Chemistry - Earth's Crust (AQA Syllabus A)

In GCSE Science students will look at the chemical changes which have taken, and do take place on the Earth and in its atmosphere. This is the first of three quizzes on that topic and it looks specifically at the Earth's core and mantle, and at the plates which form the Earth's crust.

The Earth has a layered structure, from the core at its centre, then the mantle above the core, and finally the outer layer, the crust which is divided into several tectonic plates. The crust provides a lot of the raw materials that are used to make things we use and take for granted every day, such as metals, stones and oil. The surface of the Earth is slowly changing but these geological changes are slow, taking tens of thousands of years to become noticeable.

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The changes are caused by plate tectonics and weathering. As mentioned before, the Earth's crust is broken into large plates. These plates move around and this movement is thought to be caused by convection currents in the next layer down, the mantle. In some places, like the middle of the Atlantic Ocean, plates move apart and new material erupts from below to fill the gap. In some places they collide head on, creating mountain ranges like the Andes, the Himalayas and the Alps. Here plates are destroyed as they are forced back into the mantle, creating devastating earthquakes and volcanoes. In other places, they slide past each other causing earthquakes but not volcanoes, like the San Andreas fault in the USA.

Moving plates create earthquakes because rocks don't move smoothly past each other. They stick for a while and when enough tension has built up, they suddenly release which is the earthquake. Earthquakes that occur under the sea can cause a tsunami - a fast moving powerful wave of water that can travel a long way inland causing a lot of damage and loss of life. The plates move very slowly, at about the rate your fingernails grow, which is perhaps why scientists took so long to recognise and understand how it all happened.

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  1. Which of the layers of the Earth is the hottest?
    Since the core is at the very centre of the Earth, it is well insulated and heat can only escape very slowly
  2. What is thought to cause the plates to move around?
    The mantle has the properties of a solid, but can flow very slowly
  3. When seen in cross section, the core accounts for about how much of the Earth?
    The core accounts for about 3,500km of the 6,300km from surface to centre
  4. Alfred Wegener suggested that the continents moved around on the surface of the Earth. Which of the following is not one of the observations he used?
    The fossil plants Wegener used as evidence for his theory were found in Antarctica, India and Australia so he thought that the continents had once been joined in a supercontinent that came to be known as Pangaea
  5. Wegener put forward his theory early in the twentieth century but scientists at the time refused to accept his idea until the 1960s. Why?
    One respected geologist said:
    "If we are to believe in Wegener's hypothesis we must forget everything which has been learned in the past 70 years and start all over again."
    It turned out that he was absolutely right but by the time the theory was accepted it was more like forgetting everything that had been learnt in the last 120 years than the last 70!
  6. Wegener used the similarities between fossil animals and plants as evidence for his theory. How did his critics explain it?
    Scientists can almost always find alternative explanations for anything. Sometimes these explanations are better than existing ideas, sometimes worse. The idea of convenient land bridges that have disappeared without trace seems to us even more difficult to believe than the idea that the continents move
  7. What was the main piece of evidence that convinced scientists that continental drift was right?
    The fastest plate is the Indian plate which 'rushed' northwards from near the south pole to where it is today at the massive speed of six centimetres per year
  8. Read the following passage. Choose the answer that has the missing words in the correct order.

    Where the _______ meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. __________ and volcanic eruptions happen at the _________ between plates, and the crust may ‘crumple’ to form __________ ranges.
    The heat that powers plate tectonics is thought to come from natural radioactivity deep in the Earth
  9. Why is it so difficult to predict when, where and how strong an Earthquake will be?
    There is a lot of friction between the rocks of moving plates so they actually move in a series of 'jerks'
  10. Why do major earthquakes and volcanoes occur in very narrow and well defined areas?
    Earthquakes are recorded all over the world but the big destructive ones only occur where the tectonic plates meet. Where one plate is being pushed underneath another, you get volcanoes too

Author: Kev Woodward

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