Hazards 1 Flashcards
(23 cards)
Basic Structure of the earth
Lithosphere
Asthenosphere
Mesosphere
Outer Core
Inner Core
Lithosphere
Crust and upper mantle
Breaks into tectonic plates
Oceanic-50-100km
Continental-300km
Asthenosphere
Solid mantle rock
Can flow plastically under great pressure- partially molten
Allowing for convection currents that drives tectonic plate movements
Role of convection currents
They drag tectonic plates across earth’s surface as hot magma rises and cool magma sinks, it carries plates with it
Like conveyor belt
What drives tectonic plates
Ridge push
Slab Pull
Convection currents
Explain Ridge push process
Newly formed plates at oceanic ridges are warm and so has a higher elevation than colder, denser plates
Due to gravity, the higher plate at oceanic ridges push the colder, denser plates away from the ridge
Explain Slab Pull Process
Older plates sink at subduction zone as they are colder and denser than underlying mantle, it then pulls the rest of the plate along behind it
What is now considered the main driving force of tectonic plates and why
Slab pull- the plates moving fastest have the longest subducting margin or has the most edges being subducted
It would not be convection currents or else the plates with largest surface area would be moving the fastest
Ridge push is only effective when there is no slab pull happening
What are the evidence of sea floor spreading
Paleomagnetism
1960s ocean drilling programme
Explain Paleomagnetism
Ancient record of changes in earth’s polarity
Igneous rocks originated from lava flow contains iron particles which as the rock cools, the magnetic orientation is locked in the rocks
Earth’s polarity is not constant and the changes in polarity can be recorded in the rocks
Rocks with same magnetic orientation would correspond to a time period between each magnetic reversal
The symmetrical pattern of the magnetic orientation on each side of the mid ocean ridge suggests that as fresh molten rocks reach the surface, older rocks gets pushed away - ridge push process
Age of the rock increases as it moves further from the mid ocean ridges
1960s ocean drilling programme
It discovered that thickest and oldest sediments are found nearest to continents- suggesting the plates gets pushed away from one another
Nowhere in the oceans have rocks older than 200 million years- suggesting that ocean crust was constantly recycled- slab push process
Appearant Fit
1620 Francis Bacon noted that the continents seem to fit together like jigsaw puzzles- South America and Africa
Evidence of continental drift by Alfred Wegner
1912
Theory of super continent- Pangaea
Apparent Fit
Fossil correlation- animals, coal
Mountain Ranges
Glacier Striation
Explain the fossil correlations
Mesosaurus- reptile- found in both South America and Africa
Brachiopods-found in both Australia and India limestones
Bituminous Coal- formed in warm and wet conditions are found in Antarctica
Explain Glacier Striation
As glaciers move across the land, they leave marks which were found in South America and Africa- suggesting that they used to be in colder areas that are further from the equator
Explain Mountain Ranges
Same type of rock is found in mountains in both northeast Canada and Northern Scotland
How fast do plates move
1-10cm per year
Global Pattern of plates and plate boundaries
Continental drift and plate tectonics gained acceptance in 1950s
Nuclear tests conducted underground in mid 1960s- seismic activity were picked up by seismometer
Maps shown that earthquakes concentrated in narrow bands- there are large areas in between that generate few earthquakes
Lithosphere and crust are broken down into tectonic plates
Plate tectonics are still subject of research
Features of constructive plate boundaries
Plates pulled apart- lithospheric crust is stretched and thins, reducing pressure on asthenospheric mantle
Ridge push dominant
Macro feature:
Mid ocean ridges- formed as plates move apart and new oceanic crust is created as magma rises
Rift Valley- as plates move apart, the plate get stretched and have faults and cracks in them, they drop down into the mantle, the sinking of the block forms rift valley- the sinking part is the graben
Black Smokers- when cold sea water seeks into cracks of ocean floor (rifts), and get superheated, rises to the surface and chemical reactions
Micro feature:
Pillow Lava- magma erupting underwater and cooled rapidly forming rounded mounds
Features of convergent plate boundaries (oceanic continental)
Oceanic plate is denser and subducts underneath continental plate into the asthenosphere
Slab Pull Dominant
Mountain chains- as oceanic plate converges- sediment and rock is crumpled, uplifted- Andes Mountain
Benioff Zone- increases pressure and friction, faults and fracturing occurs , angle of 45 degrees- create intra plate earthquakes- shallow focused earthquakes
Volcanoes- form as oceanic plate melts when subducting, causing pressure and the melted material is less dense than surrounding and rises
Oceanic Trench- formed along the subduction zone
Features of convergent plate boundaries (oceanic-oceanic)
When two oceanic plates meet, the one that is slightly denser would subduct under the other
Ocean Trench- formed at subduction zone
Benioff zone- where plates faults and fractures and increases friction forming intra plate earthquakes
Island Arcs- as the subducting plate melts, magma rises to surfaces forming chains of volcanic islands
Features of convergent plate boundaries (continental continental)
When two continental plate meets- causing uplift and crumpled- producing mountains
Fold Mountains- metamorphism
Violent earthquakes
E.g Himalayas
Features of transform plate boundaries
Plates slide past each other
E.g Saint Andrea’s Fault
Friction causes build up of pressure causes rock to fracture- forms earthquakes
Transform Faults are formed by fracture of rocks
