Subduction, Accretion, and the Geologic Evolution of Costa Rica

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Costa Rica is not a piece of North America in the same way that everything to the north is. From Nicaragua on north, if you drill deep enough, you'll encounter rocks hundreds of millions of years old and more. Everything north of Costa Rica is the result of breaking chunks off the ancient North American Plate and shuffling them around by plate tectonics. Costa Rica is more like Panama, Cuba, or the other Greater Antilles, a large land mass created by long continued subduction.

This page is not an attempt to reconstruct the history of Costa Rica in detail. Rather, it is intended to explain how plate tectonics can create a large land mass at a long-lived subduction zone.

Formation of an Island Arc

The 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.

When the descending plate reaches about 100 kilometers deep it begins to be heated.

The descending plate has been soaking in sea water for many millions of years and is wet. Heating drives water into the surrounding mantle.

Water doesn't affect just the earth's surface but its deep interior as well. Water lowers the melting point of rocks and causes the adjacent mantle to begin melting. Magma rises upward.

Magma ponds beneath the crust. Light minerals rise and dense minerals sink, a process called differentiation. The magma that rises further into the crust is more silica rich than the mantle.

Some magma penetrates the crust and builds a volcano. Other magma may intrude the crust but not reach the surface. At this point the arc is very much like the Mariana island arc in the Pacific or the Aleutians.

Obduction

Continued 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.

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.

At times, a sliver of oceanic crust may break off and ride onto the other plate as well to form an ophiolite.

Submarine volcanic plateaus may also collide with the volcanic 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.

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.

Actually, Cuba, the other Greater Antilles, Costa Rica and Panama got a big head start in this process because they were constructed not on ordinary ocean crust, like the Marianas or Aleutians, but on the edges of a thick submarine volcanic plateau.


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Created 18 January 2008, Last Update 14 December 2009

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