Interior of Earth | Geography for UPSC CSE PDF Download

Introduction

The interior of the Earth is organised in concentric layers that differ in chemical composition, physical state and mechanical behaviour. Commonly used divisions are the crust, mantle and core (chemical classification), and the lithosphere, asthenosphere, lower mantle, outer core and inner core (mechanical classification). Except for the liquid outer core, most layers are predominantly solid; the asthenosphere is partially molten and mechanically weak (ductile or semi-viscous). Variations in temperature, pressure and density with depth produce the distinct properties and behaviour of these layers.

• To understand the structure and composition of the Earth (crust, mantle, core) and how internal forces (heat, convection, seismic activity) control surface change.
• To explain the origin, evolution and future development of the Earth's surface and landforms.
• To account for geophysical phenomena such as earthquakes, volcanism and mountain building.
• To understand the origin and behaviour of the Earth's magnetic field and its role in shielding the atmosphere from solar wind.
• To compare internal structures of other planetary bodies in the solar system.
• To reconstruct the evolution and present composition of the atmosphere and hydrosphere.
• To aid mineral and resource exploration and to locate geothermal energy resources.

• Many landforms and geological features are the result of processes working at or near the surface.
• Forces driving these processes come both from outside the Earth (solar energy, weathering, erosion - exogenous) and from inside the Earth (heat, tectonic stresses - endogenous).
• Major features such as mountains, plateaus and basins are largely produced by endogenous processes: folding, faulting and other tectonic movements driven by forces from the interior.

• Catastrophic events like earthquakes and volcanic eruptions originate from movements and energy release within the Earth.
• Earthquakes result from sudden slip or deformation on faults; the energy driving plate motions is supplied by heat-driven convection in the mantle and by gravitational interactions of plates.
• Volcanism occurs where molten rock (magma) from the mantle or lower crust reaches the surface through conduits, vents and fissures created by tectonic activity.

• The Earth behaves like a giant magnet because of electric currents generated by convective motion in the conductive, liquid outer core. This mechanism is known as the geodynamo.
• The magnetic field is crucial for life because it helps deflect charged particles in the solar wind and reduces atmospheric erosion.

Direct access is extremely limited: the deepest boreholes and mines reach only a few kilometres, while the Earth's centre lies about 6,371 kilometres beneath the surface. Therefore, most knowledge about the deep interior comes from indirect scientific evidence combined with the study of materials brought to the surface by natural processes.

• Direct observation is constrained by technological limits: the deepest oil well is about 8 km, the deepest mines (e.g. past depths at Robinson Mine in South Africa) are less than 4 km, and deep continental and ocean drilling projects have reached only a few tens of kilometres in most places.
• Large international projects such as the Deep Sea Drilling Project and the Integrated Ocean Drilling Program have extended knowledge of crustal structure by recovering cores from below the sea floor.
• The Kola Superdeep Borehole (Arctic region) reached about 12 km and supplied important information on temperature and rock properties at depth.
• Volcanic eruptions provide direct samples of material originating at depth (magma and xenoliths), but estimating the exact source depth of erupted material is not always straightforward.

Indirect Sources

• Artificial and measurable physical parameters (temperature, pressure, density, seismic velocities and gravity).
• Evidence from theories of the Earth's origin and formation (planetary accretion, differentiation and early heating).
• Natural sources such as volcanic rocks, meteorites, seismic waves from earthquakes and observations of the Moon and other bodies.

Temperature

• Temperature increases with depth (the geothermal gradient). Measurements from boreholes show a general increase in temperature as we go deeper; volcanic activity and molten lava are further evidence of high temperatures at depth.
• Typical near-surface gradients can be about 15-30 °C per km in the upper few tens of kilometres, but the gradient varies with depth and geological setting.
• Approximate temperature estimates: base of the crust ~ 1,000 °C; bottom of the mantle ~ 3,500 °C; core temperatures up to ~ 5,000 °C or higher in the inner core.
• Heat sources include residual heat from planetary accretion, latent heat from inner-core crystallisation, and radioactive decay of elements (U, Th, K) in the crust and mantle.

Density

• The Earth's average density is about 5.5 g cm⁻³. Density increases with depth because higher pressure compresses materials and because heavier elements (iron, nickel) concentrate toward the centre.

Gravitational Force and Gravity Anomalies

• Surface gravity (g) varies with latitude and local mass distribution. Gravity is slightly greater at the poles than at the equator because the equatorial radius is larger and centrifugal force reduces apparent gravity at the equator.
• Local deviations of observed gravity from the value predicted by a smooth reference Earth are called gravity anomalies. These anomalies provide information about lateral variations in mass distribution in the crust and upper mantle (e.g. dense bodies such as ore deposits, low-density sedimentary basins, or mantle upwellings).

Gravity anomalies

• Gravity anomalies are used to infer the distribution of mass in the crust and upper mantle.
• They also help detect subsurface molten or partially molten regions and variations in crustal thickness.

Magnetic Surveys

• The Earth generates a magnetic field because of moving conductive material in the outer core. Magnetic surveys measure variations in the field to map magnetic minerals in the crust and to identify structures such as buried intrusions.
• The magnetic poles do not coincide exactly with the geographic poles and the field changes through time (secular variation and polarity reversals). Magnetic surveys therefore give clues to crustal composition and geological history.

Meteorites

• Meteorites are remnants of the early solar system and are studied as natural samples of planetary building materials. They fall to Earth after their passage through the atmosphere and can be classified broadly as stony (rocky) and metallic.
• Metallic meteorites are rich in heavy elements such as iron and nickel. Comparison of meteorite compositions with Earth's bulk composition supports the idea that the core is composed largely of iron with nickel and other siderophile elements.
• Because meteorites and the Earth formed from the same protoplanetary material, they provide constraints on the elemental composition of the deep interior.

