilometres down, where [magma](#def-g7-volcanoes-magma) gathers before an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption).

**Definition 19.3 (Vent and crater).**

A *vent* is an opening at the surface through which [magma](#def-g7-volcanoes-magma), gas and ash come out. A *crater* is the bowl-shaped hollow around a vent, usually at the top of the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).

![A volcano cut open (a drawing, not to scale). Magma gathers in the magma chamber and rises through a pipe to the main vent, at the bottom of the crater; a branch reaches a side vent on the flank. Each eruption adds a layer of lava or of ash to the cone.](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/fig-42582f9034b8.svg)

*A [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) cut open (a drawing, not to scale). [Magma](#def-g7-volcanoes-magma) gathers in the [magma chamber](#def-g7-volcanoes-magma-chamber) and rises through a pipe to the main [vent](#def-g7-volcanoes-vent), at the bottom of the [crater](#def-g7-volcanoes-vent); a branch reaches a side [vent](#def-g7-volcanoes-vent) on the flank. Each [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) adds a layer of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) or of ash to the cone.*

So the same molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) has two names. Underground it is *[magma](#def-g7-volcanoes-magma)*; once it has come out of a [vent](#def-g7-volcanoes-vent), it is *[lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption)*. The difference is not only a matter of words: [magma](#def-g7-volcanoes-magma) still holds its gas, while [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) has lost most of it into the air. We shall see that this gas decides what kind of [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) has.

How hot is it? Scientists of the Hawaiian Volcano Observatory measure the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) as it comes out of the [vents](#def-g7-volcanoes-vent) of Kilauea, one of the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) of Hawaii: about $1170{}^{\circ}\mathrm{C}$. Hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) glows, and the colour of the glow depends on its temperature: special cameras read the temperature of a flow from far away by measuring the colours of its glow. Even the eye gets a rough idea, as the table shows.

| **colour of the glowing [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)** | **temperature** |
| --- | --- |
| yellow | about 1000 to $1200{}^{\circ}\mathrm{C}$ |
| orange | about 800 to $1000{}^{\circ}\mathrm{C}$ |
| red | about 600 to $800{}^{\circ}\mathrm{C}$ |

**Remark 19.4 (A skin that cools in a second).**

When [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) first meets the air, its surface cools by hundreds of degrees every second: a dark skin forms at once on the glowing [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Under the skin the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) stays hot for a long time. In Hawaii, [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) has flowed in tunnels under its own hardened roof for $10\,\mathrm{km}$, from a [vent](#def-g7-volcanoes-vent) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and still reached the shore at about $1140{}^{\circ}\mathrm{C}$: only about 30 degrees cooler than at the [vent](#def-g7-volcanoes-vent).

![A volcanologist in a heat-reflecting suit takes a sample of lava with a long steel pole (AI-generated illustration of a generic scene). Only trained scientists, with this kind of equipment, go close to flowing lava; everyone else watches from where the authorities allow.](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-c0cf4de12a77.jpg)

*A volcanologist in a heat-reflecting suit takes a sample of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) with a long steel pole (AI-generated illustration of a generic scene). Only trained scientists, with this kind of equipment, go close to flowing [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption); everyone else watches from where the authorities allow.*

## 19.2 Two ways to erupt

Mount St. Helens blew itself apart. The [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) of Hawaii, on the contrary, usually pour out [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) which flows away down their slopes. Geologists sort [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) into two kinds.

**Definition 19.5 (Effusive and explosive eruptions).**

In an *effusive eruption* the [magma](#def-g7-volcanoes-magma) comes out quietly as [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption), which flows away from the [vent](#def-g7-volcanoes-vent), sometimes for kilometres, or spurts up in fountains of glowing drops. In an *explosive eruption* the [magma](#def-g7-volcanoes-magma) is blown to pieces by its own gas: a column of ash, gas and lumps of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is thrown high into the sky.

