homework encyclopedia

36 Earth

Planet Earth is a dense, rocky ball about 12,750 km

(7,920 miles) across. It is one of eight planets circling our

local star, the Sun. The Earth is the only planet we know

that supports life. From space, its surface looks blue

and cool, but inside it is so hot that rock can melt.

IS THE EARTH THE SAME ALL THE WAY THROUGH?

Our planet is made of four main layers. The outer

layer is called the crust. Below is the mantle,

which is solid near the top and molten

below. Temperatures become hotter

near the Earth’s centre. The outer

core is a mass of molten rock.

The temperature of the inner

core is up to 4,700°C (8,492°F).

HOW WAS THE EARTH FORMED?

The Sun began to form around 5,000 million years

ago out of a cloud of whirling dust and gas in space.

As it formed, the Sun’s gravity gradually pulled the

dust and gas into lumps, which became the planets.

At first, the Earth was a ball of molten rock. Its

surface rock slowly began to cool and harden

around 4,200 million years ago.

1 EARTH’S STRUCTURE

The crust is made up of oceanic

crust, below the oceans, and

continental crust, which carries the Earth’s

land. It is the Earth’s thinnest layer, at only

6–70 km (4–43 miles) thick. The mantle, at 2,900

km (1,800 miles) deep, is the thickest layer. The outer

core is around 2,000 km (1,240 miles) thick. The inner core at

the centre of the Earth is about 2,740 km (1,702 miles) across.

YOUNG PLANET

Around 4,500 million years ago,

the young Earth was bombarded

from space by huge rocky

meteorites and asteroids. Molten

(liquid) rock from the Earth’s fiery

centre erupted from thousands

of volcanoes. As the Earth slowly

began to cool, clouds of gas and

water vapour escaped from cracks

in the crust (the planet’s thinner,

outer layer) and formed the first

atmosphere. The water vapour

eventually formed the oceans.

EARTH’S ATMOSPHERE

The Earth’s atmosphere is a

layer of gases about 700 km

(430 miles) thick. Without the

atmosphere there could be no

life on the Earth. It protects us

from harmful rays in sunlight,

and prevents the Earth from

becoming too hot or too cold.

PLANET EARTH

Solid inner core

of white-hot

nickel and iron

Atmosphere

Crust

Molten

outer core

Mantle

Planet Earth