Were you ever curious of your “tectonic” heritage?  To ask it differently: what is the geological history of your native land?  A statement made in our last lecture prompted wonderful memories of the beautiful landscape where I spent the first 12 years of my life.  Not only did it generate memories, but it stirred in me a desire to find the tectonic explanation for the development of the island.

Dr. Kitchen explained that mountain building occurs when large pieces of material on the sub ducting plate are pressed into the overriding plates.  In the small Island of St. Croix (where I grew up) Mount Eagle stands proudly in the land scape.  The island is about 82 square miles in area.  Mount Eagle climbs to approximately 1, 096 feet (334.06 meters) above sea level.  As we sat on the front porch, my cousins and I would stare at the mountain and come up with all sorts of wild stories about living on the mountain.  Was the mountain still growing?  Was there a volcano on it?  How funny it is that things in life seem to come full circle.  Here I am an adult; once again musing about the mountain!  This time, I venture back to the island looking through the lenses of tectonics.

                Subduction zones are not only known by mountain building.  They are also known by the levels of volcanism and earthquakes.  Severe weather activity in the island comes in the form of hurricanes; not volcanoes and earth quakes.  But does this mean that Mount Eagle did not rise as a result of subduction?  How did the island come into being?

                St. Croix and the eastern Greater Antilles are on a belt of transitional lithospheres that may have been tied mainly either North American plate or the Caribbean plate (Speed, 2004).  Alternately, the belt may take up distributed plate boundary motion.  Plate tectonics provides few constraints on the relationship of St. Croix to nearby terrains.  The geological evolution of St. Croix permits or supports several ideas on present and past plate tectonics of the northeastern Caribbean.  First, the eastern Greater Antilles has probably undergone trans tension & the horizontal extension caused the breakup of the Puerto Rico and Virgin Island platforms, The Anegada Gap, and the present Puerto Rico trench.  That Puerto Rico and St. Croix were connected is pretty exciting. My parents are Puerto Rican and travel between the two islands is quite frequent.   Secondly, early Paleogene and/or late Cretaceous convergence affected the eastern Greater Antilles, probably due to a down going slab at the precursor to the present Puerto Rico Trench.  Thirdly, convergence occurred on the southern wall of the eastern Greater Antilles in the Late Cretaceous, causing the buildup that is now the Cretaceous basement of St Croix.

Mount Eagle Geography

                As it turns out, St. Croix occupies an important tectonic position in the Caribbean region, near the southern edge of the Greater Antillean ridge and its curve toward the south as the ridge becomes the Lesser Antilles.  Also, it sits near the boundary between the Greater Antillean transitional lithosphere and the oceanic lithosphere of the Venezuelan Basin.  Unfortunately, the plate tectonics of the Caribbean region are so little constrained that is difficult to assign paleographies and paleotectonics to a small terrain such as the St. Croix platform.  However, the tectonic evolution of St. Croix provides conditions for models of the plate-tectonic history of the larger northeastern Caribbean.

Works Cited:

     Speed, R. C. (2004).  Tectonic Evolution of St. Croix: Implication for Tectonics of the Northeastern Caribbean. Evanston, IL : Northwestern University.

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