this will covered in the test
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.
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category
&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7
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.
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
http://bcs.whfreeman.com/understandingearth/content/cat_110/ch18/earth4e_1817.html?v=category&i=18110.01&s=00110&n=18000&o=%7C00510%7C06000%7C14000%7C17000%7C20000%7C23000%7C22000%7C18000%7C
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
deformation.
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
blueschist.
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.
http://www.google.com/url?sa=i&source=images&cd=&docid=uA1dSaSoS40LYM&tbnid=hNIG7jhYbcPWpM:&ved=0CAgQjRwwAA&url=http%3A%2F%2Fwww.lacusveris.com%2FThe%2520Hi-Line%2520and%2520the%2520Yellowstone%2520Trail%2FThe%2520Bois%2520Brule%2FThe%2520Canadian%2520Shield.shtml&ei=0TspUdOPNqiB2gW6ioHYBw&psig=AFQjCNGds7s5c72uaEPSstz2WN9FyFpYXw&ust=1361743185948322
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.