Plate tectonics and volcanism in the Caribbean and Bahamas

This discussion topic submitted by Susan Zoladz ( on 2/17/98.

Susan Zoladz
Tropical Marine Ecosystems
Plate tectonics and volcanism in the Caribbean and Bahamas

My discussion topic discusses what plate tectonics are and how they work, and it also discusses volcanism and how that relates to plate tectonics. I hope to teach the class about how the plates formed and what role they play on this earth. I also hope to show how plate tectonics are responsible for volcanism, and how this relates to the Caribbean and Bahamas.

I feel that my topic is important because it deals with the creation of our earth and what the earth is made of, and it is important to know what the earth consists of because it is responsible for so many of the changes that we see and even don't see. Plate tectonics relate to a lot more than what we may think, therefore it is important that the class understand plate tectonics and volcanism(which is due to plate tectonics).

Plate Tectonics and Volcanism
in the
Caribbean and Bahamas

I. Tectonic Plates
A. The crust of the earth is a spherical shell of rock that consists of a few rigid
1. There are only eleven giant plates at present, and many smaller plates.
a. Two of the giant plates appear to be in the process of splitting
B. These tectonic plates drift about continually, shifting position and jostling each
other. Consequently, the boundaries between them are marked by earthquakes.

II. Theory of plate tectonics
A. The earth's surface is broken into a number of hard plates. These plates are
not fixed in one place, they move slowly around the planets surface, pushed and pulled by
the flow of hot material deeper in the earth. This movement of the surface plates is
responsible for phenomena such as earthquakes and volcanoes, for the creation of
mountains, and for the larger features on the entire planet.

III. The plates
A. The hard, rigid plates that form the outermost portion of the earth are about 60
miles(100km.) thick. These plates include both the earth's crust and the upper
B. The rocks of the crust are composed mostly of minerals with light elements,
like aluminum and sodium, while the mantle contains some heavier elements, like
iron and magnesium form the surface plates are called the lithosphere. The plates
are supported by a weak plastic layer of the lower mantle called the asthenosphere.

IV. The edge of the plates
A. The movement of the lithospheric plates is responsible for earthquakes,
volcanoes, and earth's largest mountain ranges.
1. Volcanoes result from heat deep beneath the Earth's surface, which
melts rock material to form lava.
B. The sliding of one plate over another is not a smooth, continuous movement.
Sharp jerks occur as one plate grinds its way over the lower one, and these sudden
movements create earthquakes. These quakes originate down to about 430
miles(700km.) beneath the Earth's surface. Below this depth, the rocks are
probably too hot to be rigid, and they just ooze past one another.
C. A plate with ocean crust that is forced down under the upper plate heat up.
Eventually the rocks that form the plate will melt and become magma, the hot,
molten rock beneath the earth's surface. Magma may work its way up toward the
surface, where it flows out as lava and builds volcanoes. Thus, earthquakes and
volcanoes are both associated with collisions between lithospheric plates.

V. Formation of new crust
A. As collisions destroy lithospheric plates at their edges, new crust is created
where the plates pull apart. Places where new material is forming are called rift
zones or spreading centers. As plates pull away from each other, hot magma rises
into the break between the plates. This magma cools into rock and is slowly
carried away as the pates continue to pull apart. More magma then flows toward
the rift from below, and the process continues. This process of plate tectonics,
often called sea-floor spreading, helps explain sea floor studies showing that the
youngest rocks are found along the mid-ocean ridges- the spreading center.
B. Magmas are being formed at both ends of the moving lithospheric plates, both
at the oceanic ridges where new crust is being formed and along the subduction
zone where crust is apparently being consumed.
C. The three major ocean basins of the Earth- the Pacific, Atlantic, and Indian
oceans-were created as the sea floor spread, opening up between the surrounding
land masses.

VI. Volcanism
A. Volcanism is of five major types.
1. island -arc volcanoes
2. continental flood basalts
3. abyssal flood basalts
4. spreading centers
5. ocean-basin volcanoes
B. The subduction of lithospheric plates produces very long lines of regularly
spaced volcanoes parallel to oceanic trenches. At sea, the volcanoes form lines of
remarkably regular conical islands that extend over the horizon in a regular
geometrical perspective.

VII. Darwin's theory of the origin of atolls
A. He conceived that volcanic islands and that, in coral seas, the subsidence
transforms the reefs that fringe the volcanoes into barrier reefs and later into atolls.
Thus, different types of islands represent stages of development. With less
certainty, island can be compared and an atoll can be taken to be older than an
extinct volcano with a barrier reef.

VIII. Sources

1. Ballard, Robert D., Exploring Our Living Planet. The National Geographic Society,
Washington D.C., 1983.

2. Menard, H.W., Islands. Scientific American Books, Inc., New York, 1986.

3.Rossbacher, Lisa A., Recent Revolutions in Geology. Franklin Watts, New York,

4. Allegre, Claude, The Behavior of the Earth, Continental and Seafloor Mobility.
Harvard University Press, Massachusetts, 1988.

5. Raymo, Chet, The Crust of Our Earth. Prentice-Hall, Inc., New Jersey, 1983.

6. Bullard, Fred M., Volcanoes of the Earth. University of Texas Press, Texas, 1984.

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