![Both photographs are in the public domain (via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-4ced1e66f25f.jpg)

![Both photographs are in the public domain (via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-78c72a3a7c7c.jpg)

*Effusive: a fountain of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) at Kilauea, Hawaii, 1983. Photograph: J. D. Griggs, USGS.*

*Explosive: the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) column of Mount Pinatubo, Philippines, 12 June 1991. Photograph: Dave Harlow, USGS.*

*Both photographs are in the public domain (via Wikimedia Commons).*

What makes the difference? Two things: how runny the [magma](#def-g7-volcanoes-magma) is, and how much gas it holds.

**Runny or sticky.** Some [magma](#def-g7-volcanoes-magma) is runny, almost like warm honey: this is the case of the [magma](#def-g7-volcanoes-magma) that cools into dark basalt, as in Hawaii. Other [magma](#def-g7-volcanoes-magma) is thick and sticky, like cold honey or toothpaste. Through runny [magma](#def-g7-volcanoes-magma), gas bubbles rise easily and escape at the surface, a few at a time. In sticky [magma](#def-g7-volcanoes-magma) the bubbles are trapped.

**Little or much gas.** A bottle of fizzy water holds a lot of gas dissolved in the water, kept there by the pressure inside the closed bottle; you cannot see it. Open the bottle and the pressure drops: bubbles appear everywhere, and if the bottle has been shaken, the foam bursts out. [Magma](#def-g7-volcanoes-magma) deep underground also holds gas dissolved under high pressure. As it rises, the pressure drops and the gas comes out of solution as bubbles.

Put the two together. Runny [magma](#def-g7-volcanoes-magma) with little gas lets its bubbles go and comes out as flowing [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption): the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) is effusive. Sticky [magma](#def-g7-volcanoes-magma) full of gas keeps its bubbles until they swell so much that they tear the [magma](#def-g7-volcanoes-magma) into shreds: the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) is explosive. The shreds of frothy [magma](#def-g7-volcanoes-magma) freeze in the air into ash and into pumice, a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) so full of bubbles that it can float.

**Example 19.6 (Pinatubo, 1991: the flat soda and the shaken one).**

In June 1991 the first [magma](#def-g7-volcanoes-magma) reached the surface of Mount Pinatubo, in the Philippines. On its way up it had lost most of its gas, “like a bottle of soda pop gone flat”, in the words of the scientists who were watching. It oozed out slowly and piled up into a mound over the [vent](#def-g7-volcanoes-vent): no explosion. Then, on 12 June, [magma](#def-g7-volcanoes-magma) still full of gas reached the surface and exploded — the column in the photograph above. On 15 June, even more gas-rich [magma](#def-g7-volcanoes-magma) arrived, and the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) had one of the largest [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) of the twentieth century: it threw out more than five cubic kilometres of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and ash, its ash [cloud](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) rose $35\,\mathrm{km}$ high, and the top of the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) collapsed into a hollow $2.5\,\mathrm{km}$ across. The same [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), the same week: the gas made the difference.

The shape of a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) keeps the memory of its [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption). Runny [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flows far before it hardens, so layer after layer it builds a broad [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with gentle slopes, shaped like a warrior’s shield lying on the ground: a *shield [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano)*. Mauna Loa, in Hawaii, is one. Sticky [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) and ash pile up close to the [vent](#def-g7-volcanoes-vent), into a steep cone made of layers of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) and ash: a *stratovolcano* (*strata* means layers). Mount St. Helens and Pinatubo are two of them.

![The two kinds of eruption and the volcanoes they build (drawings, not to scale).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/fig-a9067a0da08b.svg)

*The two kinds of [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) and the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) they build (drawings, not to scale).*

**Method 19.7 (Effusive or explosive? Four clues).**

To tell from a description or a photograph which kind of [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) has, look for these clues.

1. *What comes out?* [Lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flowing or spurting in fountains: effusive. A column of ash, lumps of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) thrown out: explosive.
2. *The shape of the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).* A broad [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with gentle slopes: mostly [effusive eruptions](#def-g7-volcanoes-eruption-types) . A steep cone: often explosive.
3. *The [magma](#def-g7-volcanoes-magma).* Runny, little gas: effusive. Sticky, much gas: explosive.
4. *The [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) left behind.* Dark, solid [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flows: effusive. Layers of ash and pumice: explosive.

