Plate Tectonics Flashcards
(74 cards)
Describe the structure of the earth from inside to out.
Inner core (solid) - 1200km thick
Outer core (liquid) - 2250km thick
Mantle (semi solid)- 2900km thick
Lithosphere - 100km thick
Crust - 5 to 40km thick
Who was Alfred Wegner?
When and what did he believe?
A scientist that believed in the theory of continental drift and the separation of Pangea in the early 1900s.
What was Alfred Wegner’s evidence for continental drift? x3
Jigsaw fit - outlines of the continents fit together to form the supercontinent Pangea
Fossil remains - The Mesosaurus a fresh water reptile is found in both Africa and South America. As it is freshwater it could not of swam across the Atlantic so the continents must of been joined together.
Geology - coal is found in Antarctica and couldn’t of been formed at its current lattitude.
What discoveries formed the evolution of plate tectonic theory? x4
1960 the plate tectonic theory evolved due to:
Extensive mapping of the ocean floor lead to discoveries of ocean ridges and trenches.
The crust was probed leading to the discovery that the oceanic crust is thinner than the continental crust.
Hugo Benioff discovered that the depth of the earthquakes (at collision boundaries) increases with distance from the epicentre due to subduction.
Paleomagnetism
How did paleomagnetism affect the plate tectonic theory?
It confirmed sea floor spreading:
The earths magnetic field’s change direction around every 400,000 years and when lava cools the minerals inside the newly formed rock line up with the earth’s magnetic poles.
When studying ocean ridges it was discovered that the same pattern of magnetic direction was discovered on both sides of the ridge.
This could only occur if the new plates/rock were formed at the same time on both sides
What 4 processes drive tectonic plates?
Mantle convection - convection currents that move in circles drive the plates in the same direction. -
Slab Pull - Newly formed oceanic crust at mid ocean ridges becomes denser and thicker as it cool. This causes it to sink into the mantle and pulls the rest of the plate with it
Subduction - Where two plates move towards each other (O and O or C and O) the denser slides under the other and into the mantle. It melts at an area called the subduction zone
Sea Floor Spreading - Mid - Ocean Ridges form where hot magma rises up from and hardens forming new oceanic crust. This forces the plates apart. - evidenced by paleomagnetism
Describe the processes at destructive O-C plate margins.
Oceanic collides with continental and is subducted at the Benioff zone as the oceanic is denser. Deep Ocean Trenches mark the point of subduction
The collision leads to fold mountain formation as the continental plate is pushed up.
Friction at the Benioff Zone leads to the release of earthquakes (deep and intermediate) when pressure is released.
As the oceanic plate slides down it melts and forms magma which rises up through the faults in the continental plate leading to explosive volcanic eruptions.
Describe the processes at destructive O-O plate margins.
The denser or faster is subducted forming a deep ocean trench.
Earthquakes occur at the Benioff Zone
The subducted plate melts at the Benioff Zone and the rising magma lead to submarine volcanos.
Over millions of years the volcanos grow leading to volcanic islands which are found in curved arcs called island arcs.
Eg. 2004 Indian Ocean Tsunami
Describe the processes at destructive C-C plate margins.
When continental plates collide a collision margin occurs. Due to similar densities neither are subducted.
Instead the sediment between is crumpled and forced up to form high fold mountains eg Himalayas.
However, there can be earthquakes due to continual pressure leading to sudden releases of pressure along faults eg. Nepal EQ 2015
Describe the processes at constructive plate margins.
Two plates are moving apart:
In oceans this forms mid ocean ridges as lava rises, cools and forms new rock or on continents it forms rift valleys.
Regular breaks along ocean ridges lead to minor earthquakes.
Submarine volcanos are also formed which sometimes grow above sea level to form volcanic islands eg. Iceland.
Describe the processes at conservative plate margins.
Also known as transform:
When plates move in opposite directions or move in the same direction at different speeds.
Friction occurs and pressure builds up. When the pressure is released as a fault powerful earthquakes occur eg. San Andreas Fault
When can volcanos occur away from plate margins?
Due to mantle plumes.
When an area of the mantle experiences increased temperatures large amounts of magma are upwelled beneath the crust leading to volcanos at areas called hot spots.
The mantle plume is static so overtime the volcano migrates away from the mantle plume as the plate moves and becomes dormant.
When can earthquakes occur away from plate margins?
Intra-plate Earthquakes
They occur along faults where plates fracture. This can be due to:
-Melting glaciers leading to the release of pressure.
- Fracking, rock disturbance
What are the 3 types of plate boundary?
Convergent/Destructive - two plates move towards each other
Divergent/Constructive - Two plates move away from each other
Conservative/Transform - Two plates move past each
What are the main tectonic plates called?
North American
South American
Eurasian
African
Caribbean
Nazca - west coast of South America
What is the focus and epicentre of an earthquake?
Focus = Point inside the crust where the energy is released from.
Epicentre = the point on the surface directly above the focus
Different types of seismic waves?
Primary
Secondary
Love
Characteristics of primary waves?
Body waves
The are fastest - 6km/s
Move through solids and liquids
Longitudinal movement - compressing and decompressing
Characteristics of secondary waves?
Body waves
Second fastest - 3km/s
Only travel through solids
Transverse movement
Characteristics of love waves?
Surface waves
Slowest velocity
Only travel through solid
Most destructive
Complex rolling motion - cause vertical ground movement
Primary effects of EQ?
Ground shaking - Building collapse
Crustal fracturing - leaving gaps in earth’s surface
Secondary effects of EQ?
Liquefacation:
Surface rocks loose strength due to shaking leading to ground behaving more like liquid. Foundations sink and buildings collapse. Eg. Haiti 2010
Landslides:
Causes stress on slopes so they fail and collapse.
Can prevent access eg. Nepal 2015
Tsunami
Japan 2011
Causes of aftershocks?
Danger?
Result of the crust ‘readjusting’ along the fault line.
Can be very dangerous due to already weakened communities = more vulnerable.
Eg. 6.3 mag aftershock in Christchurch, New Zealand
Primary hazards of volcanic eruptions?
Lava Flows:
Extremely hot but slow moving so are generally not a threat to humans. Will destroy any infrastructure in its path
Pyroclastic Flows:
Mixture of rock, lava, ash and gases ejected from the volcano. Extremely hot and move up to 100km/h
Tephra and Ash falls:
Tephra is pieces volcanic rock blasted into the air.
Ash fall can also cause damage and disruption. eg Eyjafjallajokull
Volcanic Gas
SO2 and CO2
