Structure and composition of the Earth and distribution of elements -2 | Geology Optional for UPSC PDF Download

Crust

Accounts for less than 1% of Earth's mass.
Two types:

• Oceanic Crust:
• Formed by magma that erupts on the seafloor, solidifying into basalt lava or cooling to form gabbro.
• Covered with sediments, such as mud and remnants of marine organisms like shells.
• Continental Crust:
• Composed of igneous, metamorphic, and sedimentary rocks, primarily granite.
• Less dense and thicker than oceanic crust, causing it to rise higher on the mantle.

Mantle

• Constitutes about 68% of Earth's mass.
• Composed predominantly of ultramafic rock, peridotite, which is rich in iron and magnesium silicates.
• Exhibits high temperatures due to heat conduction and convection.

Core:

• Makes up approximately 31% of Earth's mass.
• Composed mainly of metallic elements, with iron (85%) and nickel (15%) being the most abundant.

Mechanical Properties of Earth's Layers

Lithosphere

• The outermost rigid layer, encompassing the crust and upper brittle portion of the mantle.
• Moves as tectonic plates, enabling processes like earthquakes and mountain building.
• Example: Similar to the hard shell of an egg, it behaves as a solid, breaking or deforming under stress.

Asthenosphere

• Located beneath the lithosphere.
• Partially molten and exhibits plasticity, allowing it to flow.
• Drives plate tectonics by enabling the lithosphere to move above it.
• Example: Comparable to the egg white beneath a shell, flowing and shifting under applied force.

Earth's Crust and Lithosphere

1. Earth's Crust

Outer Layer Characteristics:

• Thin, cold, and brittle layer of rock.
• Composed of varying materials depending on location (oceanic or continental crust).

Types of Crust

Oceanic Crust

• Basaltic in composition, denser than continental crust.
• Thin and formed by volcanic activity at mid-ocean ridges.

Continental Crust:

• Granitic composition, less dense and thicker than oceanic crust.
• Includes diverse rock types, contributing to varied landscapes like mountains and plateaus.

2. Lithosphere

Characteristics:

• Includes both crust and upper mantle.
• Approximately 100 kilometers thick.
• Rigid and brittle, responding to stress through earthquakes and faulting.

Mantle

Composition

• Composed of peridotite, an ultramafic rock rich in iron and magnesium silicates.
• Rarely encountered on Earth's surface but inferred from volcanic materials and seismic data.

Heat Transfer in the Mantle

Conduction:

• Heat flows from the core to the mantle through atomic collisions.

Convection:

• Heat-driven movement within the mantle resembles boiling water.
• Hot materials near the core rise, cool at the surface, and sink again, forming convection cells.
• These currents are crucial for plate tectonics and mantle dynamics.

Core

Composition

• Primarily composed of dense metals like iron (85%) and nickel (15%).
• The core's metallic nature is inferred from Earth's density and meteorite studies.

Magnetic Field Generation

• Earth's magnetic field originates from the iron-rich outer core's movement.
• Without the metallic core, the magnetic field essential for navigation and shielding from solar radiation would not exist.

Core's Structure and State

Outer Core:

• Liquid and responsible for generating Earth's magnetic field through its dynamic flow.

Inner Core:

• Solid due to immense pressure, despite its high temperature.
• S-waves (seismic waves) ceasing at the boundary indicate the transition from liquid (outer core) to solid (inner core).

Illustrative Examples and Analogies

• The lithosphere is like the hard shell of an egg, brittle and solid, while the asthenosphere resembles the flowing egg white beneath it.
• Mantle convection can be visualized as boiling water, with heat causing upward movement, cooling at the top, and sinking as it loses energy.
• The core’s role in creating the magnetic field is akin to a dynamo, where moving conductive materials generate magnetism.