**Example 19.8 (Using the clues).**

A traveller describes a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano): “a huge, gently rounded [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), black [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flows everywhere on its slopes, some of them only a few years old”. Clue 2 (gentle slopes) and clue 4 (dark [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flows) both point to [effusive eruptions](#def-g7-volcanoes-eruption-types): it is a shield [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), like Mauna Loa.

**In the lab — A volcano in a bottle (in class, with the teacher).**

The teacher takes two closed bottles of fizzy water. One has been left standing; the other has been gently shaken. Both are opened outdoors, over a basin, pointing away from everyone. The still bottle hisses and lets a few bubbles rise; the shaken one bursts into foam that shoots out. Which bottle is like the [magma](#def-g7-volcanoes-magma) of Hawaii, which like Pinatubo’s? Then drop a piece of pumice and a [pebble](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) into a bowl of water: the pumice floats. Look at it through a magnifying glass: it is full of holes, the bubbles of gas frozen into the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) as the [magma](#def-g7-volcanoes-magma) burst.

## 19.3 What comes out of a volcano

An [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) throws out much more than [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption): gases, ash, lumps of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and, in the worst cases, avalanches of hot ash.

The most abundant gas, by far, is water vapour. Large [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) release huge amounts of gas in a short time: on a single day of 1991, Pinatubo is thought to have sent more than 250 million tonnes of gas into the upper atmosphere.

**Definition 19.9 (Pyroclastic flow).**

A *pyroclastic flow* is a fast avalanche of hot gas, ash and pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), from grains of dust to boulders, which rushes down the slopes of a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) during an [explosive eruption](#def-g7-volcanoes-eruption-types). (*Pyro* means fire and *clastic* means made of broken pieces.)

[Pyroclastic flows](#def-g7-volcanoes-pyroclastic-flow) usually move faster than $80\,\mathrm{km}/\mathrm{h}$, and the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and gas inside them are generally between 200 and $700{}^{\circ}\mathrm{C}$. They generally follow the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), like water, and knock down, burn or bury nearly everything in their path. On 18 May 1980, the [pyroclastic flows](#def-g7-volcanoes-pyroclastic-flow) of Mount St. Helens raced at 80 to $130\,\mathrm{km}/\mathrm{h}$, at a temperature of at least $700{}^{\circ}\mathrm{C}$, and reached $8\,\mathrm{km}$ from the [crater](#def-g7-volcanoes-vent).

The table sums up the products of a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).

| **product** | **what it is** | **the danger** |
| --- | --- | --- |
| [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flow | molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) flowing on the ground, then hardening | burns and buries what it meets, but usually slow enough for people to move out of the way |
| ash | tiny pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and glass, blown into the air | falls far away; roofs collapse under its weight; it damages aircraft |
| volcanic bombs | lumps of [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) thrown out, which harden as they fly | crush what they hit, near the [vent](#def-g7-volcanoes-vent) |
| gases | mostly water vapour, then carbon dioxide and sulfur dioxide | some are poisonous; carbon dioxide gathers in hollows |
| [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow) | an avalanche of hot ash, [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and gas | the deadliest: too fast and too hot to escape |
| mudflow | ash and [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) mixed with water from [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) or melted [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) | rushes down [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), sometimes far from the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) |

**Method 19.10 (How long does it take?).**

To find how long something moving at a known speed takes to cover a distance, divide the distance by the speed:

$$
\text{time} = \frac{\text{distance}}{\text{speed}}.
$$

With a distance in kilometres and a speed in kilometres per hour, the time comes out in hours; multiply by 60 to get minutes.

**Example 19.11 (Racing a pyroclastic flow).**

A [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow) moving at $120\,\mathrm{km}/\mathrm{h}$ heads for a village $6\,\mathrm{km}$ down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). It takes $6 \div 120 = 0.05$ hour, that is $0.05 \times 60 = 3$ minutes. Three minutes: nobody can run, or even drive, out of the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) in time. People are only safe if they have left *before* the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption).

**Remark 19.12 (Mudflows: the danger that comes later).**

An [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) leaves the slopes covered with loose ash. When [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls on it, or when the heat melts the [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and ice of the summit, the ash turns into mudflows that rush down the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). On 18 May 1980, the mudflows of Mount St. Helens damaged 27 bridges and nearly 200 homes. Around Pinatubo, the heavy [rains](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) of the following years turned the ash into giant mudflows which buried whole villages: they did more damage than the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) itself.

