11.3 Mountain Formation
I. Mountain Building at Convergent Boundaries
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Most mountain building occurs at convergent plate boundaries.
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Plates collide creating compressional stresses that fold, fault, and metamorphose layers of sediment.
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Oceanic-Oceanic Convergence:
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Subduction occurs.
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The melting of oceanic crust and subsequent rise of this new magma generates volcanic island arcs.
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Oceanic-Continental Convergence:
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Subduction occurs here, but generates continental volcanic arcs.
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A second mountain building process also occurs at the edge of the continental plate:
• Sediment scraped from the subducting oceanic plate is stuck on the landward side of the
subduction trench.
• This accumulation is called an accretionary wedge, and it can eventually build up tall enough
to be above sea level.
• The accretionary wedge consists of folded, faulted rocks resulting in folded mountains.
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Continental-Continental Convergence:
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A collision between continental plates creates folded mountains.
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EX: Himalayas, Ural Mountains in Russia
II. Mountain Building at Divergent Boundaries
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The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains.
III. Non-Boundary Mountains
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Volcanic mountains such as the Hawaiian islands form over hot spots far away from any plate boundary.
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Fault-block mountains can occur away from plate boundaries where regional extension or stretching is
occurring.
IV. Continental Accretion
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Accretion is the process by which smaller crustal fragments collide and merge with continental margins.
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Many Pacific rim mountains have formed this way.
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Accreted crustal blocks are called terranes.
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Have a distinct geologic history different from surrounding terranes.
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The accretion of larger crustal fragments such as a mature island arc may result in a mountain range.
V. Principle of Isostasy
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Isostasy is the concept that Earth’s crust is floating in gravitational balance upon the material of the
mantle.
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As material is added or subtracted, a new equilibrium is reached through isostatic adjustment: the
crust floats higher or sinks deeper as needed
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Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain
building and for a long period afterward.
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Erosion and uplift carve a mountainous landscape.