The earth’s crust averages 16 miles thick and rests and what is called the mantle. This layer, about 750 miles thick is belief to be semi-viscous or plastic nature rather than solid.
Thus, both continental and oceanic areas could be said float on the mantle, with continental areas pushing deeper into the mantle than do oceanic areas where the crust is thinner. Constant erosion moves dirt and rock into ocean areas where they settle. These areas gradually sink deeper into the mantle.
Eventually creating pressure that thrust new land up forms surrounding areas. Mountains formed by this process are made of sedimentary rock and are called folded or domes.
Earthquakes occur where the crust shifts or creaks apart sharply under pressure; the resulting breaks or fissures are called fault mountains created by these faults fault blocks.
On weak spot on the earth’s crustgenerally near sea coasts-gases and molten rock under tremendous pressure break through to form conical shaped mountains.
These we know as volcanic. No sooner are new mountains and flat lands created by pressure than water, ice, and wind begin to wear them away thus creating new valleys, river courses and coastlines.
The mantle is _____________
A. Pliable substance under the crust B. A solid layer of the earth’s crust C. A thin layer of earth under the ocean D. The semi-solid exterior of the earth
