Earth’s Dynamic Structure and Plate Tectonics

Posted on Oct 8, 2024 in Geology

Earthquakes are violent events caused by sudden movements of the Earth’s crust.

Hypocenter (Focus): The point within the Earth where an earthquake originates.
Seismic Waves: Oscillations/vibrations that start at the hypocenter and travel through the Earth’s inner layers.
- Types: P-waves and S-waves

Epicenter: The point on the surface that waves reach first (directly above the hypocenter).
Surface Waves: Spread from the epicenter, sometimes with catastrophic effects.

Crust: Aluminum silicates (primarily)
- Continental: Heterogeneous, granite is abundant
- Oceanic: Homogeneous, basalt mainly
Mantle: Igneous rocks rich in iron and magnesium silicates (mainly peridotite)
Core: Almost pure iron mixed with iron sulfides and nickel

Lithosphere: Includes the crust and the upper part of the upper mantle. Rigid (solid) and divided (fractured) into large blocks called lithospheric plates. These plates are dragged along by the mantle underneath, resulting in horizontal movements due to isostatic adjustment and vertical movements.
Asthenosphere (or Sublithospheric Upper Mantle): Includes the rest of the upper mantle and the transition zone (lies between the lithosphere and mesosphere). Plastic (malleable) with a tendency to flow as the lithosphere moves.
Mesosphere: The lower mantle (from approximately 670 km deep to the D” layer). Semi-solid and flows slowly (cm/year).
D” Layer: One of the most dynamic layers of the Earth. Liquid (due to accumulated heat from the outer core). Hot magma escapes from this layer, creating mantle plumes that break through the mantle and lithosphere, creating hotspots at the Earth’s surface.
Outer Core: Liquid (due to the heat of the inner core). Flows in violent convection currents, generating the Earth’s magnetic field (invisible, dynamic lines of force that cross the Earth between the two magnetic poles, which vary).
Inner Core: Solid (due to the pressure of overlaying materials). The center is 6378 km deep, and the temperature is higher than 6000°C (geothermal energy).

Subduction Zones: Occur when one or both plates are composed of oceanic crust. The denser plate is subducted under the less dense plate, melting and destroying the subducted plate.
- Oceanic-Oceanic: Island arcs and oceanic trenches form.
- Oceanic-Continental: The denser oceanic plate is subducted, often forming mountain ranges on the continent (e.g., the Andes).
Obduction Zones: Occur where continental crust meets continental crust. Both crusts are too light to subduct, resulting in a continent-continent collision that creates large mountain ranges (e.g., the Himalayas).

The space created fills with new material from molten magma below, creating new lithosphere.
On Land: Initially produce rifts, which form rift valleys.
Under the Sea: Most active divergent boundaries are between oceanic plates, often called mid-oceanic ridges.

Plates slide past each other horizontally.
Can occur underwater or on land.
Crust is neither destroyed nor created.
Friction causes stress buildup in both plates. When stress exceeds the threshold of the rocks, the energy is released, causing earthquakes.