Tectonics Flashcards
Describe the distribution of world volcanic activity
Usually found along plate boundaries, otherwise near the centre of some plates where there are hotspots. Many located around Pacific Ring of Fire where destructive boundaries dominate.
Describe the distribution of earthquakes
Main earthquake zones are found along plate boundaries. Around 70% of all earthquakes are in the ring of fire. Most powerful earthquakes are associated with destructive or conservative boundaries. Intra plate earthquakes also occur.
Explain destructive/convergent plate margins
Plates move towards each other. When oceanic plate meets continental plate, the oceanic plate slides under into the mantle and melts, forming a deep ocean trench and fold mountains. E.g. Mt St Helens When oceanic plate meets oceanic plate , one subducts, and deep ocean trenches form, while the subducted plate melts, creating magma which eventually forms underwater volcanoes. When continental plate meets continental plate, they collide and form fold mountains. E.g. Mt Everest
Explain constructive/divergent plate margins
Plates are moving apart. In oceans, this forms mid ocean ridges, and on continents it forms rift valleys. E.g. Mid-Atlantic ridge
Explain conservative plate margins/transform
Two plates sliding past each other. Associated with powerful earthquakes. E.g. San Andreas Fault
Which plate boundaries form which volcanoes?
convergent - composite
divergent - shield
How does an intra plate earthquake form?
Tension builds up in the crust as plates move past each other; there is more stress on ancient fault lines; this causes a sudden release of tension, earthquakes are created at great distances from plate boundaries, which makes them dangerous a they are unpredictable.
How do volcanic hotspots form?
Radiation from the core causes convection currents; plumes of hot magma rise from the mantle under the force of convection; a reduction in pressure under the crust allows the magma to melt; cracks form in the lithosphere where the magma penetrates the crust and rises; the magma erupts onto the sea floor and solidifies into rock under the water forming a volcano; the tectonic plate moves over the hotspot, carrying the volcano away from the hotspot; another volcano forms above the hotspot, and eventually a chain of volcanoes is formed as the cycle continues
What is the Wadati-Benioff zone?
the subduction zone
What is mantle convection?
When heat produced by the decay of radioactive elements in the earth’s core heats the lower mantle, creating convection currents which move in circles in the asthenosphere, causing plates to move
What is slab pull?
When newly formed crust at mid ocean ridges cools, it becomes denser and thicker, causing it to sink into the mantle, pulling the rest of the plate further down with it.
What is paleomagnetism?
The study of past changes in the Earth’s magnetic field, determined from rocks, sediment or archaeological records. Iron particles in lava are aligned with the Earth’s magnetic field. At regular intervals the polarity of the Earth reverses; this results in a series of magnetic stripes with the sea-floor rocks aligned alternately towards north and south poles. This striped pattern, which is mirrored exactly on either side of a mid-oceanic ridge, suggests that the ocean crust is slowly spreading away from the boundary. Also, the ocean crust gets older with distance from the mid-ocean ridge.
Describe volcanoes at destructive plate boundaries
Composite volcanoes, with highly explosive eruptions , ejects a wider range of volcanic products, andesitic, dacitic or rhyolitic lava which has higher viscosity and higher silica content so more violent eruptions, extensive ashfall, pyroclastic blasts, lahars
Describe volcanoes at constructive plate boundaries
shield volcanoes, low explosivity, low viscosity basaltic lava flows, more frequent gentle eruptions, tephra, viscous lava flows
What causes earthquakes?
the ground being subjected to so much force that it breaks
describe p waves
fastest waves, made of compressions and rarefactions, travels in crust, away from focus, causing little damage
describe s waves
second fastest waves, shake in a sideways motion, travels in crust, away from focus, causing little damage
describe love waves
slowest waves, travel like water waves over the surface, shaking ground from side to side, away from focus, causing most damage
describe raleigh waves
slower, travel in circular motion, shaking ground up and down, on the surface away from the focus, causing some damage
primary hazards of volcanoes
pyroclastic flow, tephra (volcanic bombs), volcanic gases, ash clouds, lava flows
secondary hazards of volcanoes
lahars, jokulhlaup (glacial outburst floods)
primary effects of earthquakes
ground shaking, crustal fracturing
secondary effects of earthquakes
liquefaction, landslides, avalanches, tsunamis
describe the formation of a tsunami
where subduction is occurring, there is a sudden shift along a fault, causing an upthrust of water, which has a long wavelength which shortens due to friction with shallow water, and the amplitude increases
