---
title: "Ice on Earth"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 27
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 27 — Ice on Earth

The two photographs below were taken from the same place, on a lagoon in Alaska, 26 years apart. In 1990, a wide tongue of ice came down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) all the way to the water. In 2016, the ice had pulled back up the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and grown narrower; trees and bare [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) filled the space it had left. Between 1984 and 2021, satellite images show that the front of this [glacier](#def-g8-ice-on-earth-glacier), Pedersen [Glacier](#def-g8-ice-on-earth-glacier), moved back about $3.2\,\mathrm{km}$.

![Pedersen Glacier, Kenai Fjords National Park, Alaska, seen from the same place on 13 August 1990 (left, photograph by M. Tetreau) and on 30 August 2016 (right, photograph by D. Kurtz). U.S. National Park Service, public domain (via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/img-6da2659b9d39.jpg)

![Pedersen Glacier, Kenai Fjords National Park, Alaska, seen from the same place on 13 August 1990 (left, photograph by M. Tetreau) and on 30 August 2016 (right, photograph by D. Kurtz). U.S. National Park Service, public domain (via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/img-d67c1b618676.jpg)

*Pedersen [Glacier](#def-g8-ice-on-earth-glacier), Kenai Fjords National Park, Alaska, seen from the same place on 13 August 1990 (left, photograph by M. Tetreau) and on 30 August 2016 (right, photograph by D. Kurtz). U.S. National Park Service, public domain (via Wikimedia Commons).*

Ice covers about a tenth of the land on Earth today. It lies in [mountain](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), in two huge sheets over Antarctica and Greenland, on the polar [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and, hidden, inside the frozen ground of the far north. This chapter looks at each of them in turn, at how ice shapes the land, and at the times when ice covered much more of the Earth than it does now.

**You already know.**

[Snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) is one kind of [precipitation](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation): ice crystals that grow in a [cloud](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-cloud) and fall without melting ([Chapter 7](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#ch-g4-clouds-rain-snow)). Running water erodes the land, carries the pieces away and deposits them further down; a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) cuts a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)). When [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water freezes, the salt is left behind in the water around the ice ([Chapter 26](https://one-course.com/books/earth/1/en/chapter/26-the-oceans#ch-g8-the-oceans)). In physics, you learn that water expands when it freezes, so ice is less dense than liquid water, and that a body lighter than the water it pushes aside floats.

## 27.1 Glaciers

High in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), more [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls in winter than melts in summer. Year after year, new [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) buries the old [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and presses it down. The crystals pack together, the air between them is squeezed out, and after about a year the [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) has become *firn*, a grainy material between [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and ice. Buried deeper and deeper, the firn turns into hard, solid ice; for most [glaciers](#def-g8-ice-on-earth-glacier) this takes more than a hundred years.

**Definition 27.1 (Glacier).**

A *glacier* is a mass of ice, formed from [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) piled up over many years, that moves slowly downhill over the land under its own weight.

A [glacier](#def-g8-ice-on-earth-glacier) has two parts. In its upper part, the *accumulation area*, more [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls each year than melts: the [glacier](#def-g8-ice-on-earth-glacier) gains ice. In its lower part, the *ablation area*, more ice melts each year than [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls: the [glacier](#def-g8-ice-on-earth-glacier) loses ice. The ice gained above keeps flowing down to replace the ice lost below. If the two balance, the front of the [glacier](#def-g8-ice-on-earth-glacier), its *snout*, stays in the same place.

![A valley glacier seen in a long section (a sketch, not to scale). Above the dashed line, snow piles up faster than it melts; below it, the ice melts faster than snow falls. The ice flows down from one part to the other.](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-dc345f26c9d4.svg)

*A [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) [glacier](#def-g8-ice-on-earth-glacier) seen in a long section (a sketch, not to scale). Above the dashed line, [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) piles up faster than it melts; below it, the ice melts faster than [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls. The ice flows down from one part to the other.*

How can solid ice flow? Under the weight of the ice above it, the ice deep in a [glacier](#def-g8-ice-on-earth-glacier) slowly deforms, the way a thick layer of cold honey spreads. The [glacier](#def-g8-ice-on-earth-glacier) also slides over the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below it, on a thin film of water or on wet [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). The bottom of the [glacier](#def-g8-ice-on-earth-glacier), rubbing on the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), moves more slowly than its top. Most [glaciers](#def-g8-ice-on-earth-glacier) move slowly and steadily; a few suddenly *surge*. In 1986, Hubbard [Glacier](#def-g8-ice-on-earth-glacier) in Alaska surged forward about $10\,\mathrm{m}$ a day, and the fastest surge on record, Kutiah [Glacier](#def-g8-ice-on-earth-glacier) in Pakistan in 1953, averaged about $112\,\mathrm{m}$ a day for three months.

