---
title: "Plate Tectonics: The Moving Puzzle"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 18
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 18 — Plate Tectonics: The Moving Puzzle

Spread a [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 world on a table and look at the two shores of the Atlantic [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). The great bulge of Brazil seems made to slide into the hollow of Africa’s west [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), as if someone had cut a single piece of land in two and pushed the halves apart. The first people to print good [maps](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of both [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) noticed it at once. For three centuries it remained a curiosity. In the twentieth century it became the starting point of the theory that now explains why the ground shakes in Japan, why the Andes are so high, why Iceland is slowly growing wider — and why 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 the table is only a snapshot of a world that is moving.

![South America next to Africa: on a puzzle, the fit is a game; on the real Earth, it is a clue.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/img-5dd6f6a439de.jpg)

*South America next to Africa: on a puzzle, the fit is a game; on the real Earth, it is a clue.*

**You already know.**

The Earth has three main layers: a thin, rocky *[crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers)*, a thick *[mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers)* of hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), and a metallic *[core](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers)*; temperature rises with depth ([Chapter 16](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#ch-g6-inside-the-earth)). A place on the [globe](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-globe) is located by its [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) and its [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) ([Chapter 14](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#ch-g6-latitude-longitude)).

## 18.1 Continents that once fitted together

**Definition 18.1 (Continental drift).**

*Continental drift* is the idea that the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) move slowly across the surface of the Earth, so that their positions, and the shapes of the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) between them, have changed over the ages.

**Definition 18.2 (Supercontinent).**

A *supercontinent* is a single landmass made of all, or almost all, of the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) joined together. The last one, *Pangaea* (“all lands” in Greek), began to break up between about 225 and 200 million years ago.

The scientist who defended [continental drift](#def-g7-plate-tectonics-continental-drift) as a real theory, with evidence, was Alfred Wegener. He gathered four kinds of clues.

1. **The shapes of the [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).** South America and Africa fit together like two pieces of a jigsaw, and so do several other coastlines.
2. **Fossils.** The same fossil animals and plants are found on [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) that are now separated by thousands of kilometres of [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) . *Mesosaurus* was a small reptile that lived in [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) : it could not have swum across an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) , yet its fossils lie both in Brazil and in southern Africa.
3. **[Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).** [Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the same kinds and ages, and old [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) chains of the same shape, stop at one [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and start again on the opposite one, as the lines of print on a torn newspaper do.
4. **Ancient [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate).** Coal, made from the plants of warm, swampy forests, is found in Antarctica; traces left by ancient glaciers are found in parts of Africa that are now hot and dry. These lands must once have lain in other [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) , at other [latitudes](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) .

![The southern continents put back together. Each coloured band joins the places where one fossil species is found today: the land reptiles Cynognathus and Lystrosaurus, the freshwater reptile Mesosaurus and the fern Glossopteris. Separated, the continents scatter the bands; joined, the bands run on without a break. Map: U.S. Geological Survey, This Dynamic Earth (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/img-c85ca9dad517.jpg)

*The southern [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) put back together. Each coloured band joins the places where one fossil species is found today: the land reptiles *Cynognathus* and *Lystrosaurus*, the freshwater reptile *Mesosaurus* and the fern *Glossopteris*. Separated, the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) scatter the bands; joined, the bands run on without a break. [Map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map): U.S. Geological Survey, *This Dynamic Earth* (public domain).*

**History — Wegener’s idea, 1912–1930.**

Alfred Wegener, a 32-year-old German meteorologist, set out the theory of [continental drift](#def-g7-plate-tectonics-continental-drift) in two articles in 1912. Most geologists rejected it. The strongest objection was that Wegener could not say *what* moved the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent): he thought they ploughed through the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, and the physicist Harold Jeffreys showed that solid [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) could not do that without breaking up. Wegener went on collecting evidence until he died on the Greenland ice cap in 1930, during an expedition. The answer he lacked came from the floor of the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), explored after the 1950s: it is told in a later year.

