Plate Tectonics Flashcards
Continental drift
Alfred Wegener, 1912, meteorologist.
All continents joined to form supercontinent Pangaea.
Had geological evidence but no evidence to support mechanism of moving continents.
Evidence for continental drift
Geology - S. America and S. Africa
Fossil records - Glossopteris, Meseosaurus (fresh water animal)
Climatology - past climates were similar but now 1000 miles apart
Paleomagnetism - earths magnetic field, reverse polarity = sea floor spreading
Paleomagnetism
Polarity changes every 200,000 years or so.
Magma erupts - magnetic minerals align - cool and fix.
So alternating magnetic strips along sea floor - crust is older further away from mid ocean ridge - plates moving apart.
Constructive margin landforms
Mid-ocean ridge e.g. Mid-Atlantic Ridge. Underwater volcanoes erupt along these ridges and can build up to surface e.g. Iceland.
Rift Valley e.g. African Rift Valley.
Destructive plate margins
O-C: oceanic subducts = deep sea trench e.g. Chile trench. Fold mountains.
O-O: denser one subducts =deep sea trench, earthquakes and eruptions. Underwater eruptions = island arcs e.g. Mariana islands.
Intrusive volcanic activity
Beneath the surface
Forms large magma chambers and magma forced into crust
Extrusive volcanic activity
On surface
Major form is eruptions
Minor forms: hot springs, geysers, boiling mud
Intrusive volcanic activity landforms
Dykes
Sills
Batholiths
Dykes
Where the magma has flowed into cracks in the crust and cools vertically.
Discordant to strata.
Sills
When magma flows into gaps in crust and cools between layers of rock (horizontal).
Concordant to strata.
Batholiths
When large chambers of magma cool underground they form domes of igneous rock.
E.g. Dartmoor batholith spreading across south west England. 309 million years old.
Joints
Cracks in the magma formed from when the magma cools.
Basaltic/basic lava
At constructive margins.
Low silica content.
Low viscosity/runny - gases escape easily.
Over 950C temp.
Effusive eruption, frequent, long duration.
Andesitic/intermediate lava
Destructive margins. Medium silica content. Medium viscosity. 750-950C temp. More explosive, more gases (don't escape easily). Pressure can build from trapped lava. Intermittent and short-lived.
Rhyolitic/acidic lava
Destructive margins. High silica content. Very viscous. Less than 750C temp. Lots of pressure - gas and trapped lava. Very violent. Intermittent and short-lived.
Types of volcanoes- shape
Dome Composite Caldera Shield Fissure
Dome volcano
Central vent.
Layers of lava.
Steep sides from v viscous lava that flows short distances.
Destructive margins
Rhyolitic/andesitic lava
Composite volcano
Same to dome except:
Alternating layers of lava and ash/cinder.
Caldera volcano
Layers of lava or lava/ash/cinders.
Central part of volcano collapsed as magma chamber below emptied.
Wide, circular crater, can be several km across.
Destructive margins
Andesitic/rhyolitic lava
Shield volcano
Layers of lava
Central vent
Gently sloping sides cause by runny lava flowing long distances
Constructive margins
Basaltic lava
Fissure volcano
Layers of lava
Long, linear vent, few metres wide but several km long
Fairly flat surface due to runny lava that flows long distances
Constructive margins
Basaltic lava
Hot springs
Where groundwater emerges at surface.
Close to intrusive volcanic activity.
Temps from 20-90C.
High mineral content as hot water can hold more dissolved solids.
E.g. Blue Lagoon
Geysers
Type of hot spring - hot water and steam ejected from surface. Near intense intrusive volcanic activity.
Groundwater heated above boiling point by magma deep in crust.
Hot water becomes pressurised, forces its way to surface along cracks in the rocks.
Sprays out of a vent - periodically.
E.g. Stokkur, Iceland.
Boiling mud pools
Type of hot spring.
Form in areas with fine-grain soil - mixes with water = boiling mud pool.
Can be brightly colourd due to minerals (iron and sulfur rich - purple, orange, yellow).
E.g. Iceland and Yellowstone National Park
Hotspots
Caused by a magma plume that rises up from the mantle.
Volcano forms.
Magma plume does not move but plate does, new volcanoes form = chain of islands e.g. Hawaii.
Earthquake causes
Tension
Jerking movement sends out seismic waves from focus through to epicentre.
Tree main types of seismic waves
Body:
Primary waves- compressional, through solids and liquids, fastest
Secondary waves- move 90degree to wave direction, through solids not liquids
Surface: slower
Love- through solids, move side to side, damaging due to shearing effect
Rayleigh- through solids and liquids, move in rolling motion
Seismometers
Measure amount of energy released (magnitude).
Measure duration and direction of vibrations.
Richter scale
Measures magnitude.
Logarithmic (each level of magnitude increases by x10).
No upper limit.
Each value increases by x30 for strength.
Merchalli scale
Measure impacts. Using observations. 1-12 scale. 1 felt only by instruments. 12 complete destruction.
Tsunamis
Convection currents cause plates to move (reverse or thrust fault)
Which displaces the water upwards causing a large wave
In deep water the tsunami moves very fast (800km/hr)
As the wave gets to shallow land, it slows but increases in height.
This is because there is friction at the bottom of the wave against the sand, but no friction at the top so it grows.
What determines the impact of an earthquake
- distance from epicentre (power dec further away)
- landscape and rock type
- time of day and day of week
- weather and season
- urban or rural area
- emergency services and response plans
- economic development of location
What determines the impact of an earthquake
Landscape ad rock type
soil: liquefied during earthquake (Japan 1995)
rock: granite - won’t liquefy
topography:
low lying = more damage from tsunamis
mountainous = landslides block roads
What determines the impact of an earthquake
Time of day and day of week
Time: night: in bed = safer day: in town so more vulnerable Day: working: more vulnerable in urban area weekend: home = safer San Fran 1989: 5pm = few died Japan 1995: 7000 buildings destroyed, 300 fires
What determines the impact of an earthquake
Weather and season
bad: hampers rescue efforts
cold: vulnerable - frost bite. hypothermia
warm: spread disease faster, speeds up decomposition, hygiene problems
What determines the impact of an earthquake
Urban or rural area
Rural: small pop density, less affected, fewer issues
Urban: high pop density, more impacts/vulnerable
San Fran 1989: millions dollars damage
Middle of Alaska: almost 0 affected
What determines the impact of an earthquake Emergency Services and Response Plans
More developed: plans and drills, swift, organised, prompt
e.g. Japan/CA: millions dollars spent
Less developed: can’t afford plans
e.g. Gujarat, India 2001: no plans, education but no money for it
What determines the impact of an earthquake
Economic Development of Location
MEDCs: money to building design to withstand earthquakes (rubber pads - shock absorbers, deep foundations, built on hard rock and spread out), plans and drills, stores of medicine, saved money, food, shelter, self reliant
e.g. Kobe, Japan 1995: 7.2 quake killed 5000 / Turkey 1999, smaller quake and killed 17,000
LEDCs: can’t afford structured buildings, plans and supplies (rely on international aid), LEDCs not close to help/resources
