F4 key idea 2 Flashcards
(18 cards)
plate tectonic theory
plate tectonic theory suggests that lithospheric plates have moved to their present positions over a period of time and that they are still moving
believed there was a large southern land mass called Gondwanaland around 250Ma and began to break up in the jurassic, drifting apart to form the present arrangement
convergent plate boundaries
plates moving into each other
volcanic activity
island arcs around the edges of ocean basins
batholiths
where continental crust is melted by magma, most of the material will remain separate because of the different viscosities
melted silicic material forms large granite batholiths deep within fold mountains
fold mountains
compressional features made of folded and faulted sediments that have been scraped off the descending plate, subject to regional metamorphism
trenches
Benioff zone
sloping plane of the boundary is marked by increasingly deep focus earthquakes
types of convergent plate boundaries
oceanic-oceanic
andesitic and basaltic lava
oceanic continental
silicic magma, not enough to cause partial melting of the continental crust
some may reach surface forming explosive volcanoes
some will mic with rising mafic magma to create large volumes of intermediate lava
continental-continental
fold mountains (orogeny), earthquakes
crustal shortening and thickening
no volcanoes
island arcs
when an area of seafloor spreading reaches another plate, if oceanic, the colder, denser and larger of the two descends under the smaller plate
as the subjecting plate sinks, it heats up releasing water into the overlying mantle, lowing the melting point of mantle and creating magma
this magma is less dense than surrounding rock so rises through overriding plate and erupts on the sea floor creating volcanoes
overtime, repeated irruptions build up into an island
lithosphere at MORs
on either side of the MOR core sampling of the new crust shows a consistent structure
basaltic pillow lava has small crystals which cooled rapidly under water
dolerite dykes are the vertical feeder pipes from the magma chamber, supplying the volcanoes and has medium sized crystals so cooled slower
gabbro cools very slowly, forming coarse crystals in the magma chamber
seismic evidence shows a change in physical properties at the Moho where the mafic crust changes to ultramafic peridotite mantle
divergent boundary (constructive)
Mid-Atlantic ridge
plates move away from each other, shallow-focus earthquakes occur and volcanoes
when the plates move apart, pressure is reduced, allowing the ultra-magic peridotite mantle to partially melt into mafic magma
the mafic material adds on to the trailing edge of the spreading plate, creating new oceanic crust
upwelling magma beneath the MOR and expansion of hot rock help to increase the height of the ridge
the thicker the sediments, the longer they have had to be deposited, and the older the crust, thickness increases with distance from the ridge
transform boundary (conservative)
San Andreas Fault
plates slide past each other with a horizontal movement, causing shear stress on the rocks
seismically active due to friction
locked sections with no movement, strain builds up over many years until there will be a sudden break
no subduction
no volcanoes
no creation or destruction of the crust
evidence of continental drift
fit of the continents
jigsaw fit, however unreliable
sea level is constantly changing, coastlines are a temporary feature
deposition and erosion causing gaps and overlaps
better fit using the edge of the continental shelf
rock types
proving that two rocks on either side of an ocean were once part of the same outcrop
distinctive characteristics of mineral composition and physical features
age, as determined by radiometric dating
mountain chains
linear features hundreds of kilometres
Gondwanaland shows how one precambrian fold mountain chain crosses from Africa to South America and back to Africa as a continuous belt
to the two continents must have been joined together in the precambrian
fossils
such animals and plants that lived on land or shallow sea floors, would be unable to spread across a wide ocean
having similar fossil records would indicate that they were once together
glaciation
in both South America and Africa, there are sedimentary deposits of angular, poorly sorted and scratched pebbles (clasts)
glacial striations are used to trace the movement of the glaciers from one common source
Gondwanaland probably occupied a position near the South Pole as ice sheets cannot extend to the equator
Africa and South America are now much further north, indicating they have moved
convection cells
convection cells in the mantle were thought to drive the motion of the plates
there is no correlation between plate area and the velocity of plate movement suggesting that the process is not driven by movement in the mantle
ridge push
passively upwelling mantle at a divergent plate boundary is buoyant and raises the sea floor to form a mid ocean ridge (MOR)
gravity acts on the elevated lithosphere and the two plates are pushed apart
additionally by the rising magma as it cools and thickens
slab pull
the mass of the cold, dense lithosphere sinks downward at ocean trenches and pulls the rest of the oceanic lithosphere with it
Global Positioning System (GPS)
satellites are used to measure the exact location of a series of fixed receivers on the earths surface
from these positions, the relative movement of the plates is calculated
seismic tomography
technique in which a virtual 3D model is built up by analysing the behaviour of seismic waves as they pass through sections (tomo = section) of a body (earth)
magnetic anomalies
the earths magnetic field undergoes complete reversal, so that north becomes south, vice versa
as new magma is erupted at a MOR, the iron particles line up parallel to the earth’s existing magnetic field and as the rock cools down, the particles remain permanently magnetised
the result is a striped magnetic anomaly pattern on the sea floor
