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The Shaking Earth: Plate Tectonics (Tectonic Cycle (Hot rock rises, melts…
The Shaking Earth: Plate Tectonics
Origin of an Earthquake
Event in which the earth quakes and vibrations are felt, most commonly by movement of Earth across a fault
Fault: deep fractures between the plates of the lithosphere
Movements cause stresses that build up and accumulate until a fracture or slippage, causing an earthquake
Plate: crust and mantle are broken into plates, which move around on top of the mantle like rafts. All made of lithosphere
**Not all earthquakes are on plate boundaries
Elastic Rebound Theory
Explanation for how energy is spread during earthquakes
As plates shift, they accumulate energy, and deform until strength is exceeded
Sudden movement occurs on fault, releasing accumulated energy
After energy is released, rocks try to snap back to undeformed shape
History of Tectonic Plate Distribution
Continental Drift Hypothesis(debunked)
Continents move by plowing through rocks at the bottom of the ocean (only continents move)
New Theory
Lithosphere moves laterally
Continents are set within oceanic crust and ride along plates
Tectonic Cycle
Hot rock rises, melts into liquid magma.
Buildup of magma causes lithosphere to uplift and fracture at the spreading centre
Asthenosphere melts and rises to fill the fractures, creating new oceanic lithosphere
New oceanic lithosphere becomes colder and denser as time passes
Seafloor Spreading: new oceanic lithosphere will diverge from zone of formation
When two plates comes into contact: colder, denser plate will be forced underneath the younger, hotter, less dense plate(called subduction)
Subducted slab goes back into mantle, where cycle repeats after 250 million years
Seafloor Features and Plate Boundaries
Mid-ocean ridges = mountain ranges, associated with spreading centres. Youngest area of seafloor
Fracture zones: deep cracks in seafloors that cut across the ridge
Deep sea trenches: deep depressions that can reach up to 11km below the surface, found at subduction zones
Features mark the boundaries between plates under oceans
Three Types of Plate Boundaries
Divergence Zones
Two plates pull apart from each other, producing tensional stress along a normal fault, lithosphere is created
Rock gets thin in the middle as it's pulled apart (gum)
On a normal fault: breakage, rupture, slippage & rock drops down as it breaks
Most lie below the ocean and are associated with seafloor spreading
Transform Faults
Two plates slide past each other and lithosphere is neither created nor destroyed(like two trains passing, causing the ground to shake)
Produces shear stress, along a strike-slip fault (transverse)
Potential earthquake when the rock snaps from the pressure
Connect spreading centres to subduction zones, movement along a transform faults is controlled by the spreading that occurs at mid-ocean ridges
Convergence Zones
Plates come against each other and old lithosphere is recycled or destroyed
Produces a stress called compression along a reverse fault
Appears as deep sea trenches/mountains
Rock forced upwards as it is squeezed
Types of Convergence Zones
Subduction Zones: One plate pushes its way back down into the mantle, beneath another plate, there is at least one oceanic plate involved.
Oceanic plate gets recycled into the mantle, continents are too thick and buoyant to be subjected
Collision Zones: When two plates carrying continents collide
Continental upheaval is the result(marked by mountain chains)
Geological Hazards along Plate Boundaries
Earthquakes: sudden movement of rocks along faults
Tsunamis
Volcanoes
Landslides
Severity of Quakes
Convergence Zones
Subduction Zones: Lots of energy buildup-> big quakes
Collision Zones: Rupture along large fault segments-> Big Quakes. Deadliest earthquakes occur here, because they involve two continents, meaning more people will be affected.
Transform Fault Zones
Near mid-ocean ridges: small to moderate quakes.
On continental crust: big quakes, because rocks are colder and thicker so more energy needs to accumulate.
Divergence Zones
never as large as those in subduction zones, easier to pull apart blocks of crust than push blocks together, or past each other
Don't build-up as much energy as transform and convergent plate boundaries