Plate Tectonics and Earth's Interior Flashcards
Outer to inner layers of Earth:
crust (solid), mantle (solid), outer core (liquid), inner core (solid)
What are earthquakes caused by?
A sudden release of energy from rocks breaking
How is the energy of earthquakes dispersed?
through seismic waves, which are recorded by seismographs
Different types of seismic waves travel through the Earth at:
different velocities and move in different ways
P-wave
- compression-extension (side-to-side movement) like pushing water
- travels through fluids
S-wave
- shear
- does not travel through fluids
Seismic wave velocities are changed by:
changes in density, which deflect the waves, causes paths to change direction at layer boundaries
No S-waves are recorded on the other side of the planet after an Earthquake, therefore:
there must be a liquid layer in the Earth
Scientists have modelled the layered internal structure of the earth by:
Assessing the changes in speed and amount of deflection of different seismic waves
Composition of the crust
- 46% O, 28% Si, Al 8%, Fe 6%, Mg 4%, etc.
- is stiff and solid
Composition of the whole Earth
- Fe 35%, O 30%, Si 15%, Mg 13%, Ni 2.4%, etc.
How do we know the composition of the whole Earth?
- Seismic data, tells us about density
- Samples of volcanic rock that contain pieces of the mantle
- Samples of meteorites that likely reflect the internal composition of Earth
Earth differentiated itself based on:
density, less dense = float (like O), and more dense sink
How did the layers of the Earth form?
- Heating from bombardment
- Gravitational compression (increases with mass)
- Radioactive decay produced melt
Two types of crust are:
- Continental crust (10-70km thicc), thick and rich in Si and Al, low density
- Oceanic crust (5-7km thicc), thin, rich in Fe and Mg, higher density
As we go further down in the Earth:
Pressure, temp, and density increase leading to differences in phase and mechanical behaviour
Compositional layers:
Continental crust, oceanic crust, mantle, core
Mechanical layers:
From top to centre of the earth: lithosphere (10-200km) aesthenosphere (200-660km) mesosphere (660-2900km) outer core (2900-5150km) inner core (5150-6396km)
Mantle characteristics:
- Solid
- Uppermost mantle: stiff
- Aesthenosphere: soft, can flow (like plastic)
- Mesosphere: stiffer than the asthenosphere, but can still flow
Lithosphere characteristics:
- Solid and stiff
- Composed of uppermost mantle and crust
- Uppermost mantle is stuck to the base of the crust
- Earth’s outer shell is broken into lithospheric plates
- Plates move and interact in response to forces within the earth
- Plates move 0-10cm/year
What are plate boundaries?
The place where two plates meet
Plate boundaries are named based on:
The relative motion of the plates on either side of the boundary
Transform boundary:
- The plates grind past each other laterally
- Commonly associated with spreading centres (divergent boundaries), they connect spreading ridges
- No crust created or destroyed
Divergent boundary:
The plates move away from each other
Convergent boundary:
The plates move towards each other
Position of volcanoes and earthquakes are strongly linked to:
Plate boundaries
Volcanoes typically occur at what type of boundaries?
Divergent and convergent boundaries
Earthquakes appear at what type of boundaries?
All 3 types
Divergent boundary characteristics:
- Can exist on land, but mostly exist on the seafloor
- Seafloor spreading produces a continuous row of volcanoes called a mid-ocean ridge that wraps around the world
- Rocks get older (and COLDER/denser) as you move perpendicular away from the ridge (new rocks forming at ridge b/c lava is cooling, HIGH topography)
Land-ocean convergent boundary
- Subducting plates go under continental plate b/c continental crust is less dense
- Accretionary prism: loose materials gathering from being scraped off top of subducting plate
- Partial melting (from water lowering melting point) creates less dense liquid rock = TURNS INTO MAGMA POCKETS
- As the plate is subducting, LOTS of earthquakes = easy to map
Ocean-ocean convergent boundary
- More dense oceanic plate will subduct under less dense oceanic plate
Continent-continent convergent boundary
- No plate is subducted = “car crash” = MOUNTAINS
Convection
- Hot water rises, cool sinks = loop
- Heat is transferred through the mantle via convection
- Heat is one of the sources of plate movement
Plates are likely to move because:
- Asthenosphere is weak so lithosphere can slip over it
2. Oceanic plates are dense so they dive under continental plates
Dominant forces causing plate movement seem to be:
A) Plates pull down into trenches (slab pull)
B) Plates driven down the slope from ridge to trench by gravity (ridge push)
- Rising magma pushes the plate apart, the weight of the falling lithosphere pulls the plate along, gravity causes the plate to slide down the gentle slope
Opening an ocean
- Plates are pulled apart
- Rift valley form
- Magma erupts from the mantle, adding new oceanic crust to the plate margin
- Lows fill with water
- Continued spreading creates new ocean
Closing an ocean
- Plates recycled into the mantle via subduction
- Plates brought gradually closer together
- Continents collide, the ocean is closed
