Plate tectonics, earthquakes and volcanoes

Last time

Before we start: last time, in Ecosystems, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

The moving Earth

The ground under our feet is moving, a few centimetres a year. Where the giant plates of the Earth's crust meet, earthquakes and volcanoes happen. This lesson covers the Earth's structure, plate boundaries, earthquakes, volcanoes, and why people live in danger zones.

The structure of the Earth

The Earth has a solid inner core and a liquid outer core, both very hot. The mantle is a thick layer of hot, slowly moving rock. The crust is the thin outer layer, broken into plates: thicker continental crust and thinner, denser oceanic crust.

Why plates move

Heat from the core makes the mantle rock rise, spread and sink in slow convection currents. These currents drag the plates above them. Earthquakes and volcanoes are found mainly in narrow belts along plate boundaries, such as the Pacific Ring of Fire.

Plate boundaries

The three main types of plate boundary and what happens at each.

Destructive boundaries

Where oceanic crust meets continental crust, the denser oceanic plate is forced down into the mantle: subduction. It melts, and the magma rises to form explosive volcanoes. Friction causes strong earthquakes. Fold mountains and deep ocean trenches also form here.

Volcanoes

Magma rises from a magma chamber through a vent and erupts from the crater as lava, ash and gases. Composite, or strato, volcanoes are steep, made of layers of ash and lava, and erupt explosively. Shield volcanoes are wide and gently sloping, with runny lava and gentle eruptions.

Earthquakes

Pressure builds as plates lock together, then releases suddenly, sending out seismic waves. The focus is the point underground where it starts; the epicentre is the point on the surface above it. Earthquakes are measured on a magnitude scale; each whole number is much stronger than the one below.

Effects and responses

Effects: collapsing buildings, deaths and injuries, fires, landslides, tsunamis, and damaged roads and water supplies. Responses: earthquake-resistant buildings, monitoring volcanoes, warning systems, evacuation plans and emergency drills. Richer countries can usually afford better protection, so fewer people die.

Why live near volcanoes?

Volcanic ash makes very fertile soil for farming. Volcanoes attract tourists, and provide geothermal energy and minerals. Many people also have family ties, or cannot afford to move, and eruptions may be rare.

Mistakes that lose marks

Mistake one: confusing the focus with the epicentre. Mistake two: saying conservative boundaries have volcanoes. Mistake three: labelling a volcano diagram without the vent and magma chamber. The fix: correct terms, correct boundary, full labels.

The full-marks tip

The full-marks tip: for each boundary, give the plate movement and the hazards it causes. When comparing volcanoes, use the headings given: shape, eruption, lava.

On one card

So, on one card: plate tectonics. Keep it for revision.