The Appalachian Mountains May Have Once Been as Lofty as the Himalayan-Tibetan

Multiple Choice

The Appalachian Mountains may have once been as lofty as the Himalayan-Tibetan Mountain belt is today.Why are they not this high now?

A) They developed a dense crustal root following collision,and isostasy forced them to sink to their present elevation.
B) They formed long ago,and erosion has beveled them to their present low elevation.
C) The mountains cooled following the collision,which increased the density of the of the rocks by cooling,and isostasy forced the mountains to sink.
D) Opening of the North Atlantic Ocean converted lithosphere to asthenosphere beneath eastern North America,forcing eastern North America to sink to low elevations.