Meteorite

Two commonly used schemes to describe the interior are:

• Chemical layers: Crust, Mantle, Core.
• Mechanical or rheological layers: Lithosphere (rigid outer shell), Asthenosphere (weak, ductile layer), Lower mantle, Outer core (liquid) and Inner core (solid).

The Crust

• The crust is the thin, outermost solid layer of the Earth. It is brittle and relatively low in density compared to deeper layers.
• Thickness varies: oceanic crust averages about 5 km; continental crust averages around 30 km and can reach up to 70 km beneath high mountain belts such as the Himalaya.
• Chemically the crust is commonly divided into two parts: SIAL (silica- and aluminium-rich rocks, dominant in continents; average density ~ 2.7 g cm⁻³) and SIMA (silica- and magnesium-rich rocks, dominant beneath oceans; average density ~ 3.0 g cm⁻³).
• The crust's total volume is small relative to the Earth as a whole (roughly 1%), but it hosts all surface processes, ecosystems and human activities.

The Mantle

• The mantle extends from the base of the crust (the Mohorovičić discontinuity, or Moho) down to about 2,900 km depth and constitutes the largest volume of the Earth (about 84% by volume).
• Seismic P-wave velocities increase abruptly across the Moho. This change in seismic velocity marks the crust-mantle boundary and was first recognised by Andrija Mohorovičić.
• The mantle is composed mainly of dense silicate minerals rich in oxygen, magnesium and iron (peridotitic composition). Estimated densities range from about 3.5 g cm⁻³ near the upper mantle to about 5.5 g cm⁻³ toward the base of the mantle.
• Temperatures in the mantle vary roughly from 900 °C in the upper parts to around 2,200 °C or more near the lower mantle. Partial melting in the upper mantle produces magma that feeds many volcanoes.
• The upper mantle includes the mechanically weak asthenosphere (see below); mantle convection within the mantle is the principal driver of plate tectonics, supplying energy for continental drift, earthquakes and volcanic activity.

The Core

• The core begins at about 2,900 km depth (the Gutenberg discontinuity) and extends to the Earth's centre at roughly 6,371 km depth.
• The core has two parts: the outer core (from ~2,900 km to ~5,150 km) and the inner core (from ~5,150 km to the centre).
• Seismic observations show that S-waves (shear waves) do not propagate through the outer core, indicating that it is liquid or partially liquid; P-waves (compressional waves) slow down on entering the outer core and then speed up entering the inner core.
• The outer core's average density is of the order of ~10 g cm⁻³. The inner core is denser still (estimated densities ~ 12-13 g cm⁻³) and behaves as a solid under extreme pressure.
• The core contains most of the Earth's mass concentrated in heavy metallic elements, primarily iron and nickel; this iron-nickel region is sometimes referred to as NiFe or nife (Ni = nickel, Fe = iron).
• By mass, the core constitutes about 32% of the Earth's total mass and about 16% of its volume.

Lithosphere

• Definition: The lithosphere is the rigid outer shell of the Earth formed by the crust plus the uppermost mantle. It behaves elastically on geological timescales and is broken into tectonic plates.
• Thickness: Average thickness about 100 km, but thickness varies: under oceans the lithosphere is commonly less than 50 km thick; beneath older continental shields it can exceed 200-300 km in some interpretations (thicker roots beneath some mountain ranges).
• Role: Lithospheric plates move relative to each other and interact at plate boundaries (divergent, convergent, transform), producing earthquakes, volcanism and mountain building.

Asthenosphere

• Definition and properties: The asthenosphere is the mechanically weak, ductile part of the upper mantle that lies beneath the lithosphere. It is hotter and partially molten in places, allowing lithospheric plates to move over it.
• Depth and thickness: The asthenosphere generally starts at about 100 km depth below the surface and extends to depths between roughly 350 km and 650 km; estimates of average thickness commonly range between 180-220 km though values vary with tectonic setting.
• Seismic signature: Because seismic waves travel more slowly through this region, it is often called the Low-Velocity Zone (LVZ).
• Composition: Dominated by peridotite (rich in olivine and pyroxene). Partial melting and a high degree of ductility permit convective flow within the mantle and allow plate tectonics to operate at the surface.

• Seismic waves generated by earthquakes are the primary tool for probing the Earth's interior. The two principal body waves are P-waves (primary, compressional) and S-waves (secondary, shear).
• P-waves travel through solids and liquids; S-waves travel only through solids. The disappearance of S-waves on the far side of the Earth's core provided key evidence that the outer core is liquid.
• Changes in seismic wave speeds and the reflection/refraction of waves at boundaries reveal discontinuities such as the Moho and the Gutenberg discontinuity and allow estimation of layer depths, densities and elastic properties.

• Understanding internal structure underpins earthquake hazard assessment and seismic risk mitigation.
• Knowledge of crustal thickness, density contrasts and tectonic structure is essential for mineral and petroleum exploration.
• Geothermal resource evaluation depends on knowledge of heat flow, crustal composition and deep temperatures.
• Understanding the geodynamo and past magnetic field behaviour aids palaeomagnetic reconstructions used in plate reconstructions and in dating geological events.

The Earth's interior comprises chemically distinct zones (crust, mantle, core) and mechanically distinct shells (lithosphere, asthenosphere, lower mantle, outer and inner core). Direct observation reaches only a few kilometres, so most knowledge comes from seismic waves, gravity and magnetic surveys, laboratory studies of rock properties, analysis of volcanic products and meteorites, and drilling projects. Temperature, pressure and compositional variations with depth explain the physical states and dynamics of the layers-dynamics that drive plate tectonics, earthquakes, volcanism and the geodynamo that produces the Earth's magnetic field.