---
title: "Volcanoes and Earthquakes: A First Look"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 8
exercises: 11
source: https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 8 — Volcanoes and Earthquakes: A First Look

On Monday evening, the news shows a [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) on an island far away. In the dark, a fountain of glowing orange [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) spurts from a crack in the ground, and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of fire flow slowly down the slope into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), hissing and steaming. On Thursday, the news shows another place: a small town where, at dawn, the ground shook for half a minute. Chimneys fell into the street, and one old house slid off its base.

Two very different events. Yet both come from deep inside the Earth, and, as we shall see, both happen far more often in some parts of the world than in others.

**You already know.**

[Mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) are the shapes of the land ([Chapter 2](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#ch-g1-landscapes-around-us)). The Earth’s land is gathered in seven [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), between five [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); the Pacific is the largest [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([Chapter 6](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#ch-g3-continents-and-oceans)).

## 8.1 A volcano erupts

Deep under our feet, the Earth is very hot. In some places, [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) down there is so hot that it has melted. This 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 slowly rises, like a cork in water. Where it finds a way out to the surface, a [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) is born.

**Definition 8.1 (Volcano).**

A *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 the molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) builds there, layer after layer.

**Definition 8.2 (Eruption, lava and volcanic ash).**

An *eruption* is the time when a [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) throws out molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), ash and gas. *Lava* is molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that pours out onto the surface; it flows, then cools and hardens into new [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). *Volcanic ash* is made of 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 by an eruption; it falls back like a grey [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation).

![A volcano erupting (drawn, not to scale). Molten rock rises from deep below and comes out at the top: it flows down as lava, while ash and gas are blown up into the sky. Each eruption adds a new layer to the mountain.](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/fig-e664eef903bc.svg)

*A [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) erupting (drawn, not to scale). Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) rises from deep below and comes out at the top: it flows down as [lava](#def-g4-volcanoes-earthquakes-first-look-eruption), while ash and gas are blown up into the sky. Each [eruption](#def-g4-volcanoes-earthquakes-first-look-eruption) adds a new layer to the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).*

Not all [eruptions](#def-g4-volcanoes-earthquakes-first-look-eruption) look alike. In some, the [lava](#def-g4-volcanoes-earthquakes-first-look-eruption) is runny: it pours out quietly in glowing [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that can flow for kilometres, sometimes all the way to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). In others, the molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is thick and full of gas, like a shaken bottle of fizzy drink: when it bursts out, it explodes into a huge column of ash that rises high into the sky.

![](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/img-daa0f1a80619.jpg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/img-d01f075a5735.jpg)

*A runny [eruption](#def-g4-volcanoes-earthquakes-first-look-eruption): [lava](#def-g4-volcanoes-earthquakes-first-look-eruption) pours down a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) cliff into the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), in Hawaii. Photograph: USGS Hawaiian Volcano Observatory (public domain).*

*An explosive [eruption](#def-g4-volcanoes-earthquakes-first-look-eruption): Mount St. Helens, United States, 18 May 1980. Photograph: Austin Post, USGS (public domain).*

**Example 8.3 (Mount St. Helens, 1980).**

In the spring of 1980, a bulge grew on the side of Mount St. Helens, as molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) pushed up inside the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). On the morning of 18 May, a small [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) shook the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the bulge slid away in a giant landslide, and the [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) exploded sideways and upwards. The top 400 metres of the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) were gone: the summit became a huge crater.

## 8.2 The ground shakes

The [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) deep under our feet are not perfectly still. Very slowly, they are pushed and pulled. They bend a little, like a ruler you push at both ends — until, suddenly, they break and slip. The jolt sends the ground shaking for kilometres all around.

**Definition 8.4 (Earthquake).**

An *earthquake* is a sudden shaking of the ground, caused by [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that break and slip, deep underground.

An [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) usually lasts only a few seconds, sometimes a minute or two for the largest. Most [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) are so small that nobody feels them: only instruments notice them. A strong one can make the ground roll like the deck of a ship, crack roads, and bring down walls and houses.

