Continental And Oceanic Crust

On the basis of information presented in this and following chapters, the major differences between continental and oceanic crust can be summarized as follows:

1 Layering. The large-scale layering of the continental crust is ill defined and highly variable, reflecting a complex geologic history. In places there is a broad subdivision by the

Conrad discontinuity, but this is not globally developed. By contrast, the layering of the majority of oceanic crust is well defined into three distinct layers. However, the nature of these layers, in particular layers 2 and 3, may change quite markedly with depth.

2 Thickness. The thickness of continental crust averages 40 km but is quite variable, thinning to only a few kilometers beneath rifts and thickening to up to 80 km beneath young mountain belts. Most oceanic crust has a remarkably constant thickness of about 7 km, although layer 1, the sedimentary layer, increases in thickness towards ocean margins that are not characterized by ocean trenches. Differences in the thickness and the creep strength (Section 2.10.4) of continental crust make the lower crust of continental regions much more likely to deform pervasively than in the lower layers of oceanic crust (Section 2.10.5).

3 Age. Continental crust is as at least as old as 4.0 Ga, the age of the oldest rocks yet discovered (Section 11.1). On a very broad scale the oldest crust consists of Precambrian cratons or shield areas that are surrounded by younger orogenic belts, both active and inactive. Oceanic crust, however, is nowhere older than 180 Ma, and progressively increases in age outwards from oceanic ridges (Section 4.1). Oceans are consequently viewed as essentially transient features of the Earth's surface. About 50% of the surface area of the present day ocean floor has been created during the last 65 Ma, implying that 30% of the solid Earth's surface has been created during the most recent 1.5% of geologic time.

4 Tectonic activity. Continental crust may be extensively folded and faulted and preserves evidence of being subjected to multiple tectonic events. Oceanic crust, however, appears to be much more stable and has suffered relatively little deformation except at plate margins.

5 Igneous activity. There are very few active volcanoes on the great majority of the continental crust. The only major locations of activity are mountain belts of Andean type (Section 9.8). The activity within the oceans is very much greater. Ocean ridges and island arcs are the location of the Earth's most active areas of volcanic and plutonic activity. Oceanic islands are a third distinct, but less prolific oceanic setting for igneous activity.

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