Earth Science - Writing Assignment 2

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1 Written Assignment 2 Kristi Stevenson Thomas Edison State University EAS-1010-OL009: General Earth Science Dr. Lemaire March 3, 2024

2 Convergent Plate Boundaries Convergent boundaries are regions where two or more tectonic plates collide. It can occur between oceanic plates or continental plates. There are three types of boundaries, oceanic-continental, oceanic-oceanic, and continental-continental. Oceanic-continental convergence is a type of boundary that is formed when an oceanic plate collides with a continental plate. Where the oceanic and continental plates meet there is a trench that is formed that is thousands of kilometers long that cuts deep into the ocean floor. These trenches are formed by subduction where the denser oceanic plate is pushed beneath the continental plate (U.S. Geological Survey, 2014). This creates a subduction zone and a volcanic arc, a chain of volcanoes hundreds of miles long. Many of earth’s active volcanoes are located in volcanic arcs. For example, the subduction zone of the Nazca plate under the South American plate has uplifted the Andes and created numerous volcanoes (Webb, 2020). This is evident on the western coast of the Americas, which is part of the ring of fire, “a path along the Pacific Ocean characterized by active volcanoes and frequent earthquakes” (National Geographic, n.d.-b). The second type of convergent boundary is the oceanic-oceanic boundary. This occurs when two oceanic plates collide. The older and denser plate subducts beneath the other plate, forming a trench deep in the sea. As the subducting plate sink into the mantle, it melts and forms magma. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island arc (U.S. Geological Survey, 2014). Other features include a forearc basin, marine depositional basins on the trench side of the arcs, and a backarc basin, sites of significant hydrothermal activity, on the opposite side of the arc (Shaw, 2018). An example of this type of boundary is the convergence of the Pacific and Mariana plates which includes the Mariana Islands arc and the Mariana Trench, the deepest part of the world’s ocean (Shaw, 2018).

3 The third type of convergent boundary is the continental-continental boundary. This occurs when two continental plates collide. But neither plate subducts because they are both light and resist downward motion. Instead, they continue to push against each other, causing the crust to thicken and fold, forming high mountain ranges. Since the continental lithosphere is too light to subduct deeply, a subduction zone or trench is not formed (Shaw, 2018). An example is the collision of the Indian plate with the Eurasian plate which formed the Himalayas. This slow conversion over millions of years pushed up the Himalayas to a towering height of 8,854 meters above sea level, forming the highest continental mountains in the world (U.S. Geological Survey, 2014).

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4 Characteristics of Continental and Oceanic Crust The two major types of crust that make up the earth’s lithosphere are continental and oceanic crust. They have different characteristics due to their composition and how they are formed. The chemical composition of oceanic and continental crusts is different. Continental crust is primarily composed of granite, also referred to as ‘sial’ by geologists. Sial stands for silicate and aluminum. Oceanic crust differs as it is mainly composed of basalts, known as ‘sima’ rocks. Sima stands for silicate and magnesium (National Geographic, n.d.-a) The thickness, density, and age vary between the ocean and continental crusts. The thickness of the continental crust is typically 35-40 kilometers. Its density is 2.7 grams per cubic centimeter. Its age is generally older than oceanic crust. It is formed when tectonic plates collide and continue to push into each other resulting in the formation of mountains. The oldest and thickest parts are at the world’s tallest mountain ranges. The oceanic crust is thinner and is generally 10 kilometers thick. It is denser and has a density of 3 grams per cubic centimeter. Oceanic crust is less than 200 million years old. It is formed at mid-ocean ridges where tectonic plates are moving apart. As a result, its age and density increase with distance from the mid- ocean ridges (National Geographic, n.d.-a). The differences between the continental and oceanic crust arise from their distinct formation process. The continental crust is formed when tectonic plates collide, while oceanic crust is formed at mid-ocean ridges where the tectonic plates are moving apart. The difference in composition between the mantle and the two layers of crust is maintained by a process known as partial melting (Bada, 2018). The formation processes lead to differences in thickness, density, and composition.

