Page 39 - Laboratory Manual in Physical Geology

Global Isostasy Granite (light-colored, coarse-grained igneous rock)
The average elevation of the continents is about 0.84 km and basalt (dark-colored, fine-grained igneous rock) make

above sea level (+0.84 km), but the average elevation of up nearly all of Earth’s crust. Basaltic rocks form the crust
the ocean basins is 3.87 km below sea level (-3.87 km). of the oceans, beneath a thin veneer of sediment. Granitic

Therefore the difference between the average continental rocks form the crust of the continents, usually beneath a
and ocean basin elevations is 4.71 km! If the continents did thin veneer of sediment and other rock types. Therefore,
not sit so much higher than the floor of the ocean basins, you can think of the continents (green and brown) in

then Earth would have no dry land and there would be no FIGURE 1.11A as granitic islands surrounded by a low sea
humans. What could account for this elevation difference? of basaltic ocean crust (blue). All of these rocky bodies rest
One clue may be the difference between crustal granite and on mantle rock called peridotite . Could differences among
basalt in relation to mantle peridotite. the three rock types making up the outer edge of Earth’s
geosphere explain its bimodal global topography?

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