## 18.2 The plates

Wegener’s clues were right, but his [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) were the wrong pieces. It is not only the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) that move: the whole outer shell of the Earth, under the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) and under the [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) alike, is broken into pieces.

**Definition 18.3 (Tectonic plate).**

A *tectonic plate* (or simply a plate) is a large, rigid slab of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that forms part of the Earth’s outer shell. A plate is made of the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) and of the uppermost, cold and rigid part of the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers), together at least 80 kilometres thick on average; it moves slowly over the hotter and softer [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) beneath it.

**Definition 18.4 (Plate tectonics).**

*Plate tectonics* is the theory that the outer shell of the Earth is divided into [plates](#def-g7-plate-tectonics-plate) that move relative to one another, and that most [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), [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 [mountain ranges](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) are produced where [plates](#def-g7-plate-tectonics-plate) meet.

**Remark 18.5 (How many plates?).**

About a dozen [plates](#def-g7-plate-tectonics-plate) are large; many more are small. A detailed [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) used by geologists counts 52 of them. A [plate](#def-g7-plate-tectonics-plate) may carry a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), or both: the South American [plate](#def-g7-plate-tectonics-plate) carries the whole of South America and the western half of the South Atlantic [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). The Pacific [plate](#def-g7-plate-tectonics-plate), the largest, carries almost no land at all.

![The main tectonic plates. Red: plates moving apart; blue with triangles: one plate sinking under the other (the triangles are on the plate that stays on top); grey: plates pushing together across a wide zone of mountains; dark red: plates sliding past each other. Boundaries after P. Bird (2003); coastlines from Natural Earth.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-0795e186b27c.svg)

*The main [tectonic plates](#def-g7-plate-tectonics-plate). Red: [plates](#def-g7-plate-tectonics-plate) moving apart; blue with triangles: one [plate](#def-g7-plate-tectonics-plate) sinking under the other (the triangles are on the [plate](#def-g7-plate-tectonics-plate) that stays on top); grey: [plates](#def-g7-plate-tectonics-plate) pushing together across a wide zone of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform); dark red: [plates](#def-g7-plate-tectonics-plate) sliding past each other. Boundaries after P. Bird (2003); coastlines from Natural Earth.*

How do we know where one [plate](#def-g7-plate-tectonics-plate) ends and the next begins? Look at where the Earth is restless.

**Proposition 18.6 (Earthquakes and volcanoes trace the plate boundaries).**

[Earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) and [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 not scattered at random over the Earth: most of them lie along narrow belts, and these belts are the edges of the [plates](#def-g7-plate-tectonics-plate). Of the 7 031 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of magnitude 6 or more recorded between 1976 and 2025, 6 472 — that is 92 % — happened less than $300\,\mathrm{km}$ from a [plate boundary](#def-g7-plate-tectonics-plate-boundary).

![Large earthquakes and recent volcanoes. Compare with the map of the plates above: the dots and the triangles draw the plate boundaries. Data: U.S. Geological Survey earthquake catalogue; Smithsonian Global Volcanism Program.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-c09072b12cf3.svg)

*Large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) and recent [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). Compare with 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 [plates](#def-g7-plate-tectonics-plate) above: the dots and the triangles draw the [plate boundaries](#def-g7-plate-tectonics-plate-boundary). Data: U.S. Geological Survey [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) catalogue; Smithsonian Global Volcanism Program.*

## 18.3 Three kinds of plate boundaries

**Definition 18.7 (Plate boundary).**

A *plate boundary* is the line, or the narrow zone, where two [plates](#def-g7-plate-tectonics-plate) meet. According to how the two [plates](#def-g7-plate-tectonics-plate) move with respect to each other, a boundary is of one of three kinds:

- at a *divergent boundary* the two [plates](#def-g7-plate-tectonics-plate) move apart;
- at a *convergent boundary* the two [plates](#def-g7-plate-tectonics-plate) move towards each other;
- at a *transform boundary* the two [plates](#def-g7-plate-tectonics-plate) slide past each other, side by side.