5 Age Difference Between the Seafloor and Continents The difference between the seafloor and continents is mostly due to plate tectonics and how they react to each other. The two main processes that lead to the differences in age are sea floor spreading and subduction. Oceanic crust is formed by seafloor spreading while continental crust is formed by subduction. Seafloor spreading is a processes where tectonic plates at mid-ocean ridges move apart due to mantle convection, a slow churning motion of the earth’s mantle. As magma rises through the cracks in the mid-ocean ridges the plates continue to diverge leading to the creation of new oceanic crust. This process which has resulted in a 50,000-kilometer-long system of mid- ocean ridges over millions of years. This process explains why oceanic crust is generally less than 200 million years old (National Geographic, n.d.-c) Subduction occurs when two oceanic crusts collide or when an oceanic crust collides with a continental crust. In the case of the two oceanic crusts the older denser plate subducts beneath the younger lighter plate. When an oceanic crust collides with a continental crust, the denser oceanic plate subducts beneath the lighter continental plate (Alden, 2020). As the subducted oceanic crust is pushed into the mantle, it melts due to the mantle’s heat, leading to its destruction or recycling. The continental crust is lighter and does not subduct and is therefore more stable and less likely to be destroyed or recycled. This stability along with the continuous creation and destruction of the oceanic crust results in the significant age difference between the seafloor and continents, with the continental crust being much older.

6 North American Plate’s and The Geological Activity The North American plate, which is the second largest tectonic plate on earth, encompasses North America, parts of the Atlantic Ocean, and extends to the North Pole. It also includes Siberia, the northern island of Japan, Greenland, Cuba, and the Bahamas (Earthhow, 2023). On the western edge of the North American Plate there is a region of active tectonics and volcanism due to the subduction of the Pacific Plate under the North American Plate. The subduction leads to the creation of volcanoes and magma which can rise and cause eruptions. The movement of the plates also results earthquakes. This occurs when a plate moves past another plate leading to earthquakes. Mountains are formed when the North American plate collides with other tectonic plates. The eastern boundary of the North American plate is a divergent boundary where the Eurasian and African plates meet at the mid-Atlantic ridge (Earthhow, 2023). Here the plates are moving apart creating new oceanic crust through sea floor spreading. In Alaska, which is a part of the Pacific Ring of Fire, the Pacific Plate is subducting beneath the North American Plate which leads to frequent earthquakes and volcanic activity. The volcanoes in Alaska are mostly stratovolcanoes and shield volcanoes, with notable examples being the Redoubt Volcano and Mount Spurr (Alaska.org, n.d.). The Pacific Northwest, including Washington and Oregon, is also part of the Pacific Ring of Fire. The Juan de Fuca Plate, a small oceanic plate, is subducting beneath the North American Plate. This subduction caused the formation of the Cascade Range and the associated volcanic activity. The region is also subject to earthquakes, landslides, and floods. The Cascade volcanoes are mostly stratovolcanoes, such as Mount St. Helens and Mount Hood (U.S. Geological Survey, 2021).

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7 California is marked by the San Andreas Fault, a transform boundary where the Pacific Plate is moving northward past the North American Plate. This movement causes frequent earthquakes. The largest recorded earthquake in California was the Great San Francisco Earthquake that occurred in 1906 with magnitude of 7.9 on the Richter scale. California also has both extinct and active volcanoes, which include stratovolcanoes, shield volcanoes, and cinder cone volcanoes. Notable examples are Mount Shasta and Lassen Peak (A, 2020). The East Coast of North America is considered a passive margin, generally free of volcanoes and significant earthquake activity due to its position within the North American Plate. There has been come volcanic activity in the past, such as approximately 50 million years ago when cinder cone volcanoes dotted Virigina. The region also experiences relatively large earthquakes. For example, the 2011 Virigina earthquake (Biello, 2024). The location and movement of the North American Plate significantly influence the geological activity in North America. This includes the formation of mountains, occurrence of earthquakes, and the creation of volcanoes.

