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

Physical Geography

Plate Tectonics

The Composition and Structure of Earth

• Composition of Earth's Layers:
  • Crust, Mantle, and Core: Earth's divisions are based on composition. The crust, which is less than 1% of Earth's mass, includes oceanic and continental crusts. The continental crust is typically richer in felsic rock.
  • Mantle: This layer, comprising about 68% of Earth's mass, is characterized by high temperatures.
  • Core: Mostly composed of iron metal, the core constitutes approximately 31% of Earth's mass.
• Mechanical Properties of Earth's Layers:
  • Lithosphere: The lithosphere consists of the rigid crust and the upper mantle's brittle portion. It behaves as a solid and is responsible for tectonic plate movements.
  • Asthenosphere: The asthenosphere, located beneath the lithosphere, is a partially molten layer of the upper mantle. It exhibits plastic behavior and can flow, influencing tectonic activities.
• Illustrative Example: Think of the lithosphere as the hard shell of an egg, while the asthenosphere is like the egg white beneath it. The lithosphere remains intact and solid, similar to how the crust and upper mantle behave, while the asthenosphere flows and moves like the egg white does.
• Visual Representation: Check out this animation by Earthquide to visualize Earth's layers based on both composition and mechanical properties.

Crust and Lithosphere

• Earth's Crust: The outer surface of the Earth is called the crust, which is a cold, thin, and brittle layer made of rock. It is relatively thin compared to the size of the planet.
• Types of Crust:
  • Oceanic Crust: This type of crust is formed from magma that erupts on the seafloor, creating basalt lava flows or cooling to form gabbro. The seafloor is covered with sediments like mud and shells of sea creatures.
  • Continental Crust: Made up of various igneous, metamorphic, and sedimentary rocks, the continental crust's average composition is granite, which is less dense than oceanic crust rocks. It is thicker and less dense, rising higher on the mantle compared to oceanic crust.
• The Lithosphere: The lithosphere is the outermost mechanical layer of the Earth, acting as a rigid, brittle solid. It includes the crust and uppermost mantle, both behaving in a brittle manner. It is approximately 100 kilometers thick and responsible for earthquakes when stressed.

Mantle

• Composition of the Mantle:
  • The mantle is composed of solid rock, primarily ultramafic rock called peridotite.
  • Peridotite is rich in iron and magnesium silicate minerals, rarely found on Earth's surface.
• Temperature of the Mantle:
  • The mantle is known to be hot due to heat flow and its physical properties.
  • Heat transfer in the mantle occurs through conduction and convection.
• Conduction in the Mantle:
  • Conduction is the transfer of heat through rapid collisions of atoms, happening in solid materials.
  • Heat from the core conducts into the mantle, contributing to its high temperature.
• Convection in the Mantle:
  • Convection currents in the mantle resemble those in a pot of boiling water.
  • Heating near the core causes material to rise, cool, and sink back down, forming convection cells.

Core 

Earth's Core Composition

• Earth's core is located at the planet's center and is primarily composed of dense metallic elements.
• The core is predominantly made up of around 85 percent iron metal, with nickel metal accounting for much of the remaining 15 percent.
• Scientists infer the core's metallic nature from various indicators, such as the overall density of the planet and the presence of metallic meteorites.

Magnetic Field Generation

• The existence of Earth's magnetic field is directly linked to the metallic composition of the core, particularly the presence of iron.
• If Earth's core were not metallic, the planet would not exhibit a magnetic field, crucial for various natural phenomena.

Core State and Structure

• The core consists of an outer liquid core and an inner solid core, with distinct characteristics and functions.
• S-waves ceasing at the inner core boundary indicate a transition from liquid to solid state.