Revision Year 3 Test 2 Semester A

The 3 main types of plate boundaries/margins

Divergent or constructive margins 4-A mid-ocean ridge is formed by the new crust. 6-Earthquakes are caused by volcanic eruptions or by tensional forces. 5-Submarine volcanoes appear along the ridge. Some may grow to form islands, e.g. Iceland. Oceanic crust Oceanic crust 1-Convection currents pull plates apart. 2-This leads to gaps being formed in the crustal rocks. 3-As the plates move apart, magma from the mantle rises to fill the gaps and forms new oceanic crust.

Destructive/convergent margins: Ocean-to-continent margin 7-The collision causes folding and uplift of rocks which form fold mountains. 6-The Peru-Chile trench forms where the oceanic plate is being subducted. 5-The melting plate creates lighter magma that rises towards the surface to form volcanoes. 1-The Nazca Plate and the South American Plate move towards each other due to convection currents. Andes fold mountains Pacific Ocean Trench Volcano Nazca Plate (Oceanic) South American Plate (Continental) Mantle Earthquakes Subduction zone 4-The heat from the mantle causes the oceanic plate to be destroyed. 2-The Nazca Plate (oceanic) is being subducted below the South American Plate (continental). 3-Friction and pressure cause earthquakes to occur along the subduction zone

Destructive/convergent margin b- Continental-to-continental margin 2- This results in intense folding and uplift and leads to the formation of fold mountains. 4- However, earthquakes occur due to the collision of the plates. 1- When two continental plates meet at a destructive boundary, a slow collision takes place as both plates have a low density. 3- There is no subduction. Therefore, there are no volcanoes.

Conservative [transform] margins Plates just slide past each other with crust neither created nor destroyed. Friction builds up and energy is released when the plates ‘jerk’, causing earthquakes. The San Andreas Fault in California marks the junction of the North American and Pacific plates. Both plates are moving north-west but at different speeds. Instead of slipping smoothly past each other, they tend to ‘stick’. The pressure builds up until suddenly the plates move forward and an earthquake occurs. There is no volcanic action because the crust is not being destroyed.

Activities page 91

Activity 1 Plate movement Pushing into each other Pulling apart Sliding past each other …gives earthquakes All plate movements give earthquakes …gives volcanoes Whenever oceanic crust is pushed down into mantle Magma wells up between them No rock melts! …builds mountains Whenever rock is pushed and folded upwards Volcanoes may grow into underwater mountains No rock is pushed upwards!

Activity 3 Mountains formed by plates pushing into each other; rock at the plate edges gets folded upwards, making mountains. i) Peru is on the South American plate (continental crust), near the boundary with the Nazca plate (oceanic crust). The two plates are pushing into each other. The denser Nazca plate is diving downwards, causing earthquakes and volcanoes. Rock is getting squeezed upwards too, giving fold mountains: the Andes ii) Italy is on the Eurasian plate, but very close to the boundary with two other plates. All three plates are pushing into each other. Heavier oceanic crust is diving under the continental crust, causing earthquakes and volcanoes. Land has been folded upwards too, giving Italy’s Apennine Mountains.

Activity 4 Although the ocean floor is getting wider in places, ocean floor is being destroyed in other places, where it is diving under continental crust!

Tectonic hazards

What is a hazard? It is an event, such as earthquakes and volcanic eruptions, which has the potential to cause loss of life or damage to property.

What happens in an earthquake? Earthquakes occur when the tension is released from inside the crust. This happens because plates do not move smoothly- sometimes they get stuck. When this happens a great deal of pressure builds up. Eventually this pressure is released and an earthquake occurs.

Focus and epicentre . The seismic waves spread out from the focus. The strongest waves are found near the centre of the earthquake. This means that the most severe damage caused an earthquake will happen close to the epicenter- the point on the Earth’s surface directly above the focus.

Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismometers

The Richter scale The amount of energy released by an earthquake is called its magnitude.

What is the focus and epicentre of an earthquake? The focus of an earthquake is its central point-the point deep below the surface where the earthquake starts. Seismic waves are released and then spread in all directions. The point on the surface directly above the focus is called the epicentre. It is the place on the surface that is nearest to the focus and therefore where the greatest effects will be felt.

Questions page 93

Activity 1 When enough tension builds up in a rock it will suddenly give way, like an elastic band snapping. The result is a (sudden) earthquake.

Activity 2 Seismic waves are waves of energy given out during an earthquake; they pass through Earth in all directions and cause everything to shake. The focus of an earthquake is the point where the rock moved, setting the earthquake off. The epicentre of an earthquake is the point on Earth’s surface directly above the focus. The magnitude of an earthquake means how much energy it gives out; it is measured on the Richter scale.

Activity 4 At A, since it is nearer the epicentre. At B, since buildings can collapse and kill people. A lot more damage, since it releases a lot more energy. 30 times. At 5 am, since people will be asleep. By 10.30 am, many will have left for work and home. That does not mean they are out of danger. But they will have more chance to protect themselves if they are awake, for example by moving away from windows, and following the emergency earthquake drill.

Activity 5 Chile is near the edge of the South American plate, close to where the heavier Nazca plate is sliding under it. So Chile has volcanoes, and many earthquakes. Chile is a long, narrow country. Its cities and towns are all close to the coast. So they would be badly affected by a large earthquake at the plate edge. Chile has been quite poor in the past, so it is likely that many homes were badly built, and could not survive being shaken. The earthquake also set off a tsunami, which destroyed homes along the coast. The earthquake set off a tsunami in the Pacific Ocean which travelled across the ocean, and drowned people in Japan. (It also drowned people in Hawaii and did extensive damage in Hawaii and Japan, and some damage on the west coast of the USA.)