Earth’s Interior & Plate Tectonics: A Comprehensive Guide
Questionnaire: Earth’s Internal Structure
Methodology
External: Composition of surface rocks, research surveys, and laboratory experiments.
Internal: Seismology, magnetic methods, gravimetry, and astronomical methods.
Seismic Waves
Seismic waves are categorized as superficial (surface) or deep (body) waves.
Surface Waves:
- R-waves: Vibrate vertically along the direction of wave propagation.
- L-waves: Vibrate horizontally, perpendicular to the direction of propagation.
Body Waves:
- P-waves: Vibrate parallel to the direction of propagation. They are the fastest and reach the surface first.
- S-waves: Transverse, secondary waves that vibrate perpendicular to the direction of propagation. They arrive after P-waves and cannot travel through fluids.
Discontinuities
A discontinuity is a zone within Earth’s interior where seismic wave velocities change due to variations in chemical composition or phase transitions.
- Conrad Discontinuity: Between the upper and lower crust.
- Mohorovičić (Moho) Discontinuity: Between the crust and mantle.
- Repetti Discontinuity: Between the upper and lower mantle.
- Gutenberg Discontinuity: Between the mantle and core.
- Wiechert-Lehmann Discontinuity: Between the outer and inner core.
Static Earth Model
- Crust: Thin, rocky outer layer (0-70 km).
- Mantle: Dense layer composed of rocks. Divided into the upper mantle (70-670 km) and lower mantle (670-2900 km).
- Outer Core: Fluid, molten layer (2900-5150 km).
- Inner Core: Solid, metallic layer (5150-6378 km).
Dynamic Earth Model
- Lithosphere (0-100 km): Rigid, outermost layer composed of the crust and uppermost mantle. Fragmented into tectonic plates.
- Asthenosphere (100-350 km): Fluid, malleable part of the upper mantle beneath the lithosphere. Subject to convection currents.
- Mesosphere (350-2900 km): The lower mantle.
- Endosphere (2900-6378 km): The Earth’s core, comprising the outer and inner core.
Crustal Structure
- Continental Crust: Forms continents and shallow sea areas.
- Oceanic Crust: Found beneath deep ocean areas.
Questionnaire: Plate Tectonics
Theory of Plate Tectonics
Proposed by Alfred Wegener, the Theory of Plate Tectonics states:
- The lithosphere is divided into tectonic plates.
- Plates can be composed of continental crust, oceanic crust, or both.
- Plates move atop the asthenosphere.
- Plate movement causes geological processes like volcanism, earthquakes, and magmatism.
- Plate interactions create different types of plate boundaries.
Plate Composition
Tectonic plates can be made of oceanic or continental crust.
Plate Boundaries
Plate boundaries can be passive, divergent, or convergent.
Oceanic-Oceanic Collision
The denser, older plate subducts, creating volcanic island arcs and few earthquakes.
Oceanic-Continental Collision
The oceanic plate subducts beneath the continental plate, forming mountains, strong earthquakes, and magmatism.
Continental-Continental Collision
Minimal subduction occurs, forming mountain ranges like the Himalayas, Alps, and Pyrenees.
Plate Tectonics and Seismic/Volcanic Activity
Earthquake and volcano distribution correlates with subduction zones, particularly where oceanic plates subduct beneath continental plates.
Divergent Boundaries
Divergent boundaries are mid-ocean ridges where new seafloor is created. A central rift zone marks the thinnest area of the lithosphere. Rift evolution can split a plate, forming new oceanic crust and a mid-ocean ridge. While mostly submerged, some ridges rise above sea level, like in Iceland.