Tim Konrad – “I have heard that the Adirondack mountains of New York were once the size of the Himalayas. Is this true? What other mountain ranges were once far larger then their current size?”
Tim, the answer is “sort of.” There once was a mountain range similar in stature to the stately Himalaya that played a part in the formation of today’s Adirondacks, however this tall, ancient mountain range did not directly become the Adirondacks. Today’s Adirondacks are the uplifted root of this ancient mountain range.
The Adirondacks are one of the main physiographic regions of the larger Appalachian Mountain chain, which extends from Georgia to Maine. Roughly dome shaped, the Adirondack Mountains are ~125 miles in diameter, and lie in northeastern or “upstate” New York. They are geologically distinct from the Appalachians. The Adirondacks have at least 5,300 feet of structural relief (Isachsen, 1981), are made almost entirely of middle-Proterozoic aged metamorphic rocks, and are surrounded by younger sedimentary rocks.
The metamorphic rocks that makeup the Adirondacks were formed at great pressures and temperatures far beneath the surface of the earth, as much as 30 km below ground. In order for these rocks to have been cooked to their present form, they must have spent a great deal of time at or near the bottom of a very thick column of crust. This crust would have had to have been about twice as thick as normal continental crust, such as the crust found under today’s Himalayas at the plate boundary between India and Asia. In order for this to happen, the rocks that make up today’s Adirondacks must have been buried beneath a mountain range the size of the Himalayas that was subsequently eroded away (to the tune of 25,000 meters of flattened mountains!), and then the Adirondacks were uplifted to the surface where we see them today.
As I mentioned a few weeks ago, mountain ranges are formed as a result of disequilibrium between two processes – uplift, and erosion – in that mountains are formed when uplift exceeds erosion. In order for the Adirondacks to have been uplifted from the depths at which their rocks were formed, they “skyrocketed” to the surface at rates of as much as 3 mm per year (Isachsen, 2000) over a period of about 5 million years. The source of this uplift is the subject of ongoing debate – some scientists contend that it was the result of a magmatic hotspot that heated up the crust beneath the proto-Adirondacks and caused the rocks to become buoyant relative to those around them and rise to the surface. Others contend the Adirondack dome was uplifted as a result of tectonic processes related to regional continental collision.
Tim also asked what other mountain ranges were once as tall as the Himalaya. The first range that comes to mind is the Appalachians. These mountains were formed in three separate periods of uplift, the most recent being the Alleghenian Orogeny of 350-300 million years ago. The ancient North American proto-continents Laurasia and Gondwana crashed into each other during the formation of the super-continent Pangea, thickening the crust and forming both a towering mountain range called the Central Pangean Mountains and a high plateau behind these mountains. The eroded remnants of these mountains form many of the thick sedimentary rocks found in and just beyond today’s Appalachians, which are what give geologists the clues to conclude that the Appalachians were once so enormous. Along the same lines, some of the enormous sandstones of the Colorado Plateau were once towering mountain ranges as tall as or taller than the Himalayas.
Next week I’m going to school you on geologic time and how insignificant humanity really is in the face of how old all the rocks on the planet are.
References
Geology of New York : A Simplified Account. New York State Museum’s Educational Leaflet # 28. Isachsen, Yngvar, 2000.
McLelland, J, and Isachsen, Y., 1980, Structural Synthesis of the southern and central Adirondacks: A model for the Adirondacks as a whole and plate tectonic interpretations: GSA Bulletin, v. 91, p 208-292.