Answer to Question 1
The region is underlain with low-density buoyant mantle.
Answer to Question 2
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.