Tectonics Flashcards
constructive/divergent plate margins
plates move apart, convection currents push apart and cause a gap between plates.
Magma rises to fill this gap and new crust is created.
conservative/transform plate margins
Plates slide past each other horizontally, crust is not created or destroyed but edges are jagged so catch onto each other. Friction and pressure builds until plates can’t take the stress and slip past, causing both plates to move and the ground shaking.
convergent/destructive plate margins
when oceanic plate slides beneath a continental plate as they’re denser. Crust is destroyed as this happens (subduction zone).
rocks catch against each other and pressure builds until plates can’t take the stress. Plates slip past each other causing both plates to move and the ground shakes.
oceanic plates
make up ocean floor
High density
Basaltic rock
7-10km thick (only)
sink (subduct) under other plates
continental plates
Make up Earths landmasses
Thicker (25-75km)
Less dense
Granitic rock
Do not sink (subduct)
Tectonic hazards
Natural/geophysical event with potential to threaten life and property. (volc, EQs Tsunamis)
Happens at plate boundaries and the distribution of these are uneven.
intraplate EQs And volcs - HOTSPOTS
Mantle plumes create weakness in plates known as hotspots and result in volcanoes. At hotspots there is a hot mass of rising heat under a weakness in a plate. magma rises through this. eg Hawaiian islands formed due to mid-Pacific hotspot.
Intraplate Volcanoes
Volcanoes can happen at hotspots (Hawaii)
Isolated plumes of converting heat (mantle plumes) rise towards surface generating basaltic volcanoes.
Plume remains stationary while plate moves slowly over it and over time produces chain of volcanic islands with extinct ones furthest from plume location.
Intra plate EQs
EQs can happen in mid-plate settings, associated with ancient fault lines being reactivated by tectonic stresses ( minor EQs in UK)
zone of weakness are created as plates move and stress builds.
Earths core
Central part of Earth
made of inner (5100km thick) and outer core (2900km)
Inner is solid, outer liquid although both made from iron and nickel.
core is source of radioactive heat.
The mantle and asthenosphere
semi molten body of rock between the earths crust and its core.
Asthenosphere is part of mantle below lithosphere which floats on top of it.
Lithosphere (crust and upper mantle)
form tectonic plates. together ~80-90 km thick
Crust thickness:
oceans - 6-10km thick
continents - 30-40km thick
Theory- sea floor spreading
1960s Harry Hammond suggested sea floors crack open at mid ocean ridges and new sea floor forms. New material then moves from the ridge. He believed sea moved and this brought continents with it.
Theory- sea floor spreading
EVIDENCE?
Palaeomagnetism, age of ocean floor, subduction zones:
Mid- Atlantic ridge found 1000km wide and made of volcanic rock.
palaeomagnetism - magnetic surveys of ocean floor show symmetrical pattern of paleo magnetic striping suggesting when lava erupts magnetic domains in lava minerals are aligned with magnetic field. lava then cools and remains proving Hammond right as crust does form at ocean ridges which then pushes plates apart and sea floor spreads.
Age of ocean Floor
Very young rock on/ near ridges (Iceland) only 1 million years old
Older rock on continental masses are 200million
Subduction zones/ocean trenches
where oceanic crust has been pushed into the earth and destroyed by the subduction zones (Wadati-benioff zone has deep EQ depth suggesting subduction of basaltic plates into asthenosphere)
Mechanism for continental movement
higher core temps (heat released by radioactive decay of elements) and heat released by mantle help create convection currents.