## 19.4 Where are the volcanoes?

The Smithsonian Institution, in Washington, keeps a list of all the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) known to have erupted in the last 12 000 years: 1 214 of them, on land and under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). For each one the list says where it stands with respect to the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate). The [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) below colours them in this way.

![The volcanoes active in the last 12 000 years, coloured by where they stand. Faint lines: the plate boundaries where plates move apart (red) and where one plate sinks under another (blue). Data: Smithsonian Global Volcanism Program; boundaries after P. Bird (2003); coastlines from Natural Earth.](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/fig-6c947c6087c9.svg)

*The [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) active in the last 12 000 years, coloured by where they stand. Faint lines: the [plate boundaries](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart (red) and where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another (blue). Data: Smithsonian Global Volcanism Program; boundaries after P. Bird (2003); coastlines from Natural Earth.*

![How many of the 1 214 volcanoes stand in each setting.](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/fig-f4e5f4ad9a23.svg)

*How many of the 1 214 [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) stand in each setting.*

**Proposition 19.13 (Most volcanoes stand on plate boundaries).**

Of the 1 214 [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) active in the last 12 000 years, 838 (69 %) stand where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another and 221 (18 %) where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart; 149 (12 %) stand inside a [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate), far from its edges. For the last 6 (0.5 %) the setting is not known.

**Where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart.** Along the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ridges, and in rifts such as the one that is tearing East Africa apart, the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) pull away from each other. [Magma](#def-g7-volcanoes-magma) rises from the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) into the widening cracks and fills them: new [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) is made. Most of the ridges lie under the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), but Iceland stands right on the Mid-Atlantic Ridge: there, these [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) can be seen on land. Their [magma](#def-g7-volcanoes-magma) is mostly runny basalt, and their [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) are mostly effusive.

**Where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another.** More than two [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) in three stand here, along lines that follow the edges of the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) — the “Ring of Fire” — and a few other places. Deep down, along the sinking [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate), some of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) partly melts; scientists still discuss exactly which [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) melts, the sinking [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) or the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) above it, or both. The [magma](#def-g7-volcanoes-magma) that rises there is often sticky and full of gas: these are the explosive [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), like Mount St. Helens and Pinatubo. The data show it: of the 660 steep cones (stratovolcanoes) in the list, 564 — that is 85 % — stand where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another.

**Inside a [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate).** A few [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) are far from any boundary. The most famous are those of Hawaii, in the middle of the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), more than $3200\,\mathrm{km}$ from the nearest edge of a [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate).

**Definition 19.14 (Hotspot).**

A *hotspot* is a place, deep in the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), which stays hot for millions of years and keeps feeding [magma](#def-g7-volcanoes-magma) to [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) at the surface, even far from the edges of the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate).

The [hotspot](#def-g7-volcanoes-hotspot) stays where it is, while the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) above it moves. A [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) grows above the [hotspot](#def-g7-volcanoes-hotspot); then the moving [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) carries it away, and it dies, while a new [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) grows over the [hotspot](#def-g7-volcanoes-hotspot) behind it. In time, the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) carries a whole line of [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), the oldest farthest from the [hotspot](#def-g7-volcanoes-hotspot). That is just what we see in Hawaii: the oldest [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the island of Kauai, at the north-west end of the inhabited islands, are about 5.5 million years old, and deeply worn down by [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and waves; on the Big Island, at the south-east end, the oldest [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) at the surface are less than 0.7 million years old, and new [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) still comes out.

**History — A geophysicist and a line of islands.**

Why should a chain of volcanic islands rise in the middle of the largest [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) on Earth? In 1963 the Canadian geophysicist J. Tuzo Wilson proposed an answer. The Hawaiian islands, he suggested, are a line because the Pacific [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) moves over a deep, fixed source of heat in the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), which partly melts the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) above it. If he was right, the islands should be older and more worn down the farther they are from the active [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) of the Big Island. The ages of their [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) show exactly that. A single idea explained both the line and the order of the ages. (Wilson is also the scientist who discovered the [transform boundaries](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary), where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) slide past each other.)

