Plate Tectonics: A Comprehensive Overview
Plate Tectonics
Expansion of the Seafloor
In 1962, geophysicist Harry Hess proposed that mid-ocean ridges were areas where new oceanic crust is created. This new crust is pushed sideways, causing the ocean floor to expand. Hess observed that the basalt of the seafloor was older and thicker further away from the ridge, with the oldest crust found near the continents. The current speed of seafloor spreading is estimated to be a few millimeters per year, a rate that has varied throughout history.
The Asthenosphere
Initially, it was believed that the asthenosphere, a layer beneath the lithosphere, allowed for the sliding of the lithosphere and was the site of convection currents. Current research has shown that convection affects the entire mantle, with the lithosphere being the part that subducts downward. This mantle convection drives the movement of the lithospheric plates.
Lithospheric Plates
- Oceanic plates: Consist of oceanic lithosphere (e.g., Pacific, Cocos, Nazca plates).
- Continental plates: Consist of continental lithosphere (e.g., Arabian plate).
- Composite plates: Composed of both oceanic and continental lithosphere (e.g., North American, South American, African, Eurasian, Indo-Australian, Antarctic plates).
- Microplates: Small fragments of lithosphere that move independently, often pushed by larger surrounding plates (e.g., islands).
Plate Boundaries
Plate boundaries are areas where two or more plates interact, resulting in intense geological activity such as volcanism, seismicity, and compression. The type of activity depends on the relative motion of the plates at the boundary.
Types of Plate Movement
- Divergent (Constructive): Creates new oceanic lithosphere. Characterized by moderate seismicity, intense volcanism, and seafloor spreading.
- Convergent (Destructive): Destroys oceanic lithosphere. Characterized by intense seismicity, volcanism, formation of volcanic landscapes, and continental collisions leading to seismicity, rock folding, and mountain building.
- Shear (Passive or Conservative): Characterized by seismicity.
Geological Processes at Plate Boundaries
Constructive Boundaries: Ocean Ridges
Oceanic ridges are constructive plate boundaries characterized by:
- Fracture zones thousands of miles long where hot mantle material rises to the surface, causing fissure volcanic activity.
- Extensive basalt volcanism, creating new oceanic crust.
- Convection currents that exert tensional forces, separating the plates and allowing basaltic magma to rise.
- Magma pressure that forms ridges. The fractures are called rifts.
- Thin and fractured crust in the rift zone, allowing seawater to enter and be ejected as hydrothermal vents.
Passive Boundaries: Transform Faults
The expansion of the ocean floor creates shear zones called transform faults. These faults are characterized by active movement and strong seismicity.
Destructive Boundaries: Subduction Zones
Subduction zones occur where oceanic lithosphere bends and sinks into the mantle, leading to the destruction of oceanic lithosphere. The subducting plate is always oceanic, while the overriding plate can be oceanic or continental.
- Oceanic lithosphere is destroyed.
- The force of the subducting plate causes earthquakes.
- Magmatism is produced by the melting of the subducting basalt plate.
- The lighter basalt generates volcanic mountain ranges (island arcs).
- Metamorphism occurs due to increased pressure and temperature.
Collision Boundaries: Orogenic Collision
When two continental plates collide, one overrides the other, and the convergent movement stops.
- The thickness of the continental lithosphere increases, potentially doubling.
- Sediments are deformed, fractured, and piled up at the suture zone, forming a collisional orogen (mountain range).
- Isostatic uplift of the orogen occurs due to the thickened lithosphere’s buoyancy.
- Compression causes metamorphism and magmatism.
- The collision generates major fractures in both plates, causing strong seismicity.