When a [glacier](#def-g8-ice-on-earth-glacier) loses more ice below than it gains above, its snout moves back up the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform): the [glacier](#def-g8-ice-on-earth-glacier) *retreats*. A retreating [glacier](#def-g8-ice-on-earth-glacier) still flows downhill, like a conveyor belt; it just melts faster than it flows. Muir [Glacier](#def-g8-ice-on-earth-glacier), in Alaska, filled its whole [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) down to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) in 1941. By 2004, its snout had moved back $12\,\mathrm{km}$, and the ice surface had come down by more than $800\,\mathrm{m}$.

## 27.2 How ice shapes a valley

A [glacier](#def-g8-ice-on-earth-glacier) is a powerful tool of [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion). Its ice freezes onto blocks of loose [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and plucks them away; the stones frozen into its base scrape the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below like sandpaper. The [glacier](#def-g8-ice-on-earth-glacier) carries all this [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), from fine dust to huge boulders, down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) like a conveyor belt, and drops it where the ice melts.

![The same mountain valley in cross-section, before and after a glacier (a sketch). The ice scours the narrow river valley into a wide trough with a broad floor and steep sides, and leaves ridges of rock debris along its edges.](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-f969482044a9.svg)

*The same [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) in cross-section, before and after a [glacier](#def-g8-ice-on-earth-glacier) (a sketch). The ice scours the narrow [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) into a wide trough with a broad floor and steep sides, and leaves ridges of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) debris along its edges.*

**Definition 27.2 (U-shaped valley and moraine).**

A *U-shaped valley* is a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with a broad, flat floor and steep sides, carved by a [glacier](#def-g8-ice-on-earth-glacier) that widened and deepened a narrow V-shaped [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). A *moraine* is a ridge of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) debris, from dust to boulders, left by a [glacier](#def-g8-ice-on-earth-glacier): a *lateral moraine* along its side, a *terminal moraine* at its end.

[Moraines](#def-g8-ice-on-earth-glacial-landforms) are useful clues. A terminal [moraine](#def-g8-ice-on-earth-glacial-landforms) far down a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), with no ice anywhere near it, shows how far the [glacier](#def-g8-ice-on-earth-glacier) once reached; a lateral [moraine](#def-g8-ice-on-earth-glacial-landforms) high on a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side shows how thick the ice once was. A large boulder resting alone on a field, made of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) found nowhere nearby, is another clue: it is an *erratic*, a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) carried far from its home by a [glacier](#def-g8-ice-on-earth-glacier) and left behind when the ice melted. Erratics range from [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) to blocks larger than a house.

**In the field — Reading a valley shaped by ice.**

On a walk in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), or in a photograph of a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), look at the shape of the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) in cross-section: a narrow V, or a wide U with steep walls? Look across the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor for long ridges of jumbled stones and soil running along its sides or across it: they may be [moraines](#def-g8-ice-on-earth-glacial-landforms). Look for big boulders that do not match the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) around them, and for smooth [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) surfaces scratched by long parallel lines. Stay on the paths, keep away from the edges of steep slopes and from the ice itself, and draw what you see in your notebook.

## 27.3 Ice sheets, ice shelves and icebergs

The largest masses of ice on Earth are not in [mountain](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): they cover whole lands. The Antarctic [ice sheet](#def-g8-ice-on-earth-ice-sheet) spreads over almost $14$ million $\mathrm{km}^{2}$ and is nearly $4.9\,\mathrm{km}$ thick at its thickest. The Greenland [ice sheet](#def-g8-ice-on-earth-ice-sheet) covers about $1.7$ million $\mathrm{km}^{2}$, four-fifths of the island, and is over $3\,\mathrm{km}$ thick. Together they hold more than 99 % of the ice on land, and more than two-thirds of all the [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) on Earth.

