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Theories of continental drift and plate tectonics - Coggle Diagram
Theories of continental drift and plate tectonics
Wegener's theory of continental drift
1) The fit of the continents:
Wegener used the shape of a coastline to make the fit
a good example of a fit is coastlines of South America & Africa
they were not an exact fit (as sea level changed, coastlines temporary, deposition & erosion since 167Ma leaving gaps & overlaps)
Bullard's reconstruction of the fit was taken at -2000m contour, essentially the edge of the continental shelf, this is the best fit to date, Wegener was unaware of this feature
2) Mountain chains and shield areas (cratons) match across continents:
they are linear or elongated (chains) & mostly occur along the edge of continents and within continents
Wegener noticed the trend of mountain ranges of the same age can be traced across several continents (suggesting they were formed at the same time on one continent e.g. Greenland/Norway 440-400Ma old)
cratons (shield areas) shapes, rock types and ages match across continents (a cartoon is an area of old rocks of the same age & composition) - Wegener matched them across continents
3) Global glaciation:
evidence from ancient glacial depositis
ice occurs in the tropics and along the equator
rock sequences containing glacial deposits such as in Africa & South America could not have formed near the equator
caused by continents fitting together on the South Pole
also, the direction of striations (give flow directions) point to S.Africa - this must be the pole; demonstrates movement of glaciers
the orientation of striations seen in bedrock of Australia, South America & India suggest glaciers formed in the oceans and move on land
4) Fossil evidence:
if Africa & South America were separated they would find completely different fossil records, e.g. Mosasaurus is only found in both & freshwater reptile so couldn't have been able to swim across Atlantic
land based reptiles and plants cannot spread across vast oceans
Glossopteris fauna plant, found in South America, India, Australia, Africa & Antartica, would not have been able to survive in a wide range of environments (tropical to polar), seeds too large to have been spread by win & would not have been able to survive floating in seawater
fossils of land reptile Lystrosaurus are found in the Gondwanan continents
5) Matching climate-sensitive rock types:
evidence from rock sequences
certain rock types are deposited in certain climatic conditions
coal in tropical rainforests & swamps
desert sandstones in hot deserts
salt & gypsum (evaporates) in hot arid climates (tropics)
glacial deposits on the poles
Wegener matched these across continents, suggesting formed when in one plate
distributed of Permian tropical coal deposits, subtropical desert deposits etc, only makes sense in Pangea formation
Continental drift theory
= theory that the position of continents has changed overtime, and this movement indicates great forces at work
Alfred Wegener
(theories from 1915-29), argued that there was once a single supercontinent called Pangea, that separated & broke up to form several land masses
1915, The Origin of Continents, set out evidence for continental drift, not accepted
he died in 1930s
1950s, evidence from palaeomagnetism and exploration of seafloor has led to acceptance
he argued in the Carboniferous period, Pangea existed and slowly broke into 1 land masses of Laurasia (North) and Gondwanaland (South)
this spreading & separated has continued
Evidence of seafloor spreading
Evidence of seafloor spreading:
pioneering work in the ocean by 'Glomar Challenger' (1960s-80s) providing new data
magnetic measurements across the mid-Atlantic ridge, by magnetometers
Magnetic signature of seafloor:
found magnetic patterns
1) elongate parallel to the ridge
2) symmetrical about the ridge
3) negative & positive
magnetism preserves in ancient rock is called paleomagnetism
Magnetic reversals:
seafloor rocks recorded periodic changes in the Earth's magnetic field
negative & positive measurements indicated Earth's magnetic field had switched at times in the past
as plates move apart, new basalts inherit the magnetic signatures of Earth at particular times - symmetry shows movements is equal on each sides & parallel
width is a function of time & rate spreading
MOR magma rises and cools to form new oceanic crust, preserving a record of earth's polarity at the time, as it cools magnetic minerals (containing iron) become aligned with Earth's magnetic field - symmetrical
Other evidence: age of ocean crust:
as crust moves away from ridge, it gets older, indicating new material is created & moves away from the ridge
Other evidence: age & thickness of sediment:
as crust moves away from ridge, more sediment accumulates over time
sediment is thicker & older further of the ridge
Palaeomagnetism:
tech designed to measure submarines measured very small variations in Earth's magnetic field
the magnetic field showed up as a striped pattern across the ocean floor
as lava erupts the magnetic orientation of the iron particles is locked into the rock depending on the Earth's polarity
every 400,000-500,000 years the polarity changes, this is recorded by the ocean floor In 1960s the idea of sea floor spreading was born
symmetrical pattern of geomagnetic reversals on either side of mid-ocean ridges indicate fresh molten rock from the asthenosphere reached the ocean bed, older rock was 'pushed' away from the ridge (like a 'conveyor belt')
eventually it reaches an ocean trench where material subducted
plates moved by sea-floor spreading from mid-ocean ridges to subduction at ocean trenches
The age of sea-floor rocks:
1960s ocean drilling programme established looking at ocean sediments and crustal rocks deep in ocean floor
this recovered cores in water up to 7000m deep and cores revealed spatial pattern of sediments supporting sea-floor spreading
thickest and oldest sediments nearest to continents
nowhere in the ocean rock older than 200m years, showing ocean crust recycled
Lithospheric plates
What is a lithospheric plate?