**Example 18.8 (Divergent boundaries: Iceland and East Africa).**

Down the middle of the Atlantic [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) runs a chain of undersea [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the Mid-Atlantic Ridge, where the [plates](#def-g7-plate-tectonics-plate) on either side move apart. Hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) from the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) rises into the gap, melts in part, and becomes new [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor: a [divergent boundary](#def-g7-plate-tectonics-plate-boundary) builds new [plate](#def-g7-plate-tectonics-plate). Iceland is a piece of this ridge that rises above the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): there, North America moves westward and Eurasia eastward, and the island is slowly torn open. Between 1975 and 1984, the cracks near the Krafla [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), in the north of the island, widened by about 7 metres in all. A [divergent boundary](#def-g7-plate-tectonics-plate-boundary) can also cut through a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent): along the East African Rift, Africa is beginning to split, and the Red [Sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is a young [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) that has already separated Arabia from Africa.

**Example 18.9 (Convergent boundaries: the Andes and the Himalaya).**

Off the west [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of South America, the Nazca [plate](#def-g7-plate-tectonics-plate), which carries [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), moves towards the South American [plate](#def-g7-plate-tectonics-plate), which carries a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent). The [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate), heavier, bends and sinks beneath the [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), deep into the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers). Along the line where it goes down, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor plunges into a deep trench; above the sinking [plate](#def-g7-plate-tectonics-plate), the land is shaken by [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) and lined with [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 Andes. In the western Pacific, where one [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate) sinks under another, the trenches are the deepest places on Earth: the Mariana Trench is nearly $11\,000\,\mathrm{m}$ deep. When two [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) meet, neither sinks: the [crust](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) crumples and thickens. India collided with Asia about 50 million years ago, and the two [plates](#def-g7-plate-tectonics-plate) are still pushing; that is how the Himalaya and the Tibetan Plateau were raised.

**Example 18.10 (A transform boundary: the San Andreas fault).**

In California, the Pacific [plate](#def-g7-plate-tectonics-plate) slides north-westwards past the North American [plate](#def-g7-plate-tectonics-plate) along the San Andreas fault, about $1300\,\mathrm{km}$ long. No [plate](#def-g7-plate-tectonics-plate) is made or destroyed there; the two [plates](#def-g7-plate-tectonics-plate) grind past each other, and the jolts of that grinding are [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake).

![Above: a cut through two plate boundaries (not to scale; the depths are much exaggerated). At the ridge the plates move apart and hot rock rises from the mantle; at the trench the ocean plate sinks beneath the continent, and the rock that melts above it feeds volcanoes. Below: a transform boundary seen from above.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-37040b2450d4.svg)

![Above: a cut through two plate boundaries (not to scale; the depths are much exaggerated). At the ridge the plates move apart and hot rock rises from the mantle; at the trench the ocean plate sinks beneath the continent, and the rock that melts above it feeds volcanoes. Below: a transform boundary seen from above.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-e42a14387c21.svg)

*Above: a cut through two [plate boundaries](#def-g7-plate-tectonics-plate-boundary) (not to scale; the depths are much exaggerated). At the ridge the [plates](#def-g7-plate-tectonics-plate) move apart and hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) rises from the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers); at the trench the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate) sinks beneath the [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), and the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that melts above it feeds [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). Below: a [transform boundary](#def-g7-plate-tectonics-plate-boundary) seen from above.*

![The San Andreas fault crossing the Carrizo Plain, California, seen from an aeroplane: the long straight valley is the boundary between the Pacific plate (left) and the North American plate (right). Photograph: Robert E. Wallace, U.S. Geological Survey (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/img-4027fe8fc9e1.jpg)

