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Roger Weller, geology instructor
Puerto Rico Trench
by Jose A Hance
The Puerto Rico Trench
Living in Puerto Rico is a wonderful experience but people should know of all possible threats around a person home even when it is miles under water. Many people are not aware of what an oceanic trench is or how it is created. The Puerto Rico Trench is located at the deepest point of the Atlantic Ocean. The hazards posed by the Puerto Rico trench that threaten Puerto Rico and the Virgin Islands.
An interesting geological discovery is, the Puerto Rico Trench. Many people have very little knowledge of an oceanic trench as well as the dangers that can be caused by these trenches that are created beneath the sea.
Most of the habitants of planet earth are not aware of all the natural things that occur within the planet until it is too late and has caused an earthquake or a volcano to erupt. As stated in a recent article:
Ocean trenches happens at areas where two plates collide (converge). As convectional currents in the upper mantle converge and sink, this causes one plate to move towards the other and two plates collide. Plates collision causes denser oceanic crust (oceanic plate) to sink or slip beneath lighter continental crust (continental plate). Hence this is a zone of subduction, compression and destruction. Oceanic crust/plate is made up of heavy material (baslatic rocks). Along a collision plate boundary, thinner, denser oceanic crust is squeezed down and subducted along the subduction zone and melts in the magma of the upper mantle (asthenosphere). It is consumed, hence destroyed in the mantle beneath the continent (along the Benioff zone). Subduction produces a deep Ocean Trench on the ocean floor, parallel to the plate boundary. (Ocean)
Plates are constantly shifting and when two plates collide, they do not just crush which causes a trench that can lead to earthquakes and even tsunamis.
With an understanding of how trenches are formed, we can focus on one specific trench, the Puerto Rico Trench. The Atlantic oceans deepest part is located near the island of Puerto Rico and it holds the deepest trench giving it the name, the Puerto Rico Trench. Uri ten Brink states:
The Puerto Rico Trench is the deepest part of the Atlantic Ocean, with water depths exceeding 8,400 meters (figure 1). Its depth is comparable to the deep trenches in the Pacific Ocean. Trenches in the Pacific are located in places where one tectonic plate subducts or slides under another one. The Puerto Rico Trench, in contrast, is located at a boundary between two plates that slide past each other with only a small component of subduction. The trench is less deep where the component of subduction is larger. The unusually deep sea floor is not limited to the trench, but also extends farther south toward Puerto Rico. The Puerto Rico Trench is also associated with the most negative gravity anomaly on earth, -380 milliGal, which indicates the presence of an active downward force. Finally, a thick limestone platform, which was originally deposited in flat layers near sea level, is now tilted northward at a uniform angle. Its northward edge is at a depth of 4,200 m, and its southern edge can be found on land in Puerto Rico at an elevation of a few hundred meters. Many tectonic models have been proposed to explain this geologically fascinating, tectonically active region; however, none have gained acceptance, and the region remains poorly understood, largely because its underwater location makes it difficult to study.
Exploring the oceans has been an ongoing mission for centuries but the equipment used for underwater exploration can only withhold underwater pressure up to certain depths which has left scientist limited to the research that can be conducted under water.
Figure 1: Bathymetry of the northeast corner of the Caribbean plate.
Figure 2: Map of the North American - Caribbean tectonic plate boundary. Colors denote depth below sea level and elevation on land. Bold numbers are the years of moderately large (larger than about M7) historical earthquakes written next to their approximate location. Asterisk - Location of the January 12, 2010 earthquake. Barbed lines- boundary where one plate or block plunges under the other one. Heavy lines with half arrows - faults along which two blocks pass each other laterally.
The Puerto Rico Trench has been studied and the United States is aware of its presence but there are many dangers that can be caused by this specific trench. In some cases we have been told that the deadliest disasters that can happen is that of the one we do not hear nor see coming. As stated by Brink:
A survey to map tsunami and earthquake hazards in the northeastern Caribbean was carried out between March 28 and April 17, 2005, aboard the Spanish research vessel Hesperides and the Puerto Rican commercial tugboat Kruger B. The survey was conducted jointly by the University of Madrid, the Spanish Royal Naval Observatory, the U.S. Geological Survey (USGS) Woods Hole Science Center, and the Puerto Rico Seismic Network (operated by the University of Puerto Rico). It included detailed sea-floor mapping of a 24,000-km2 area south and southwest of Puerto Rico known as the Muertos Trough, where the Caribbean plate is possibly being thrusted or subducted under Puerto Rico and the Dominican Republic. The area has never been mapped before, and so the level of recent tectonic activity there is unknown. The second part of the survey was centered in a section of the Puerto Rico Trench north of the Virgin Islands, previously mapped by the USGS (see "Mapping of the Puerto Rico Trench, the Deepest Part of the Atlantic, Is Nearing Completion," Sound Waves, October 2003). The purpose of this part of the survey was to understand the reason for the high level of earthquake activity in the area and its potential hazard to Puerto Rico and the Virgin Islands. Ten USGS ocean-bottom seismometers were deployed here to 6,000-m depth to record acoustic waves generated by the Hesperides. The seismometers will stay on the sea floor for a period of 7 months to record local seismicity. This deployment will also help calibrate the University of Puerto Rico Seismic Network to locate earthquakes in the Puerto Rico Trench.
Above: USGS-WHOI ocean-bottom seismometer, one of 10 instruments deployed in the Puerto Rico Trench.
Seismic-reflection data, which provide vertical cross sections through the crust, were also collected and will augment ongoing USGS research in the area.
The third part of the survey included the deployment of land seismometers on the Dominican Republic, which recorded acoustic waves generated by the Hesperides south of the island. The survey included two stops, one in Ponce, Puerto Rico, and one in Roadtown, British Virgin Islands, where press conferences were held and school groups visited the ship. The USGS was represented by Uri ten Brink, Thomas O'Brien, and Edward Sweeney. Woods Hole Oceanographic Institution (WHOI) technicians Vic Bender and Dave DuBois and University of Puerto Rico scientist Jay Pulliam (also affiliated with the University of Texas Institute for Geophysics) deployed the ocean-bottom seismometers.
The trenches that run beneath the ocean floors pose a dangerous threat to the habitants of earth and constant studies continue to reveal how dangerous they can be. Earthquakes are constantly shaking the earth but we just don’t realize it until it is big enough to rattle our homes.
The Puerto Rico Trench is undergoing constant surveillance and exploration to come to an understanding of why there are constant earthquakes and anomalies within this specific trench.
Work Cited Page
"The Ocean Trench" Oracle Think Quest. 2011. Web. November 2011
Brink ten Uri. "The Puerto Rico Trench: Implications for Plate Tectonics and Earthquake and Tsunami Hazards “Revised December 04, 2006 by the Ocean Explorer. Web. November 29, 2011
Brink ten Uri. “Joint Spanish-United States Cruise Investigates Tsunami and Earthquake Hazards in the Northeastern Caribbean”