Earth.Parts #20 - Continental collisions & mountain-building by orogeny
Channel: Earth.Parts
Date: 2017-02-05

What happens when continental crustal plates collide? When ocean crust collides with continental crust the ocean plate will subduct below the continent and be destroyed, forming a subduction-related mountain range and more continental crust. What about continents?

Continents cannot sustainably subduct one below the other, because continents are made of granite and andesite that are less dense than ocean crust or mantle rock. The continents can't be buried. Instead they grind together and pile up thick masses of crust that drive up the highest mountains on Earth.

Collisions of continents raise mountains by regional orogeny - mountain-building - that involves slow but violent folding, faulting and compression of rock over hundreds to thousands of kilometers. The Himalayan mountains of Asia are the direct result of India colliding with Asia starting about 55 million years ago, and include the tallest mountains in the world; Mount Everest and K2. 

Orogeny resulting from continental collisions drive rock to high pressures, even at low to moderate geologic temperatures, resulting in forced recrystallization of the rock's minerals into alternate forms, other minerals, comprised of the same material but reconstituted into forms stable under those intense orogenic conditions.

Regional metamorphism, pressure & temperature-related recrystallization of minerals in rock, can produce highly altered, folded and twisted expressions of natural rock that hosts rare or remarkable minerals.

The Alps are another example of collisional orogeny, one that started about 300 million years ago when Africa and Europe collided, and continuing sporadically since then. Epic scenery such as now found in Switzerland at Jungfraujoch, or the Matterhorn which is comprised of rock from two continents. 

The Appalachian mountains of eastern North America are eroded remains of a long history of orogeny, beginning about 450 million years ago when an ancient subduction-related volcanic island chain collided with proto-North America, raising a vast range of mountains, the Taconic orogeny. 

Then, around 375 million years ago an island continent we now call Avalonia collided with the Taconic mountain coast, raising up a more northerly range of peaks, the Acadian mountains. Then around 300 million years ago Africa and proto-North America collided, raising the Appalachian mountains during what is called the Alleghenian orogeny around 250 million years ago. 

Around 180 million years ago Africa split from North America, and erosion since then has worn down the ancient Taconic, Acadian and Alleghenian ranges to form the gentler slopes of the modern Appalachians. Today the highest Appalachian peak, Mt. Mitchell, may not look like Mt. Everest, but long ago it and its sibling heights were perhaps even higher still.