*The San Andreas fault crossing the Carrizo [Plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), California, seen from an aeroplane: the long straight [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) is the boundary between the Pacific [plate](#def-g7-plate-tectonics-plate) (left) and the North American [plate](#def-g7-plate-tectonics-plate) (right). Photograph: Robert E. Wallace, U.S. Geological Survey (public domain).*

**Method 18.11 (Recognising a plate boundary on a map).**

To decide what kind of boundary separates two [plates](#def-g7-plate-tectonics-plate):

1. find, on a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of [plate](#def-g7-plate-tectonics-plate) motions, the arrows on either side of the boundary;
2. if they point away from the boundary, it is divergent; if they point towards it, convergent; if they are parallel to it and point in opposite directions, transform;
3. check with the [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) : a ridge in the middle of an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) or a rift across a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) goes with a [divergent boundary](#def-g7-plate-tectonics-plate-boundary) ; a deep trench with a 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) , or a high [mountain range](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) , with a convergent one; a long straight fault with neither, with a [transform boundary](#def-g7-plate-tectonics-plate-boundary) .

## 18.4 How fast do plates move?

**Proposition 18.12 (A few centimetres a year).**

Two neighbouring [plates](#def-g7-plate-tectonics-plate) move relative to each other at a speed of a few centimetres per year: from about 2 centimetres per year across the North Atlantic to more than 14 centimetres per year across the East Pacific Rise, the fastest-spreading ridge on Earth.

![How fast plates move at six boundaries: averages along each boundary computed from P. Bird’s plate model (2003).](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-0cf96fd578f0.svg)

*How fast [plates](#def-g7-plate-tectonics-plate) move at six boundaries: averages along each boundary computed from P. Bird’s [plate](#def-g7-plate-tectonics-plate) model (2003).*

**Method 18.13 (From centimetres per year to kilometres per million years).**

A kilometre is $100\,000$ centimetres. A speed of $1\,\mathrm{cm}$ per year, kept up for a million years, therefore covers $1\,000\,000$ centimetres, that is $10\,\mathrm{km}$. Hence:

$$
1\ \text{cm per year} = 10\ \text{km per million years}.
$$

Multiply a speed in centimetres per year by 10 to get kilometres per million years; then multiply by a number of millions of years to get a distance in kilometres, or divide a distance by the speed to get a time.

**Example 18.14 (The Atlantic grows).**

On average, the two sides of the Mid-Atlantic Ridge move apart at about $2.5\,\mathrm{cm}$ per year, that is $25\,\mathrm{km}$ per million years. In 100 million years that makes $2500\,\mathrm{km}$: kept up long enough, a speed far too slow to notice in a lifetime opens an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Remark 18.15 (Measuring the motion today).**

Such slow motions are now measured directly. Permanent stations fixed to the [bedrock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) receive the signals of positioning satellites and work out their own position, day after day, to a few millimetres. Over a few years, stations on different [plates](#def-g7-plate-tectonics-plate) are seen to move apart or closer together, at the speeds found from the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) over millions of years. Satellite positioning itself is the subject of [Chapter 32](https://one-course.com/books/earth/1/en/chapter/32-maps-of-the-whole-earth#ch-g9-maps-of-the-earth).

![A permanent positioning station: its antenna is bolted to the rock, so it moves with the plate.](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/img-504cfc0c42ce.jpg)

*A permanent positioning station: its antenna is bolted to the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), so it moves with the [plate](#def-g7-plate-tectonics-plate).*

## 18.5 Exercises

**Exercise 18.1 ★.**

What is [continental drift](#def-g7-plate-tectonics-continental-drift)? Give the four kinds of clues Wegener used to defend it.

**Solution of Exercise 18.1.**

[Continental drift](#def-g7-plate-tectonics-continental-drift) is the idea that the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) move slowly across the Earth’s surface, so that their positions have changed over the ages. Wegener’s clues: the fit of the coastlines (South America and Africa); the same fossils on [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) now separated by [oceans](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); the same [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and old [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) chains on either side of an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); traces of ancient [climates](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) out of place (coal in Antarctica, glacial deposits in hot, dry Africa).

