What are the two major kinds of igneous rocks? How do they differ?
Rochelle Gunby 3. 7. Why is the oceanic crust composed of gabbro and basalt? True oceans have abyssal depths (seas are comparatively shallow). The Mohorovicic discontinuity [see Chapter xx.x], which demarks lower boundary of the crust, shows that, wherever the ocean is a true ocean, the oceanic crust is a rocky layer 5 to 10 km thick. Deep sea drilling has confirmed that the oceanic crust beneath relatively thin layer of covering sediments is igneous and is basaltic in composition. Seismic studies have shown oceanic igneous crust is two layered. Remarkably in some places the entire ocean crust and mantle on which it rests has been raised above sealevel (by obduction) and the up-ended sequence of rocks (the whole called an ophiolite) is exposed in horizontal section (by erosion). These exposures show that beneath oceanic sediments, the upperpart of the oceanic igneous crust is made of basalt flows and feeder dikes and the underpart is a thicket of gabbro feeder dikes and interspersed si
