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Plate Tectonics PLATE TECTONICS Figure 1 Plate tectonics is the process that drives the recycling of Earth’s oceanic crust and the drift of continents on its surface. Plate tectonics is caused by heat flow within the Earth’s interior as well as density differences between the Earth’s crust and mantle. Many major geological processes like volcanoes, earthquakes, mountain formation, and the arrangement of tectonic plates (Figure 1) are all results of plate tectonics. Many of these processes occur constantly, but at such a slow rate they appear static from a human perspective. On a geologic time scale of about 200+ million years; however, plate tectonics can disassemble and then reassemble the world’s continents whilst recycling all of the oceanic crust between. This rearrangement is known as the Wilson cycle and its steps will form the basis for our understanding plate tectonics. The main steps are as follows: heat convection, rifting, sea floor spreading, subduction, and continental collision. HEAT CONVECTION Let us start with a simple idea: Earth’s interior is hot and space is cold. This thermal gradient drives the slow dispersion of heat from our planet. Some of the internal energy finds its origin in Earth’s formation approximately 4.6 billion years ago. The rest of this energy arises from radioactive decay of uranium, thorium and potassium within the mantle. In the metallic, liquid outer core heat can disperse easily due to rapid convection and conduction, but the mantle drastically hinders this cooling. The mantle is composed of solid, less conductive silicate rock. Although it’s solid, mantle material can rise upward very slowly (a few centimeters per year) in convection cells (Figure 2). This Figure 2 “fluid” behavior allows heat to eventually reach the asthenosphere (upper mantle). With the thermal energy is near the crust, the next step in plate tectonics can begin. RIFTING Typically, the heat of the asthenosphere slowly conducts through the solid crust and radiates into space, but in some special locations, known as hot spots, heat is concentrated enough to melt and break through the crust. Hot spots that occur in continental crust are key to our second step, rifting. Rifting occurs where continental crust thins and faults to form rift basins (Figure 3). In order to create a rift basin that divides an entire continent, multiple adjacent hotspots are required. The thinning of crust occurs above and between each hot spot until a large rift basin forms. Some rifts fail, but others continue to spread further apart until the sea floods the basin, forming two separate continents. The thinning of the crust continues under this new ocean until the asthenosphere finally reaches the surface. With little crustal mass overlying the asthenosphere, the hot rock near the center of the rift is no longer compressed into a solid and melts. A modern example of this rifting process is the African Rift Valley. Figure 3 SEAFLOOR SPREADING The third step in plate tectonics is seafloor spreading. The newly formed ocean fills what was a rift basin, but is now an ocean basin. The decompression melting described in the previous step allows molten mantle material to reach the surface and solidify into new oceanic crust. This crust formation occurs symmetrically around a series of linear ridges which lie at the center of the basin. The existence of these ridges is due to low density of the hot asthenosphere, which lifts the thin oceanic crust above it (Figure 4). As rock continues to solidify on either side the ridge, the oceanic crust moves away from the divergent boundary analogous to a conveyer belt. Unlike a conveyer, the crust also sinks downward with respect to distance from the ridge due to cooling of the lithospheric mantle attached below the crust. Since the oceanic crust is connected to each of the rifted pieces of continental crust, they continue to separate as new crust is placed between them. After at least 100 million years of cooling and thickening, the mean density of the oceanic plate furthest from the ridge becomes greater than the density of the Figure 4 asthenosphere and it starts to sink. At this point the next step of plate tectonics, subduction, can begin. SUBDUCTION Subduction is a process that occurs at a convergent boundary where either an oceanic plate sinks under a continental plate or under a younger, less dense oceanic plate. To maintain the consistency of the steps described thus far, we will focus on the first case. Also for simplicity, we will assume the continental plate on the right of Figure 5 is stationary relative to the subducting oceanic plate on the left. As the oceanic plate sinks beneath the continent, it tugs on the rest of plate in a process known as slab pull. Most geologists currently agree that slab pull is the primary force Figure 5 driving sea floor spreading, not