Mountain Building

Mountain building is a very complex subject. To simplify, some mountains are a direct result of volcanic activity but most mountains are the result of tectonic forces causing layers of rock to fold, elevate and fault.

All mountains will fall into one of five categories: Volcano, Fault-Block, Folded and Thrust-Fault (complex), Erosion, or Dome.


Volcanoes also can be divided into groups as well: Cone, Shield, Cinder Cone, Dome, or Fissure.

The classic Cone volcano is represented by Mount Saint Helen in the Pacific North Western United States. This type of volcano is most often located above subducting plates that by their nature feed lava to the volcano.

The other widely recognized volcano system is the island chain of Hawaii. These islands formed over “Hot Spots” where the undersea crust is thin and molten material flows freely to the surface to form Shield volcanos.

Notable Cinder Cone volcanoes are found in Northern Chile. These volcanoes are formed from light and loosely packed material, such as cinders and ash that erupt from the earth. These form a loosely structured cone as opposed to a regular cone volcano that forms solid walls from flowing lava.

Dome volcanoes occur often in old Cone volcanoes such as Mount St. Helen after an eruption blows away part of the top of the volcano. Thick and sticky magma then pushes into the old area to form a new dome.

Fissure Volcanoes can be observed in Hawaii and most notably in Iceland. Most of the volcanic activity in the world takes place under the surface of the ocean in fissure volcanoes. This activity happens above the spreading boarders of the continental plates.

Fault-block, Folded and Thrust-Fault (complex), Erosion, and Dome Mountains.

The United States has two of the major mountain systems in the world. These mountain systems are the North American Cordillera in the West and the Appalachians in the East.

Fault-Block formation was part of the early development of the Sierra Nevada Mountains and the Grand Teton Range. This type of formation happens chiefly due to large sections of the earth’s crust moving along faults.

Later in geologic time, Complex or Folded-Thrust Fault mountains, such as our Appalachian mountains on the east coast, were formed as tremendous forces compress the crust. The crust of the earth was also exposed to high temperatures from the magma below, as it folded and broke into layers that overrode underlying rock layers. These forces, combined with intrusion of minerals in liquid are the basic forces of metamorphic rock formation.

Erosion Mountains form as uplifted areas, such as the Black Hills in South Dakota, erode and weather away to form complex mountains and valleys.

Dome Mountains, such as the Adirondack Mountains of New York, form when the earth’s crust heaves upward without folding or faulting into a rounded dome. The Adirondacks today bear little resemblance to the original dome shape due largely to erosion action.

Another interesting geological phenomenon that helps build mountains is called isostatic rebound. As the plates collide and stack up or as a volcano erupts and deposits material, tremendous weight is added to that area of the earth’s crust. This tremendous weight literally pushes down on the elastic upper mantle of the earth’s crust causing it to drop in elevation. Over millions of years, wind and water erode and remove material thus making the mountain lighter, which in turn allows the mantle to rebound upward, thus elevating the remaining harder material that did not weather away. In some cases, this rebound can actually cause sections of the crust to break into blocks that fault upward building and changing the shape of the mountains.

Interestingly, the longest mountain range in the world is located totally under water at the Mid-Atlantic Ocean Ridge. This range is approximately 6,000 miles long.

With the exception of newly formed volcanoes, almost all mountain building is the result of combinations of several mountain building forces not just one set of forces. Our own Green Mountains in Vermont were originally formed 400 million years ago when the North American continent and the African Continent collided. Since that early faulting and lifting, many geological forces such as glaciations and erosion forces have shaped our landscape into the wonderful mountains we all love.