**Definition 27.3 (Ice sheet, ice shelf and iceberg).**

An *ice sheet* is a mass of ice covering more than $50\,000\,\mathrm{km}^{2}$ of land, like a huge dome that spreads outwards in all directions. Where ice flowing off the land reaches the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and goes on floating on it, still joined to the ice behind it, it forms an *ice shelf*. An *iceberg* is a large block of ice, more than $5\,\mathrm{m}$ across, that has broken off a [glacier](#def-g8-ice-on-earth-glacier) or an ice shelf and floats in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

![The edge of an ice shelf near Adelaide Island, off the Antarctic Peninsula, seen from an aircraft on 28 October 2012. The thick, wrinkled white ice on the right is the ice shelf; on the left, broken pieces of thinner sea ice float on the dark water. NASA, Operation IceBridge (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/img-0bd32d427352.jpg)

*The edge of an [ice shelf](#def-g8-ice-on-earth-ice-sheet) near Adelaide Island, off the Antarctic Peninsula, seen from an aircraft on 28 October 2012. The thick, wrinkled white ice on the right is the [ice shelf](#def-g8-ice-on-earth-ice-sheet); on the left, broken pieces of thinner [sea ice](#def-g8-ice-on-earth-sea-ice) float on the dark water. NASA, Operation IceBridge (public domain, via Wikimedia Commons).*

The water locked in the [ice sheets](#def-g8-ice-on-earth-ice-sheet) came from the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): it evaporated, fell as [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and piled up on land. If all the ice of Greenland melted, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) would rise by about $7.4\,\mathrm{m}$; if all the ice of Antarctica melted, by about $58\,\mathrm{m}$. All the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) [glaciers](#def-g8-ice-on-earth-glacier) of the world, more than $200\,000$ of them, would add less than half a metre.

![How much the sea would rise if each kind of land ice melted completely (values from the National Snow and Ice Data Center; the glaciers’ value is an upper limit, “just under half a metre”).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-89e0627694a9.svg)

*How much the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) would rise if each kind of land ice melted completely (values from the National Snow and Ice Data Center; the [glaciers](#def-g8-ice-on-earth-glacier)’ value is an upper limit, “just under half a metre”).*

Floating ice is different. An [ice shelf](#def-g8-ice-on-earth-ice-sheet) or an [iceberg](#def-g8-ice-on-earth-ice-sheet) already floats on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): it has already pushed aside its own weight of water. When it breaks up or melts, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) does not rise. But an [ice shelf](#def-g8-ice-on-earth-ice-sheet) holds back the [glaciers](#def-g8-ice-on-earth-glacier) behind it, like a cork in a bottle; when the Larsen B [ice shelf](#def-g8-ice-on-earth-ice-sheet) in Antarctica broke apart in 2002, the [glaciers](#def-g8-ice-on-earth-glacier) behind it sped up, some to eight times their former speed, and poured more land ice into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**In the lab — Floating ice and ice on land.**

Indoors, put a small stone in each of two clear glasses so that it sticks out above the water when you fill them. In the first glass, float a few ice cubes on the water. In the second, balance the same number of ice cubes on the stone, out of the water. Fill both glasses with water to the same level and mark it with a felt pen. Wait until all the ice has melted, then compare the water levels with the marks. Which glass behaves like the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) when an [iceberg](#def-g8-ice-on-earth-ice-sheet) melts, and which like the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) when an [ice sheet](#def-g8-ice-on-earth-ice-sheet) melts?

## 27.4 Sea ice

[Sea ice](#def-g8-ice-on-earth-sea-ice) is something else again: it is the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) itself, frozen. Each winter the surface of the polar [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) freezes over a vast area; each summer much of this ice melts again. In the Arctic, the ice is smallest at the end of the summer, in September, and largest at the end of the winter, in March. Satellites have measured its *extent*, the area of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) where at least 15 % of the surface is covered with ice, every day since the late 1970s.

**Definition 27.4 (Sea ice).**

*Sea ice* is frozen [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water: it forms, grows and melts in the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) itself. It is different from [icebergs](#def-g8-ice-on-earth-ice-sheet) and ice shelves, which float on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) but were made of [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) fallen on land.

