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Describe the characteristics, explain the formation, of deep sea trenches and island arcs
Island arcs and trenches are major structural features, together with oceanic ridges, of ocean
basins
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The arc is
convex toward the ocean and concave toward the continent with a deep trench running parallel to
the arc along the convex (ocean) side
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Ocean trenches are long, narrow depressions on the seafloor
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Ocean trenches are found in
every ocean basin on the planet, although the deepest ocean trenches ring the Pacific as part of
the so-called “Ring of Fire” that also includes active volcanoes and earthquake zones
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In particular, ocean trenches are a feature of convergent plate boundaries, where two or more
tectonic plates meet
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When the leading edge of
a dense tectonic plate meets the leading edge of a less-dense plate, the denser plate bends
downward
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Some ocean
trenches are formed by subduction between a plate carrying continental crust and a plate carrying
oceanic crust
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Ocean trenches formed by this continental-oceanic boundary are
asymmetrical
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On the inner slope (continental side), the trench walls are much
more steep
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The Peru-Chile Trench off the west coast of South America is formed by the oceanic crust of the
Nazca plate subducting beneath the continental crust of the South American plate
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island arcs are
formed where two lithospheric plates converge
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An island arc is built up from the surface of
the overriding plate by the extrusion of basalts and andesites
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These magmas then rise through fractures and melt their way through the overlying crust to be
extruded in the form of volcanoes
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If the
overriding plate is oceanic, then volcanoes are extruded underwater and may eventually rise high
enough to become islands
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