Unit 2.15, Exercise 1, Page 66.0:01
Standing on solid ground.0:15
Our planet is made up of four main layers.0:28
The center is called the core and is the hottest part of the planet.0:31
It is divided into two layers, the inner core and the outer core.0:36
Around the core is a layer of liquid rock known as the mantle.0:42
The final layer is a thin layer of solid rock known as the crust.0:47
It is thin compared to the other parts, but in reality, that still means it is between 10 and 45 km thick.0:53
It is not one uniform surface, but is broken up into many smaller independent pieces called plates.1:03
Like a cracked eggshell or a jigsaw, powerful currents in the mantle cause the plates to move around constantly.1:11
The plates usually move at a rate of 1-10 cm a year.1:21
But this tiny movement has incredible effects.1:26
Where the plates meet.1:30
The towering Himalayan mountains were formed by two plates pushing into each other.1:32
And the fiery volcanoes of Iceland are the result of two plates moving apart.1:38
Earthquakes, however, tend to occur at the third kind of boundary, where plates slip alongside each other.1:45
This is because two plates never slide past each other smoothly.1:54
The rocks catch on each other so that the plates are still pushing.1:59
Pressure gradually builds up until the rocks can no longer stand the pressure and they suddenly give way.2:04
This movement travels to the surface in waves that shake the ground.2:12
Most earthquakes are so tiny that we don't even feel them.2:18
Small earthquakes are useful because they release the dangerous pressure between the plates.2:22
Every few years, however, enough pressure builds up to produce a large earthquake, often with catastrophic results for human life and buildings in the affected area.2:29
Afterwards, the plates start moving once more, and then until they get stuck again.2:42
And so the whole process starts over.2:46
Such earthquakes are nature's reminder that we don't stand on solid ground.2:50
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