[Sea ice](#def-g8-ice-on-earth-sea-ice) matters far beyond the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). Open [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water is dark: it reflects only about 6 % of the sunlight that reaches it and absorbs the rest. [Sea ice](#def-g8-ice-on-earth-sea-ice) reflects 50 to 70 %, and [sea ice](#def-g8-ice-on-earth-sea-ice) covered with [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) up to 90 %. When the ice melts, the dark water underneath absorbs more of the Sun’s energy and warms, which melts more ice.

![The extent of the Arctic sea ice each September, 1979–2025, and the straight line that best follows the points. Data: NSIDC Sea Ice Index, version 4 (F. Fetterer and others, 2025).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-5d89d8f8ef51.svg)

*The extent of the Arctic [sea ice](#def-g8-ice-on-earth-sea-ice) each September, 1979–2025, and the straight line that best follows the points. Data: NSIDC Sea Ice Index, version 4 (F. Fetterer and others, 2025).*

The points go up and down from one year to the next: the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) of each summer counts. But over the years they clearly come down. To see the change through the ups and downs, scientists draw a *trend line*, the straight line that passes as close as possible to all the points.

**Method 27.5 (Reading a trend on a graph).**

1. Read the axes: what is measured, in which unit, over which years.
2. Look at the points: is there a general rise, a general fall, or neither, behind the ups and downs?
3. Read the trend line at the first year and at the last year.
4. Take the difference, and divide by the number of years (or of decades) between them: this is the average change per year (or per decade).
5. Do not read too much into one year: a single point far from the line is [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) , the line is the trend.

**Example 27.6 (The Arctic’s September ice).**

On the graph, the trend line is at about $7.6$ million $\mathrm{km}^{2}$ in 1979 and about $4.1$ million $\mathrm{km}^{2}$ in 2025. The change is $7.6 - 4.1 =
3.5$ million km$^2$ in $2025 - 1979 = 46$ years, that is 4.6 decades: $3.5 \div 4.6 \approx 0.76$. The September [sea ice](#def-g8-ice-on-earth-sea-ice) has shrunk by about $0.76$ million $\mathrm{km}^{2}$ per decade: in 46 years, the trend line has lost almost half of its 1979 value. The lowest point, $3.57$ million $\mathrm{km}^{2}$ in 2012, sits well below the line: that summer was an exceptional one.

Measurements show that the summer [sea ice](#def-g8-ice-on-earth-sea-ice) of the Arctic has become both smaller and thinner since the 1980s, as the Arctic has warmed. [Chapter 28](https://one-course.com/books/earth/1/en/chapter/28-the-earths-climate-past-and-present#ch-g9-climate-past-and-present) looks at how the Earth’s [climate](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) is changing, and why.

## 27.5 Permafrost

In the far north, the cold reaches deep into the ground. Under the tundra of North America, Greenland and northern Asia, the soil and [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) stay frozen all year round, and have often been frozen for thousands of years. Only a thin top layer, the *active layer*, thaws each summer and freezes again each winter; plants grow in it, and water that cannot drain into the frozen ground below lies in pools on the surface.

**Definition 27.7 (Permafrost).**

*Permafrost* is ground, soil or [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), that stays at or below $0{}^{\circ}\mathrm{C}$ for at least two years in a row.

[Permafrost](#def-g8-ice-on-earth-permafrost) lies under about $23$ million $\mathrm{km}^{2}$, a quarter of the land of the [northern hemisphere](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), and under about 85 % of Alaska. Frozen ground full of ice is as hard as [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock); when its ice melts, the ground turns to [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and sinks. Roads crack, buildings tilt, and pipelines can break: where it crosses [permafrost](#def-g8-ice-on-earth-permafrost), the Trans-Alaska Pipeline runs above the ground for $676\,\mathrm{km}$. Thawing [permafrost](#def-g8-ice-on-earth-permafrost) also lets the remains of plants frozen in it for thousands of years decay, and release carbon dioxide and methane into the air.

