The Earth -- Introduction

Class Notes - Mountain Building

Introduction
Mountains rise several hundred meters or more above the surrounding landscape. [I suppose that someone has a definition that distinguishes between a hill and a mountain!]. Some mountains are isolated masses - Kilimanjaro volcano, for example, rises to an elevation of about 6000 meters (20,000 feet) above sea level.

Other mountains are part of extensive mountain belts such as the Alps shown below.

Some owe their uplift to faulting:
whereas others have been uplifted as the terrain was folded.

Most geologists argree that the continents have gradually grown larger by the addition of linear mountainous terrains to their flanks.

Mountain Belts
Orogenesis is the name for the set of processes that collectively produce mountain belts. You should be able to locate the following mountain belts:

Alps
Urals
Himalaya
Appalachians
Caledonian
Andes
North American Cordillera (including the Sierra Nevada and the Rocky Mountains
Although folding is often the most conspicuous sign of these forces, faulting, metamorphism and igneous activity are present in varying degrees.

Isostasy and Crustal Uplift
The authors note that there are two major questions that should be addressed in studying a mountain belt:

what is the origin of the enormous horizontal forces tht deformed these crustal rocks?

what keeps the mountains elevvated above the surrounding lowlands?
Geroge Airy argued that the Earth's crustal rocks float on the denser, more plastic mantle. He argued that the crust must be thicker under mountains than under the adjacent lowlands. Mountainous terrains are supported by light crustal material that extends as roots into the denser mantle.
The concept of a floating crust in gravitational balance is called isostacy. When a ship is loaded, the ship settles into the water. When the ship is unloaded, it "floats higher". During the Pleistocene glaciation a 3-kilometer sheet of ice covered the Hudson Bay region in Canada. The underlying crust was depressed. Removal of the ice has resulted in uplifting of some 300 meters in the last 10,000 years. Convection may also help keep crustal areas uplifted.

Mountain Building
Mountain belts consist of roughly parallel ridges of folded and faulted sedimentary and volcanic rocks. These rocks may exceed 15 kilomters in thickness and represent accumulation in deep marine to shallow continental shelf deposits. An extensive period of quite deposition along a continental margin is then followed by a dramatic episode of deformation.
Mountain belts take a long interval of time from start to uplift. Many studies show that deformation began in the deep water, marine margin and proceeded toward the interior of the continent. Volcanism often accompanied the deformation of these deeper-water sequences.
The shallow, near shore deposits are often deformed by both folding and thrust faulting as deformation proceeds.

Convergent Boundaries

Aleutian-type Subduction
When two oceanic plates converge, one will be subducted beneath the other. Partial melting and the upward migration of the resulting andesitic magma produces Island Arcs that often develop mountainous terrains as they are eroded.

Andean-Type Subduction
When an oceanic plate meets a continental plate, the oceanic plate (denser) is subducted beneath the continental plate.
Passive Margins are the first stage in the development of Andean-type mountains. During this stage a thick wedge of shallow-water sediments accumulate adjacent to a continental mass (such as off the east coast of North America at the present time. Turbidity currents may transport sediment out into deeper water.
Active Margins may eventually replace passive margins and the oceanic plate begins to be subducted beneath the continental plate. Sediment derived from the land and from the downgoing slab is plastered against the landward side of the trench to form an acretionary wedge. Partial melting and the production of andesitic magma results in active volcanos at the surface.
The Sierra Nevada and the Coast Ranges are good examples of inactive Andean-type orogeny. At the time of formation, the Pacific plate was being subducted beneath the North American continent. The Sierra Nevada batholith of granitic rocks represents the core of the continental volcanic arc that formed during subduction. The chaotic rocks of the Franciscan Formation of California represent an accretionary wedge of mixed origin that accumulated in the trench.

Continental Collisions occur when both plates carry continental crust. Continental crust is too bouyant to be subducted.

the Himalayas
About 45 million years ago India collided with the Eurasian plate. Watch the following animation showing the break-up of Pangea. Note that about 200 million years ago there was a large "wedge-shapped" ocean - the Tethys - at the equator on the right hand side of the image. What happens to the Tethys? Follow the assembly of the southern margin of Europe and Asia.
Reload your page to start the animation
After collision, the subducted oceanic plate probably decouples from the rigid continental plate and continues its descent into the asthenosphere.
India continues to migrate north a few centimeters per year. Should a subduction zone develop south of India, the growth of the Himalayas would cease.

the Urals
Plate tectonics provided an understanding as to how a mountain range could form within a continental interior. The Urals formed when proto Europe collided with proto Asia and the two were sutured or welded together.

the Applachians
Mountain ranges now in Greenland and Scandanavia formed at the same time as the Applachians. The orogeny lasted about 300 million years (the Paleozoic). The animation above begins with the breakup of a supercontinent. Welding of the supercontinent produced the Applachians and Caledonians.

Orogenesis and Continental Acretion
Small crustal fragments collide and merge with continental margins -- the Assembly of California.
These fragments may have been island arcs or microcontinents. When oceanic plates move they transport the microcontinents or island arcs. The upper portions of these thickened zones may be trhust from the descending plate as relatively thin sheets onto the adjacent continental block. This material increases the width of the continents. These accreted crustal blocks are called terrane.

The Origin and Evolution of Continental Crust

Early Evolution of the Continents - proposes that almost all continental crust formed early in the history of the Earth and has been recycled many times.

Gradual Evolutiion of the Continental Crust - proposes that continental crust have formed over time with a gradual increase in continental material.
_________________________________________________________________________________________________
| jbutler@uh.edu |ClassListserv |Textbook Home Page |Glossary of Geologic Terms|
|Search These Pages|Other Courses|Resources|Grade Book|
_________________________________________________________________________________________________
Return
Copyright by John C. Butler, July 29, 1995