**Exercise 18.2 ★.**

Name the three kinds of [plate boundary](#def-g7-plate-tectonics-plate-boundary) and say, for each, how the two [plates](#def-g7-plate-tectonics-plate) move with respect to each other.

**Solution of Exercise 18.2.**

Divergent: the [plates](#def-g7-plate-tectonics-plate) move apart. Convergent: they move towards each other. Transform: they slide past each other, side by side.

**Exercise 18.3 ★.**

Look at 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 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) and [volcanoes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano). Name two regions of the world with many large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake), and one [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) that has almost none in its middle.

**Solution of Exercise 18.3.**

For example, all around the Pacific [Ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (Japan, Indonesia, the Andes), and the belt running from the Mediterranean to the Himalaya. Almost no large [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) in the middle of Africa or of Australia (also of North America east of the Rocky [Mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), or of Brazil).

**Exercise 18.4 ★.**

Fossils of *Mesosaurus*, a small reptile that lived in [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh), are found in Brazil and in southern Africa. Why is this a clue that the two [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) were once joined?

**Solution of Exercise 18.4.**

An animal of [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh) could not cross thousands of kilometres of salt [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). Finding it on both sides means that the two places were once joined by land, in one [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), when *Mesosaurus* lived.

**Exercise 18.5 ★★.**

Coal is found in Antarctica. What does this tell us about the [climate](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) in which the coal formed, and what can we conclude about where Antarctica was at that time?

**Solution of Exercise 18.5.**

Coal forms from the plants of warm, wet, swampy forests: the [climate](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-climate) was warm and humid. Such forests cannot grow near the [South Pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles) today, so Antarctica must once have been much closer to the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator): it has moved.

**Exercise 18.6 ★★.**

Say which kind of boundary is found at each of these places: the Mid-Atlantic Ridge; the Andes; the San Andreas fault; the Himalaya; the East African Rift.

**Solution of Exercise 18.6.**

Mid-Atlantic Ridge: divergent. Andes: convergent (an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate) sinks under a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent)). San Andreas fault: transform. Himalaya: convergent (two [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) collide). East African Rift: divergent (a [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) splitting).

**Exercise 18.7 ★★.**

Convert a speed of $2.5\,\mathrm{cm}$ per year into kilometres per million years. How far apart would two points on either side of the boundary move in 60 million years at this speed?

**Solution of Exercise 18.7.**

$2.5 \times 10 = 25$ kilometres per million years; in 60 million years: $25 \times 60 = 1500\,\mathrm{km}$.

**Exercise 18.8 ★★.**

Between 1975 and 1984 — nine years — the cracks near the Krafla [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano) in Iceland widened by about $7\,\mathrm{m}$ in all. What was the average widening per year, in centimetres? Compare it with the average speed of the Mid-Atlantic Ridge, about $2.5\,\mathrm{cm}$ per year, and suggest why the two numbers are so different.

**Solution of Exercise 18.8.**

$7\,\mathrm{m} = 700\,\mathrm{cm}$, and $700 \div 9 \approx 78$: about $78\,\mathrm{cm}$ per year, some 30 times the average of $2.5\,\mathrm{cm}$ per year. The [plates](#def-g7-plate-tectonics-plate) do not slide apart smoothly at the surface: the strain builds up for years and is released in short bursts, when the cracks open by metres at once. Over long periods, quiet years and bursts average out to a few centimetres per year.

**Exercise 18.9 ★★.**

Wegener’s theory was rejected for decades. What question could he not answer? What does the theory of [plate tectonics](#def-g7-plate-tectonics-plate-tectonics) change in his picture of moving [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent)?

**Solution of Exercise 18.9.**

He could not say what force moves the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent), and the motion he imagined ([continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) ploughing through the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor) was impossible. In [plate tectonics](#def-g7-plate-tectonics-plate-tectonics), the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) do not plough through anything: they are carried, with the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor around them, by [plates](#def-g7-plate-tectonics-plate) that move as whole pieces of the Earth’s outer shell.

