Crustal Roots??

Question

Describe the gravity related evidence for mountain chains having deep crustal roots and why should ocean basins have shallow crust beneath them

Answer 1

When mountains form by collision of lithospheric plates, uplift of the Earth's surface is accompanied by thickening of the crust, and the buoyancy of these deep crustal roots relative to the surrounding mantle is thought to contribute to the support of mountain topography.

Once active tectonism ceases, continuing erosion will progressively wear away surface relief.

This study examines how crustal roots respond to erosional unloading over very long timescales. In old collisional mountain belts, surface relief relative to the magnitude of the underlying crustal root is observed to be smaller than in young mountains.

Based on gravity data, this trend is best explained by a decrease in the buoyancy of the crustal root with greater thermo-tectonic age. Such an increase in crustal root density is consistent with metamorphic reactions produced by longterm cooling. Observed average root densities are in general comparable to the densities required to isostatically compensate surface relief, suggesting that the continental lithosphere remains weak enough to permit exhumation of crustal roots in response to erosion of surface topography for hundreds of millions of years. However, the amount of such uplift appears to be significantly reduced by progressive loss of root buoyancy.

Oceanic crust is generally composed of dark-colored rocks called basalt and gabbro. It is thinner and denser than continental crust, which is made of light-colored rocks called andesite and granite. The low density of continental crust causes it to “float” high atop the viscous mantle, forming dry land. Conversely, dense oceanic crust does not “float” as high—forming lower-lying ocean basins. As oceanic crust cools, it becomes denser and ultimately sinks back into the mantle under its own weight after about 200 million years.