© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2005 1 of 32 KS4 Physics Earth’s Structure.

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 32

KS4 Physics

Earth’s Structure


Earth’s Structure

Contents

Earth and earthquakes

Seismic waves

Continental drift

Summary activities

More about plate boundaries


crust

mantle

outer core

inner core

Structure of the Earth


The Earth’s crust is made of sections called tectonic plates. Where two plates meet is called a plate boundary.

Tectonic plates

Convection currents in the mantle plates move these tectonic plates, which can move towards each other, move away from each other and or move past each other.


plate A

plate B

Two plates can slide past each other, but this doesn’t always happen smoothly. The large friction forces involved mean that strain builds up and the plates move suddenly when the strain gets too much.

This sudden movement of plates is called an earthquake and it can be very destructive.

Plate boundaries 1 – sliding plates


Plate boundaries 1 – conservative boundary


Why do earthquakes happen?


Features of an earthquake


outer core

crust

mantle

inner core

Which part of this cross-section of the Earth should each label point to?

Cross-section of the Earth – labels


plate A plate B

An earthquake has occurred in the area shown in this picture.

If the plates moved in the directions indicated by the arrows, what would the location would look like after the earthquake?

Effect of an earthquake


Earth’s Structure

Contents


Seismic waves

Continental drift

Summary activities



During an earthquake there are two types of seismic waves that are released from the epicentre. These are called primary waves (P waves) and secondary waves (S waves).

Primary or P waves are the faster type of seismic waves. They are longitudinal waves and when they hit the Earth’s surface they make objects and buildings vibrate vertically. P waves can travel through solids and liquids.

Secondary or s-waves are the slower type of seismic waves. They are transverse waves and when they hit the Earth’s surface they make objects and buildings vibrate horizontally. S waves can only travel through solids.

Primary and secondary waves


P-waves arrive first and shake buildings vertically.

This causes little damage.

S-waves arrive next and shake buildings horizontally.

This can cause a lot of damage.

Effects of different seismic waves


Waves within a layer change direction gradually because there is a gradual change in density.

S wave shadow - only P waves are detected in this region.

Seismic waves inside the Earth

earthquakefocus

At a layer boundary there is a big change in direction because there is a big change in density between layers.


epicentre

primary waves

secondary waves

The faster type of seismic wave, that is longitudinal and makes buildings vibrate up and down.

The location where the shift in plates occurred producing seismic waves.

The slower type of seismic wave, that is transverse and makes buildings shake from side to side.

Words about seismic waves


P wave S wave

Type of wave

Speed

Media wave can travel through

Damage caused on Earth’s

surface

Comparing seismic waves

Complete this table comparing the properties of the two types of seismic waves.


outer core

mantle

wave A wave B

1. What type of wave is wave A? How can you tell this?

2. Explain the path of wave B in terms of density.

3. What is the S wave shadow?

Questions about seismic waves


Earth’s Structure

Contents


Seismic waves

Continental drift

Summary activities



What happens at a plate boundary where an oceanic plate and a continental plate collide?

The thinner, more dense oceanic plate is driven down into the mantle.

convection currentconvection current

melted oceanic plate

This is known as subduction.

thicker, less dense continental plate thinner, more dense

oceanic plate

Plate boundaries 2 – colliding plates


Plate boundaries 2 – destructive boundary


convection current

convection current

oceanic plate oceanic plate

What happens at a plate boundary when oceanic plates move apart due to convection currents in the Earth’s mantle?

Magma moves up to the

surface, cools and forms new oceanic plate.

This is known as sea-floor spreading.

Plate boundaries 3 – plates moving apart


Plate boundaries 3 – constructive boundary


subduction

sea-floor spreading

friction

The force that causes earthquakes.

When an oceanic plate is pushed down into the mantle as it collides with a continental plate.

When two oceanic plates move apart to create new crust.

Words about plate boundaries


Earth’s Structure

Contents


Seismic waves

Continental drift

Summary activities



It is thought that all the continents were once joined together in a huge supercontinent called Pangaea.

Over millions of years the continents drifted apart because the tectonic plates they were on moved apart due to convection currents in the Earth’s mantle.

What is continental drift theory?


Continental drift theory animation


1. The shapes of the continents fit like a jigsaw.

continent A continent B

Evidence for continental drift theory

There are a number of pieces of evidence to support the theory of continental drift:

2. The rock strata on separated continents are identical.

3. The fossil records on separated continents are identical.


Earth’s Structure

Contents


Seismic waves

Continental drift

Summary activities



Glossary (1)

continental drift – The theory that millions of years ago all the continents formed a huge supercontinent and have been slowly drifting apart ever since.

core – The central region of the Earth below the mantle. The outer part is liquid and the inner part is solid.

crust – The thin, rocky, outer layer of the Earth.

earthquake – A sudden movement of the Earth’s crust, which causes vibrations that travel through the Earth.

mantle – The layer of the Earth, between the crust and the core, which is mostly solid rock that can flow very slowly.

P waves – These are the fastest type of seismic waves. They are longitudinal waves and can travel through solids and liquids.


Glossary (2)

plate boundary – An area where two tectonic plates meet.

S waves – These are the slowest type of seismic waves. They are transverse waves and can only travel through solids.

sea-floor spreading – This occurs under oceans where plates move apart and magma rises to the surface and cools to form new crust.

seismic waves – Vibrations that travel through the Earth as a result of an earthquake.

subduction – This occurs where plates move together and one of the plates is driven down into the mantle.

tectonic plates – Large sections of the Earth’s crust and upper mantle that move slowly over the surface of the Earth.


Anagrams


Multiple-choice quiz

© Boardworks Ltd 2004 1 of 20 © Boardworks Ltd 2005 1 of 32 KS4 Physics Earth’s Structure.

Documents

secondary waves slide

primary waves p waves

secondary waves s waves

longitudinal waves

transverse waves

earthquake slide

earth slide

types of seismic waves