![Left: a house tilting on thawing permafrost in the tundra (AI-generated illustration of a generic scene). Right: the ground under the tundra (a sketch, not to scale).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/img-e009c25c0ef4.jpg)

![Left: a house tilting on thawing permafrost in the tundra (AI-generated illustration of a generic scene). Right: the ground under the tundra (a sketch, not to scale).](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-de2f92d0711e.svg)

*Left: a house tilting on thawing [permafrost](#def-g8-ice-on-earth-permafrost) in the tundra (AI-generated illustration of a generic scene). Right: the ground under the tundra (a sketch, not to scale).*

## 27.6 The ice ages

Ice has not always covered the same part of the Earth. About $20\,000$ years ago, [ice sheets](#def-g8-ice-on-earth-ice-sheet) covered much of North America and of northern Europe, including Scandinavia. [Glaciers](#def-g8-ice-on-earth-glacier) then covered about a quarter of the land, against about a tenth today. So much water was locked up in this ice that the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) stood more than $120\,\mathrm{m}$ lower than today: parts of today’s continental shelves were dry land. From about $15\,000$ years ago the [ice sheets](#def-g8-ice-on-earth-ice-sheet) melted back, and the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) rose; it reached roughly its present level about $8000$ years ago.

**Definition 27.8 (Ice age).**

An *ice age* is a long cold time in the Earth’s history when [ice sheets](#def-g8-ice-on-earth-ice-sheet) spread far beyond the polar regions and cover large parts of the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent). The last ice age was at its coldest about $20\,000$ years ago and ended about $12\,000$ years ago.

The clues of the last [ice age](#def-g8-ice-on-earth-ice-age) are everywhere in the lands it covered: [U-shaped valleys](#def-g8-ice-on-earth-glacial-landforms) far from any [glacier](#def-g8-ice-on-earth-glacier) today, terminal [moraines](#def-g8-ice-on-earth-glacial-landforms) across [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), erratic boulders lying on fields. And it was not the only one. Over the last few hundred thousand years, the [ice ages](#def-g8-ice-on-earth-ice-age) and the warmer times between them have followed one another again and again; we live in one of the warm times. [Chapter 28](https://one-course.com/books/earth/1/en/chapter/28-the-earths-climate-past-and-present#ch-g9-climate-past-and-present) tells how scientists read this long story in the ice of Antarctica itself.

## 27.7 Exercises

**Exercise 27.1 ★.**

Using the long section of a [glacier](#def-g8-ice-on-earth-glacier) in this chapter, name: (a) the part of the [glacier](#def-g8-ice-on-earth-glacier) where it gains ice; (b) the part where it loses ice; (c) the front of the [glacier](#def-g8-ice-on-earth-glacier); (d) the ridge of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) debris at its end.

**Solution of Exercise 27.1.**

(a) The accumulation area. (b) The ablation area. (c) The snout. (d) A (terminal) [moraine](#def-g8-ice-on-earth-glacial-landforms).

**Exercise 27.2 ★.**

True or false? Correct the false sentences. (a) A [glacier](#def-g8-ice-on-earth-glacier) is made of frozen [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water. (b) [Snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) takes about a year to become firn. (c) When a [glacier](#def-g8-ice-on-earth-glacier) retreats, its ice flows uphill. (d) [Sea ice](#def-g8-ice-on-earth-sea-ice) is smallest in the Arctic in September.

**Solution of Exercise 27.2.**

(a) False: a [glacier](#def-g8-ice-on-earth-glacier) is made of [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) fallen on land and turned into ice; frozen [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) water is [sea ice](#def-g8-ice-on-earth-sea-ice). (b) True. (c) False: its ice still flows downhill; the snout moves back because the ice melts faster than it flows. (d) True.

**Exercise 27.3 ★.**

Give the word for each description: (a) a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with a broad floor and steep sides, carved by ice; (b) a boulder carried far from its home by a [glacier](#def-g8-ice-on-earth-glacier); (c) a floating plate of ice still joined to the ice on land; (d) ground that stays frozen for at least two years in a row.

**Solution of Exercise 27.3.**

(a) A [U-shaped valley](#def-g8-ice-on-earth-glacial-landforms). (b) An erratic. (c) An [ice shelf](#def-g8-ice-on-earth-ice-sheet). (d) [Permafrost](#def-g8-ice-on-earth-permafrost).

