The theories of continental drift and plate tectonics tell us that the position of continents as we observe them today is a rather recent phenomenon. The continents have moved across the face of the globe for most of the Earth’s history, and the evidence for that movement and its effects are seen in the rock record. The continents not only move, but they collide with each other forming new, larger continents, and large continents split into smaller ones.
This cycle is continuous, and all continents show the effects of a number of collisions and subsequent breakups. Plate tectonic motions control the distribution and frequency of phenomena such as earthquakes and volcanoes, the distribution of rocks, minerals, and natural resources, and even the distribution of living organisms.
We know that North America experienced a collision some 1.2 to 1.1 billion years ago. Evidence for this is seen in the folded and crumpled Precambrian rocks of the Adirondack region and western Vermont. Ancient sediments were deformed, metamorphosed and uplifted, forming a large mountain range. The process of collision and mountain-building is called orogeny, and this collision is referred to as the Grenville Orogeny.
The Grenville collision and other similar collisions elsewhere caused all the continents at that time to be assembled into a single supercontinent. Eventually, about 660 million years ago, this Precambrian supercontinent split apart, and much of the northeastern U.S. was covered by a shallow sea called the Iapetus Ocean. About 550 million years ago an island arc began to form off the coast of ancient North America. This chain of volcanic islands would have appeared much as the island arc of Japan does, near the modern coast of Asia. Plate tectonic processes eventually caused the island arc to collide with ancient North America in another mountain-building event called the Taconic Orogeny. The Taconic Orogeny occurred about 460 million years ago. Sands and muds eroded from the rising Taconic mountains were deposited in ocean basins both east and west of the mountain range.
Recall that plate tectonic processes occur all the time–even as the Taconic mountain range was being eroded, more continents were headed for collision with North America. This collision (410-380 million years ago) produced a new mountain range, the ancient Acadian mountains. During the Acadian Orogeny, parts of Europe and a smaller continent called Avalon collided with North America. Just like the Taconic mountains before them, the Acadian mountains were eroded. A huge quantity of sediment was carried by rivers flowing westward from the Acadian mountains and deposited in the shallow inland sea that covered much of North America. The sediments formed delta deposits at the river mouths, and are now referred to as the Catskill Delta. These delta sediments became the rocks that form most of the scenery in central New York today.
During the period from 310 – 250 million years ago more continental collisions again caused all the Earth’s continents to become assembled as a single supercontinent. In eastern North America this is referred to as the Alleghenian Orogeny, and the supercontinent that is created is called Pangea. Pangea is derived from the Greek word for “all lands,” and the continent of Pangea was surrounded by a large ocean called Panthalassa, “all seas.” Approximately 220 million years ago Pangea began to split, or rift apart. As Europe and Africa drifted away from North America the Atlantic Ocean began to form, and the world gradually grew to look more and more as it does today.
One final and very important event left its mark on North America about 18 thousand years ago. This was the ice age, in which massive glaciers formed in arctic regions and slowly spread south, covering all of Canada and the northern U.S. While the landscape of central New York is built of rocks deposited hundreds of millions of years ago, it is the much more recent ice age that was responsible for sculpting the modern landscape. On our field trip we will observe rocks deposited during the Taconic and Acadian orogenies and the ice age, and we will consider how processes such as continental collision and continental glaciation have affected the landscape that we observe today.