the “slab push” that occurs at the ridge. The oceanic plate that sinks will continue descending into Earth’s interior until its recycled back into mantle material. This completes the mantle convection process mentioned in step one. Subduction is also responsible for other tectonic phenomena including volcanism and earthquakes. The volcanism stems from the unique manner at which subduction produces molten rock. Some of the ocean water trapped in the oceanic crust is brought down into the asthenosphere with it. Because of the higher temperature, the water evaporates from the oceanic crust and dissolves into the wedge of asthenosphere that lies between the subducting slab below and continent plate above (Figure 5). The solution of water vapor and mantle rock has a lower melting point than dry mantle rock, so melting occurs. This is similar to the freezing point depression that occurs when salt melts water ice. Now, the new magma rises through the continental crust due to its lower density. This process occurs parallel to the length of the convergent boundary, forming an inland volcanic arc. One well known example of an inland arc is the Cascades. If the magma does not make it to the surface, a non-volcanic mountain range is formed instead. The Sierra Nevada Mountains were created in this way. Earthquakes are another common feature of subduction. Where the oceanic and continent plates contact, significant pressure can build due to the static friction between the plates. This frictional “locking” does not halt the movement of the entire oceanic plate, but instead stops portions of the crust at a time. When the pressure is released, the locked area rapidly slips back like a spring to continue moving with the rest of the plate. Each such slip event is an earthquake. Earthquakes can occur at a variety of depths along the subducting slab, but typically near the surface. CONTINENTAL COLLISION The final step in plate tectonics is continental collision. Technically, plate tectonics was complete (with respect to oceanic crust) when the subducting slab was recycled in the previous step. Nonetheless, it’s appropriate to end plate tectonics with continental collision because it is the final step in the Wilson cycle; the cycle being synonymous to plate tectonics with respect to the continents. As a result of the complex interaction of multiple tectonic plates (Figure 1), the rate of seafloor spreading in a particular ocean basin may not match the rate of subduction in the same basin. This means that in some parts of the world continents are separating and in other parts they are converging together. In the latter where subduction is the dominant process (like of the Pacific Plate, Figure 1), opposite continents are expected to collide. Upon collision the two continental plates will halt the subduction after the last portion of oceanic crust sinks in the mantle. The continental plate on the left (Figure 5) is saved from subducting with the oceanic plate due to its lower density (greater buoyancy on the Figure 6 asthenosphere). Instead, the two continental plates fold and crinkle up into a mountain range as they suture together. This variety of mountain formation is often responsible form some of the highest mountain ranges in history like the Himalayas and the Appalachians. PLATE TECTONICS AS A WHOLE Over long time scales, plate tectonics plays a part many of geological processes that shape our planet. The order of steps chosen to describe plate tectonics logically starts with heat convection in the mantle, but in reality the original step in the cycle is not well constrained. Plate tectonics is an example of the chicken and egg paradox. Whether the upwelling of hot mantle to the surface drives the sea floor spreading or whether the cold, subducting plate drives the convection is unknown. The current consensus is that subduction drives the overall convection, but note that plate tectonics is a theory and will likely be modified in the future. Regardless, plate tectonics has created and destroyed oceanic crust for more than 4 billion years and will continue doing so for the next few billion. All the while, the continents have collided into super continents (like Pangea) and rifted apart (like present day) as they ride upon the relatively fluid mantle below. Many common phenomena like mountain building, volcanism, earthquakes, etc. are all results of the interaction of continental and oceanic tectonic plates. Figure 7 provides a holistic diagram of the steps in plate tectonics (excluding mantle convection). It shows hot spots, rifting, sea floor spreading, subduction, and other tectonic processes occurring simultaneously. This is just like the real Earth, where the cycle of plate tectonics is occurring on many locations; and at each site, displaying a different step in the process. Figure 7 REFERENCES Kious, W.
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