![After a strong earthquake in California in 1989: this wooden house was thrown sideways off its concrete base. Photograph: J. K. Nakata, USGS (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/img-a78f7ea39919.jpg)

*After a strong [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) in California in 1989: this wooden house was thrown sideways off its concrete base. Photograph: J. K. Nakata, USGS (public domain).*

**Remark 8.5 (How often?).**

Geologists keep a list of all the large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) on Earth. Over the 50 years from 1976 to 2025, the list counts 7 031 large ones (strong enough to damage buildings near them): about 141 in a year, or almost 3 every week. The very largest, which can destroy whole towns, are about 14 in a year. Over the same Earth, about 1 200 [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) have erupted in the last twelve thousand years or so.

## 8.3 Where volcanoes and earthquakes happen

If [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) and [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) happened anywhere at random, the dots on a [world map](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-world-map) would be sprinkled evenly over it. They are not.

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 shows a striking pattern. Most [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) and most large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) lie in long, narrow belts. The longest of all runs round the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): from New Zealand up through Indonesia, the Philippines and Japan, across to Alaska, then all the way down the west [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the Americas to the tip of Chile. Because of its many [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano), it is often called the “Ring of Fire”.

**Example 8.6 (Counting on the map).**

We counted the dots that lie in the shaded ring (within 500 kilometres of its middle line). Of the 1 214 [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano), 837 are in the ring: about 2 out of every 3. Of the 696 very large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake), 532 are in the ring: about 3 out of every 4. The ring covers only a small part of the Earth, but it holds most of its [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) and [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake).

![Volcanoes that have erupted in the last twelve thousand years or so, and the very largest earthquakes of 1976–2025. Most of them lie along a ring of island chains and mountain ranges around the Pacific Ocean (shaded). Data: Smithsonian Global Volcanism Program; U.S. Geological Survey earthquake catalogue; coastlines Natural Earth.](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/fig-edde10d8f2a7.svg)

*[Volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) that have erupted in the last twelve thousand years or so, and the very largest [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) of 1976–2025. Most of them lie along a ring of island chains and [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) ranges around the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (shaded). Data: Smithsonian Global Volcanism Program; U.S. Geological Survey [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) catalogue; coastlines Natural Earth.*

Other belts are smaller: one crosses southern Europe and Asia, from the Mediterranean [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) to the Himalaya; others run down the middle of the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), where [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) are hidden under the water. Some places, like Hawaii, have [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) far from any belt. Why the belts are where they are is a question for [Chapter 18](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#ch-g7-plate-tectonics): the answer explains the shape of the whole [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map).

## 8.4 Staying safe

Nobody can stop an [eruption](#def-g4-volcanoes-earthquakes-first-look-eruption) or an [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake). But knowing what to do saves lives.

![What to do as soon as the ground starts to shake: drop, cover and hold on.](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/fig-497edb7d039a.svg)

*What to do as soon as the ground starts to shake: drop, cover and hold on.*

**Safety.**

**In an [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake).** As soon as you feel the ground shake: **drop** onto your hands and knees; **cover** your head and neck with your arm, and crawl under a sturdy table or desk if one is near; **hold on** to it until the shaking stops. Do not run outside while the ground shakes, and do not stand in a doorway: falling glass and objects are the main danger. When it stops, leave the building calmly, as you practise in school drills.

**Near a [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano).** Scientists watch [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) that may wake up. If they warn that an [eruption](#def-g4-volcanoes-earthquakes-first-look-eruption) is coming, the authorities tell people to leave the area. Follow their instructions at once, and never go closer to watch.

![An earthquake drill: the pupils practise “drop, cover and hold on” under their tables.](https://one-course.com/images/onecourse/chapters/earth-1/g4-volcanoes-earthquakes-first-look/img-ac9a98ef4398.jpg)

*An [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) drill: the pupils practise “drop, cover and hold on” under their tables.*

## 8.5 Exercises

**Exercise 8.1 ★.**

[Lava](#def-g4-volcanoes-earthquakes-first-look-eruption), ash or gas? (a) a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of glowing molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) flowing down a slope; (b) tiny pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) blown into the sky, that fall back like grey [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation); (c) something invisible that escapes from the [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) with the ash.

