Earth’s Crust

Overview: The crust is the outermost and thinnest layer of the Earth.

• Divisions of the Crust:
  • Upper Crust:
    • Composed of granitic rocks, it forms the continental landmasses.
    • It is rich in silica and alumina, earning the name “Sial.”
  • Lower Crust:
    • Made up of dense basaltic rocks, it constitutes the ocean floors.
    • Primarily composed of silica and magnesium, it is termed “Sima.”
• Density:
  • The continental crust (Sial) has a lower density of 2.7 g/cm³ compared to the oceanic crust (Sima), which has a density of 3.5 g/cm³.
• Thickness:
  • The crust beneath oceans averages 5 km, while beneath continents it is approximately 30 km.

Earth’s Mantle

Overview: The mantle lies between the crust and the core, extending from the Moho discontinuity to a depth of 2,900 km.

• Density:
  • The mantle has an average density of about 4.5 g/cm³.
• Composition:
  • Predominantly composed of minerals like pyroxene, olivine, garnet, plagioclase, and amphibolite.
• Divisions of the Mantle:
  • Upper Mantle:
    • Depth: Extends from approximately 403 to 660 km below the crust.
    • Temperature: Ranges from 500°C to 900°C.
    • Features: Less pressure makes it more viscous compared to the lower mantle.
  • Lower Mantle:
    • Depth: Extends from 660 km to 2,891 km.
    • Temperature: Significantly higher, reaching up to 7,400°C.
• Special Zones within the Mantle:
  • Asthenosphere:
    • Located within the upper mantle, it extends to a depth of 400–500 km.
    • It is the primary source of magma during volcanic activity.
  • Lithosphere:
    • Comprises the crust and the rigid uppermost mantle above the asthenosphere.

Earth’s Core

• Extent:
  The core stretches from 2,900 km to approximately 6,371 km below the surface.
• Composition:
  • Made up of heavy materials like nickel and iron, collectively called “Nife” (Ni for Nickel, Fe for Iron).
• Divisions of the Core:
  • Outer Core:
    • Thickness: About 2,200 km.
    • Temperature: Ranges from 4,500°C to 5,500°C.
    • Density: Between 12.6–13 g/cm³.
    • The outer core is believed to be in a liquid state.
  • Inner Core:
    • Composition: Primarily iron.
    • Temperature: Around 5,200°C.
    • Density: Ranges from 9.9–12.2 g/cm³.
    • The inner core is assessed to be solid.

Seismic Discontinuities

Earth’s Interior: Temperature, Pressure and Density

Temperature:

• Increases with depth, but the rate of change varies.
• Geothermal Gradient: Refers to the rate of temperature change with depth.
• Sources of Heat:
  • Residual heat from Earth’s formation.
  • Radioactive decay.

• Pressure:
  • Caused by the immense weight of overlying rocks.
  • Pressure increases with depth, reaching 364 GPa at Earth’s center.
• Density:
  • Density also rises with depth.
  • The core’s density ranges from 9.5–14.5 g/cm³.

PYQ:
In the structure of planet Earth, below the mantle, the core is mainly made up of which one of the following? (2009)
Options:
(a) Aluminium
(b) Chromium
(c) Iron
(d) Silicon
Answer: (c) Iron