**Exercise 18.10 ★★.**

Look at the cut through two [plate boundaries](#def-g7-plate-tectonics-plate-boundary). Where is the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor highest, and where is it deepest? Where is new [plate](#def-g7-plate-tectonics-plate) being made, and where does a [plate](#def-g7-plate-tectonics-plate) go back into the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers)?

**Solution of Exercise 18.10.**

Highest at the ridge, deepest at the trench. New [plate](#def-g7-plate-tectonics-plate) is made at the ridge, where hot [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) rises from the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers); the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate) goes back into the [mantle](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) at the trench, where it sinks beneath the [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent).

**Exercise 18.11 ★★★.**

Across the East Pacific Rise the [plates](#def-g7-plate-tectonics-plate) move apart at about $14.2\,\mathrm{cm}$ per year, and across the North Atlantic ridge at about $2.2\,\mathrm{cm}$ per year. How many times faster is the East Pacific Rise (round to the nearest whole number)? How long does each ridge take to open $1000\,\mathrm{km}$ of new [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor?

**Solution of Exercise 18.11.**

$14.2 \div 2.2 \approx 6.45$: about 6 times faster. In kilometres per million years the speeds are 142 and 22. Times for $1000\,\mathrm{km}$: $1000 \div 142 \approx 7$ million years for the East Pacific Rise; $1000 \div 22 \approx 45$ million years for the North Atlantic ridge.

**Exercise 18.12 ★★★.**

Of the 7 031 [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) of magnitude 6 or more recorded from 1976 to 2025, 4 578 happened less than $100\,\mathrm{km}$ from the line of a [plate boundary](#def-g7-plate-tectonics-plate-boundary) drawn 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), 6 472 less than $300\,\mathrm{km}$ and 6 786 less than $500\,\mathrm{km}$. (a) How many happened between $100\,\mathrm{km}$ and $300\,\mathrm{km}$ from the line? (b) Work out, to the nearest whole number, the percentage of all these [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) that happened less than $100\,\mathrm{km}$ from the line. (c) Many of 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) between $100\,\mathrm{km}$ and $300\,\mathrm{km}$ from the line lie on the same side of a deep [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) trench. Using what happens at a [convergent boundary](#def-g7-plate-tectonics-plate-boundary), explain why an [earthquake](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) caused by a [plate boundary](#def-g7-plate-tectonics-plate-boundary) can happen so far from the line drawn 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).

**Solution of Exercise 18.12.**

(a) $6472 - 4578 = 1894$ [earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake). (b) $4578 \div 7031 \approx 0.651$, that is 65 %. (c) The line 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) marks the trench, where the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [plate](#def-g7-plate-tectonics-plate) starts to sink. The [plate](#def-g7-plate-tectonics-plate) does not go straight down: it sinks slanting beneath the other [plate](#def-g7-plate-tectonics-plate), deeper and deeper as it goes under it. [Earthquakes](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-earthquake) happen all along the sinking [plate](#def-g7-plate-tectonics-plate), so many of them lie under the overriding [plate](#def-g7-plate-tectonics-plate), on the same side of the trench, tens or hundreds of kilometres from the line. A [plate boundary](#def-g7-plate-tectonics-plate-boundary) is a zone, not a thin line.

## 18.6 Problem: Opening the South Atlantic

**Problem 18.1.**

Weekend problem — measuring an ocean on the map and finding how long it took to open

On a [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 South Atlantic, a student follows the [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) at [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) $23{}^{\circ}$ South. It leaves the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of South America near Rio de Janeiro, at [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) $43.2{}^{\circ}$ West, crosses the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and the Mid-Atlantic Ridge, and reaches the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Africa near Walvis Bay, at [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) $14.4{}^{\circ}$ East. On this [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), one degree of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) measures $102\,\mathrm{km}$. Across the ridge, at this [latitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude), the South American and African [plates](#def-g7-plate-tectonics-plate) move apart at $3.4\,\mathrm{cm}$ per year.

