Читать книгу Geology For Dummies - Alecia M. Spooner - Страница 71

The boundary between mantle rock and crustal rock in the lithosphere is labeled the Moho discontinuity, named after the scientist who discovered it, Andrija Mohorovičić. This boundary, illustrated in Figure 4-4, is where the composition of rocks changes from the more dense mantle rocks to the much lighter crustal rocks, which are composed primarily of silica.

The layer of crust covering the earth comes in two types: continental and oceanic. These two types of crust vary in thickness and are composed of slightly different materials:

 Continental crust: The crust that composes the continents is pretty thick. At its thinnest sections, continental crust is about 12 miles thick; at its thickest sections (where there are mountains), it is up to about 45 miles thick. The rocks that compose the continental crust are primarily granites (see Chapter 7).

 Oceanic crust: This crust, which lies under the earth’s oceans, is thin — only about 5 miles thick. This type of crust is composed of dark, dense silicate rocks such as basalt and gabbro (see Chapter 7). Oceanic crust is relatively young, being created even now from the eruption of molten rock along ridges in the sea floor (see Chapter 10).