8 African Rift The African Rift, also known as the East African Rift, is an active continental rift zone in East Africa. This region is where the earth’s tectonic forces are currently attempting to create new plates by splitting apart existing ones (Wheeler, 2024). The African Rift is a divergent plate boundary, which means that it’s a region where two tectonic plates are moving away from each other. The African Plate is splitting into two new tectonic plates, the Somali Plate and the Nubian Plate (Wheeler, 2024). The rate at which the African Rift is widening varies, but it is estimated that the eastern and western rifts are opening at a rate of more than 6.35 millimeters per year. Other sources suggest that the African Plate is splitting at a rate of 6-7 millimeters per year (Wheeler, 2024). In East Africa, spreading processes have already separated Saudi Arabia from the rest of the African continent which formed the Red Sea. The African Plate and the Arabian Plate are actively splitting and meet at the triple junction, where the Red Sea meets the Gulf of Aden. (U.S. Geological Survey, 2015). Geologists believe that East Africa could be the site of the next major ocean. If spreading continues, the three plates that meet at the edge of the today’s African continent will separate completely and allow the Indian Ocean to flood the area and turning the easternmost corner of Africa, the Horn of Africa, a large island (U.S. Geological Survey, 2015). This ongoing geological activity continues to shape the landscape of the Great Rift Valley, leading to the formation of numerous valleys, faults, fractures, and volcanoes (Wheeler, 2024).

9 References A, A. (2020, September 24). A guide to California’s volcanoes . California.com. https://www.california.com/guide-californias-volcanoes/ Alaska.org. (n.d.). Alaska Volcanoes Directory: List of over 40 volcanoes . ALASKA.ORG. https://www.alaska.org/guide/alaska-volcanoes-directory Alden, A. (2020, January 17). Subduction: The sinking of tectonic plates . ThoughtCo. https://www.thoughtco.com/what-is-subduction-3892831 Bada, F. (2018, August 3). Are there differences between continental crust and oceanic crust? . WorldAtlas. https://www.worldatlas.com/articles/are-there-differences-between- continental-crust-and-oceanic-crust.html Biello, D. (2024, February 20). When was the last time volcanoes erupted on the East Coast? . Scientific American. https://www.scientificamerican.com/article/recent-east-coast-volcano/ DiPietro, J. A. (2013). Volcanic arc . ScienceDirect Topics. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/volcanic-arc Earthhow. (2023, September 25). North American plate: Tectonic boundary map and movements . Earth How. https://earthhow.com/north-american-plate/ National Geographic. (n.d.-a). Crust . Education. https://education.nationalgeographic.org/resource/crust/ National Geographic. (n.d.-b). Ring of fire . Education. https://education.nationalgeographic.org/resource/ring-fire/ National Geographic. (n.d.-c). Seafloor spreading . Education. https://education.nationalgeographic.org/resource/seafloor-spreading/ Shaw, E. (2018, April 20). Three types of convergent boundaries . Sciencing. https://sciencing.com/three-types-convergent-boundaries-7501192.html U.S. Geological Survey. (2014, September 15). Understanding plate motions . The mechanism behind Plate Tectonics. https://ucmp.berkeley.edu/geology/tecmech.html U.S. Geological Survey. (2015, September 15). Understanding plate motions . Understanding plate motions. https://pubs.usgs.gov/gip/dynamic/understanding.html U.S. Geological Survey. (2021, March 30). Pacific Northwest geologic mapping: Northern Pacific Border, cascades and Columbia . Geology, Minerals, Energy, and Geophysics Science Center. https://www.usgs.gov/science/pacific-northwest-geologic-mapping- northern-pacific-border-cascades-and-columbia

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10 Webb, P. (2020, July 14). 4.6: Convergent plate boundaries . Geosciences LibreTexts. https://geo.libretexts.org/Bookshelves/Oceanography/Introduction_to_Oceanography_ %28Webb%29/04%3A_Plate_Tectonics_and_Marine_Geology/ 4.06%3A_Convergent_Plate_Boundaries Wheeler, K. (2024, March 3). What plate boundary caused the Great Rift Valley in Africa? . Geographic Pedia. https://www.ncesc.com/geographic-pedia/what-plate-boundary- caused-the-great-rift-valley-in-africa/

Earth Science - Writing Assignment 2

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