Maps and place knowledge, Year 10: natural hazards and plate margins

people

提示Ask who, if anyone, is in the way.

为什么An earthquake, a flood or a storm is a natural event. It is called a natural hazard when it could cause death, injury, damage or disruption. The same event in an empty place is not a hazard.

Hazard risk

提示Two words. The second is about probability, not about damage already done.

为什么Risk depends on the event and on the people. It rises with how often and how strongly the event strikes, how many people live there, and how well they can prepare and cope. Two places hit by the same event can face very different risks.

tectonic

提示The same adjective describes the moving slabs that make up the Earth's surface.

为什么Tectonic hazards come from movement of the Earth's plates. Hazards caused by the weather, such as tropical storms, droughts and heatwaves, form a second group, usually called atmospheric or weather hazards.

It has less money for strong buildings, warning systems and emergency services

提示Think about what wealth can pay for before and after the ground shakes.

为什么Wealth is one of the main factors in hazard risk. A richer country can enforce building rules, train rescue teams and equip hospitals. A poorer one often cannot, so more buildings collapse and help arrives more slowly.

More people and buildings are packed into the place it hits

提示Compare one storm crossing empty farmland with the same storm crossing a crowded district.

为什么Urbanisation concentrates people, homes, roads and businesses in a small area. A single event therefore affects far more lives and far more property. Risk is highest where housing is poorly built or on unsafe land such as steep slopes and floodplains.

It is denser

提示Compare a small pebble with a large block of polystyrene in water.

为什么Oceanic crust is thin, about 5–10 km, but made of heavy rock. Continental crust is much thicker but lighter. When two plates converge, the heavier one goes under.

slab pull

提示The first word means a thick flat block; the second is the opposite of 'push'.

为什么As the edge of a plate sinks into the mantle, its weight tugs the whole plate after it. Research shows this is the main force moving most plates. A second force, ridge push, acts where new, raised crust at a mid-ocean ridge slides away under gravity.

Convection currents in the mantle

提示Warm air above a radiator rises, cools and sinks in the same way.

为什么Rock in the mantle is solid but flows very slowly over millions of years. Hot rock rises, spreads, cools and sinks again. For a long time these flows were thought to carry the plates along; they are now seen as one of several forces, with slab pull the most important.

centimetres

提示About the speed at which your fingernails grow.

为什么The movement is far too slow to feel, but it never stops. Over millions of years it opens oceans and builds mountain ranges. Stress builds where plates stick, and is released suddenly as an earthquake.

Pacific

提示It is the largest of them all, lying between Asia and the Americas.

为什么The plates beneath this ocean are being subducted under the plates around it almost all the way round. That gives a nearly continuous line of destructive margins, with volcanoes and powerful earthquakes from New Zealand through Japan to the Andes.

A conservative plate margin (boundary)

提示Its name suggests that nothing is gained and nothing is lost.

为什么At this kind of margin the plates move sideways past each other, or in the same direction at different speeds. No magma is formed, so there are no volcanoes. The plates stick, then slip, so earthquakes can be severe.

subduction

提示It starts with the same three letters as 'submarine'.

为什么The sinking plate heats up and partly melts. The magma formed is less dense than the rock around it, so it rises and can erupt as a volcano. Friction between the two plates also causes very powerful earthquakes.

Neither plate is subducted, so no magma is formed

提示Both slabs are too light to go down. So what never gets made?

为什么Continental crust is too light to sink into the mantle. Instead the rocks crumple and are pushed upwards into fold mountains, as in the Himalayas. Without a sinking, melting plate there is no supply of magma.

The plates lock together, pressure builds, then they slip suddenly

提示Push two rough bricks past each other. Do bricks glide smoothly?

为什么Friction holds the two plates still while the rest of each plate keeps moving. Stress builds for years until the rocks give way and jerk past each other. The energy stored is released at once as seismic waves.

It is thick and sticky, so gas is trapped in it

提示Think of blowing bubbles through treacle instead of through water.

为什么At a constructive margin the magma is runny, so gas escapes easily and lava flows out gently, building low shield volcanoes. At a destructive margin the magma is viscous. Pressure builds until it blasts out as ash and rock, building steep composite volcanoes.

trench

提示The same word names the long ditches dug by soldiers in the First World War.

为什么The trench marks the line where the plate begins to sink. These are the deepest places in the oceans. Finding one beside a line of volcanoes is good evidence of a destructive margin.