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Chapter20 Mountain Belts understanding how mountains are created

Mountain belts

are several linear ranges

of mountains. These are

typically on the edges of

the plates, but can be

found in the center of

plate boundaries. The

interior mountain ranges

are remnants of ancient

continent to continent

convergent plate

boundaries (exception

Rocky Mountains).

Mountain belts, even in the center of continents, are associated with earthquakes and the belts on

edges of continents are also associated with volcanoes

Mountain built creation is a combination of three processes: 1. Increase deformation (plate

tectonics), 2. Weathering and erosion and 3.Isostasy. These three different processes interact

differently depending on the location of the mountainous region and the climate to make each

mountain built unique.

Deformation and mountain

building is the shortening of the

continental crust. As plates collide

under the pressure of convergence

the crust is shortened. This

shortening creates folds and faults

with reverse faults being common.

The fold and thrust belts (thrust

faults are shallow angled reverse

faults) found at convergent plate

boundaries are composed of thrust

faults stacked on another with the

rock in between being highly folded.

The book gives a great example of

the impact of this crustal

shortening. The alps are composed of crustal material that (if unfolded and unfaulted) would extend out

500 km wide (310.69 miles) and now has been compressed to a width of 200 km (124.72 miles) a 40%

change in width!

Isostasy is the most difficult concept to understand because it is a balance of forces, gravity

(weight) pulling down on the mountain and the asthenosphere pushing up. Remember the

asthenosphere is part of the upper mantle and is plastic. As a mountain is

formed the weight of it pushes down on the asthenosphere and moves

mantel material away, like sitting on an air

mattress. Your weight pushes the air away from

where you sit. You have equilibrium between

your weight (or the mountains weight,

lithosphere) and the amount of air pushed away

(asthenosphere). This is not a static condition

but will respond to changes in the deformation

due to plate tectonics and weathering and erosion removing material from

the mountain. (page 449 in book). The shortening of the crust from above

creates a tremendous localized increase in weight that then triggers the

isostatic adjustment. Examples of mountains that have been weathered down

and undergone isostatic adjustment are the Appalachians.


C22000%7C18000%7C (isostasy video)

Weathering and erosion is the great leveler. Material is removed from the mountains and

transported to other areas of the crust, either continental or ultimately oceanic. The weight removal

then triggers further isostatic adjustment uplifting the root of the mountain higher into the crust. This

continues until the continental crust becomes a uniform thickness. Mountain Building

In the Americas mountain belts run parallel to the coast lines but in Asia they are central in the

continents, such as the Himalayas, the Alps and the Pyrenees. The Appalachian Mountains are rising

from isostatic rebound and are not actively building. The interior plains between the Appalachians and

the Cordillera are the remains of Proterozoic continent building from continent to continent plate

boundaries. The sedimentary rock overlay of these ancient deep seated mountain roots. These are

considered stable and are called the Craton. These rocks are seen in the Grand Canyon, Black Hills and

the Ozark dome as well as some of the Rockies. The figure above is from the Wilson Cycle of plate

tectonics and what the Craton looks like beneath the sedimentary rock.

The sedimentary rock on the Craton is thin, less than 1,000-2,000

meters (0.62-1.24 miles) while the sedimentary rock in the mountain belts is

over 10,000 meters (6.21 miles). This thickness is due to the deformation

with folds and reverse faults. This represents crustal shortening and


The Canadian Shield of the North American continent date back to the

more ancient times, the Archean. The advance of the glaciers during the

Pleistocene removed all of the sedimentary rocks. These are complex

metamorphic and plutonic rocks that date back over a billion years. This figure to

the left is the shield.

Continent building takes place with the island arc. These

mountains have little if any

sedimentary rock. Instead

the metamorphic rock

forms from metamorphism

of ocean crust. Once the

volcano starts to build by

rising plutons forming

magma chambers erosion now takes

placed on the volcano creating an

accretionary wedge. This area

undergoes metamorphism forming


There is a complex of

metamorphic and igneous rock

(plutonic rock) found in the heart of

major mountain belts. The

metamorphic rocks are both sedimentary and igneous rock that were deeply buried and now exposed.

This is often record of convergent plate boundary assembly. The images here are convergent plate

boundaries. The mountain range in the Cascade Range and the Andes are this type of plate boundary.

The “wiggled” lines indicate metamorphic rocks. Multiple types of metamorphism represented here.

This last image is continent-continent collision. Here there

are two continents joining. This type of collision created the

Appalachians starting back 550 million years ago. This is the same

boundary that can be seen in the Alps, Pyrenees, and the Himalayas.

These events took place starting in the Mesozoic and are still active

in some of the chains. As you can see the mountain building includes

compression of the crust as well as the rising up of batholiths.

The faulting involved are large thrust fault belts but you can

also find normal faults present. Normal faults indicate extension.

The top of large mountains are

overcome by gravitational

collapse and the igneous rock on

the mountain top is forced to

flow downward by gravity and

joins the molten material rising

(batholiths) upward from the

subducting plate.

Once the mountain range is formed they proceed to weather away. And this is where the

isostacy takes over and the mountains once eroded flat are

uplifted by the flow of the asthenosphere. This is not an

instantaneous event but can be in several decades to

millennium. The Appalachians formed in the Paleozoic time

starting in the Ordovician (488-444 ma) and completed in the

Permian (251 ma). During the Mesozoic Era the Appalachians

weathered away and are now reemerging due to isostacy.

Isostacy will continue the uprising until the continental crust

once more reaches balance.

One possibility for this is a process called

delamination. Here the mountain root heats

(lithosphere) to the point it becomes hot and molten. It

is still colder than the asthenosphere and, therefore,

denser. It breaks apart and sinks into the

asthenosphere and asthenosphere on either side of the

root flows into the void. This phenomena causes uplift and extension with normal faults forming. This is

what is thought to be happening in the basin and range region.

This extension and melting of continental crusts

causes a variety of volcanism, with stratovolcanoes and

basalt flows as well as the material from the mantel rises.

As you can see from the picture above this explains lava

flows found Utah, Nevada, a portion of Oregon and

California, as well as Arizona, New Mexico and Texas.

Mountain formed with island and continent

accretions building our own continent but others. We

know of this by finding areas with geologic material

different than the surrounding land terranes. Many of

these terranes come from the breakdown of mountains created by

orogeny and essentially form in place. Suspect terranes are

terranes that don’t appear to have formed in place. If the terrane is

shown to not have formed on the present continent they are called

accreted terrane and are the result of collision of islands or mini-

continents the size of New Zealand. If they can be shown to have

traveled great distances by fossil assemblage or paleomagnetic

poles they are called exotic terranes. The Carolina terrane forming

the Appalachian Mountains has trilobites associated with England

but not the United States. The image on the left shows the building

of our own western continent.

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