Ch. 11.3
Mountains and Plates
Lecture Outline
Mountain building still occurs in many places worldwide.
Examples:
Teton Range in Wyoming still rising
Appalachians in the Eastern U.S. are deeply eroded.
Convergent Boundary Mountains
With the development of the theory of plate tectonics, suggests that most mountain
building occurs at convergent plate boundaries..
Colliding plates produce the compressional forces that fold, fault and
metamorphose the thick layers of sediments deposited at the edges of
landmasses.
The partial melting of mantle rock also provides a source of magma that
intrudes into and further deforms these deposits.
Ocean-Ocean Convergence
The convergence of two oceanic plates mainly produces volcanic mountain.
Volcanic island arcs form along the subduction zones around a shrinking ocean basin.
Example…Philippine Islands
Ocean-Continental Convergence
The convergence of an oceanic plate and a continental plate produces volcanic
mountains and folded mountains.
A continental volcanic arc forms when an oceanic plate is subducted beneath a
continental plate. The belt of mountains that results is made up of volcanoes and
intrusive rocks mixed with metamorphic rocks.
Example…Andes of South America
The Andes formed through subduction of the Nazca Plate beneath
the South American Plate.
Ch. 11.3
Mountains and Plates
Lecture Outline
p.2
Another process forms a belt of coastal mountains made up of folded and faulted
rocks.
During subduction, sediment is eroded from the land and scraped from the subducting
plate. This sediment becomes stuck against the landward side of a trench.
Along with scraps of oceanic crust, the sediment forms an accretionary wedge.
A long period of subduction can build an accretionary wedge that stands
above sea level.
Example…California’s Coast Ranges formed by this process.
Continental – Continental Convergence
At a convergence boundary, a collision between two plates carrying continental
crust will form folded mountains.
The reason for this is that continental crust is not dense enough, compared with
denser rock of the mantle, to be subducted downward.
Example…India collided with Eurasian Plate 45 million years ago forming
The Himalaya Mountains and the Tibetan Plateau.
Before that event, India was part of Antarctica.
The closing up of an the ocean between India and the Eurasian plate is an
example of how place motions can destroy a sedimentary basin.
Divergent Boundary Mountains
Most mountains are formed at convergent boundaries, but some are formed at divergent
boundaries, usually on the ocean floor.
These mountains form a chain that curves along the ocean floor at the ocean ridges.
This mountain chain is over 70,000 kilometers long and rises to 2000 to 3000 meters
above the ocean floor.
The mountains that form along ocean ridges at divergent boundaries are fault-block
mountains made of volcanic rock.
Ch. 11.3
Mountains and Plates
Lecture Outline
p.3
The mountains are elevated because of isostacy. Rock at the ridge is hotter and less
dense, so it rises higher than older, colder oceanic crust.
Non-Boundary Mountains
Volcanic mountains at hot spots, as well as some upwarped mountains and fault-block
mountains, can form far from plate boundaries.
Example:
Hawaiian Islands from hot spots
Southern Rocky Mountains and mountains of the Basin and Range region from
upwarping and faulting.
Southern Rockies began to form about 60 million years ago with the subduction of
an oceanic plate more than 1600 kilometers away.
The subducting plate separated the lithosphere above, allowed hot rock to
upwell from the mantle, pushing up the crust and forming the Southern
Rockies.
As the crust bent upward, tensional forces stretched and fractured it,
forming the fault-block mountains of the Basin and Range Region.
Continental Accretion
The process of accretion enlarges continental landmasses and forms
mountains along the edges of continents.
When fragments of crust collide with a continental plate, they become stuck to or
embedded into the continent through accretion.
Many of the mountainous regions rimming the Pacific have been produced
through collisions and accretion.
Terrane
Geologists refer to accreted crustal blocks as terranes.
A terrane is any crustal fragments that has a geologic history distinct from that of
the adjoining terranes.
Ch. 11.3
Mountains and Plates
Lecture Outline
p.4
How does the process of accretion work?
As oceanic plates move, they carry the embedded volcanic island arcs and
microcontinents along with them.
Eventually a collision between the crustal fragment and the continent occurs.
Small crustal pieces are peeled from the oceanic plate at a subduction zone. Thin
sheets of the crustal fragment are thrust onto the continental block. This newly
added material increases the width and thickness of the continental crust.
Mountains from Accretion
The accretion of larger crustal fragments, such as mature island arc, may result in a
mountain range.
These are small mountains that result from continent-continent collision.
Because of its buoyancy, or ability to float, an island arc will not subduct beneath
a continental plate. Instead, it will plow into the continent and deform both
blocks.
Example:
Areas in the mountains of Alaska and western Canada were found to
contain rocks, fossils, and structures that were different from those in
surrounding areas. These areas have been accreted to the western
margin of North America.