The Rock Cycle and Earth’s Structure: A Comprehensive Guide
The Rock Cycle
Surface Processes
Weathering breaks down rocks into sediments. These sediments are transported by geological agents and accumulate in sedimentary basins.
Internal Processes
Within the Earth’s crust, materials are subjected to high temperatures and pressures, transforming sediments into rocks.
Inside the Earth’s Crust
- Pressure: Increases with depth due to the weight of overlying rocks.
- Temperature: Increases with depth.
- Stress and Strain: Caused by movements in the mantle, compressing and stretching the crustal material.
Rock Transformations
- Diagenesis: Transformation of sediments into sedimentary rocks due to pressure and temperature.
- Metamorphism: Changes experienced by rocks subjected to high temperatures and pressures without melting, forming metamorphic rocks.
- Magmatism: Melting of rocks forms magma, which cools and solidifies to create igneous rocks.
The rock cycle encompasses the processes that transform rocks into sediments and back into new rocks.
Earth’s Internal Heat and Geothermal Gradient
The geothermal gradient refers to the increase in temperature with depth inside the Earth.
Sources of Internal Heat
- Meteorite impacts: Kinetic energy converts to heat energy.
- Radioactive decay: Produces high-speed electrons and neutrons.
- Gravitational settling of denser materials.
Composition and Structure of Earth
Crust
- Continental Crust: Composed primarily of granite, often covered by sedimentary rocks and unconsolidated sediments.
- Oceanic Crust: Forms the seafloor and is composed of basalt and gabbro.
Mantle
- Upper Mantle: Extends from the base of the crust to 670 km deep.
- Lower Mantle: Extends from 670 km to the core-mantle boundary.
Core
- Outer Core: Liquid layer.
- Inner Core: Solid sphere with a radius of about 1220 km.
Seismic Discontinuities and the Lithosphere
- Mohorovičić Discontinuity (Moho): Separates the crust and mantle.
- Repetti Discontinuity: Separates the upper and lower mantle.
- Gutenberg Discontinuity: Separates the mantle and outer core.
- Lehmann Discontinuity: Separates the outer and inner core.
Lithosphere
The rigid outer part of the Earth, composed of the crust and uppermost mantle.
- Continental Lithosphere: Continental crust and upper mantle.
- Oceanic Lithosphere: Oceanic crust and upper mantle.
Continental Drift and Plate Tectonics
Alfred Wegener
In 1912, Alfred Wegener proposed the theory of continental drift, suggesting that continents were once joined in a supercontinent called Pangaea and have since moved apart.
Harry Hess and Seafloor Spreading
Harry Hess proposed that mid-ocean ridges create new oceanic crust, pushing older crust away and causing seafloor spreading.
Magnetic Stripes on the Seafloor
Earth’s magnetic field periodically reverses. These reversals are recorded in volcanic rocks containing magnetic minerals, creating magnetic stripes on the seafloor.
The Asthenosphere
Joseph Barrell proposed the existence of the asthenosphere, a plastic layer beneath the lithosphere.
Lithospheric Plates
- Oceanic Plates: Composed of oceanic lithosphere (e.g., Pacific, Cocos, Nazca).
- Continental Plates: Composed of continental lithosphere (e.g., Arabian).
- Mixed Plates: Contain both continental and oceanic lithosphere (e.g., Eurasian, North American, South American, African, Antarctic, Australian).
Plate Boundaries and Relative Movements
Plate boundaries are zones where plates interact, resulting in geological activity like volcanism, earthquakes, and deformation.
Constructive Plate Boundaries (Mid-Ocean Ridges)
Characteristics
- Fracture zones where hot material rises from the mantle, creating new oceanic crust through volcanic activity.
- Basaltic volcanism.
- Divergent extensional forces separate the plates.
- Upwelling magma creates the ridge topography.
- Hydrothermal vents form where ocean water interacts with hot rocks.