![A plate moving over a hotspot (a drawing, not to scale; the islands in between are left out). The volcano over the hotspot is young and active; the plate carries the older ones away to the north-west, where they die, wear down and finally sink below the waves.](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/fig-370c57a6c6e6.svg)

*A [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) moving over a [hotspot](#def-g7-volcanoes-hotspot) (a drawing, not to scale; the islands in between are left out). The [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) over the [hotspot](#def-g7-volcanoes-hotspot) is young and active; the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) carries the older ones away to the north-west, where they die, wear down and finally sink below the waves.*

![Mauna Loa, a shield volcano on the Big Island of Hawaii, seen from the air: a huge mountain with gentle slopes, built by thousands of runny lava flows. Photograph: J. D. Griggs, U.S. Geological Survey, 1985 (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-volcanoes/img-a28174b1f5eb.jpg)

*Mauna Loa, a shield [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) on the Big Island of Hawaii, seen from the air: a huge [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with gentle slopes, built by thousands of runny [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flows. Photograph: J. D. Griggs, U.S. Geological Survey, 1985 (public domain, via Wikimedia Commons).*

## 19.5 Living with volcanoes

Many people live near [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano): about 3 million lived in the region around Pinatubo in 1991. Luckily, a large [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) often gives warning signs. Rising [magma](#def-g7-volcanoes-magma) pushes the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) aside: the ground cracks and shakes with small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), the slopes swell, gas escapes. At Mount St. Helens, the first small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) came on 16 March 1980; by 17 May there had been more than 10 000, and the north side had swollen outwards by at least $137\,\mathrm{m}$. Scientists measured all this and the area was closed; even so, 57 people died on 18 May.

At Pinatubo, in 1991, the scientists who watched the [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), the gas and the swelling of the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) gave the warning in time: people living near the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) left, and at least 5 000 lives were saved.

**Remark 19.15 (Never go near an erupting volcano).**

Watching an [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) is only for the places that the authorities have opened to the public. Gases, falling [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), fresh lava crust that breaks under your feet and [pyroclastic flows](#def-g7-volcanoes-pyroclastic-flow) can kill, often without warning. [Volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) observatories publish what is safe to visit: always follow them.

## 19.6 Exercises

**Exercise 19.1 ★.**

Match each word with its meaning: [magma](#def-g7-volcanoes-magma); [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption); [magma chamber](#def-g7-volcanoes-magma-chamber); [vent](#def-g7-volcanoes-vent); [crater](#def-g7-volcanoes-vent). (a) A bowl-shaped hollow at the top of a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). (b) Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) underground. (c) An opening through which [magma](#def-g7-volcanoes-magma) comes out. (d) Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that has come out at the surface. (e) A large pocket of [magma](#def-g7-volcanoes-magma) a few kilometres under a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano).

**Solution of Exercise 19.1.**

(a) [crater](#def-g7-volcanoes-vent); (b) [magma](#def-g7-volcanoes-magma); (c) [vent](#def-g7-volcanoes-vent); (d) [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption); (e) [magma chamber](#def-g7-volcanoes-magma-chamber).

**Exercise 19.2 ★.**

Effusive or explosive? (a) [Rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of glowing [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) flow down a gentle slope. (b) A column of ash rises $35\,\mathrm{km}$ into the sky. (c) A fountain of glowing drops spurts from a crack. (d) An avalanche of hot ash and gas rushes down a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).

**Solution of Exercise 19.2.**

(a) Effusive. (b) Explosive. (c) Effusive: a [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) fountain throws out glowing drops which fall back and flow; the [magma](#def-g7-volcanoes-magma) is not blown into ash. (d) Explosive: it is a [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow).

**Exercise 19.3 ★.**

True or false? Correct the false ones. (a) [Magma](#def-g7-volcanoes-magma) and [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) are two names of the same molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), underground and at the surface. (b) The most abundant volcanic gas is carbon dioxide. (c) A [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow) is a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of liquid [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption). (d) The Hawaiian [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) stand on a [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary).

