Plate tectonics help us to explain the process of folding. According to the theory of plate tectonics, the earth’s crust is broken up into sections called plates. These plates float on the semi-molten mantle. Thermal convection currents in the mantle drag these plates in different directions resulting in tectonic activity.
Folding occurs when rock layers that were originally horizontal are bent into a series of wave-like folds. As a result of this collision of the earth’s tectonic plates, the rocks are folded and uplifted and fold mountains are created.
This process is known as orogeny. There have been three major periods of fold mountain building, the Caledonian, Armorican and Alpine periods.
The Caledonian period of folding took place about 400 million years ago when the Eurasian plate and the American plate collided. The in-between ocean floor was subducted under both continents and the seafloor sediments were buckled up to form the sedimentary rocks of the Caledonian Fold Mountains.
The Appalachian mountains in North America, the mountains of Norway, Sweden and Scotland, and in Ireland the Dublin-Wicklow mountains and the mountains of the West and North-west were formed as a result of this collision.
The Armorican period of folding occurred about 250 million years ago when plate tectonics resulted in a collision between the Eurasian and African plates. Examples of Armorican fold mountains include the Vosges mountains in France and the Black Forest mountains in Germany. These mountains have an East-West trend as the compression came from the South. The ridge and valley landscape of Munster is a result of Armorican folding. During the Armorican foldings sedimentary rocks in Munster were folded to form ridges of sandstone and valleys of limestone. The limestone was easily eroded from the fold anticlines and they are seen today as sandstone mountain ridges such as the Mac Gillycuddy Reeks.
Plate movement is also responsible for the formation of the Himalayan mountains in Asia, the Alps in Europe, the Andes in South America and the Rocky mountains in North America. These fold mountains were formed as the African plate collided with the Eurasian plate. These are the youngest fold mountains, formed 60 million years ago. They are known as Alpine fold mountains. The Himalayan mountains are still increasing in size and volcanic and earthquake activity in the region shows that these tectonic plates are still colliding.