Philips Atlas of the Universe

THE SOLAR SYSTEM

▲An earthquake occurs

along a fault line when the

crust on either side is being

forced to move in different

directions. The focus, where

the fault gives, can be up

to 700 km (450 miles) below

the surface. The epicentre

is the point on the surface

directly above the focus

where the damage is

usually most severe.

Seismic waves.

P waves are compression

waves that travel through

solid and fluid alike. S waves

are transverse waves that

only travel through solids.

▲ Seismic activityhas

allowed scientists to study

the inner structure of the

Earth. The crust is only

on average 10 km (6 miles)

thick beneath the oceans

and 50 km (30 miles) thick

beneath the land. Below

is the 2900-km-thick

(1800 miles) mantle of

hot, plastic rock. Inside

that is an outer liquid core,

2100 km (1300 miles) thick,

with a solid core inside it,

2700 km (1700 miles) in

diameter.

Volcanoes

Volcanoes form where tectonic plates

meet. Pockets of magma force themselves

up from the mantle through weak

points in the crust. The molten

magma may bubble inside

the crater or give off

clouds of ash and gas.

Magma may also find its way to the

surface via side vents. A volcano may be

inactive for a considerable time,

allowing the magma to solidify

near the surface. Huge pressure

can then build up beneath it,

often with devastating results.

Lower mantle

Rock

strata

Ash and

gas cloud

Neck or pipe

Lava

Cinders

Crater

Side vent

Inner core

Transition zone Outer core

Upper mantle Crust

Secondary

(S) Waves

Primary

(P) Waves

Magma

chamber

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