**Exercise 27.4 ★.**

(a) Which two lands are covered by [ice sheets](#def-g8-ice-on-earth-ice-sheet) today? (b) Which of the two would raise the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) more if all its ice melted, and by how much? (c) What share of the land on Earth is covered by ice today?

**Solution of Exercise 27.4.**

(a) Antarctica and Greenland. (b) Antarctica, by about $58\,\mathrm{m}$ (Greenland: about $7.4\,\mathrm{m}$). (c) About a tenth (10 %).

**Exercise 27.5 ★★.**

Use the graph of the Arctic [sea ice](#def-g8-ice-on-earth-sea-ice) in September. (a) Read the extent in 1979 and in 2012. (b) By how many million km$^2$ did it shrink between these two years? (c) What fraction of the 1979 extent had gone in 2012? Round to a simple fraction. (d) Why is it more reliable to compare the trend line than two single years?

**Solution of Exercise 27.5.**

(a) About $7.05$ million $\mathrm{km}^{2}$ in 1979 and $3.57$ million $\mathrm{km}^{2}$ in 2012. (b) $7.05 - 3.57 = 3.48$: about $3.5$ million $\mathrm{km}^{2}$. (c) $3.48 \div 7.05 \approx 0.49$: about half. (d) A single year depends on the [weather](https://one-course.com/books/earth/1/en/chapter/1-watching-the-weather#def-g1-watching-the-weather-weather) of that summer: 2012 is far below the line. The trend line uses all the years, so one unusual summer changes it very little.

**Exercise 27.6 ★★.**

Muir [Glacier](#def-g8-ice-on-earth-glacier)’s snout moved back $12\,\mathrm{km}$ between 1941 and 2004. (a) How many years is that? (b) What was its average retreat per year, in metres? Round to the nearest ten metres. (c) Pedersen [Glacier](#def-g8-ice-on-earth-glacier) moved back $3.2\,\mathrm{km}$ between 1984 and 2021. What was its average retreat per year? (d) Which [glacier](#def-g8-ice-on-earth-glacier) retreated faster?

**Solution of Exercise 27.6.**

(a) $2004 - 1941 = 63$ years. (b) $12000 \div 63 \approx 190.5$: about $190\,\mathrm{m}$ a year. (c) $2021 - 1984 = 37$ years; $3200 \div 37 \approx
86$: about $86\,\mathrm{m}$ a year. (d) Muir [Glacier](#def-g8-ice-on-earth-glacier), more than twice as fast.

**Exercise 27.7 ★★.**

Explain, in a few sentences and in order, how [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falling high on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) becomes the ice of a [glacier](#def-g8-ice-on-earth-glacier)’s snout, far down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). Use the words *firn*, *accumulation area*, *flows* and *ablation area*.

**Solution of Exercise 27.7.**

In the *accumulation area*, high on the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), more [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls each year than melts. New [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) buries and presses the old [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), which turns into *firn* after about a year, then, deeper down and much later, into solid ice. Under its own weight the ice *flows* slowly down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). Lower down, in the *ablation area*, it melts faster than [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) falls; the ice that reaches the snout is ice that formed high up, often long ago.

**Exercise 27.8 ★★.**

Two glasses are filled to the same level. In glass A, ice cubes float on the water; in glass B, the same ice cubes rest on a stone above the water. (a) After the ice has melted, in which glass has the level risen? (b) Which glass is a model of an [iceberg](#def-g8-ice-on-earth-ice-sheet) melting in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)? Of an [ice sheet](#def-g8-ice-on-earth-ice-sheet) melting on land? (c) Why does the breaking up of an [ice shelf](#def-g8-ice-on-earth-ice-sheet) still matter for the level of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)?

**Solution of Exercise 27.8.**

(a) In glass B: the melted ice runs into the water and raises it; in glass A the level stays where it was. (b) Glass A is the [iceberg](#def-g8-ice-on-earth-ice-sheet) (ice already floating in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean)); glass B is the [ice sheet](#def-g8-ice-on-earth-ice-sheet) (ice on land). (c) An [ice shelf](#def-g8-ice-on-earth-ice-sheet) holds back the [glaciers](#def-g8-ice-on-earth-glacier) behind it. When it breaks up, these [glaciers](#def-g8-ice-on-earth-glacier) speed up and pour more land ice into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and that raises it.

