Download 11.1 Where Mountains Form

11.1 Where Mountains
Form
Key Idea:
Events at plate boundaries and at the
continental margins result in the
formation of mountains.
Objectives:
1. Explain how some of the Earth’s major
mountain belts formed.
2. Compare and contrast active and passive
continental margins.
What is a mountain?
A mountain is a large mass of rock that rises
a great distance above its base.
Most mountains result from forces
associated with processes that occur at
convergent plate boundaries.
Mountain Belts
• Look at a physical map of the world. All the
mountain ranges that you identify on the map
(dark brown areas) have formed as a result of
plate tectonics interaction.
• Examples:
-The Ural Mountains (this is the natural border
between Europe and Asia)
-The North American Cordillera
-The Andes
-The Himalayan Mountains, etc.
Mountain Belts
Mountain Belts
• Most of the world mountains form along belts which tend
to follow convergent plate boundaries.
• These mountain belts are regions where mountains are
forming or have formed in the past.
• The Himalayan mountains are result of the Indian
subcontinent colliding with South Asia. They are young
mountains, which are still rising.
• The Appalachian or Ural mountains, on the other hand,
are very old and they have stopped rising long time ago.
They show the suture ( connection line) between older
plate tectonics, which have been active in in past.
A Model of Mountain Belts
Formation
Continental Margins
•
Continental margins are boundaries
between continental crust and oceanic
crust.
There are two types of continental margins:
1. Passive continental margins which do
not occur at plate tectonics boundaries.
2. Active continental margins, which occur
at the contact of a oceanic plate and a
continental plate.
Passive Continental Margin
Passive Continental Margins
• Passive continental margins are found along the
remaining coastlines. Because there is no collision or
subduction taking place, tectonic activity is minimal and
the earth's weathering and erosion processes are
winning. This leads to lots of low-relief (flat) land
extending both directions from the beach, long river
systems, and the accumulation of thick piles of
sedimentary debris on the relatively wide continental
shelves. Again South America provides a great example.
The Amazon River, whose source is in the Andes
Mountains (the active margin) drains east across the
interior of South America to the coast, where it enters the
Atlantic Ocean and deposits the tremendous volume of
sedimentary materials it eroded from the continent
Passive Continental Margins
• Passive continental margins do not occur at
plate boundaries. For example, the along the
east coast of North America the continental crust
thins out and changes progressively into oceanic
crust. Both continental and oceanic crust are
the makeup pf the North American plate, which
ends at the Mid Atlantic Rift.
• There areas are stable, and they accumulate
large amounts of sediments.
Active Continental Margins
• Mountain building takes place near active
continental margins.
• The active continental margins involve
subduction; the oceanic plate goes under
the continental plate, and all the sediments
are compressed, folded and risen.
• Let’s remember that these areas are also
accompanied by volcanic activity and
seismic activity as well.
Active Continental Margins
Active Continental Margins
• An active continental margin is found on the
leading edge of the continent where it is
crashing into an oceanic plate. An excellent
example is the west coast of South America.
Active margins are commonly the sites of
tectonic activity: earthquakes, volcanoes,
mountain building, and the formation of new
igneous rock. Because of the mountainous
terrain, most of the rivers are fairly short, and the
continental shelf is narrow to non-existent,
dropping off quickly into the depths of the
subduction trench.
Are Passive Margins and Active
Margins Related?
• Yes, they are!
• The passive margins provide the materials that
form mountains; a lot of sediments eroded from
the continent are carried and deposited at the
continental margin. The margin will become
heavier and heavier and it will sink.
• At some point, the plate will break apart and the
oceanic crust will sink and undergo subduction;
this area will eventually become an active
continental margin. The material accumulated by
erosion and transport will become the material
that will form the new mountains.