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Earth consists of three distinct layers: the crust, mantle, and core, each with unique physical and chemical characteristics.

The crust, composed of silicate solids, forms the Earth's outermost shell. Beneath it lies the mantle, a layer of viscous molten rock. Deeper still is the core, divided into an outer layer of viscous liquid and an inner layer of dense solid. These layers interact to shape the Earth's structure and influence its geological processes.

Different layers of the Earth

Earth's layers are categorised into distinct strata, including the lithosphere, asthenosphere, mesospheric mantle (a segment beneath the lithosphere and asthenosphere), outer core, and inner core, each distinguished by unique physical and chemical attributes. These layers are further divided into the crust, upper mantle, lower mantle, outer core, and inner core, delineating the chemical composition of the Earth's structure.

Crust of the Earth

The Earth's crust, the outermost layer of our planet, plays a crucial role in shaping its surface and supporting life.

Composition and Density

• The crust comprises primarily solid rocks and minerals.
• It accounts for a small percentage of the Earth's volume and mass, approximately 0.5-1.0% of its volume and less than 1% of its mass.
• Density increases with depth, with an average density of around 2.7 g/cm³, compared to the Earth's average density of 5.51 g/cm³.

Thickness and Variation

• The thickness of the crust varies significantly depending on location.
• In oceanic regions, the crust ranges from 5 to 30 kilometres thick, while in continental areas, it can extend from 50 to 70 kilometres.
• Major mountain systems, like the Himalayas, can have exceptionally thick continental crust, reaching depths of 70-100 kilometres.

Temperature Profile

The temperature of the crust increases with depth.

Near the boundary with the underlying mantle, temperatures range from approximately 200°C to 400°C.

In the upper crust, temperature rises by as much as 30°C for every kilometre in depth.

Layered Structure

• The crust consists of several layers, including sedimentary material forming the outermost layer.
• Beneath the sedimentary layer lie crystalline, igneous, and metamorphic rocks, contributing to the Earth's diverse geological composition.
• The lowest layer of the crust comprises basaltic and ultrabasic rocks, adding to the structural complexity of the crust.

Mohorovic Discontinuity (Moho)

• The Moho is a seismic velocity discontinuity marking the boundary between the crust and the asthenosphere, the upper portion of the mantle.
• Depth Variation: It exists at different depths beneath ocean basins (around 8 kilometres) and continental surfaces (approximately 30 kilometres).
• Cause: The Moho is believed to be caused by a change in rock composition, transitioning from feldspar-containing rocks to feldspar-free rocks, leading to variations in seismic wave velocities.

Lithosphere

• The lithosphere, Earth's outermost rigid layer, plays a pivotal role in shaping its geological features. 
• The lithosphere ranges from 10 to 200 kilometers in thickness.
• It encompasses both the crust and the upper mantle.

Tectonic Plates

• The lithosphere is divided into tectonic plates, also known as lithospheric plates.
• These plates are in constant motion, generating significant geological changes such as folding and faulting.

Heat Sources

• Primordial heat from Earth's formation, along with radioactive decay of elements like uranium, thorium, and potassium in the crust and mantle, drive plate tectonics.

Asthenosphere

The asthenosphere, lying beneath the lithosphere, is a crucial layer with distinct properties:-

• Depth and Characteristics
• Extending from 80 to 200 kilometres below the surface, the asthenosphere is softer and more plastic than the lithosphere.
• Its viscous, brittle, and ductile nature allows the lithospheric plates to move easily over it.
• During volcanic eruptions, the asthenosphere serves as the primary source of magma.

The Mantle

The mantle, comprising the majority of Earth's volume and mass, is a complex layer with diverse features:-

Composition and Density

• It constitutes approximately 83% of Earth's volume and 67% of its mass.
• Density varies within the mantle, ranging from 2.9 to 5.7 g/cm³.

Chemical Composition

• Silicate rocks high in iron and magnesium dominate the mantle's composition.
• Basic elements include oxygen (45%), silicon (21%), and magnesium (23%).

Temperature Profile

• Temperatures within the mantle range from 200°C at the upper border to nearly 4,000°C at the core-mantle boundary.
• Convection currents, driven by temperature differentials, facilitate material circulation within the mantle.

Seismic Activity

• Despite high-pressure conditions inhibiting seismicity, earthquakes have been detected in subduction zones up to 670 kilometres below the surface.

The Earth’s Core

The core, Earth's innermost layer, is primarily composed of heavy materials, predominantly nickel and iron. 

Composition

• Mostly it consists of nickel and iron, often referred to as the NIFE layer.
• Divided into the outer core and the inner core.

Core-Mantle Boundary

• Located at a depth of 2900 km.
• Marked by the Gutenberg discontinuity, characterised by a sudden density increase from 5.5 g/cm³ to 10 g/cm³.

The Outer Core

The outer core surrounds the inner core and extends from 2900 to 5100 kilometers below the Earth's surface.

Composition

• Comprises iron with nickel and trace amounts of lighter metals.
• It remains in a liquid state due to insufficient pressure to solidify, despite sharing a similar composition with the inner core.

Magnetic Field Generation

• The Earth's magnetic field is believed to be generated by convection currents in the outer core, coupled with the Coriolis effect according to dynamo theory.

The Inner Core

Extending from the Earth's center to 5100 kilometers below the surface, the inner core is solid and exhibits distinct characteristics:-

Solid State

• The inner core is solid, capable of transmitting shear waves (transverse seismic waves).
• Rotation: It rotates slightly faster than the Earth's surface.
• Magnetic Field: Due to its high temperature, a persistent magnetic field cannot be maintained in the solid inner core.
• Volume and Mass: Despite occupying only around 16% of the Earth's volume, the core (inner and outer combined) accounts for 33% of its mass.

Seismic Discontinuities

Seismic discontinuities are abrupt changes in seismic wave velocities within the Earth's interior. They provide crucial insights into Earth's structure and composition. Notable discontinuities include:-

Mohorovicic Discontinuity (Moho)

• Marks the boundary between the Earth's crust and mantle.
• Named after Andrija Mohorovičić, it represents a transition from low-velocity crustal rocks to denser mantle rocks.

Gutenberg Discontinuity

• Separates the Earth's mantle from the outer core.
• Characterised by a significant drop in velocity for primary seismic waves (P-waves) and the absence of secondary seismic waves (S-waves) in the outer core.

Conclusion

Earth's layers, comprising the crust, mantle, and core, form a dynamic system essential for understanding our planet's geological and physical processes. From the outer crust supporting life to the inner core generating Earth's magnetic field, each layer plays a distinct role. Seismic discontinuities like the Moho and Gutenberg Discontinuity provide valuable insights into Earth's internal structure. Together, these layers interact to shape Earth's surface features, drive tectonic activities, and influence environmental phenomena, highlighting the intricate complexity of our planet's composition and behaviour.