most plates are composed of oceanic & continental crust
7 large plates & several smaller ones
new crust formed at MORs (plates move apart) & subduction zones (plates together), causes earthquakes
interior of plates are not involved in these motions, aseismic regions
zones of earthquakes, chains of volcanoes, high heat, trenches, rift valleys, fold mountain belts, all mark plate boundaries
Role of convection currents
Earth's outer layer of lithosphere is rigid & brittle & is composed of a jigsaw of plates
underlying asthenosphere acts as ductile material & flows, flows driven by large-scale convection currents
hot magma rises and sinks
Why do plates move?
they allow heat to escape from the deep interior with temps reaching 5000C
earth is large and rocks inside conduct heat poorly, removing heat by conduction is extremely slow causing convection
the rocks that make up the outer 2/3 of our planet, the mantle flow at only a rate of a few cm / year (meaning flow transports better than conduction)
Plates as part of the mantle convection system:
convection works with heat added from metallic core at the base but also decaying radioactive elements inside the mantle
when mental rocks cool at surface, they become stiff and form hard plates, breaking when stresses are high enough
when plates break, this relives stress on remaining plate fragments, which remain stiff while deformation concentrates along plate boundaries leading to plate tectonics
this plate moves apart, upward-flowing mantle reaches surface along boundary between plates, then cooling forming a new plate
when plates move away from hot boundaries, they cool, contract and become increasingly dense
subduction occurs when plates move towards each other and the denser one slides under the other
the subducting plate sinks into warmer, lighter mantle, it pulls rest of plate at surface
Where does the heat in the Earth come from?
from convection / tectonics, 45 terawatt is released through the surface (3x energy use of all people on our planet)
below plates is temp of 1,300C, 5000C in Earth’s centre
part of the heat inside the Earth is left over from its formation
heat is produced from decay of radioactive elements
it has taken 2b years for mantle to cool by 100 degrees
Rising and sinking flows in the mantle:
rising plumes of hot mantle reach the top and can trigger mantle and crustal melting and volcanism – this is called ‘hotspots’
only 10-15% of heat released from mantle comes out of the core
plumes are only a small apart of mantle convection and not very effective at pushing plates, but can help them split
85-90% of heat inside mantle is mostly released by cooling the mantle at the surface
the cold sinking plates cool the mantle and form the sinking part of mantle convection, they can pull plates apart forming ridges e.g., Mid Atlantic Ridge
Melting where plates meet and part:
2,900km depth mantle is hot enough to flow but not melt
shear waves emitted by earthquakes travel through mantle but can’t through fluids
mantle material does melt in a few places but only the minerals with the lowest melting temperature melts
this is called partial melting and forms a mush of solid crystals and liquid melt, any melt that erupts from such mush is different composition from original solid rock
Spreading ridges:
most common form of mantle melting occurs under spreading ridges
plates spread and lid is taken off mantle, allowing it to flow almost to the surface where pressures are very low
melting temp decreases when pressure decreases, material of mantle temp can suddenly melt
the partial melts formed under the spreading ridges form the thin basaltic crust found on ocean floor
Subduction zones:
mantle melts formed when water is added to the mantle and the melting temp is lowered (water added to mantle in subduction zones)
subducting plate sinks into the hotter mantle, it heats up and starts to release the ocean water that seeped into its cracks and was part of minerals while the plate was still at the surface
when water rises up into the hotter mantle above plate, it lowers the melting point causing partial melting of the mantle
the subducting plate doesn’t melt as it is too cold
rocks conduct heat poorly, so it will take millions & millions of years until plates that are sinking through the mantle have heated up to the same temp as the surrounding mantle
when the plates sink to bottom of the mantle, they may deform and buckle under increasing pressures
water released by subducting plates allows mantle above the sinking plate to melt
