Earth’s Layers, Seismic Waves, and Plate Tectonics
Earth’s Compositional Layers
Crust: Composed of less dense rocks rich in silicon and aluminum.
Mantle: Made up of denser rock rich in peridotite and magnesium.
Core: Principally made up of iron.
Earth’s Physical Layers
Lithosphere: The rigid surface layer.
Asthenosphere: The area of the lithosphere where the mantle is ductile and partially molten.
Lower Mantle: Solid but flexible and ductile.
Core: Consists of a molten outer layer and a solid inner part.
Seismic Waves
Primary Waves (P): The fastest waves, arriving first at the surface. They move through solids and liquids.
Secondary Waves (S): Slower waves, registered after P-waves. They only move through solids.
Sudden changes in the speed of seismic waves indicate changes in the composition of the material they travel through. Three key discontinuity boundaries exist: Mohorovičić, Gutenberg, and Wiechert-Lehmann.
Discontinuity Boundaries
Mohorovičić Discontinuity: Marks the boundary between the crust and the mantle, located 30-40 km below continents and 10 km under the ocean.
Gutenberg Discontinuity: Found at a depth of 2900 km. S-waves disappear at this boundary, while P-waves are refracted and slow down. This marks the boundary between the mantle and the molten outer core.
Wiechert-Lehmann Discontinuity: Located at a depth of 5150 km, marking the boundary between the inner and outer core.
Continental Drift Theory
This theory proposes that continents (made of lighter material) slide over a thicker layer, the ocean floor. It suggests that over 200 million years ago, the continents were joined as one supercontinent, Pangea.
Evidence for Continental Drift
- Paleontological Evidence: Identical fossils of land-based organisms have been found on continents far apart.
- Geological Evidence: Continents fit together along coastlines and continental shelves.
- Paleoclimatic Evidence: Continents situated at Pangea’s south pole show glacial moraines of the same age. Northern hemisphere continents have large coal deposits, indicating they were once covered by tropical forests.
Plate Boundaries
Divergent Boundaries (Constructive): Zones where plates move apart, creating new oceanic lithosphere. Example: Mid-Atlantic Ridge.
Convergent Boundaries (Destructive): Zones where continents collide or where an ocean floor subducts into the mantle. Example: Mariana Trench.
Transform Boundaries (Passive): Fractures (transform faults) where plates slide horizontally. This causes seismic activity but little volcanic activity. They are passive because they don’t create or destroy oceanic lithosphere.
Benioff Zone
The area of seismic activity in a subduction zone. Earthquakes farther from the trench are deeper.
Additional Information
Keywords: nebular hypothesis, nebula, supernova, nuclear fusion, gravitational attraction, volatile, refractory
Fossil Order: trilobite, giant fern, graptolite, armored fish, ammonite, pterosaur, ichthyosaur