4 . 原文填空
The Earth is made up of three layers. In the centre is the core, which is as big as the Mars and has a high 1 of over 7,000˚C. 2 the core is a thick layer called the mantle, 3 lower part is solid rock, 4 portion is molten rock called the magma. Floating on top of the magma is the Earth’s rocky crust.
The crust is made up of tectonic plates. These huge plates are moved slowly by magma currents below, sliding past each other with 5 . If a plate is forced down 6 another and gets 7 , pressure will build up until the stressed rocks suddenly 8 apart, causing 9 ground movements called earthquakes. The point where the rocks suddenly jolt or break is called the epicentre, from which, powerful shock waves 10 out in all directions.