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Environmental Science
Nick Eyles

What is inside the Earth? It was the study of seismic refraction and seismic reflection that enabled scientists to plot the three main zones of the Earths interior. - The crust is the outer layer of rock, which forms a thin skin on Earths surface. o Below the crust lies the mantle, a thick shell of rock that separates the crust above from the core below. The core is the central zone of earth. It is probably metallic and the source of earths magnetic field. The Crust: Studies of seismic waves have shown: 1. That the crust is thinner beneath the oceans than beneath the continents. 2. That seismic waves travel faster in oceanic crust than in continental crust. Because of this velocity differences, it is assumed that the two types of crust are made up of different kinds of rock. Seismic P waves travel through oceanic crust at about 7km per second, which is also the speed at which they travel through basalt and gabbro (the coarse-grained equivalent of basalt) . Samples of rocks taken from the sea floor by oceanographic ships verify that the upper part of the oceanic is basalt and suggest that the lower part is gabbro. The oceanic crust averages 7 km in thickness, varying from 5 to 8 km. Seismic P waves travel more slowly through continental crustabout 6 km per second, the same speed at which they travel through granite and gneiss. Continental crust is often called granitic, but the term should be put in quotation marks because most of the rocks exposed on land are not granite. The continental crust is highly variabl
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