Download 1 Mountains and Mountain Ranges

Mountains and Mountain
Ranges
• Orogeny – the process of mountain building
– Crustal thickening
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Subduction zones
Sub-plate magmatic activity
Continental collision
Compressive forces “squeeze” crust together
– Thickening leads to isostatic adjustment
• Rising of less-dense continental crust
• Rising increases erosion, redistributes mass
Island Arcs
Andes(6,000 m) are the world’ s second highest
mountain chain (Bolivia)
Island arcs
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The Andes
• 190 mya (Jurassic) South America moves
west
• By 140 mya, Pacific plate had subducted to
accommodate motion
• By 130 mya, descending plate creates lots of
basaltic magma
The Andes: Subduction at a continental
margin
As the South American lithospheric plate
moved westward in early cretaceous time,
about 140 million years ago, a subduction
zone and a trench formed at the west coast
of the continent.
The Andes
• Rising magma
– Some rises to form volcanoes
– Some cools inside forming plutons
– Thickening leads to isostatic rise
– Once thick enough, soft rock beneath oozed
outwards creating thrust faults
• Sediments eroding from risen landscape
makes sedimentary rocks on both sides of
mountains
The Andes: Subduction at a continental
margin
By 130 million years ago, igneous
activity began and a subduction
complex and forearc basin formed.
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The Andes: Subduction at a continental
margin
The Himalayas rose
when a plate carrying
oceanic crust
converged with a plate
carrying continental
crust, followed by a
continent–continent
collision.
Pictured here is Lamoshe, in china’s
Sichuan Province.
By 90 million years ago, the trench and region of igneous
activity had both migrated eastward. Old volcanoes became
dormant and new ones formed to the east.
About 200 million years ago, India was part of a
large continent located near the South Pole.
The Himalayas
• 80 mya, India lithosphere fragment splits from
South Polar landmass
• India “gallops” (20 cm/yr) north, creates
subduction zone beneath Asia
– Creates “Andean” margin – granitic plutons,
stratovolcanoes, etc.
At that time India, southern Asia, and
the intervening ocean basin were parts
of the same lithospheric plate.
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By 80 million years ago, India was migrating
northward, approaching the equator.
The Himalayas
• 40 mya – oceanic lithosphere
between landmasses consumed.
Continents collide.
– Velocity decreases to ~5 cm/yr
– Both continental crust, neither can sink.
– India thrusts beneath Asia, crustal
thickness doubles
When India began moving northward,
the plate broke and subduction began
at the southern margin of Asia. By 80
million years ago, an oceanic trench
and subduction complex had formed.
Volcanoes erupted and granite plutons
formed in the region now called Tibet.
By 40 million years ago, India had
moved 4,000 to 5,000 kilometers
northward and collided with Asia.
When India collided with Tibet, the
leading edge of India was underthrust
beneath southern Tibet.
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The Himalayas
• Today:
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Contains all three rock types
Many sedimentary rocks from old sea-floor
Plateau is ~4000m, mountains rise from this
Crustal mass caused faulting as it spread
• Extensional (normal) faults in mountains
• Compressional (reverse/thrust) faults in foothills
continued plate movement has doubled
the thickness of continental crust,
creating the high Tibetan Plateau and
the Himalayas.
– India still moving north
– Alps, Urals, Appalachians all happened under similar
circumstances
Question
Question
• ________ is a fracture with
displacement.
• The right graph is showing a _____.
A.
B.
C.
D.
A.
B.
C.
D.
Fold;
Fault;
Joint;
None of the above
anticline;
Syncline
dome
basin
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