Thanks for asking!
The current day location of the Rocky Mountains in relation to
continental or tectonic boundaries may be explained by two processes:
orogeny and accretion.
Continents consist of two different types of structural units: cratons
and orogenic belts, or mountain ranges. Cratons are large regions
occupying the interior regions of continents. Cratons have achieved a
high degree of geologic stability. Cratons are regions that haven't
changed much over vast periods of geologic time, hundreds of millions
of years. Orogenic belts are elongated areas of continental crust that
have been deformed, or folded, faulted, and exposed to extremes of
heat and pressure. Orogenic belts have also been intruded by massive
magma infusions, which cool beneath the surfaces during repeated
cycles of mountain building.
Cratons have vast age. They are surrounded by continental shields,
which contain the cores of younger mountain ranges (orogenies) that
have grown outward from the center. Within the United States, the
Great Plains, between the Rockies and the Appalachians make up the
continental shield. Most generally, the further away from a craton,
the younger the region. The youngest portions of a continent are at
the edges of the ocean. The Rockies are older than the Sierra Nevada
and Cascade ranges. However, the shape and size of the North American
continent differed greatly at the time the Rockies were born.
Orogenies themselves, the process of mountain building, continue over
a hundred million years of time. The rock structures common to
orogenic sequences indicate that the rock deformation proceeds in
landward directions, starting from the sea and working its way inland.
This type of orogeny begins when volcanoes create an island arc or
belt along the continental margin. Sediments from the sea floor
between the island arc and the continental margin are squeezed,
crumpled and uplifted.
During the process, thickening of the crust occurs, and causes
thrusting of continental shelves inward toward the continental
interior, producing giant thrust faults hundreds of miles long, as if
one block of continental crust was simply pushed over the top of
another one by a giant hand. This uplift, tilting, and the erosion and
sedimentary processes of the rock cycle cause deposition of material
on both sides of the mountains.
A second method of continent building accounts for much of the current
land mass between the Rockies and the Pacific Ocean. Continents are
also built by accretion, an event which joins together two separate
land masses. Lightweight continental fragments, islands and sea mounts
are scattered among the world's oceans. Because they are buoyant, they
cannot be subducted. As these fragments collide with a continental
mass they are "glued" to the edges by metamorphic and igneous
activity. The position of the offshore continental subduction zone
moves to the seaward side of the added region.
Much of Western North America consists of these continental fragments,
sea mounts and island arcs which have attached themselves, or accreted
to the continent during the last 150 million years.
No matter how they originate, mountain belts along tectonic boundaries
cease to grow when subduction ends. As new exotics accumulate at the
edges, the continent expands seaward from the former edges, and
mountain building processes also move outward.
The link provided below by robertskelton-ga leads to a number of
representations of the Southwestern portion of North American
continent over time. The following maps represent the changing western
edge of the continent over geologic time periods corresponding to the
birth of the Rockies and subsequent geologic events.
Silurian Paleogeography
http://jan.ucc.nau.edu/~rcb7/silpaleo.html
Devonian Paleogeography
http://jan.ucc.nau.edu/~rcb7/devpaleo.html
Mississipian Paleogeography
http://jan.ucc.nau.edu/~rcb7/mispaleo.html
Pennslyvanian Paleogeography
http://jan.ucc.nau.edu/~rcb7/penpaleo.html
Permian Paleogeography
http://jan.ucc.nau.edu/~rcb7/perpaleo.html
Triassic Paleogeography
http://jan.ucc.nau.edu/~rcb7/tripaleo.html
Jurassic Paleogeography
http://jan.ucc.nau.edu/~rcb7/jurpaleo.html
Cretaceous Paleogeography
http://jan.ucc.nau.edu/~rcb7/crepaleo.html
Tertiary Paleogeography
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FURTHER STUDY:
Geology/Geophysics 101 Mountainbuilding
http://www.hcc.hawaii.edu/hcconline/gg101/program8%20MountainBuilding/program8.html
Tectonics in the Rocky Mountains
http://talc.geo.umn.edu/courses/1002/Lecture38_CordTect.html
The Secret of Colorado 14ers
http://www.cmc.org/cmc/tnt/956/whycolo14ers.htm
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Google Search Terms:
mountainbuilding explanation
laramide orogeny
laurasia geology
I've enjoyed the "hunting and gathering" of information. Should you
have any questions about the material or links provided, please, feel
free to ask.
--larre |
Clarification of Answer by
larre-ga
on
27 Mar 2003 11:44 PST
The western boundary of the North American plate is defined by the
subduction zone near the edge of the continent where it collides with
the Pacific plate. So in effect, yes, the plate boundary moves seaward
(west) as the continent grows in size. The shape, orientation and
position of the North American Plate has changed over geologic time.
Tectonics and Plate Movement Over Time - Maps
http://www.scotese.com/earth.htm
At the time of the Laramide Orogeny, 70MYA, the area which was to
become the Rockies was indeed further inland than most mountain
ranges, however, the subduction zone at the boundaries of the North
American and Pacific plates is thought to be unique. Rather than
subducting at a steep angle, the oceanic crust is presumed to be
sliding under the North American plate at a much shallower angle,
thus carrying the mountain building activity further inland on the
continental mass. This theory of origin is explained and diagramed by
the USGS at:
USGS Rocky Mountain System
http://www.aqd.nps.gov/grd/usgsnps/province/rockymtn.html
The boundary zone of the Pacific and North American plates is
considered to be mainly a Transform Boundary, with the plates
partially sliding past one another in opposite directions, however,
there is also enough lateral convergent movement to trigger the
subduction process.
Plate Boundaries and Tectonic Processes
http://geoweb.tamu.edu/courses/geol101/lab/tectonics_1/tectonictxt.htm
I hope this helps.
--l
|
