• The Earth can be divided into two ways: mechanically or chemically.
• Mechanically (rheologically), it can be divided into:
  • Lithosphere
  • Asthenosphere
  • Mesospheric mantle
  • Outer core
  • Inner core
• Chemically, it can be divided into:
  • Crust
  • Mantle (subdivided into upper and lower mantle)
  • Core (subdivided into outer and inner core)
• Chemical division is the more popular of the two.

Sources to Study the Earth’s interior

• The sources which provide knowledge about the interior of the earth may be classified into 2 sources
• Direct Sources:
  • These materials are available normally at the surface of the earth.
  • Some examples are:
  • Surface Rock:
    • It is readily available on the surface of the Earth. It can be used to know the form of material upto a certain depth.
  • Volcanoes:
    • It can be analyzed after eruptions.
    • They come from under great depths, hence give more insight on the type of materials below.
    • However, accuracy cannot be predicted at all times.
  • Material obtained from mining and drilling areas:
    • Reveals the nature of material at a certain depth.
    • Analysis of these materials show that pressure and temperature increase towards the interior of the earth.
    • Also, density of the material increases from the surface to the interior bottom.
  • Other sources can be Deep Ocean Drilling, Integrated Ocean Drilling Project, etc.
• Indirect Sources:
  • Temperature and seismic variation:
    • Scientists use technologies like gravitation, magnetic and seismic knowledge to study the interior of the earth.
  • Meteorites:
    • These are considered an indirect source for the study of the earth’s interior, as scientists assume they were once part of a planet.
  • Gravitational force:
    • Differences in gravitational value show uneven distribution of mass within the earth.
  • Magnetic force:
    • Magnetic survey reveals the distribution of magnetic materials within the earth.
  • Seismic knowledge:
    • It provides information on the state of the interior of the earth (solid, liquid, or gaseous).

Structure of the Earth’s interior

• It is divided into three broad layers: crust, mantle and core.
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Crust

• The crust is the outermost solid part of the earth and is a thin layer with a total thickness of 30-50 km.
• Thickness:
  • It varies between oceanic and continental areas, with oceanic crust being thinner (5-30 km) and continental crust being thicker (50-70 km).
  • In areas of major mountain systems, the continental crust can be as much as 70-100 km thick (e.g., in the Himalayan region).
• The crust is only 0.5-1.0% of the earth’s volume and less than 1% of its mass.
• Temperature:
  • It increases with depth, reaching 200-400°C near the mantle-crust boundary.
  • In the higher region of the crust, the temperature increases by up to 30 degrees Celsius per kilometer.
• Material:
  • The crust’s outer layer is made of sedimentary material, and beneath it are acidic crystalline, igneous, and metamorphic rocks.
  • Basaltic and ultrabasic rocks make up the lowest layer of the crust.
  • The lighter silicates, silica + aluminum (sial), make up the continents, while the heavier silicates, silica + magnesium (sima), make up the oceanic crust.
  • The lighter (felsic) sodium potassium aluminum silicate rocks, such as granite, make up the continental crust. The oceanic crust is made of thick (mafic) iron magnesium silicate igneous rocks like basalt.
• The most abundant elements of the Earth’s Crust

Element	Approximate % by weight
Oxygen (O)	46.6
Silicon (Si)	27.7
Aluminum (Al)	8.1
Iron (Fe)	5.0
Calcium (Ca)	3.6
Sodium (Na)	2.8
Potassium (K)	2.6
Magnesium (Mg)	1.5

Lithosphere

• The lithosphere is the earth’s rigid outer layer, with a thickness of 10 to 200 km.
• It consists of the crust and the upper part of the mantle.
• The lithosphere is divided into tectonic plates.
• Plate tectonics cause large-scale changes in the earth’s geological structure (such as folding and faulting).
• The sources of heat driving plate tectonics are primordial heat from the planet’s birth and radioactive decay of elements such as uranium, thorium, and potassium in the earth’s crust and mantle.

Asthenosphere

• The asthenosphere lies below the lithosphere, with a depth ranging from 80-200 km.
• The asthenosphere is soft and plastic, allowing the rigid lithosphere to move easily over it.
• The asthenosphere is viscous, brittle, and ductile with a higher density than the crust.
• It is the primary source of magma during volcanic eruptions.
• The lithospheric shell is divided into large pieces called lithospheric plates.
• These plates can separate or collide, leading to geological changes such as ridge formation.

Mantle

• The mantle is the portion of the earth’s interior beyond the crust.
• The mantle extends from Moho’s discontinuity (35 km) to a depth of 2,900 km and makes up 83% of the earth’s volume and 67% of its mass.
• The upper part of the mantle is called the asthenosphere, which is completely solid.
• The crust and uppermost part of the mantle together make up the lithosphere, which has a thickness of 10-200 km.
• The lower mantle extends beyond the asthenosphere and is in a solid state.
• The density of the mantle is higher than the crust and ranges from 3.3 to 5.5.
• The mantle is mostly made up of silicate minerals rich in iron and magnesium.
• Temperatures in the mantle range from 200°C at the upper crustal border to almost 4,000°C at the core-mantle boundary.
• The mantle has convective material circulation due to temperature differential and this causes the motions of tectonic plates at the surface.
• Seismicity in the mantle is inhibited by high pressure, but earthquakes have been detected in subduction zones as far as 670 km below the surface.

Core

• The core is made up of nickel and iron and is sometimes referred to as the NIFE layer.
• The core-mantle boundary is located at a depth of 2900 km.
• The core lies between 2900 km and 6400 km below the earth’s surface.
• The outer core is liquid and the inner core is solid.
• The density of the core is higher than the mantle and varies from 5.5 to 13.6 g/cm3.
• The volume and mass of the core make up 16% and 32% of the total volume and mass of the earth, respectively.
• The Gutenberg discontinuity is a sudden change from 5.5 gm/cm3 of the mantle to 10 gm/cm3, located at the core-mantle boundary.
• The core is divided into two parts: the outer core and the inner core.
• Outer core:
  • The outer core is located between 2900 and 5100 km below the Earth’s surface.
  • Made up of iron, nickel, and trace amounts of lighter metals.
  • Liquid due to insufficient pressure to solidify.
  • Earth’s magnetic field is believed to be created by convection in the outer core and the Coriolis effect according to dynamo theory.
• Inner core:
  • The inner core is located from the earth’s center to 5100 km below the surface.
  • It is solid and can transmit shear waves.
  • The rotation of the inner core is slightly faster than the surface.
  • The solid inner core is too hot to maintain a persistent magnetic field.
  • The core (inner core and outer core) makes up 16% of the earth’s volume and 33% of its mass.
• 

Seismic Discontinuity

Discontinuity refers to a sharp boundary between layers in the earth’s internal structure. Across these boundaries, seismic waves undergo significant changes in terms of their direction and speed.

There are 5 layers:

1. Conorod Discontinuity– between upper and lower crust.
2. Mohorovicic Discontinuity (Moho) – separates the crust from the mantle, its average depth being about 35 km.
3. Repiti Discontinuity – between the upper and lower mantle.
4. Gutenberg Discontinuity – lies between the mantle and the outer core. Below 2900 km from earth’s surface.
5. Lehman Discontinuity- between inner and outer core.

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Earth’s Chemical Composition

• Earth’s mass is approximately 5.97×1024 kg (5,970 Yg).
• It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of trace amounts of other elements.