Interior of the Earth: Crust, Mantle and Core

Interior of the Earth

• The Earth’s interior consists of concentric layers: the crust, mantle, outer core, and inner core, each with distinct properties crucial to Earth’s dynamics.
• The crust forms the outermost layer, composed primarily of solid silicate materials.
• Beneath the crust lies the mantle, characterized by its viscous, molten rock composition.
• Deeper within the Earth is the outer core, which exists as a dense, viscous liquid.
• At the Earth’s center lies the inner core, a solid mass of immense density.
• Mechanically, the earth’s layers can be divided into lithosphere, asthenosphere, mesospheric mantle (part of the Earth’s mantle below the lithosphere and the asthenosphere), outer core, and inner core.
• Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.

The Crust

• The crust, forming the Earth’s outer layer, constitutes only about 0.5-1.0% of the planet’s volume and less than 1% of its total mass.
• Its density increases with depth, averaging around 2.7 g/cm³ (compared to Earth’s average density of 5.51 g/cm³).
• Oceanic crust typically ranges from 5-30 km thick, while continental crust spans 50-70 km, with thickness exceeding 70 km in major mountain regions like the Himalayas.
• Temperatures within the crust rise with depth, ranging from approximately 200°C to 400°C at the boundary with the mantle.
• The temperature gradient in the upper crust can increase by about 30°C per kilometer.
• The crust’s outermost layer comprises sedimentary materials, beneath which lie igneous and metamorphic rocks, predominantly acidic in nature.
• The lower crust layer consists mainly of basaltic and ultra-basic rocks.
• Historically, the crust was classified into “sial” (continental crust, rich in silica and aluminum) and “sima” (oceanic crust, with heavier silicates rich in magnesium and silica), although this classification is now considered outdated.
• Continental crust primarily consists of lighter (felsic) rocks like granite, rich in sodium, potassium, and aluminum silicates.
• In contrast, oceanic crust mainly comprises denser (mafic) rocks like basalt, dominated by iron, magnesium, and silicate compositions.

Note: In geology, felsic refers to igneous rocks that are relatively rich in elements that form  feldspar  and quartz. It is contrasted with mafic rocks, which are relatively richer in magnesium and iron. Felsic refers to rocks which are enriched in the lighter elements such as  silicon ,  oxygen,  aluminium, sodium, and potassium.

Most Abundant Elements of the Earth’s Crust:

Lithosphere

• The lithosphere, ranging in thickness from 10 to 200 km, constitutes the Earth’s rigid outer layer.
• It encompasses both the crust and the uppermost portion of the mantle.
• This rigid lithospheric layer is fragmented into tectonic plates, also known as lithospheric plates, whose movements drive significant geological phenomena such as folding and faulting.
• The dynamic motion of these tectonic plates leads to large-scale changes in the Earth’s geological structure.
• The driving force behind plate tectonics is primarily derived from two sources of heat: residual heat from the Earth’s formation process and the radioactive decay of elements such as uranium, thorium, and potassium within the Earth’s crust and mantle.

The Mantle

• The mantle, comprising about 83% of the Earth’s volume and holding 67% of its mass, extends from the Moho’s discontinuity down to a depth of 2,900 km.
• Its upper portion, the upper mantle, exhibits a density ranging from 2.9 g/cm³ to 3.3 g/cm³.
• Beyond the asthenosphere lies the lower mantle, which remains solid and features densities between 3.3 g/cm³ and 5.7 g/cm³.
• Rich in iron and magnesium compared to the crust, the mantle primarily consists of silicate rocks.
• Constituting approximately 45% oxygen, 21% silicon, and 23% magnesium (OSM), the mantle’s composition is dominated by these elements.
• Temperatures within the mantle vary from around 200°C at its upper boundary with the crust to roughly 4,000°C at the core-mantle boundary.
• Despite being solid, the mantle’s high temperatures make its silicate material ductile, leading to convective circulation within it.
• This convective motion manifests at the Earth’s surface through the movement of tectonic plates.
• Although high-pressure conditions in the mantle typically inhibit seismic activity, earthquakes are observed down to depths of 670 km (420 mi) in subduction zones.

Asthenosphere

• Known as the asthenosphere, the upper portion of the mantle lies just beneath the lithosphere, extending approximately 80-200 km deep.
• Characterized by high viscosity, mechanical weakness, and ductility, the asthenosphere’s density surpasses that of the crust.
• These unique properties facilitate plate tectonic movements and isostatic adjustments, where elevations in one area of the crust are balanced by depressions elsewhere.
• Acting as a crucial source of magma, the asthenosphere is responsible for the volcanic eruptions by supplying molten rock to the Earth’s surface.
• Mohorovicic (Moho) discontinuity forms the boundary between the crust and the asthenosphere (upper reaches of the mantle) where there is a discontinuity in the seismic velocity.

The Outer Core

• Situated between 2900 km and 5100 km beneath the Earth’s surface, the outer core envelops the inner core.
• Comprised primarily of iron mixed with nickel (NiFe) and small amounts of lighter elements, the outer core remains in a liquid state due to insufficient pressure, despite sharing a similar composition with the inner core.
• The density of the outer core varies from 9.9 g/cm³ to 12.2 g/cm³.
• Temperatures within the outer core range from 4400°C in its outer regions to 6000°C near the inner core.
• According to the dynamo theory, convection currents in the outer core, coupled with the Coriolis effect, generate Earth’s magnetic field, contributing to its formation and maintenance.

The Inner Core

• Extending from the Earth’s center to a depth of 5100 km, the inner core comprises a solid state.
• Predominantly consisting of iron (80%) and some nickel (NiFe), the inner core is capable of transmitting shear waves, also known as transverse seismic waves.
• Its solidity is evidenced by its ability to transmit shear waves, as opposed to P-waves which convert to S-waves upon encountering the outer core-inner core boundary.
• Earth’s inner core exhibits a slightly faster rotation relative to the surface.
• Despite its solid state, the inner core is too hot to maintain a permanent magnetic field.
• The density of the inner core ranges from 12.6 g/cm³ to 13 g/cm³.
• Comprising only about 16% of the Earth’s volume, the core (both inner and outer) holds a significant portion of the Earth’s mass, around 33%.
• Recent scientific findings suggest that the temperature near the Earth’s center reaches approximately 6000°C, indicating it to be 1000°C hotter than previously estimated.
• Despite reaching temperatures as high as the Sun’s surface, the immense gravitational pressure prevents the inner core’s iron from liquefying.

Note: When ambient pressure increases the melting point of solid increases, and vice versa. One exception is Ice. In the case of ice increase in ambient pressure will lower its melting point.

Seismic Discontinuities

Seismic discontinuities are the regions in the earth where seismic waves behave a lot different compared to the surrounding regions due to a marked change in physical or chemical properties.

Most Abundant Elements of the Earth