Describe the bathymetry of the ocean seafloor and how the features relate to plate tectonics
The geology of the earth’s surface has been shaped by two important processes: plate tectonics and continental drifting. Continental drifting is a concept that explains how the Earth’s continents move relative to each other. The theory was built on the hypothesis that the current earth’s continents were once a single landmass before they broke and drifted to their current positions (Thompson and a Turk p. 9).
As the continental plates drifted apart, several unique geologic features were either formed or deformed. Closely related to continental drifting is plate tectonics. Plate tectonics is a scientific theory that describes how the earth’s lithosphere moves (Thompson and a Turk p. 68). The word tectonics is derived from the Greek word τεκτονικός meaning “to build” while the plate is a geological term meaning large, rigid slab of solid rock (Thompson and a Turk p. 68).
Putting these two words and their meaning together, plate tectonics is, therefore, a theory that describes how the earth’s lithosphere is built with large, rigid solid rocks. The theory is built around the hypothesis that the earth’s lithosphere is made up of various plates that move relative to one another in three distinct ways: two plates moving in opposite direction forming divergent boundary; two plates rubbing against one another forming convergent boundary, and two plates sliding past one another forming transform boundary (Thompson and a Turk p. 73). Bathymetry, on the other hand, refers to the elevations of the ocean floor (Sandwell p. 121).
Just the same way as the land surface is covered by several mountains and valleys constituting the topography of the land, the depths of the ocean floor are also covered by several mountains and valleys constituting the bathymetry of the ocean floor. But how do these features relate to plate tectonics? As Sandwell states, plate tectonic processes are often reflected in the bathymetry of the ocean floor (p. 122). As had already been mentioned, the movement of plates results in the formation of three distinct boundaries: divergent, convergent and transform boundaries. Each boundary produces unique seafloor features.
Divergent boundaries are often associated with mid-ocean ridges also known as seafloor mountain ranges (“Plate Boundaries” par. 3). As the plates pull apart, they create a crack in between, which then triggers under sea mantle plumes (volcanic eruptions). The cooled lava attaches itself to the trailing edge of plates resulting in a ridge (“Plate Boundaries” par. 3). The process is rather slow but continuous. Further drifting apart of the plates leaves behind ridges looking like mountain ranges on the seafloor. The ridges currently present in our ocean floors must have taken many years to be formed.
Convergent boundaries, on the other hand, are associated with deep-sea trenches. Deep-sea trenches occur when two oceanic plates converge leading to subduction of one of the plates (“Plate Boundaries” par. 4). Oceanic-oceanic plate convergence is also associated with undersea volcanoes, which over the years may pile up and rise above the sea level as island arcs.
Finally, transform boundaries are associated with valleys on the seafloor. Transform boundaries occur where two plates slide horizontally past each other (“Plate Boundaries” par. 9). As the plates slide past one another, they produce zig-zag like plate margins, which often form the epicenters of shallow earthquakes. Another characteristic of a transform boundary is the faulting process, which results in the formation of undersea valleys deeper than even the rift valleys on the earth’s surface. Transform faults often occur as points of separation between ocean ridges.
The strongest part of an ocean transform fault is in the middle, capable of having the largest earthquakes. What is happening on either side of the transform fault?
Geologists have managed to draw a link between plate tectonics and significant geomorphologic phenomena such as earthquakes and volcanic eruptions. A close relationship has been observed between plate boundaries and these phenomena. For example, the Ring of Fire in the Pacific Ocean is characterized by a series of volcanic mountain ranges, oceanic trenches, and island arcs (Kious and Tilling p. 59). The region is associated with very high volcanic and earthquake activities located in the Pacific Ocean.
As had been mentioned in the previous discussion, transform boundaries (also known as transform faults) occur where two plates slide horizontally past one another (“Plate Boundaries” par. 9). Plates typically grind side by side past one another resulting in a zig-zag like fault line along the edges of the two plates. Transform faults usually offset oceanic ridges of divergent boundaries. However, there are few occasions where a transform fault disintegrates an oceanic ridge and trench. The most famous transform boundary is the San Andreas Fault, which separates the Pacific Ocean Plate and the North American Plate. Transform faults are often associated with earthquakes. San Andreas Fault is considered an earthquake hotspot in the world. But what usually happens on either side of the transform fault that causes earthquakes?
