59 A Subduction ZoneThe story begins where two oceanic plates converge and one sinks into the earth's mantle, a process called subduction. In these diagrams light green indicates basaltic oceanic crust, dark green is a layer of the earth's mantle about 100 kilometers thick that forms the base of the plate. These two units collectively are called the lithosphere. Brown represents deeper mantle and red represents young igneous rocks. An ocean trench marks the place where the descending plate sinks into the mantle.
68 Exotic TerranesContinued subduction builds a larger volcanic chain and the weight of the volcanoes causes the crust to sag. Erosion off the volcanic arc sheds sediment onto the flanks. Eventually, convergence of the plates may bring a submarine volcano, or seamount, into the subduction zone.
69 When the seamount enters the subduction zone, something has to give.
70 Often the seamount is thrust onto or beneath the arc.
71 When the seamount reaches the subduction zone, it may be shoved under the other plate or may break off and be thrust onto the other plate. Sediment accumulating in the trench also may get shoved onto the arc as well. This process is called obduction.
72 At times, a sliver of oceanic crust may break off and ride onto the other plate as well to form an ophiolite.
73 Submarine volcanic plateaus may also collide with the volcanic arc.
74 The submarine volcanic plateau begins to be thrust onto the arc The submarine volcanic plateau begins to be thrust onto the arc. It is too thick to be subducted so it will either be thrust onto the arc, or it will cause the subduction zone to relocate somewhere else.
75 Here a submarine volcanic plateau has been added to the arc Here a submarine volcanic plateau has been added to the arc. Seamounts, ophiolites and volcanic plateaus are all made of igneous rock but ophiolites, in particular, have a distinctive structure that sets them apart.
76 Eventually so much material can be added to the arc that the subduction zone clogs and a new subduction zone forms. Repeating this process over 100 million years can build up a very sizable land mass like Cuba, the other Greater Antilles, or Costa Rica and Panama.
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