Answer to Question 1
Answer: 1. The wood-block models assume a very weak crust that cannot resist deformation when any weight is place on top of it. The models also assume that faults penetrate completely through the crust so that adjacent blocks of crust readily slide up and down past one another while adjusting to changes in crustal thickness. In reality, the upper crust is strong, and the entire crust is not penetrated by faults because plastic flow occurs in the hot lower crust.
2. The mantle is almost entirely made up of solid rock, not liquid. Although the hot mantle flows like a very high viscosity fluid, this flow is much slower than the adjustment of water level to bobbing wood blocks. The crust can only move as quickly as the mantle very, very slowly flows from one place to another to equalize the pressure below regions of changing crustal weight.
Answer to Question 2
Answer: Continents grow through time by the collision and accretion of crustal fragments that cannot be subducted along convergent plate boundaries. The low-elevation regions of continents are areas of long-term tectonic stability, called cratons, where Precambrian metamorphic and plutonic rocks are present at or near the surface. Craton crust formed during mountain-building events between 1 and 4 billion years ago. These mountain-building events featured collisions of thick blocks of low-density, mostly igneous crust. North American crust is a collage of these crustal blocks. Although the Precambrian mountains were long ago eroded down and partly buried, the elevation of the continental interior of North America above sea level is a result of the crustal thickening that occurred during ancient periods of mountain building.
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