Lecture 16 - The Appalachian Mountains

Outline

I. Evolution of the Appalachian Mountains

II. Shenandoah National Park

III. Great Smoky Mountains National Park

IV. Acadia National Park

I. Evolution of the Appalachian Mountains

The Appalachian Mountain belt runs from Newfoundland, Canada to Alabama. Like the Rocky Mountains it was created by the collision of large tectonic plates in a series of orogenies or mountain building episodes. Unlike the Rockies, the Appalachians are old and eroded. The east coast of the United States and Canada has been a passive margin for the last 200 million years. During this time no tectonic activity has been taking place on the margin of the Appalachians and the only geologic events to occur are erosion that wear the mountain sdown and sedimentation as rivers carry sediment to be deposited on the continental margin.

The southern portion of the Appalachian Mountains has been divided into 4 geomorphic provinces that share geologic features, from east to west they are:

  1. The Piedmont- consists of Precambrian to Paleozoic metamorphosed sedimentary and volcanic rocks intruded by granite and eroded to low relief.
  2. The Blue Ridge- Precambrian and Cambrian (earliest Paleozoic) metamorphosed sedimentary and volcanic rocks and granites with the highest relief in the Appalachian Mountains. These rocks were thrust over rocks of the Valley and Ridge Province to the west. Both Shenandoah and Great Smoky Mountains National Parks are located in this province.
  3. Valley and Ridge- Thick Paleozoic sedimentary rocks with abundant folds and reverse (thrust) faults.
  4. Appalachian Plateau- Paleozoic sediments dipping gently to the west. Contain a lot of limestone units. Mammoth Caves National Park (discussed later) is located in this province.

The rocks of the Appalachian Mountains show evidence for 3 episodes of mountain building in Paleozoic time. The sequence of events is given below.

Precambrian Time

Africa and North America were joined into 1 super continent. The collision of these land masses over 1 billion years ago metamorphosed the original rocks producing the Pedlar gneiss and Old Rag granite observed in Shenandoah National Park. In late Precambrian time this super continent began to rift apart under tensional forces producing the Catoctin rift basalts that are also observed in Shenandoah. As they rifted apart they created a growing ocean called the proto-Atlantic or Iapetus after the father of Atlas, for whom the Atlantic Ocean is named.

Towards the end of the Precambrian, the tensional forces changed o compression and subduction began. Volcanic islands grew as a result of andesitic volcanism associated with the subduction.

Taconic Orogeny- Ordovician Time (~425 my ago)

With continued subduction and convergence, volcanic islands collided with North America and these rocks were thrust up on the continental margin. Deformation, metamorphism and magmatism accompanied this collision and gave rise to the Taconic Orogeny. Rocks metamorphosed and deformed in this event are seen in the Blue Ridge and Piedmont provinces.

Acadian Orogeny- Devonian Time (~380 my ago)

As North American, African , and Europe convergence continued, a small microplate known as the Avalon terrane, collided with North America producing a second phase of deformation, metamorphism and magmatism known as the Acadian Orogeny. This event had the largest effects in the northern Appalachians and its effects are very visiblei n Acadia National Park in Maine but not easily seen in Shenandoah and Great Smoky Mtn. National Parks to the south.

Alleghenian Orogeny- Pennsylvanian Time (~300 my ago)

With continued subduction, finally collision of the African continent with North America occurred causing intense folding (most pronounced in the Valley and Ridge province) and metamorphism. TheEuropean and South American plates were also converging with Nort hAmerica at this time to form a super continent called Pangea.

Mesozoic Time

Pangea began to break up in Jurassic time about 200 million years ago as the present Atlantic Ocean opened up. The rifting occurred at slightly different locations from the original boundaries between the North American, African, European and South American Plates leaving behind small crustal pieces of these plates attached to North America. We see these rocks exposed in Acadia National Park.

II. Shenandoah National Park

Although most parks are established to preserve pristineenvironments, Shenandoah Park was established in 1926 to restore a badly misused area. Earlier farmers had overused the land and timber companies had cut so many trees that erosion was a serious problem. Much of the wildlife was gone. In the protective hands of the Park Service, Shenandoah Park barely shows any signs of its original abuse.

Major Geologic Features

Shallow angle reverse faults (thrust faults) are the dominant feature in this park. The Precambrian and Cambrian rocks of the Blue Ridge province were thrust over younger sedimentary rocks of the continental margin. Movement along these faults is believe to be as large as 150 miles westward. Few good rock outcrops exist in the Park due to abundant vegetation and severe weathering due to the humid environment. The precambrian metamorphic and igneous rocks are the dominant rock types exposed in the park. These are the Catoctin metamorphosed basalt, the Pedlar gneiss and the Old Rag granite.

III. Great Smoky Mountains National Park

Because this park is open all year round and it is close to populated regions, it is visited by more people than any other Park. The close proximity of the Park to highly developed regions has caused abundant problems. In particular, pollutants from nearby oil and coal burning plants and abundant "acid rain" has damaged vegetation in the Park. This park was established as a Biosphere Reserve in 1976 and is being monitored for pollution and its effects on plant and animal life. The park is in the southern Blue Ridge Province of the Appalachian which contains the highest peaks in the eastern U.S. Over twenty summits in the Great Smoky Mountains are over 6,000 feet. Warm wet winds from the Gulf of Mexico discharge large quantities of rain here, making this region the wettest in the eastern U.S.

The geology of Great Smoky Mountains (Figure 48.6) is very similar to that in Shenandoah National Park with the deformation being more intense here in the south. The few rock exposures in the park consist primarily of Precambrian metamorphosed sedimentary rocks (The Ocoee Supergroup) thrust over Paleozoic sediments. Two of the major thrust faults are the Greenbrier and the Great Smoky Faults.

IV. Acadia National Park

Acadian National Park is located along the coast of Maine. Although it shares much of its history with the Shenandoah and Great Smoky Mtn. National Parks, a few major differences exist:

  1. Glaciers covered this area during the past million years and glacial erosion has stripped away much of the vegetation and soil to expose abundant rock exposures.
  2. This region significantly experienced the Acadian orogeny during Devonian time when a "suspect terrane" called the Avalon terrane, lodged itself against northern North America.
  3. Wave action has acted as an important erosional agent here.

A suspect terrane is defined as a province with geologic features that sharply contrast with those of nearby provinces. The Avalon terrane which docked itself next to North America during the Acadian orogeny fits this description. Its rocks are very different from those of adjacent regions. These rocks are exposed throughout the park and consist primarily of metamorphosed sedimentary and volcanic rocks. Abundant igneous activity associated with the Acadian orogeny produced large amounts of dioritic and granite igneous rock also exposed in the park.

[return icon]Returnto Earth Sciences 3 Lectures Home Page


This page was last reviewed on 2/24/04.