**Solution of Exercise 8.1.**

(a) [Lava](#def-g4-volcanoes-earthquakes-first-look-eruption). (b) [Volcanic ash](#def-g4-volcanoes-earthquakes-first-look-eruption). (c) Gas.

**Exercise 8.2 ★.**

What is an [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake)? Does it last hours, or seconds?

**Solution of Exercise 8.2.**

An [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) is a sudden shaking of the ground, caused by [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that break and slip deep underground. It lasts seconds (a minute or two for the very largest), not hours.

**Exercise 8.3 ★.**

Put the three steps in order: hold on; drop; cover.

**Solution of Exercise 8.3.**

Drop, cover, hold on.

**Exercise 8.4 ★.**

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) on page [51](#fig-g4-volcanoes-earthquakes-first-look-map), around which [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) do most [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) lie?

**Solution of Exercise 8.4.**

Around the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Exercise 8.5 ★.**

True or false? (a) Most [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) are too small to be felt. (b) [Lava](#def-g4-volcanoes-earthquakes-first-look-eruption) stays liquid for ever. (c) When the ground shakes, the safest thing is to run outside.

**Solution of Exercise 8.5.**

(a) True: only instruments notice most of them. (b) False: [lava](#def-g4-volcanoes-earthquakes-first-look-eruption) cools and hardens into new [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (c) False: running while the ground shakes is dangerous (you can fall, and glass and objects fall from buildings); drop, cover and hold on.

**Exercise 8.6 ★.**

Look at the drawing of the [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) on page [49](#fig-g4-volcanoes-earthquakes-first-look-volcano). Where does the molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) come from? Where does it come out?

**Solution of Exercise 8.6.**

It comes from deep below the [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano), where [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) has melted. It rises up through the middle of the [volcano](#def-g4-volcanoes-earthquakes-first-look-volcano) and comes out at the top.

**Exercise 8.7 ★★.**

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), find: (a) a country of the Americas with many [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) along its whole length; (b) a group of islands 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), with [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano), that is not on the ring.

**Solution of Exercise 8.7.**

(a) Chile (Peru, Ecuador or Mexico are also right): its [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) run along its whole length, near the Pacific [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (b) 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), far from the shaded ring.

**Exercise 8.8 ★★.**

About 141 large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) happen in a year. About how many is that in a month? (A year has 12 months; give a whole number.)

**Solution of Exercise 8.8.**

$141 \div 12$ is a little less than 12 (because $12 \times 12 = 144$): about 12 large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) a month.

**Exercise 8.9 ★★.**

About 3 out of every 4 very large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake) happen in the ring around the Pacific. Out of 20 very large [earthquakes](#def-g4-volcanoes-earthquakes-first-look-earthquake), about how many would you expect in the ring? And outside it?

**Solution of Exercise 8.9.**

Out of 20, a quarter is 5, so three quarters is $3 \times 5 = 15$: about 15 in the ring, and about $20 - 15 = 5$ outside it.

**Exercise 8.10 ★★.**

During an [earthquake](#def-g4-volcanoes-earthquakes-first-look-earthquake) drill, Leo runs to stand in the classroom doorway. What should he have done instead, and why?

**Solution of Exercise 8.10.**

He should have dropped onto his hands and knees, covered his head and neck under his table, and held on to it until the shaking stopped. A doorway does not protect him from falling or flying objects, and running while the ground shakes can make him fall.

**Exercise 8.11 ★★★.**

Of the 1 214 [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano) 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), 837 are in the ring around the Pacific. How many are outside it? Is “about 2 out of 3” a good way to describe the share in the ring? (Hint: what is two thirds of 1 200?)

**Solution of Exercise 8.11.**

Outside the ring: $1214 - 837 = 377$ [volcanoes](#def-g4-volcanoes-earthquakes-first-look-volcano). A third of 1 200 is 400, so two thirds of 1 200 is 800. The ring holds 837, a little more than 800 out of about 1 200: “about 2 out of 3” is a good description.