magmas that form this way contain much more water than magmas that form under spreading ridges
magmas formed at spreading ridges (and hotspots) tend to flow out passively as lava, but water-rich magmas at subduction zones often lead to explosive eruptions
melting at subduction zones creates magmas with a range of compositions, they evolve as they get help up on their way through the plate above, with more of the heavier, higher-melting-temperature minerals getting left behind
this forms a different type of crustal rock, these rocks make up the crust of continents
these evolved melts form lower density rocks than basaltic melts, this is why continental crust floats higher on the mantle than oceanic crust and continents emerge above sea level
also why ocean plates subduct and don’t remain at the surface for more than 100-200m years
Hotspots:
there’s only a few places where the mantle melts because it is quite hot, this happens below hotspots where hot plumes rise deep in the earth
melting only happens at quite shallow depths (>150km) where pressures are low enough to allow mantle to partially melt to produce basalts
forms volcanoes
Structure of the earth
series of concentric shells
3 layers
1) core (inner & outer)
2) mantle
3) crust
separated by boundaries known as discontinuities, sharp boundaries
Cross-section of the Earth:
crust, granite - lithosphere
discontinuity, moho, av 35km
upper mantle, periodite - lithosphere
rest of mantle, peridotite (starts at 700km) - asthenosphere
discontinuity, taylor-gutenburg, 2900km
outer core, liquid Fe, Ni, S
discontinuity, lehmen, 5100km
inner core, solid Fe, Ni, S (ends at 6371km)
Earth's layers:
inner core is solid, compressed with extremely high pressures
outer core, fluid, very high temps exceed melting temp of iron even at high pressures; fluid as water and convects vigorously 100ms / year as heated from inner core
mantle, 2,900km-thick, silicate rocks, more then 80% of earths volume, average temp of 2,000-2,500C, rocks flow slowly of few cm / year, high viscosity 10^23 times that of water
crust, thin, cooled mantle magmas, average 7km thick below oceans, 35km thick below continents
tectonic plates / lithosphere, crust plus underlying layer of cold stiff mantle can be as thick as 100-250km
How do we know what the Earth looks like inside?
deepest hold drilled in Russia reached depth of 12km (1/3 thickness of crust at that location)
mountain belts may cause rocks from deeper depths to come back to the surface, minerals here retain clues of high pressures and temps experiences at depth (up to 200km but often only a few tens)
volcanic eruptions can bring up small rock fragments (xenoliths) from great depths inside plates
meteorites also help showing original building blocks of the rocky planets still flying around solar system
we can look at interior structure of earth using seismic waves produced by volcanoes, they can perform tomography studies
all of this allows crust, mantle and core to be mapped, the subducting plates and rising mantle plumes have been images
Plate tectonics
Theory of plate tectonics:
upper part of the Earth has many sections, they move relative to eachother, carried by moving materials below (these sections are rigid lithospheric plates)
the moving part is the asthenosphere (partially molten & flows)
Theory of plate tectonics
= the deformation of the Earth’s surface in terms of the motion of a set of rigid plates with narrow deformation zones (plate boundaries) between them
plate tectonics cause features (such as volcanoes) and allow life on Earth due to cycling of water
Seafloor hypothesis & plate tectonics
Henry Hess (1960) populated the idea of seafloor spreading (continents drift apart causing wider ocean, by adding basalt & creating mid-ocean ridge)
suggested the seafloor created ridges was consumed at sea trenches, seafloor a conveyor belt
surface of sliding was the base of the lithosphere (and crust), driven by convection currents
regions can collide so strongly they form mountain ranges
Tuzo Wilson (1966), suggested Earth's surface was composed of pink units containing continental & oceanic crust (lithospheric plates)
Wegener's hypothesis rejected
rejected by many geologists in 1912, disliked idea about mechanism / driving force
Wegener said continents floated on the mantle and were driven by gravitational pull of the moon
1966, Arthur Holmes, heat released by radioactive decay of materials in the mantle caused convection currents that carried the continents like rafts (popular theory)