Transform faults occur were two faults are moving past one another in a parallel manner. Most often, transform faults separate ocean ridges, i.e., two divergent plates moving parallel to one another. At the ridge offset segment, there is a strong strip-slip abrasion along the edges of the two plates, the motion that triggers seismic waves and hence the high prevalence of earthquakes (The Open University p. 38). In short, the transform fault edges are characterized by a series of strip-slip abrasions that tend to tear the two plates apart. The two plates thus move horizontally past one another grinding and slipping against the edge of each other resulting in breakage of rocks along the edges and friction that eventually results in seismic waves and hence earthquakes.
Charles Darwin noticed that the geology of the Galapagos Islands suggested that the islands were sinking. Explain what he meant by this statement
Galapagos is an archipelago of islands located along the equator covering approximately 1000 kilometers along with the South American coast (“A Brief Introduction” par. 1). The islands are believed to be a result of mantle plumes triggered by the divergence movement of lithosphere plates (“A Brief Introduction” par. 2). Galapagos Islands were discovered in the 14th century by the English pirates who often used them as their hideouts.
Charles Darwin visited the islands in 1835 during his Voyage of the Beagle. While he was interested in studying the flora and fauna on these islands to aid his natural selection theory, he was as well moved by the geology of the Galapagos Islands and collected a series of geologic specimens. It is after he analyzed these specimens that he noticed that the geology of the Galapagos Islands suggested that the islands were sinking. His statement pointed to the submergence of islands.
Having noticed that the rocks in the highland were predominantly igneous and trachyte, he concluded that the islands must have resulted from active volcanoes. According to him, the Galapagos Islands were bound to submerge due to future volcanic eruptions that might weaken existing islands making them sink. He also suggested that the island could be eroded over time leading to a reduction in their elevations.
While Darwin’s explanation gave some insights on the submergence of islands, he failed to recognize the influence of plate tectonics in the emergence and submergence of the Galapagos Islands. The Galapagos Islands are mid-ocean ridges along the northern edge of the Nazca plate (Rothman par. 2). The Nazca plate is drifting southeast forming a divergent boundary with the Coco plate to the north and Pacific plate to the east. The Nazca plate further forms a convergent boundary with the South American Plate to the east.
This kind of movement is also characterized by transform faults, especially on the divergent boundaries. As the Nazca plate drifts further south, more mantle plumes are triggered to fill up the divergent boundaries. This will add more weight on the lithosphere forcing the mantle below to compress and hence the shrinking of islands. Another way to explain the shrinking of the Galapagos Islands is to look at it from a seafloor spread point of view.
As the Nazca plate drifts apart, it will likely take with it some materials from the islands causing it to reduce in size slowly, but continuously. The islands may also be consumed through seduction in the convergent boundary-making them reduce in size. This theory has been successful in explaining the submergence of the Hawaiian Islands that once protruded above the sea level, but are now partly submerged.
Works Cited
“A Brief Introduction to the Geology of Galapagos.” Galapagos Geology on the Web, 1997.
Kious Jacqueline and Tilling Robert. This Dynamic Earth: The Story of Plate Tectonics. Washington, DC: U.S Government Printing Office, 1996. Print.
“Plate Boundaries.” Plate Tectonics.com, 2010. Web.
Rothman, Robert. “Plate Tectonics and the Formation of the Galapagos Islands”. Formation of the Islands.
Sandwell, David. “Ocean Bathymetry and Plate Tectonics”. Our Changing Planet (121-124).
The Open University. The Ocean Basins: Their Structures and Evolution. Oxford: Open University, Walton Hall, Milton Keynes MK7 6AA and Butterworth-Heinemann, 1998. Print.
Thompson, Graham and Turk, Jonathan. Modern Physical Geology. Saunders College Publishing, 1991. Print.