Earth’s Interior: Structure, Composition, and Processes
Earth’s Interior
Characteristics
Density, Internal Heat, and Magnetism
Density increases with depth, as does internal heat. Terrestrial magnetism is also a key characteristic.
Structure and Chemical Composition
The Earth’s structure consists of concentric layers:
- Crust: The outermost layer.
- Mohorovičić Discontinuity: Boundary between crust and mantle.
- Mantle: Extends to the Gutenberg discontinuity.
- Gutenberg Discontinuity: Boundary between mantle and core.
- Core: Composed of an outer liquid layer and an inner solid layer.
Physical Composition
- Lithosphere: Solid, rigid outermost layer encompassing the crust and upper mantle.
- Asthenosphere: Semi-viscous layer beneath the lithosphere where magma is generated and convection currents occur.
- Mesosphere: Rigid layer below the asthenosphere.
- Outer Core: Liquid layer responsible for Earth’s magnetic field.
- Inner Core: Solid, innermost layer.
Other Key Discontinuities
- Conrad Discontinuity: Separates the upper and lower continental crust.
- Lehmann Discontinuity: Separates the outer and inner core.
- Repetti Discontinuity: Separates the upper and lower mantle.
Lithosphere and Asthenosphere
The lithosphere is the solid, rocky outer layer composed of the crust and upper mantle. It is divided into tectonic plates.
The asthenosphere is the semi-molten layer beneath the lithosphere (60-300 km deep). Convection currents within the asthenosphere drive the movement of lithospheric plates.
Relief Transformation
Orogeny
Orogeny is the process of mountain formation.
Glyptogenesis
Glyptogenesis refers to the wearing down of relief through erosion. Together, orogeny and glyptogenesis form the geological cycle.
Rock Types and Formation
Internal Geological Cycle
- Metamorphic Rocks: Formed by the transformation of existing rocks through heat and pressure.
- Magmatic Rocks: Formed from the cooling and solidification of magma.
External Geological Cycle
- Sedimentary Rocks: Formed from the accumulation and cementation of sediments.
Crustal Deformation
- Elastic Deformation: Temporary deformation that recovers when stress is removed.
- Plastic Deformation: Permanent deformation that does not recover when stress is removed.
- Fracture: Occurs when the internal cohesion of the material is exceeded, leading to breakage.
Earthquakes
Earthquakes are vibrations of the lithosphere caused by the sudden release of energy.
- Focus/Hypocenter: The point within the Earth where the earthquake originates.
- Epicenter: The point on the Earth’s surface directly above the hypocenter.
Seismic Waves
- P-waves (Primary): Longitudinal waves that travel through solids, liquids, and gases. They are the fastest seismic waves.
- S-waves (Secondary): Transverse waves that travel only through solids. They are slower than P-waves.
- Surface Waves: Travel along the Earth’s surface and cause the most damage.
Earthquake Measurement
- Magnitude: Measures the energy released by an earthquake (Richter Scale).
- Intensity: Measures the destructive effects of an earthquake (MSK Scale).
Volcanoes
Volcanoes are vents in the Earth’s surface through which magma and gases erupt. Different eruption styles exist (e.g., Hawaiian, Strombolian, Vulcanian).
Continental Drift and Plate Tectonics
Continental Drift (Wegener)
Alfred Wegener proposed that continents have moved over time. Evidence includes:
- Geographic: The shapes of continents appear to fit together.
- Geologic: Similar geological structures are found on different continents.
- Paleontological: Fossil distributions support the idea of continental drift.
- Paleoclimatic: The distribution of past climates suggests continental movement.
Wegener suggested centrifugal force and the gravitational pull of the sun and moon as possible causes, but these are not the accepted mechanisms today.
Plate Tectonics
The Earth’s lithosphere is divided into plates (oceanic, continental, and mixed) that move relative to each other.
- Divergent Boundaries: Plates move apart, creating mid-ocean ridges.
- Convergent Boundaries: Plates collide, leading to subduction (oceanic-oceanic or oceanic-continental) or mountain building (continental-continental).
- Transform Boundaries: Plates slide past each other horizontally (e.g., San Andreas Fault).
Plate movement is driven by convection currents in the asthenosphere.
Isostatic Movements
Isostatic movements are vertical adjustments of the Earth’s crust due to the buoyancy of the lithosphere floating on the asthenosphere.
Tectonic Features
- Mid-Ocean Ridges: Underwater mountain ranges formed at divergent boundaries.
- Rift Valleys: Formed by the stretching and thinning of the lithosphere.
- Transform Faults: Faults where plates slide past each other horizontally.
Subduction Zones
Subduction zones are characterized by:
- Oceanic Trenches: Deep depressions in the ocean floor.
- Earthquakes: Caused by the friction between the subducting and overriding plates.
- Volcanoes: Formed by the melting of the subducting plate.
Orogeny
Orogeny, the process of mountain building, can occur through:
- Pericontinental Orogeny: Subduction of an oceanic plate beneath a continent.
- Intercontinental Orogeny: Collision of two continents.
Himalayan Formation
The Himalayas formed from the collision of the Indian and Eurasian plates.
Distribution of Earthquakes and Volcanoes
Earthquakes and volcanoes are concentrated along plate boundaries.
The Rock Cycle
The rock cycle describes the continuous transformation of rocks between igneous, sedimentary, and metamorphic forms.
Petrogenetic Minerals
Petrogenetic minerals are the building blocks of rocks. Fundamental minerals include quartz, feldspars, micas, pyroxenes, amphiboles, and olivine. Accessory minerals occur in smaller quantities.