**Exercise 27.9 ★★.**

A walker crosses a wide [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with a flat floor and very steep sides. In the middle of a field she finds a granite boulder as big as a car; all the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) are limestone. Higher up, a long ridge of jumbled stones runs along the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side. (a) What shape is the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), and what carved it? (b) How did the boulder get there? What is it called? (c) What is the ridge, and what does its height on the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side tell you?

**Solution of Exercise 27.9.**

(a) A [U-shaped valley](#def-g8-ice-on-earth-glacial-landforms), carved by a [glacier](#def-g8-ice-on-earth-glacier). (b) A [glacier](#def-g8-ice-on-earth-glacier) carried it from far up the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), where there is granite, and left it when the ice melted: it is an erratic. (c) A lateral [moraine](#def-g8-ice-on-earth-glacial-landforms): its height shows how thick the [glacier](#def-g8-ice-on-earth-glacier) once was; the ice reached at least that high on the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side.

**Exercise 27.10 ★★.**

The Antarctic [ice sheet](#def-g8-ice-on-earth-ice-sheet) would raise the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) by about $58\,\mathrm{m}$ if it all melted, and the Greenland [ice sheet](#def-g8-ice-on-earth-ice-sheet) by about $7.4\,\mathrm{m}$. (a) What would a tenth of the Greenland ice add? (b) What would a hundredth of the Antarctic ice add? (c) How many times more does Antarctica hold than Greenland? Round to the nearest whole number.

**Solution of Exercise 27.10.**

(a) $7.4 \div 10 = 0.74\,\mathrm{m}$. (b) $58 \div 100 = 0.58\,\mathrm{m}$. (c) $58 \div 7.4 \approx 7.8$: about 8 times more.

**Exercise 27.11 ★★★.**

The mean extent of the Arctic [sea ice](#def-g8-ice-on-earth-sea-ice) in September was $7.21$ million $\mathrm{km}^{2}$ over the years 1980–1989 and $4.65$ million $\mathrm{km}^{2}$ over the years 2010–2019. (a) One million is $10^6$. Write both values in km$^2$, without the word “million”. (b) By how much did the mean extent fall between these two decades? (c) The two decades are 30 years apart. What is the fall per decade? Compare it with the trend line’s $0.76$ million $\mathrm{km}^{2}$ per decade. (d) Why can a straight trend line not be extended forever into the future?

**Solution of Exercise 27.11.**

(a) $7.21 \times 10^6 = 7\,210\,000\,\mathrm{km}^{2}$ and $4.65 \times 10^6 =
4\,650\,000\,\mathrm{km}^{2}$. (b) $7.21 - 4.65 = 2.56\text{ million }\mathrm{km}^{2}$, that is $2\,560\,000\,\mathrm{km}^{2}$. (c) $2.56 \div 3 \approx 0.85$: about $0.85$ million $\mathrm{km}^{2}$ per decade, a little more than the trend line’s $0.76$ million $\mathrm{km}^{2}$; each decade mean still carries the ups and downs of its own ten summers, but both ways of reading the graph show the same large loss. (d) The extent cannot fall below zero, and a trend line only describes the years that were measured: what happens next depends on how the Arctic’s [climate](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) changes, so the line is not a forecast.

**Exercise 27.12 ★★★.**

About $20\,000$ years ago the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) stood about $120\,\mathrm{m}$ lower than today. Off a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the [continental shelf](https://one-course.com/books/earth/1/en/chapter/26-the-oceans#def-g8-the-oceans-seafloor) slopes down very gently, by $1\,\mathrm{m}$ for every $1\,\mathrm{km}$ out to [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (exercise data). (a) How far out to [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) was the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) at that time? (b) Where did the water of the missing $120\,\mathrm{m}$ go? (c) Explain why a terminal [moraine](#def-g8-ice-on-earth-glacial-landforms) found today on a [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), far from any [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), is a clue to the same [ice age](#def-g8-ice-on-earth-ice-age).

**Solution of Exercise 27.12.**

(a) $120 \div 1 = 120$: the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) was about $120\,\mathrm{km}$ further out to [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (b) It had evaporated from the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), fallen as [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) on the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) and stayed there as the ice of the great [ice sheets](#def-g8-ice-on-earth-ice-sheet). (c) A terminal [moraine](#def-g8-ice-on-earth-glacial-landforms) forms at the end of a [glacier](#def-g8-ice-on-earth-glacier). One on a [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) far from any [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) shows that an [ice sheet](#def-g8-ice-on-earth-ice-sheet) once reached that [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), at a time when ice covered much more land than today.

