Ocean-Ocean Convergence - Free essay

When two plates come together, a convergent boundary is created. If at least one of the plates is oceanic, there will be subduction, the process by which part of the oceanic crust, individualized in a lithospheric plate, is submerged under another plate of a continental character. The subduction of a plate leads to the melting of the plate and the creation of volcanoes. Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arches, also known as volcanic lines.

Is your time best spent reading someone else’s essay? Get a 100% original essay FROM A CERTIFIED WRITER!

Order now

Ocean-continent Convergence

An ocean-continent convergence occurs when an oceanic plate collides with a continental plate, causing the oceanic plate’s subduction due to differences in density. The effect of this collision is the occurrence of major volcanic eruptions and intense earthquakes. An example is the 2004 Sumatra-Andaman earthquake, which occurred along a tectonic subduction zone where the India Oceanic Plate is being subducted beneath the Burma microplate, part of the larger Sunda plate.

Continent-continent Convergence

Continent-Continent convergence occurs when two continental plates collide. The collision of these plates results in the formation of metamorphic rocks and Earthquakes. Intense pressure from the collision of the two plates makes it difficult for magma to pass through the thick crust, thereby creating mountains instead of volcanoes. Continent-continent convergence creates some of the longest mountain ranges in the world. The Himalayas, the highest mountains in the world, and the Appalachian Mountains are examples of geological features formed when two continental plates collide.

2. Differences between Oceanic and Continental Crust

In the Earth’s internal structure, three layers can be distinguished: crust, mantle, and core. The crust is the outermost layer on which people live, while the core is the Earth’s center. Each of these layers has a different composition and density. The higher the density, the more a material weighs, which determines each layer’s position relative to the others.

The less-dense layers like the crust, are located above the mantle, which is denser, and hence located above the core, the densest layer of the three. Without becoming molten, the upper knob rocks have enough ductility to allow the tectonic plates to slide, as if the crust were floating on the mantle (Kious & Tilling, 1996). In this regard, density becomes one of the most important differences between oceanic crust and continental crust.

Oceanic Crust

The oceanic crust is formed by partial fusion of the mantle rocks. In areas where oceanic plates diverge, on mid-range ridges, magma flows from the mantle. When the magma solidifies, it turns into basalt rocks that form the oceanic crust. Basalt is a type of rock made up of magnesium, oxygen, and silicon. As the oceanic crust plates grow, the older rocks move away from the ocean ridges.

The oceanic crust has a density of approximately 3.0 g/cm 3, which is greater than the density of the continental crust, which partly explains why the continental crust has a higher elevation in comparison. Since the oceanic crust is denser, when it meets the continental crust, it sinks under it until it reaches the mantle and melts again, which occurs in the subduction zones.

Continental Crust

The continental crust has an average density of 2.6 g/cm 3. It is composed mainly of granite-like rock with abundant aluminum, oxygen, and silicon. Being less dense than the oceanic crust, the continental crust floats more freely on the mantle than the oceanic crust, allowing the continents to remain longer. The rate of creation and destruction of the continental crust is much slower than the oceanic crust, which is in constant recreation.

The continental crust is also considerably thicker than the oceanic crust, varying between 30 and 70 km compared to 8-10 km for the oceanic crust.

3. Age Difference between the Seafloor and Continents

The oceanic crust is subjected to a continuous recycling process by the magma that flows from the mantle through the oceanic cracks, making it always younger than the continental crust. While the oldest oceanic crust is only 200 million years old, there are areas of continental crust that are 4 billion years old. This model also explained all the geological activity throughout the western Pacific, where volcanoes and earthquakes are frequent phenomena. The argument is that the Atlantic Ocean is widening, at a rate of about two centimeters a year, while the Pacific Ocean is shrinking, which has the consequence that North America is slowly moving towards Asia.

The ascent of mantle material along the Central Ocean ridge system created new ocean floors. The convective movement of the mantle material transports the ocean floor in a way similar to how a conveyor belt moves to underwater trenches, where the ocean floor descends into the mantle. The expansion of the ocean floor occurs in the oceanic ridges, forming new oceanic crust through the activity of the central trenches and the gradual movement of the bottom away from the ridge. Therefore, as the ocean floor moves away from the ridge, it is replaced by new crust. The downward limb of a mantle convection current takes place in the vicinity of underwater trenches, with places where the oceanic crust is pushed back into the Earth’s interior. As a result, old portions of the ocean floor gradually eat away as they descend into the mantle, such as the Mid-Atlantic Ridge.

4. The location of North American plates and the geological activity occurring in North America

The North American plate is located where the Pacific oceanic plate meets the continental plate. The Cascadia fault, located at the bottom of the Pacific Ocean where the seabed meets the tectonic plate that houses the North American continent, has a length of 1126.54 kilometers and extends from Vancouver Island in Canada to Cape Mendocino in California. This fault is called subduction because the Juan de Fuca plate sinks below the North American plate forming the Cascadia subduction zone.

At a depth of about 30 kilometers, the subduction is blocked by friction between plates, but the seabed continues to sink and slowly accumulates great stress. When the subduction forces exceed the plates’ friction forces, they cause sliding of one on the other, causing an earthquake along the entire fault. The earthquakes in this subduction zone are the largest in the world, and it is the only zone that can produce earthquakes with magnitudes greater than 8.5 on the Richter scale.

The largest known earthquake in the continental United States occurred on January 26, 1700. It caused the coast to sink, drowning its forests and unleashing a huge tsunami that swept everything in its path through the Pacific, causing damage, even in Japan’s coastal towns. Its speed was such that the eastern half of the wave took approximately fifteen minutes to reach the North American coast, and the other half took ten hours to cross the ocean.

Mount Saint Helena is the most active volcano in the so-called Cascadia Volcanic Arc, an alignment of volcanoes in western North America that runs from northern California to British Columbia. The Cascadia Arc, where it is located, is over a geologically active region, in which the Juan de Fuca tectonic plate subducts under the northern part of the western edge of the North American plate. In Washington, Oregon, or California, the Volcanoes have an explosive profile and are often covered with snow and ice, projecting the danger of an eruption over long distances and into densely populated areas such as Alaska on the Island of Hawaii.

5. The African Rift

The African Rift plate exists at the boundary of two tectonic plates. The rift is widening, with several fractures up to 15 meters deep and 20 wide and kilometers are appearing as a consequence of several rifts that are separating the terrain: the East African, the Gregory, and the West, each marked by the presence of a multitude of volcanoes. The cracks have appeared in the Great Rift Valley, one of the most unstable terrains on the African continent. The fractures have appeared in the East African Valley, a fault in which the Somali and the Nubian plates are separating. The current fractures result from activity in the eastern part of the rift, in an area that runs through Ethiopia, Kenya, and Tanzania. Within tens of millions of years, these tectonic plates will separate the African continent in two, creating a large landmass larger than Madagascar.

References

Kious, W. J., & Tilling, R. I. (1996). This dynamic Earth: the story of plate tectonics. DIANE Publishing. https://pubs.usgs.gov/gip/dynamic/dynamic.html

UCMP Berkeley. Alfred Wegener (1880-1930). https://ucmp.berkeley.edu/history/wegener.html

UCMP Berkeley. Plate Tectonics: The Mechanism. https://ucmp.berkeley.edu/geology/tecmech.html