why are the elevations of recent mountain chains so high but elevations of older mountain chains so low?
Two processes are acting - mountain building and erosion.
Mountains grow at a relatively fast rate (a few mm/year) until the forces that formed them are no longer active. Even while they are growing, they are being cut down by the processes of erosion. These processes include wind blown sand eating away at surfaces, ice in glaciers scouring the surfaces, and water in streams. Water is by far the most important. Water gets into the rocks and breaks them down chemically in a process called chemical weathering, and rain drops and ice further break up the rocks with mechanical weathering, then the water washes the rock (now soil) away. Whenever you see a stream colored brown, rock is being washed away.
The Himalayas are still growing today after starting to grow about ten million years ago when India crashed into Asia, and at the same time water and ice are tearing them down. Once the processes that cause the mountains to grow stop, the forces of erosion begin to win, and the mountains are slowly eaten away until there is little left after a hundred million years or so.
Also, mountains are large enough that they push the crust of the earth into the earth's mantle forming "roots". As the mountain erodes from above, it also rises from below as the root is no longer pushed into the mantle by the weight of the mountain - much like an ice cube rises in the water as the top is melted away. Roots for high mountains can be very deep - as much as 20 km or so. This means that erosion needs to erode away more than 20 km of material to completely flatten out a mountain that was 5 km high at the start. It also explains why we get large areas of rocks at the surfce that were formed at depths of 20 km or so in places where there used to be high mountains - like the Canadian shield.
Dr. Fred Duennebier, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822