Extracted from http://gretchen.geo.rpi.edu/roecker/nys/adir_txt.html

 

We know enough about the geology of the Adirondack region to begin to piece together a history of the Middle and Late Proterozoic there. But there are many things we still don't know. We have to make some educated guesses at nearly every stage of our reconstruction.

We find the age of igneous rocks by radiometric dating. However, this task is not simple. Sometimes intense metamorphism, like that which occurred in the Adirondacks, can "reset" some or all of the radioactive "clocks" in the rock. If this resetting happens, radiometric dating will tell us when the rock was metamorphosed. It will not give us the age of the original igneous rock. Radiometric dating has been done on many Adirondack rocks, but we have to be very careful in interpreting the results.

We have found that almost all rocks in the Adirondacks are of Middle Proterozoic age. Radiometric dating of the metavolcanic rocks suggests that the oldest ones may be as much as 1.3 billion years old. We think the metasedimentary rocks were deposited as sedimentary rocks beginning at about the same time.⁶

The original sedimentary rocks of the Adirondack basement-sandstone, limestone, dolostone, and shale were probably deposited in a shallow inland sea. Although they were deposited most likely no more than 1.3 billion years ago, some contain grains of the mineral zircon that are about 2.7 billion years old. This fact tells us that the sediments that formed these rocks were eroded from a much older landmass. This landmass was probably the Superior Province, located to the west and north of the Grenville Province.  Metavolcanic rocks that occur with the metasedimentary rocks indicate that volcanoes were present in the region at that time.

Most of the metaplutonic rocks of the Adirondack Highlands are probably between 1.15 and 1.1 billion years old. Shortly before the Grenville Orogeny, large volumes of magma may have risen from the mantle into the crust. Heat from the magma partially melted the surrounding crust, producing molten rock of different compositions. The various kinds of molten rock, such as anorthosite and granite, tended to rise through the crust because they were less dense than the surrounding rocks. Some continued to rise even after they partly cooled and solidified, eventually forming balloon-like domes or spreading out as thick sheets within the crust.

At some point during the Middle Proterozoic, the rocks we now find at the surface in the Adirondack region were as much as 30 km below the surface. Remember that some of these rocks began their existence as sedimentary rocks at the surface, which means that they must have been pushed down that far. For them to be buried so deeply, the continental crust in the region had to be nearly twice as thick as normal continental crust. A modern example of double-thick crust is the Tibetan Plateau just north of the Himalayan Mountains. As India continues to collide with Asia, the collision is creating the Himalayas-the world's highest, mountains-along the collision zone, and a double thickness of continental crust under them and to the north. This double-thick crust makes Tibet the world's highest plateau region, with an average elevation of 5 km above sea level. Far below the surface, the rocks are subjected to very high temperatures and pressures.

The Grenville Orogeny, which may have been caused by a similar collision, buried the Adirondack rocks. It is difficult to say when the orogeny began. It was under way at least 1.1 billion years ago. The deformation and metamorphism appear to have peaked between 1.1 and 1.05 billion years ago. Some additional plutonic rocks may have been formed at the time, either by partial melting of the crust or by injection of new magma from below. By about 900 million years ago, the rocks had cooled again. We still don't know the details of these complex events.

Like the collision of India and Asia, the Grenville Orogeny built huge mountain ranges along the collision zone and a high plateau behind it. Over the next several hundred million years, erosion coupled with uplift levelled the mountains and stripped more than 25 km of rock from the plateau. Between 650 and 600 million years ago, the crust of eastern proto-North America was stretched and was broken by major faults. These faults are the ones that run north-northeast throughout the eastern Adirondacks. There are also many smaller faults running east-northeast, east, and southeast. Igneous rocks called diabase dikes show that molten rock was injected and hardened in narrow vertical zones, often along faults. Radiometric dating tells us that these dikes were formed about 600 million years ago.

Beginning in the Late Cambrian, the Adirondack region was gradually submerged beneath shallow seas. Sandstones with trilobite fossils were deposited over much of the region. The contact between these younger rocks. and the underlying basement is visible in several places near the outer edge of the present Adirondack dome. Sediments continued to accumulate across much of the eastern United States (with some interruptions) through the Pennsylvanian Period, but no rocks younger than Middle Ordovician remain in northeastern New York.

Later erosion in the Adirondack region stripped off nearly all of the Paleozoic sedimentary rocks. However, there are still traces of Cambrian and Ordovician rocks within the Adirondacks; this fact proves that they once covered the region. In the southern Adirondacks, we find grabens that contain Cambrian and Ordovician rocks formed in these seas. Because these blocks dropped down lower than the surrounding landscape, they were saved from erosion when the other Paleozoic layers were worn off during regional uplift. The Lower Paleozoic rocks that originally covered the region still encircle the Adirondack dome.

From the Middle Ordovician into the Tertiary Period, there is no evidence of any tectonic activity in the Adirondacks, despite three more mountain-building events that affected New England and southeastern New York. The region that is now the Adirondack Mountains was flat, just like the rest of the region west of the Appalachian Mountains. In Jurassic or Cretaceous time, some small dikes intruded in the eastern Adirondacks and Vermont.

Sometime in the Tertiary Period, the Adirondacks began to rise. Why? Our best guess is that a hot spot formed under the region near the base of the crust. This hot spot heated the surrounding material at depth, causing it to expand. This expansion raised the crust above, causing the present dome-shaped uplift. In the early 1980s, remeasurement of the elevations of old surveyors' bench marks showed that the Adirondacks may be rising at the astonishing (to a geologist!) rate of 2 to 3 mm per year. The mountains are growing about 30 times as fast as erosion is wearing them away. We suspect, however, that the present rapid uplift is a temporary spurt, and the average rate may be much less.

After the Adirondack dome began to rise, stream erosion (and much later glacial erosion) started wearing away the softer rocks and the fractured zones. Eventually, erosion carved the region into the separate mountain ranges we see today. Glacial ice entered the region about 1.6 million years ago.