The Dynamic Crust: Evidence of Crustal Movements
- Earthquakes: Sudden movements along faults.
- Volcanic Activity: Magma movement causes crustal deformation.
- Displaced Structures: Buildings and landmarks shift over time.
- Bench Marks: Permanent markers show gradual elevation changes.
- Tilted and Folded Rock Layers: Originally horizontal layers are deformed.
- High-Elevation Sedimentary Rocks: Marine fossils found on mountains indicate uplift.
- Thick Sediment Layers: Suggests crustal subsidence during sediment accumulation.
Stress can cause rocks to deform elastically (temporary change), plastically (permanent change), or fracture (break).
Deformation and Fracture
- Elastic Deformation: Temporary changes that reverse when stress is removed.
- Ductile Deformation: Permanent changes, such as folding.
- Fracture: Rocks break under stress, forming joints or faults.
High temperatures and pressures favor ductile deformation, while low temperatures and pressures increase the likelihood of fracture.
Folds
- Anticlines: Upward-arched folds.
- Synclines: Downward, valley-like folds.
- Monoclines: Single-limb bends.
- Overturned and Recumbent Folds: More complex structures.
Don't confuse joints and faults. Joints show no movement, while faults involve displacement of rock layers.
The Theory of Plate Tectonics
The Lithosphere and Asthenosphere
- Lithosphere: Rigid outer layer (~100 km thick), includes crust and upper mantle.
- Asthenosphere: Semi-fluid layer beneath the lithosphere, allowing plate movement.
Hot spots are not located at plate boundaries. They provide evidence of plate movement over stationary mantle plumes.
Interactions at Plate Boundaries
Divergent Boundaries:
- Plates move apart, creating tension stresses.
- Magma rises to form new lithosphere (e.g., mid-ocean ridges).
Convergent Boundaries:
- Plates collide, causing compression and shear stresses.
- Subduction zones form when oceanic crust sinks beneath continental crust.
- Continental collisions create mountain ranges (e.g., Himalayas).
Transform Boundaries:
- Plates slide past each other, causing earthquakes.
- Example: San Andreas Fault.
Hot spots provide a unique record of plate motion and can be used to calculate the speed and direction of plate movement.
Evidence Supporting Plate Tectonics
Continental Drift
- Matching Shorelines: Coastlines of Africa and South America fit together.
- Geological Structures: Similar rock formations on different continents.
- Fossils: Identical species found on widely separated continents.
- Past Climates: Evidence of glaciation in now-tropical regions.
* Glossopteris Fossils: These plant fossils are found in South America, Africa, India, and Antarctica, supporting the idea of a supercontinent (Pangaea) where these regions were once connected.
Seafloor Spreading
- Mid-Ocean Ridges: Underwater mountain chains with rift valleys.
- Young Oceanic Rocks: Rocks near ridges are younger than those farther away.
- Paleomagnetism: Symmetrical magnetic stripes on either side of ridges record Earth's magnetic reversals.
It's a common misconception that Earth is expanding due to seafloor spreading. In reality, new crust is balanced by subduction, maintaining Earth's size.
Subduction Zones
- Trenches: Deep oceanic trenches mark areas where crust is destroyed.
- Earthquake Patterns: Deep-focus earthquakes occur along subducting plates.
When studying subduction zones, remember that volcanic arcs (e.g., the Andes) often form parallel to trenches due to melting of the subducted plate.
The Rock Cycle and Plate Tectonics
- Magma Production: At divergent and convergent boundaries.
- Metamorphism: Rocks are altered by heat and pressure during subduction or mountain building.
- Sedimentary Basins: Formed by crustal downwarping, accumulating thick sediment layers.
