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EQ1: Why are some locations more at risk from tectonic hazards? GapFill

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Earth’s core is hot.  Some of that heat is left over from when Earth was .  It just hasn’t escaped yet.  Some is generated by friction as material moves towards the core, and some isotopes release heat as they decay.

Alfred Wegener, in 1912, published his theory that the was made up of fragments which had moved around over time.  He wasn’t able to provide a scientific process to back up his theory so it didn’t catch on immediately.  He and others noticed that, give or take a bit of erosion, the continents seemed to fit together, and Wegener noticed patterns in the types of rock and across the continents, meaning that the land on either side of oceans must have, at some point in time, been joined together.

In the 1940s, spreading was proposed as a mechanism for this continental drift, helping to validate Wegener’s theory.  Plates move apart at ocean ridges, and lava fills the gap and solidifies to rock.  It’s called pillow lava.  The iron in the rock aligns to Earth’s field as it cools, and stays that way.  Periodically throughout geological history, Earth’s magnetic field has switched polarity.  This means that bands of rock with alternating polarity are spread out on either side of mid-ocean .

The original theory of plate movement focused on currents.  Hotter parts of the mantle rise to the surface, hit the crust, spread out and sink.  As this happens, pressure is exerted, and the plates move .

More recently, pull is thought to be a cause, whereby dense oceanic plates are pulled down (subducted) at boundaries, pulling the whole plate down.

Sometimes, volcanoes are found away from plate boundaries.  These areas are called , and they’re formed when a plume of hot magma melts through the crust and forms a volcano.  The plume stays in the same place, but the plate moves across it.  The volcanoes become inactive, and there are gaps as the plume takes a while to melt through again.  This is why exists as a chain of islands.

There are several types of shockwave caused by earthquakes.  The waves are primary waves (P waves) – they form compressions and expansions, causing the ground to .  Immediately afterwards are the secondary waves (S waves) – they move up and down and cause the shaking.  At the surface are two types of surface waves.  Love waves shake side to side, and Rayleigh waves are similar to ocean waves – they have a rolling motion.

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