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Earth^39

Dry land covers just under one-third of the Earth’s surface. It is made

up mostly of seven huge landmasses called continents, plus many smaller

islands. The largest continent, Asia, is 43,608,000 sq km (16,838,365 sq

miles). The smallest, Australia, is 7,686,850 sq km (2,967,892 sq miles).

WHAT DO TECTONIC PLATES DO?

As the plates slowly drift across the planet’s surface,

they may slide past each other, grind against one

another, pull apart, or do all three. The boundary

between two tectonic plates is called a plate margin.

Mountains, earthquakes, and volcanoes usually occur

at plate margins where the Earth’s crust is thinner

than in the centre of the plates.

WHY DO THE EARTH’S CONTINENTS MOVE?

The continents (and oceans) rest on top of giant slabs

called. TECTONIC PLATES which make up the

Earth’s outer crust. These plates float like rafts on the

hot, semi-liquid mantle below the crust. Slow-moving

currents deep inside the Earth send the plates (and the

land or ocean that rests on them) slowly moving

across the surface of the planet.

HAVE THE EARTH’S CONTINENTS

ALWAYS LOOKED THE SAME?

The world looked very different millions of years ago,

when all the continents were joined in one huge block

of land, or supercontinent, called Pangaea. This was

surrounded by a vast ocean, called Panthalassa.

Over millions of years, Pangaea split into smaller

continents, which drifted across the Earth’s surface.

WHAT HAPPENS WHERE PLATES MEET?

When plates carrying continents collide, the land may

crumple up and form a massive mountain range. If one

plate is forced under the other, oceanic crust sinks into

the mantle and melts. Where plates pull apart, molten

rock rises up from inside the Earth. This cools and adds

new material to the plates.

The Earth’s outer crust is split into seven large tectonic plates

and about twelve smaller ones. Studying plate tectonics (plate

movement) helps scientists to work out why earthquakes

strike and volcanoes erupt and how mountains form.

4 MOVING CONTINENTS

Pangaea slowly split into two

huge landmasses – Laurasia and

Gondwanaland. Later, as the

Atlantic Ocean was formed, these

broke up into separate, smaller

continents. Over millions of years,

the continents slowly drifted to

their present positions and are

still moving today.

1 WORLD-SIZED JIGSAW
The Earth’s tectonic plates fit together like the pieces of a giant jigsaw.
Oceanic plates make up most of the sea floor. The Earth’s continents are
embedded in continental plates. Some plates carry both land and sea.

TECTONIC PLATES

1 PLATE BOUNDARIES

Volcanoes and earthquakes are common in zones of active plate

movement. The plates are still drifting very slowly, at around

2–20 cm (1–8 in) a year.

Continents

Long mountain chains

buckle up along fault

lines where two

plates collide

Subduction zone

where a denser oceanic plate

sinks below continental crust

on adjoining plate

Transform fault

(a crack in the

surface rock) forms

where plates slide

past one another

Mid-ocean ridge

(undersea mountain

range) forms when

Ocean trench plates pull apart

forms where

one plate sinks

below another

Molten rock

rises from the

mantle, creating

new plate material

220 MILLION YEARS AGO 180 MILLION YEARS AGO 65 MILLION YEARS AGO

South

America

PANGAEA

Atlantic

Ocean

North

America

GONDWANALAND

LAURASIA

Africa

Australia

India

Antarctica

Europe

Asia

ALFRED LOTHAR WEGENER

German, 1880-1930

Scientist Alfred Wegener put

forward the idea of drifting

continents in 1919. He

noticed that the shapes

of continents fitted together

like a giant jigsaw puzzle,

which suggested they had

once been joined. His ideas

were not generally accepted

until the 1960s.

FIND OUT MORE. Earthquakes 43 • Mountains 45 • Rocks 46–47 • Volcanoes 44

continents