## 27.8 Problem: A glacier’s long retreat

**Problem 27.1.**

Weekend problem — the front of a mountain glacier has been measured every ten years since 1960; its speed of retreat is worked out, and the valley it leaves is read

Every ten years since 1960, a team has measured the position of the snout of [Glacier](#def-g8-ice-on-earth-glacier) G, a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) [glacier](#def-g8-ice-on-earth-glacier) in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). The distance is measured up the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) from a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) marker R, painted on the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor at the snout in 1960. A hut H stands beside the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor, $1980\,\mathrm{m}$ up the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) from R. The [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) shows the snout at each measurement and the ridge just below the 1960 snout.

| year | 1960 | 1970 | 1980 | 1990 | 2000 | 2010 | 2020 |
| --- | --- | --- | --- | --- | --- | --- | --- |
| distance of the snout from R (m) | 0 | 180 | 330 | 520 | 700 | 880 | 1080 |

![](https://one-course.com/images/onecourse/chapters/earth-1/g8-ice-on-earth/fig-87179951e68b.svg)

**Part I — The snout’s positions.**

1. How far did the snout move back between 1960 and 2020?
2. Work out how far it moved back in each decade. In which decade did it move back the least? The most?
3. Has the [glacier](#def-g8-ice-on-earth-glacier) stopped flowing downhill? Explain.
4. The dark ridge just below the 1960 snout is made of jumbled [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) debris. What is it called, and how did it form?

**Part II — The speed of retreat.**

5. What was the average retreat per decade between 1960 and 2020?
6. And per year?
7. Muir [Glacier](#def-g8-ice-on-earth-glacier) moved back $12\,\mathrm{km}$ in 63 years, about $190\,\mathrm{m}$ a year. How many times faster was that than [Glacier](#def-g8-ice-on-earth-glacier) G?
8. Did [Glacier](#def-g8-ice-on-earth-glacier) G gain more ice in its accumulation area than it lost in its ablation area, or less? Explain.

**Part III — The valley it leaves.**

9. What shape would you expect the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor between R and the 2020 snout to have in cross-section? Why?
10. In 2020, how far was the snout from the level of the hut?
11. At the average speed of Part II, how many years would the snout need to reach the level of the hut? In which year would that be?
12. Give one reason why this year is not a forecast.

**Solution of Problem 27.1.**

**1.** $1080 - 0 = 1080\,\mathrm{m}$.

**2.** 1960–1970: $180\,\mathrm{m}$; 1970–1980: $150\,\mathrm{m}$; 1980–1990: $190\,\mathrm{m}$; 1990–2000: $180\,\mathrm{m}$; 2000–2010: $180\,\mathrm{m}$; 2010–2020: $200\,\mathrm{m}$. The least in 1970–1980, the most in 2010–2020.

**3.** No: a retreating [glacier](#def-g8-ice-on-earth-glacier) still flows downhill. Its snout moves back because the ice melts faster than it arrives.

**4.** A terminal [moraine](#def-g8-ice-on-earth-glacial-landforms): the ice carried [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) debris down the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and dropped it at its snout, where the ice melted, while the snout stood there.

**5.** $1080 \div 6 = 180\,\mathrm{m}$ per decade.

**6.** $180 \div 10 = 18\,\mathrm{m}$ a year.

**7.** $190 \div 18 \approx 10.6$: about 11 times faster.

**8.** Less: it lost more ice in its ablation area than it gained in its accumulation area, so its snout moved back.

**9.** U-shaped: a broad floor with steep sides, because the [glacier](#def-g8-ice-on-earth-glacier) widened and deepened the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) while it filled it.

**10.** $1980 - 1080 = 900\,\mathrm{m}$.

**11.** $900 \div 18 = 50$ years: in **2070**.

**12.** The speed changed from decade to decade (from 150 to $200\,\mathrm{m}$), and the future retreat depends on the snowfall and the summer temperatures of the years to come: a past rate is not a forecast.