If the mantle has a density of 3000 kg/m3 and a section of crust has a density of 2000 kg/m3, what ...

(Answer to question 1 )  2000 km/m3 divided by 3000 km/m3 results in 2/3rds, which means that 2/3rds (66.6) of the crust would be below water, while 1/3 (33.3) of the block would be above water.

(Answer to question 2 )  The highest mountains and thickest crust form over the overriding plate in the region marked by intrusion of plutons and compressive thickening and plastic flow. This region is 200 to 400 km inland of the trench, which marks the plate boundary.

(Answer to question 3 )  The region is underlain with low-density buoyant mantle.

(Answer to question 4 )  Precambrian rock masses (cratons) are preserved, yet usually highly metamorphosed. These rocks were emplaced in the middle to lower crust during mountain building, then exposed and later buried under sedimentary rocks. Bands of younger rock formed by accretion of material that was too light (i.e., crust) to subduct. Many are volcanic arcs and are referred to as exotic terranes.

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