Plate tectonics cont
Mid-ocean Ridge
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Mid-ocean ridges are elongate, relatively narrow chains of basaltic volcanoes that
traverse the ocean floor, rising to a relief of 2 to 2.5 km above the surrounding
abyssal plain
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In some cases, a narrow axial trough runs down the center of the ridge
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The high-heat flux at the ridge melts mantle material to form magma, which is
relatively light and rises to the surface
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Some of the magma crystallizes beneath the surface (as gabbro or in thin
basaltic dikes)
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Some erupts to form volcanic lava, which flows and ultimately solidifies to form
pillow basalt
Age of ocean sediments
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Sediments atop oceanic basalts become thicker with increasing distance from
midocean ridges as spreading continues
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The lower-most (oldest) layers become progressively older with increasing
distance from the ridge as well
Oceanic vs. continental lithosphere
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The crustal part of the oceanic lithosphere is thinner, more mafic (largely basalt),
and more dense, compared to the granitic and thicker crust of the continental
lithosphere
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The mantle is essentially identical beneath each type of crust.
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Only oceanic lithosphere is dense enough to be subducted at convergent
boundaries
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When continental lithosphere is pushed (by a ridge) and pulled (by a leading edge
of subducting oceanic lithosphere) into another continent, a mountainous
collision zone is formed
– two plates involved become sutured together
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Conversely, a single large plate can become rifted apart when its lithosphere is
stretched, thinned, and broken apart by a new mid-ocean ridge
Ocean trenches
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Deep-ocean trenches are elongate arcs where ocean depths reach down to as much
as 12 km
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The trenches border chains of volcanoes at subduction zones
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A seamount chain is an elongate series of former volcanic islands that have
subsided below sea level.
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Sediments scrape off the subducting plate to form an accretionary prism at the
edge of the overriding plate
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Behind the prism, melting associated with the subducting plate produces either a
volcanic continental arc or a
volcanic island arc
Subduction nec. on non-expanding earth
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The introduction of new crust at the mantle causes subduction to be a topological
necessity. Unless the Earth expands (or its shape is dramatically altered), surface
area remains constant
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Gravity prevents rocks from breaking off at the surface and floating into space, so
any new surficial material must be balanced by a loss of surficial material through
subduction.