Plate Tectonics and Crust Oceanic Lithosphere

Table of Content

The Lithosphere is composed of the top part of the crust and upper mantle. It varies in thickness, ranging from 31 to 62 miles (50 to 100 km) beneath oceans and 93 miles (150 km) on land. Within the lithosphere are minerals, rocks, and soils that are used for construction, metals, and agriculture. It consists of the upper crust, which is approximately 3 miles (5 km) thick in oceans and 40.3 miles (65 km) thick on land, as well as the rest of the upper mantle. The lithosphere can be divided into two categories: oceanic crust and continental crust. Oceanic lithosphere forms at spreading centers on oceanic ridges and exists as the outermost layer below oceans.

The oceanic crust is composed of multiple layers and has a thickness of approximately 6 km (4 miles) excluding the sediment above it. The topmost layer consists of basalt lavas with a thickness of about 500 meters (1,650 feet). Unlike the continental crust, the oceanic crust is thinner, denser, younger, and has a unique chemical composition. It is formed above subduction zones. In contrast, the continental crust mainly consists of lighter granite and varies in thickness from 20 to 70 kilometers.

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The areas with the thinnest parts of continental crust, which have a density of about 2.7 grams per cubic centimeter, are found in regions such as the Rift Valleys of East Africa and the Basin and Range Province in the western United States. An example of one such area is the Basin and Range Province situated in Nevada, stretching approximately 1500 kilometers wide and 4000 kilometers long (North/South). In contrast, mountain ranges possess thicker continental crust that extends into the mantle. This movement of Earth’s surface is caused by plate tectonics, where large plates constituting the surface constantly move at a rate of few centimeters per year, resembling pieces of a giant jigsaw puzzle.

The ocean floors are constantly moving, expanding from the center and sinking at the edges. Convection currents beneath the plates drive them in various directions. The heat that powers these convection currents is generated by radioactive decay occurring deep within the Earth. Plate boundaries, where the plates meet, experience intense geological activity like earthquakes, volcanoes, and mountain building. The Earth’s crust is divided into 12 major plates that move in different directions.

The movement of plates leads to their collision, separation, or rubbing against each other, creating certain Earth structures called “tectonic” features. The term “tectonic” describes how the crust deforms due to plate interaction. There are three types of tectonic boundaries: divergent (where plates move away from each other), transform (where plates slide sideways in relation to one another), and convergent (where plates move towards each other). Divergent boundaries involve magma rising from deep within the Earth’s mantle and pushing apart two or more plates.

Mountains and volcanoes are created along the seam, causing vast basins to expand and renew the ocean floor. The Earth’s oceans are connected by a single mid-ocean ridge system, which is the longest mountain range on Earth. On land, large troughs like Africa’s Great Rift Valley form when plates separate from each other. If this plate separation continues in eastern Africa, it will eventually detach from the continent and create a new landmass. In this scenario, a mid-ocean ridge would serve as the boundary between these plates. Additionally, California’s San Andreas Fault demonstrates a transform boundary where two plates move past each other through strike-slip faults.

Convergent boundaries occur when a plate of ocean subducts under a landmass, causing the overlying plate to lift up and form mountain ranges. These boundaries also often result in volcanic eruptions and the formation of deep trenches, such as the Mariana Trench. The halting motion at these boundaries can trigger large earthquakes, such as the 1906 one that devastated San Francisco.

When two tectonic plates come together, they can create different kinds of collisions with various geological effects. One result is the formation of island arcs through underwater volcanic activity. There are three main types of convergent plate boundaries: continent-continent collision, continent-oceanic crust collision, and ocean-ocean collision.

During a continent-continent collision, mountains like the European Alps and the Himalayas are formed. In a continent-oceanic crust collision, subduction occurs as oceanic lithosphere sinks beneath continental lithosphere. This process causes the oceanic lithosphere to heat up and lose moisture, leading to volcano formation. The Andes exemplifies this type of collision.

Ocean-ocean collision occurs when two oceanic plates collide, causing one plate to sink into the mantle and creating a subduction zone. This results in deep trenches in the ocean floor, such as the Mariana Trench which reaches a depth of 11 km. Volcanoes and earthquakes are often associated with these types of plate boundary collisions.

Volcanoes are elevated formations that reach into a reservoir of molten rock beneath the Earth’s surface. When pressure intensifies, eruptions occur, causing the forceful emergence of gases and rocks through openings. This results in diverse phenomena such as the spreading or filling of air with fragments of lava. Moreover, eruptions can generate lateral blasts, flowing lava, hot ash flows, mudslides, avalanches, falling ash, and floods. Volcano eruptions have even been accountable for the obliteration of entire forests.

In addition to these consequences, volcano eruptions can also activate tsunamis, flash floods, earthquakes,mudflows and rockfalls. Volcanoes can take shape via subduction or rifting processes when hot mantle plumes breach the surface within a tectonic plate.

A hotspot is an area on the Earth’s surface with continuous volcanic activity, caused by a hot mantle plume rising from the core-mantle boundary through the crust to reach the surface. The Hawaiian island chain serves as an illustration of hotspot volcanoes, where a tectonic plate sliding over a stationary hotspot forms a sequence of volcanoes. These volcanoes become progressively younger as one travels along the chain.

Earthquakes occur when there is a sudden movement in the Earth’s lithosphere, consisting of its crust and upper mantle. These movements can be caused by stress release along geologic faults or magma movement in volcanic regions. Similar to volcanoes, earthquakes are not evenly distributed worldwide but instead tend to happen at plate boundaries. The friction between plates causes them to become stuck together, and when the accumulated energy is eventually released and the plates separate, earthquakes occur. There are three main types of plate boundaries where earthquakes form: Divergent, Convergent, and Transform.

Earthquakes can happen when there is movement and slipping at various types of boundaries. The earthquake’s depth depends on the type of movement, with shallow and deep earthquakes being possible. Most tectonic earthquakes occur at depths of less than tens of kilometers. However, in subduction zones where old and cold oceanic crust goes under another tectonic plate, “Deep Focus Earthquakes” can occur at much greater depths, reaching up to seven hundred kilometers. These earthquakes occur at depths where the subducted crust should no longer be brittle due to the high temperature and pressure.

The presence of tectonic faults and magma movement within volcanoes in volcanic regions can lead to deep focus earthquakes, which serve as indicators of volcanic eruptions. Furthermore, massive earthquakes have the potential to generate tsunamis. The lithosphere plays a critical role as it houses important resources that humans rely on. Many materials essential for daily use come from the lithosphere, supplying key elements for various goods. Moreover, the lithosphere is a primary source of vital fuels like coal, petroleum, and natural gas that are necessary for life as we currently know it.

The lithosphere, hydrosphere, and atmosphere are crucial for the development of plants and animals as they provide necessary nutrients for sustaining life. Furthermore, the lithosphere acts as a protective barrier against the Earth’s extreme heat, safeguarding us and other organisms. This shield allows water to remain in its liquid state, which is essential for the survival of carbon-based life forms. Additionally, the lithosphere contributes to maintaining Earth’s stability by exerting gravitational force on inner layers. This force guides elements towards the planet’s radioactive core, promoting a steady balance through nuclear fusion.

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Plate Tectonics and Crust Oceanic Lithosphere. (2016, Oct 16). Retrieved from

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