**Solution of Exercise 19.3.**

(a) True. (b) False: the most abundant volcanic gas is water vapour; carbon dioxide comes next. (c) False: a [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow) is an avalanche of hot gas, ash and pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (d) False: they stand inside the Pacific [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate), more than 3200 km from the nearest boundary, above a [hotspot](#def-g7-volcanoes-hotspot).

**Exercise 19.4 ★.**

Use the table of glow colours. (a) [Lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) glows yellow: give its temperature. (b) A hardening flow glows dark red: give its temperature. (c) A photograph shows orange [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) at the front of a flow and yellow [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) near the [vent](#def-g7-volcanoes-vent). Where is it hotter? Why?

**Solution of Exercise 19.4.**

(a) About 1000 to 1200 degrees. (b) About 600 to 800 degrees. (c) Near the [vent](#def-g7-volcanoes-vent), where the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) is yellow (1000 to 1200 degrees) rather than orange (800 to 1000 degrees): the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) cools as it flows away from the [vent](#def-g7-volcanoes-vent).

**Exercise 19.5 ★★.**

Use the bar chart of the 1 214 [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). (a) How many [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) stand on a [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) (where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart, or where one sinks under another)? (b) What percentage of all the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) is that? (c) Check that the four percentages of the proposition add up to about 100 %.

**Solution of Exercise 19.5.**

(a) $838 + 221 = 1059$ [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). (b) $1059 \div 1214 \approx 0.872$: about **87 %**. (c) $69 + 18 + 12 + 0.5 = 99.5$, about 100 %: the small difference comes from rounding each percentage.

**Exercise 19.6 ★★.**

Of the 660 steep cones (stratovolcanoes) of the list, 564 stand where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another, 62 where [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) move apart and 33 inside a [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate). (a) Find the percentage of steep cones that stand where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another. (b) How many steep cones are left for the setting “not known”? (c) Use the four clues: what kind of [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) should we expect along the Ring of Fire?

**Solution of Exercise 19.6.**

(a) $564 \div 660 \approx 0.855$: about **85 %**. (b) $660 - 564 - 62 - 33 = 1$: one steep cone. (c) Steep cones are built by sticky [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) and ash, the mark of [explosive eruptions](#def-g7-volcanoes-eruption-types): along the Ring of Fire, where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another, we expect many [explosive eruptions](#def-g7-volcanoes-eruption-types), like those of Mount St. Helens and Pinatubo.

**Exercise 19.7 ★★.**

At Kilauea, [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) leaves the [vent](#def-g7-volcanoes-vent) at about $1170{}^{\circ}\mathrm{C}$. In a [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) tunnel it reached the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), $10\,\mathrm{km}$ away, at about $1140{}^{\circ}\mathrm{C}$. (a) By how many degrees did it cool? (b) How many degrees is that per kilometre? (c) [Lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) in the open air cools by hundreds of degrees per second at its surface. Explain why the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) in the tunnel stayed so hot.

**Solution of Exercise 19.7.**

(a) $1170 - 1140 = 30$ degrees. (b) $30 \div 10 = 3$ degrees per kilometre. (c) In the tunnel the [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) does not touch the air: the hardened roof above it keeps the heat in, like the lid of a pot. In the open, its surface loses heat to the air at once.

**Exercise 19.8 ★★.**

The oldest [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of Kauai are about 5.5 million years old; those of the Big Island, less than 0.7 million years old. (a) Which island is older? By at least how many million years? (b) Which island is more worn down? Why? (c) Which island stands over the [hotspot](#def-g7-volcanoes-hotspot) today? (d) Kauai lies north-west of the Big Island. In which direction does the Pacific [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) carry the islands?

**Solution of Exercise 19.8.**

(a) Kauai, by at least $5.5 - 0.7 = 4.8$ million years. (b) Kauai: [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and waves have been wearing it down for much longer. (c) The Big Island, where new [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) still comes out. (d) Towards the north-west: Kauai, carried away from the [hotspot](#def-g7-volcanoes-hotspot) first, is now north-west of it.

**Exercise 19.9 ★★.**

The Hawaiian islands are more than $3200\,\mathrm{km}$ from the nearest [plate boundary](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary). (a) Why is that surprising for [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano)? (b) Explain in three or four sentences where their [magma](#def-g7-volcanoes-magma) comes from. (c) On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), which colour are they?