![](https://one-course.com/images/onecourse/chapters/earth-1/g7-plate-tectonics/fig-50cabd68ec17.svg)

**Part I — The clues of the fit.**

1. On the fossil [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) of this chapter, name the fossils whose bands cross from South America into Africa.
2. Why can the fossils of *Mesosaurus* not be explained by an animal that swam from one [continent](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) to the other?
3. Which part of South America fits into which part of Africa?
4. What kind of [plate boundary](#def-g7-plate-tectonics-plate-boundary) runs down the middle of the South Atlantic today? How do the two [plates](#def-g7-plate-tectonics-plate) move there?

**Part II — Measuring the ocean.**

5. How many degrees of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) separate the two [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) along the [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) at $23{}^{\circ}$ South?
6. What is the width of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) , in kilometres, along this [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) ? Round to the nearest ten kilometres.
7. At the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) , one degree of [longitude](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) measures $111\,\mathrm{km}$ . Why is it only $102\,\mathrm{km}$ at $23{}^{\circ}$ South?
8. The ridge runs roughly down the middle of the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) . About how far from the ridge is each [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) ?

**Part III — The age of the ocean.**

9. Convert the speed at which the two [plates](#def-g7-plate-tectonics-plate) move apart into kilometres per million years.
10. If the [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) had always opened at this speed, how many million years would it have taken to reach its present width? Round to the nearest ten million years.
11. Geologists date the break-up of [Pangaea](#def-g7-plate-tectonics-supercontinent) at between about 225 and 200 million years ago, and say that the Atlantic has grown over the last 100 to 200 million years. Does your answer agree with this?
12. Give one reason why the true age of the South Atlantic may differ from the age you found.

**Solution of Problem 18.1.**

**1.** *Mesosaurus*, *Cynognathus* and *Glossopteris* (the band of *Lystrosaurus* joins Africa, India and Antarctica).

**2.** It lived in [fresh water](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-salt-fresh): it could not cross a salt [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) thousands of kilometres wide.

**3.** The bulge of north-east Brazil fits into the hollow of the Gulf of Guinea, on Africa’s west [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**4.** A [divergent boundary](#def-g7-plate-tectonics-plate-boundary), the Mid-Atlantic Ridge: the South American [plate](#def-g7-plate-tectonics-plate) moves west and the African [plate](#def-g7-plate-tectonics-plate) east, away from each other.

**5.** The [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) are on opposite sides of the [prime meridian](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude): $43.2 + 14.4 = 57.6{}^{\circ}$.

**6.** $57.6 \times 102 = 5875.2$: about $5880\,\mathrm{km}$.

**7.** The [meridians](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-longitude) come closer together as one goes from the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator) towards a [pole](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles). The [parallel](https://one-course.com/books/earth/1/en/chapter/14-latitude-longitude-and-time-zones#def-g6-latitude-longitude-latitude) at $23{}^{\circ}$ South is a smaller circle than the [equator](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-equator), so each of its 360 degrees is shorter.

**8.** About half the width: $5880 \div 2 = 2940\,\mathrm{km}$.

**9.** $3.4 \times 10 = 34$ kilometres per million years.

**10.** $5880 \div 34 \approx 173$: about 170 million years.

**11.** Yes: 170 million years lies between 100 and 200 million years, and it comes after the start of the break-up of [Pangaea](#def-g7-plate-tectonics-supercontinent), 225 to 200 million years ago. A simple calculation on a [map](https://one-course.com/books/earth/1/en/chapter/4-from-a-drawing-to-a-map#def-g2-drawing-to-map-map) gives the right order of magnitude.

**12.** For example: the speed has not been the same all the time; the [continents](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-continent) stretched and thinned before they split, so today’s [coasts](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) are not exactly the line along which they broke apart.