Geology of the Appalachian Mountains

A brief overview of Appalachian Geology

Sunrise in the Smoky Mountains
A sunrise in the Great Smoky Mountains National Park. Tony Barber/Moment/Getty Images

The Appalachian Mountain range is one of the oldest continental mountain systems in the world. The tallest mountain in the range is the 6,684-foot Mount Mitchell, located in North Carolina. Compared with the Rocky Mountains of western North America, which have 50 plus peaks over 14,000 feet in elevation, the Appalachians are rather modest in height. At their tallest, however, they rose to Himalayan-scale heights before being weathered and eroded down over the past ~200 million years.

A Physiographic Overview

The Appalachian Mountains trend southwest to northeast from central Alabama all the way to Newfoundland and Labrador, Canada. Along this 1,500-mile path, the system is split up into 7 different physiographic provinces that contain distinct geologic backgrounds.

In the southern section, the Appalachian Plateau and Valley and Ridge provinces make up the western border of the system and are composed of sedimentary rocks like sandstone, limestone and shale. To the east lie the Blue Ridge Mountains and the Piedmont, composed primarily of metamorphic and igneous rocks. In some areas, like Red Top Mountain in northern Georgia or Blowing Rock in northern North Carolina, the rock has eroded down to where one can see basement rocks that formed over a billion years ago during the Grenville Orogeny. 

The northern Appalachians are made up of two parts: the St. Lawrence Valley, a small region defined by the St.

Lawrence River and St. Lawrence rift system, and the New England province, which formed hundreds of millions of years ago and owes much of its present topography to recent glacial episodes. Geologically speaking, the Adirondack Mountains are quite different than the Appalachian Mountains; however, they are included by the USGS in the Appalachian Highland region.

 

Geologic History

To a geologist, the rocks of the Appalachian Mountains reveal a billion year story of violent continental collisions and the subsequent mountain building, erosion, deposition and/or volcanism that came with. The geologic history of the area is complex, but can be broken down into four major orogenies, or mountain building events. It is important to remember that between each of these orogenies, millions of years of weathering and erosion wore the mountains down and deposited sediment in the surrounding areas. This sediment was often subjected to intense heat and pressure as the mountains were uplifted again during the next orogeny. 

  • Grenville OrogenyThis mountain building event occurred around 1 billion years ago, creating the supercontinent Rodinia. The collision formed tall mountains along with the igneous and metamorphic rocks that make up the very core of the Appalachians. The supercontinent began to break apart around 750 million years ago and by 540 million years ago, an ocean (the Iapeus Ocean) existed between the paleocontinents. 
  • Taconic Orogeny – Approximately 460 million years ago, as the Iapetus Ocean was closing, a volcanic island arc chain collided with the North American craton.  The remnants of these mountains can still be seen in the Taconic Range of New York.
  • Acadian Orogeny – Starting 375 million years ago, this mountain building episode occurred as the Avelonian terrane collided with the North American craton. The collision did not happen head on, as it struck the northern section of the protocontinet and then slowly moved southward. Index minerals show us that the Avelonian terrane struck the North American craton at different times and with different collisional forces.
  • Alleghanian Orogeny – This event (sometimes referred to as the Appalachian orogeny) formed the supercontinent Pangea ~325 million years ago. The ancestral North American and African continents collided, forming Himalayan-scaled mountain chains known as the Central Pangean Mountains. The modern day Anti-Atlas Mountains of northwestern Africa were part of this chain. The mountain building ended some 265 million years ago, and the ancestral North American and African continents began rifting apart ~200 million years ago (and continue to do so to this day).

    The Appalachians have weathered and eroded away over the past hundreds of millions of years, leaving only remnants of the mountain system that once reached record heights. The strata of the Atlantic Coastal Plain are made up of sediment from their weathering, transportation and deposition.