Earth’s Layers: Lithosphere, Plate Tectonics, and Major Boundaries

Lithosphere

The lithosphere is the solid, outer part of Earth. The lithosphere includes the brittle upper portion of the mantle and the crust, the outermost layers of Earth’s structure. It is bounded by the atmosphere above and the asthenosphere (another part of the upper mantle) below.

• Scientists subdivide the lithosphere horizontally into tectonic plates
• There are 2 types of lithosphere:
• Oceanic: The oceanic crust associates with ocean basins and is 50–140 km thick.
• Continental: The continental crust associates with continents and 40 – 280 km thick

• The pedosphere is the uppermost layer of the lithosphere that, through the soil-forming process, chemically reacts with the atmosphere, hydrosphere, and biosphere.
• In 1911, A.E.H. Love described the concept of this layer
• Geothermal gradients in continental areas are commonly about 25 °C km⁻¹

Earth divided in 3 layers

• Crust
• Mantle
• Core

Plate Tectonics

• Plate tectonics is a scientific theory that explains how Earth’s surface is made up of large, rigid plates that move around over time. This theory also explains how these plates move and form mountain ranges, ocean basins, and volcanoes.

• Scientists define four types of boundaries between tectonic plates based on the movement of the plates: divergent and convergent boundaries, transform fault boundaries, and plate boundary zones

Divergent boundaries occur when plates pull away from each other. Scientists call the area between the plates the rift zone.

Convergent boundaries happen in places where two plates meet, which means two plates move towards each other.

Transform fault boundaries occur when two plates slide past each other, creating a transform fault or boundary.

These boundaries mostly occur on the ocean floor, although some appear on land.

•  The surface of the Earth is divided into 7 major and 8 minor plates. The largest plates are the Antarctic, Eurasian, and North American plates. Plates are on average 125km thick, reaching maximum thickness below mountain ranges. Smallest – Juan de Fuca

Image 3. world’s major and minor tectonic plates (World Atlas)

•  Seven major tectonic plates which cover nearly 95% of the Earth’s surface. 
•  The Pacific Plate is approximately 103,300,000 square kilometers in size.   Found underneath the Pacific Ocean, it is the largest of all tectonic plates.

 Structure of Earth

Crust

•  The crust is the outermost solid part of the earth.

• It is fragile.
• The thickness of the crust varies under the oceanic and continental areas.
• Oceanic crust is thinner as compared to the continental crust.
• The continental crust is thicker in the areas of major mountain systems.
• heavier rocks having a density about 3 g/cm3.
• The kind of rock seen in the oceanic crust is basalt.
• The mean density of material in the oceanic crust is 2.7 g/cm3.
• Silica (Si) and Aluminium (Al) are major constituent minerals. Hence it is often termed as SIAL.

Mantle

• The portion of the interior beyond the crust
• It is in a solid-state.
• It has a density higher than the crust portion.
• The thickness ranges from 10-200 km.
• The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
• The asthenosphere is the upper portion of Mantle.
• It is the chief source of magma that finds its way to the surface during volcanic eruptions.
• The crust + the uppermost part of the mantle = the lithosphere.
• The major constituent elements of the mantle are Silicon and Magnesium and hence it is also termed as SIMA

Core

• The core-mantle boundary at the depth of 2,900 km.
• The inner core is in the solid-state whereas the outer core is in the liquid state.
• Heavy materials, mostly nickel and iron, make up the core. Hence, people also call it the ‘nife’ layer.

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Discontinuity

There are five discontinuities inside the earth.

1. Conrad Discontinuity: Transition zone between SIAL and SIMA.
2. Mohorovic Discontinuity: Transition zone between the Crust and Mantle.
3. Repiti Discontinuity: Transition zone between Outer mantle and Inner mantle.
4. Gutenberg Discontinuity: Transition zone between Mantle and Core.
5. Lehman Discontinuity: Transition zone between Outer core and Inner core.

Most Abundant Elements in Earth’s Crust

Most Abundant Elements in Earth’s Mantle

1. Oxygen             44 %
2. Magnesium       22.8 %
3. Silicon               21.5 %
4. Iron                   5.8 %
5. Calcium            2.3 %
6. Aluminium       2.2 %

Most Abundant Elements in Earth’s Core

• Iron             85 %
• Nickel         5 %
• Other           10 %

 Overall, Most Abundant in Earth (Entire Earth)

1. Iron             35%
2. Oxygen       30%
3. Silicon         15%