The Restless Earth: Tectonic Hazards

The restless Earth beneath us

The ground feels solid and still, but the outer shell of the Earth is broken into huge pieces that are slowly moving all the time. These pieces float on a hot, slowly churning layer of rock below, drifting no faster than your fingernails grow, yet over millions of years they have rearranged whole continents. Most of the time we notice nothing. But where two of these pieces meet, the strain can be released suddenly and violently, and that is where the great natural hazards of the ground are found: shaking that flattens buildings, and molten rock that bursts to the surface. This module explains why the Earth is restless, what happens where its plates meet, why the resulting hazards strike where they do, why millions still choose to live in these places, and how the danger can be reduced.

Tectonics words

Learn these terms before the features are matched up. They name the ideas used in this topic.

Match each tectonic feature to what it is

Constructive against destructive boundaries

Two of the three kinds of plate boundary do opposite things, and telling them apart explains where different hazards happen.

What measures an earthquake?

A scientist wants to record the strength of the shaking during an earthquake. Which instrument do they use?

• A seismometer
• A thermometer
• A barometer
• A rain gauge

Why people stay near the danger

It seems strange that millions of people live in the most dangerous places on the planet, but they have good reasons. The land around old eruption sites is often astonishingly fertile, giving rich harvests that feed whole regions. Hot rocks close to the surface can be tapped for clean, cheap power and for heated water. Dramatic scenery draws visitors, and the money they spend supports many jobs. Some people simply cannot afford to leave, or will not leave the home and community their family has always known. And modern building design, warning systems and drills mean the risk, while real, can be managed far better than in the past. So people weigh the clear benefits of the land against a danger they have learned to live with and to prepare for.

Pick the true facts about the restless Earth

Select every statement about tectonic hazards that is true.

• The plates of the outer shell are slowly moving
• Most hazards happen where plates meet
• People live in hazard zones for real benefits
• The outer shell of the Earth never moves
• Eruptions happen evenly all over the Earth

Order what happens in an earthquake

Put the stages of an earthquake into the order they happen.

• Strain slowly builds where two plates meet
• The rocks suddenly slip past each other
• Energy travels outwards as waves
• The ground shakes at the surface
• Buildings may be damaged or fall

Complete the tectonics facts

The rigid outer shell of the Earth is broken into large pieces called _____ plates. These pieces slowly move because of heat currents in the _____ below. Where plates suddenly slip past each other, the ground shakes in an _____. Where molten rock rises to the surface and erupts, it builds a _____.

Tap the two tectonic hazards

Read these four natural hazards. Tap the TWO that are caused by movements of the plates.

• an earthquake
• a volcanic eruption
• a river flood
• a summer heatwave

A cracked eggshell

To picture the outside of the Earth, think of a hard-boiled egg that has been rolled on the table so the shell is cracked all over. The shell is no longer one smooth piece: it is broken into many curved plates that still fit neatly together, sitting on the soft, slippery layer just beneath. Press gently and the cracked pieces can slide and grind against one another along the cracks. The Earth is like that egg. Its firm outer layer is split into great plates that rest on a softer layer below, and almost all the action happens along the cracks where those plates meet. Away from the cracks the shell is quiet; along them, pieces push, pull and scrape, and that is where the ground gives most of its trouble.

The tectonics run

Answer each one correctly to keep your lives. Choose the best answer.

Reduce the tectonic risk

Read each situation and choose the best way to reduce the risk.

• A city sits in a zone that shakes often. How should new buildings be made?
• Scientists notice a volcano is swelling and giving off more gas. What should they do?
• A school wants people ready for the next earthquake. What helps most?

Build a tectonics point

Choose the word for each gap to complete one point about the restless Earth.

Explain the restless Earth

A friend asks why the ground is dangerous in some parts of the world. Using what you have learned, explain tectonic hazards.

• Explain why the plates of the Earth move
• Explain what happens at a constructive and a destructive boundary
• Explain how an earthquake causes damage
• Explain why people still live in hazard zones
• Explain two ways the risk can be reduced