**Solution of Exercise 19.9.**

(a) Almost all [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) stand on [plate boundaries](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate-boundary) (87 % in the list): [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) so far from any boundary are an exception. (b) Deep in the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) under Hawaii, a [hotspot](#def-g7-volcanoes-hotspot) stays hot for millions of years. It partly melts the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) above it. The [magma](#def-g7-volcanoes-magma), lighter than the solid [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) around it, rises through the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) and erupts. As the [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) moves, new [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) grow over the [hotspot](#def-g7-volcanoes-hotspot) and the old ones are carried away. (c) Orange dots (“inside a [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate)”).

**Exercise 19.10 ★★.**

In June 1991 the first [magma](#def-g7-volcanoes-magma) of Pinatubo oozed out quietly; three days later, other [magma](#def-g7-volcanoes-magma) exploded. Using the two bottles of fizzy water, explain the difference. Which of the two [magmas](#def-g7-volcanoes-magma) had lost its gas on the way up?

**Solution of Exercise 19.10.**

The first [magma](#def-g7-volcanoes-magma) had lost most of its gas on the way up, like a bottle of fizzy water left open or left standing: when it reached the surface, nothing was left to blow it apart, and it oozed out. The [magma](#def-g7-volcanoes-magma) of 12 and 15 June still held its gas, like the shaken bottle: as the pressure dropped near the surface, the gas formed bubbles all at once, which swelled and tore the sticky [magma](#def-g7-volcanoes-magma) into ash. The first [magma](#def-g7-volcanoes-magma) is the one that had lost its gas.

**Exercise 19.11 ★★★.**

A village stands $10\,\mathrm{km}$ down a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) from a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). (a) A [pyroclastic flow](#def-g7-volcanoes-pyroclastic-flow) moves at $80\,\mathrm{km}/\mathrm{h}$. How many minutes would it take to reach the village? (b) Same question at $130\,\mathrm{km}/\mathrm{h}$. (c) A family needs 20 minutes to leave by car. Explain why the warning has to come before the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) starts, not when the flow is seen.

**Solution of Exercise 19.11.**

(a) $10 \div 80 = 0.125$ hour, and $0.125 \times 60 = 7.5$ minutes. (b) $10 \div 130 \approx 0.077$ hour, about $0.077 \times 60 \approx
4.6$ minutes, so about 5 minutes. (c) The family needs 20 minutes, but the flow arrives in 5 to 8 minutes: once the flow is seen, it is too late. The family can only be safe if it leaves when the scientists see the warning signs, before the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption).

**Exercise 19.12 ★★★.**

The ash of Mount St. Helens went once around the whole Earth in 15 days. One degree of [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) is about $111\,\mathrm{km}$, and a full turn is 360 degrees. (a) About how long is a full turn of the Earth, in kilometres? Round to the nearest thousand. (b) How many kilometres did the ash travel per day, on average? (c) How many kilometres per hour is that? (d) Is the ash carried by the wind or by its own speed? Explain.

**Solution of Exercise 19.12.**

(a) $111 \times 360 = 39\,960$: about **40 000 km**. (b) $40\,000 \div 15 \approx 2667$: about 2 670 km per day. (c) $2667 \div 24 \approx 111$: about **110 km per hour**. (d) By the wind: the fine ash has no speed of its own once the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) has thrown it up; it floats high in the air and the wind carries it, while it slowly falls.

## 19.7 Problem: The day the mountain lost its top

**Problem 19.1.**

Weekend problem — the warning signs of Mount St. Helens, the eruption of 18 May 1980, and how much of the mountain was gone

Mount St. Helens stands in the north-west of the United States. Use the photographs at the start of the chapter, the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) and the numbers below, measured by the U.S. Geological Survey.

**Part I — Before the eruption.**

1. The first small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) came on 16 March 1980; the great [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) on 18 May. How many days passed between the two? (March has 31 days, April 30.)
2. By 17 May, more than 10 000 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) had been recorded, in the 62 days since 16 March. About how many is that per day, on average?
3. The north side of the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) swelled outwards by at least $137\,\mathrm{m}$ . Explain why the swelling worried the scientists.
4. On the [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of the [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) , Mount St. Helens is a blue triangle. What does that tell you about its setting? Which kind of [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) do you expect, and which shape of [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) ?

