Structure and composition of the Earth and distribution of elements -1 | Geology Optional Notes for UPSC PDF Download

Interior of the Earth: Structure and Composition

Overview

• Earth's interior comprises three main layers: the crust, mantle, and core.
• Imagine this structure similar to peeling a boiled egg: the crust is like the outer hard and thin layer, the mantle corresponds to the middle layer, and the core is akin to the innermost part.

Crust

• The Earth's crust is the outermost layer, characterized by its solid, rocky composition.
• It is divided into oceanic and continental crust, each with distinct properties.
• For instance, the continental crust is thicker but less dense compared to the oceanic crust.

Mantle

• Located beneath the crust, the mantle is a dense layer composed of silicate rocks.
• It is further divided into the upper mantle and the lower mantle based on physical properties.
• Convection currents in the mantle drive plate tectonics, leading to various geological phenomena like earthquakes and volcanic eruptions.

Core

• The core is the Earth's innermost layer, consisting primarily of iron and nickel.
• It is divided into the outer core, a liquid layer, and the inner core, a solid sphere.
• The core's high temperature and pressure conditions contribute to the generation of the Earth's magnetic field.

Thermal and Physical State of the Earth's Interior

• Temperature

  • As you delve deeper into the Earth's interior, the temperature rises by approximately 1 degree Celsius for every 32 meters descended.
  • At the core of the Earth, temperatures are projected to reach a staggering 4000 degrees Celsius due to this consistent temperature increase.
  • Within the asthenosphere, located around 48 kilometers beneath the surface, temperatures can vary between 1200 and 2000 degrees Celsius.
  • Such extreme temperatures can cause rocks and minerals to undergo significant changes, transitioning from solid to liquid states.
  • This transformation is responsible for volcanic activity, with eruptions originating from depths of about 48 kilometers below the Earth's surface.
  • However, accurate temperature measurement beyond the initial 8 kilometers becomes challenging.
  • Deeper into the Earth, beyond 100 kilometers, temperatures gradually decrease due to diminishing concentrations of heat-sensitive radioactive elements like uranium and thorium.

Pressure and Earth's Core

• Cause of High Density:
  • Traditional Explanation: The Earth's core's high density was previously attributed to the immense pressure from successive rock layers. This pressure, as depth increases, typically raises rock density. However, there is a limit to how much density can increase for a given rock type. Therefore, solely attributing density increase to overlying pressure is inaccurate.
  • True Nature of Core: Scientific studies reveal that the Earth's core comprises primarily of iron and nickel, heavy metallic elements. This composition is also supported by the Earth's geocentric magnetic field.
  • Metallic Core: The core likely consists of intense metallic substances with inherently high density, rather than just rock layers' pressure.
  • Surrounding Layer: A layer of crystalline rock strata encircles the top portions of the metallic core.

Density of Earth's Interior

• Average Density: The Earth's interior has an average density of 5.5.
• Continental Shells: The density in continental shells is 2.7, which are the upper surface layers made up of granitic rocks and sial (silica and aluminium).
• Depth and Density: Geologists suggest that materials become denser with depth. As we move deeper into the Earth, material density increases.
• Core Density: Materials in the Earth's core are about twelve times denser than water. The core consists of iron and nickel, known as 'nife'.
• Middle Layers: The density is 4.3 in the middle layers, which are composed of sima (silica and magnesium). Heavier materials have higher densities.

Earth's Interior Overview

Crust

• Earth's outermost layer is called the crust, also known as the lithosphere.
• The crust consists of two parts: the upper crust and the lower crust, with an average thickness of about 20 kilometers.
• The upper crust has an average density of 2.8, while the lower crust has a density of 3.0.
• Minerals in the upper crust formed under lower pressure compared to those in the lower crust.
• Seismic waves slow down as they travel through both the upper and lower parts of the crust.

Mantle

• The mantle is known as the mesosphere and is located between the lower crust and upper mantle.
• Seismic wave speed increases from 6.9 kilometers per second at the base of the lower crust to 8.1 kilometers per second due to a discontinuity.
• The Mohorovicic Discontinuity, or Moho Discontinuity, marks the boundary between the crust and mantle and was discovered by Yugoslavian seismologist Andrija Mohorovicic in 1909.

Mantle Structure Overview

• The mantle, constituting 83% of Earth's volume and 68% of its mass, has a mean density of 4.6 g/cm3.
• It extends nearly 2900 kilometers beneath the Earth's surface, which is less than half the Earth's radius of 6371 kilometers.

Historical Mantle Division

• Previously, the mantle was categorized into two zones based on seismic wave properties:
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  • Upper mantle: Extends from the Moho Discontinuity down to 1000 kilometers.
  • Lower mantle: Ranges from 1000 to 2900 kilometers in depth.

New Mantle Zoning

• Recent findings from the International Union of Geodesy and Geophysics (IUGG) have led to a revised division of the mantle into three zones:
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  • Moho Discontinuity to 200 kilometers in depth.
  • 200 to 700 kilometers in depth.
  • 700 to 2900 kilometers in depth.

Characteristics of Upper Mantle

• The upper mantle's top region features a gradual decrease in the speed of seismic waves.
• Within the depth of 100 to 200 kilometers, seismic wave velocity decreases to 7.8 kilometers per second, known as the "zone of low velocity seismic waves."

Core - Earth's Deep Interior

• Definition of Core

  Core refers to the innermost and most distant region within the Earth.

• Characteristics of the Core

  • Depth and Extent

    The core extends from about 2900 kilometers beneath the lower mantle to the very center of the Earth at a depth of 6371 kilometers.

  • Weichert-Gutenberg Discontinuity

    At a depth of 2900 kilometers, there is a boundary known as the Weichert-Gutenberg Discontinuity, indicating a significant change in density from 5.5 g/cm³ to 10.0 g/cm³.

  • Density and Composition

    The density of the core increases with depth, reaching values between 12.3 to 13.6 g/cm³. It is primarily composed of metallic elements like iron and nickel.

  • Inner and Outer Core

    At around 5150 kilometers deep, the core is divided into the outer core and inner core. The outer core is molten, evidenced by the absence of secondary seismic waves, while the inner core is solid with a density of 13.3 to 13.6 g/cm³.