**Part II — 18 May 1980.**

5. At 8:32 a small [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) set the swollen side sliding. The [landslide](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-landslide) moved at up to $240\,\mathrm{km}/\mathrm{h}$ . How many kilometres is that per minute?
6. The [landslide](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-landslide) uncovered the [magma](#def-g7-volcanoes-magma) like a cork pulled from a shaken bottle. Explain why the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) then exploded sideways.
7. The sideways blast moved at least $480\,\mathrm{km}/\mathrm{h}$ and reached $27\,\mathrm{km}$ from the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) . At that speed, how many minutes did it take, at most, to get there?
8. The [pyroclastic flows](#def-g7-volcanoes-pyroclastic-flow) moved at up to $130\,\mathrm{km}/\mathrm{h}$ and reached $8\,\mathrm{km}$ from the [crater](#def-g7-volcanoes-vent) . How many minutes did they take at that speed? Round to the nearest minute.

**Part III — After.**

9. The column of ash reached a height of about $24\,\mathrm{km}$ in less than 15 minutes. How many times higher than Mount Everest ( $8.8\,\mathrm{km}$ ) is that? Round to one decimal.
10. Between 1980 and 1986, sticky [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) oozed out in the [crater](#def-g7-volcanoes-vent) and built a mound $267\,\mathrm{m}$ high over the [vent](#def-g7-volcanoes-vent) . Were these [eruptions](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) effusive or explosive? Which bottle of fizzy water do they remind you of?
11. Look at the two photographs of the [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) . Describe in two sentences what changed.
12. Before the [eruption](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) , the summit was $2950\,\mathrm{m}$ above [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude) ; after it, the highest point was $2549\,\mathrm{m}$ . How many metres of height did the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) lose?

**Solution of Problem 19.1.**

**1.** From 16 to 31 March: $31 - 16 = 15$ days; then 30 days of April and 18 days of May: $15 + 30 + 18 = 63$ days.

**2.** $10\,000 \div 62 \approx 161$: more than 160 small [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) a day, on average.

**3.** The swelling showed that [magma](#def-g7-volcanoes-magma) had risen high inside the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and was pushing the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) outwards. It also made the north side steep and unstable, ready to slide.

**4.** A blue triangle stands where one [plate](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) sinks under another. We expect sticky, gas-rich [magma](#def-g7-volcanoes-magma), [explosive eruptions](#def-g7-volcanoes-eruption-types) and a steep cone (a stratovolcano) — just what the first photograph shows.

**5.** $240 \div 60 = 4$ kilometres per minute.

**6.** The swollen side was holding the [magma](#def-g7-volcanoes-magma) in, under pressure, like the cap of a bottle. When it slid away, the pressure dropped at once; the gas dissolved in the [magma](#def-g7-volcanoes-magma) turned into bubbles all at once and burst out through the open side — sideways, where the lid had gone.

**7.** $27 \div 480 \approx 0.056$ hour, and $0.056 \times 60 \approx
3.4$ minutes. Since the blast moved at least that fast, it took at most about 3.4 minutes.

**8.** $8 \div 130 \approx 0.062$ hour, and $0.062 \times 60 \approx
3.7$: about **4 minutes**.

**9.** $24 \div 8.8 \approx 2.7$: the column rose about 2.7 times higher than Mount Everest.

**10.** Effusive: the sticky [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) came out without exploding and piled up over the [vent](#def-g7-volcanoes-vent). Like the bottle that stood still, this [magma](#def-g7-volcanoes-magma) had already lost most of its gas.

**11.** Before, the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) was a smooth, pointed cone covered with [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), above green forests. After, its summit and north side are gone: a huge [crater](#def-g7-volcanoes-vent) open towards us has taken their place, and the slopes and the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) in front are grey with ash and broken [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**12.** $2950 - 2549 = 401$: the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) lost about **401 m** of height, about 400 metres, as told in grade 4.