Principles of tectonics Flashcards
What are the names of the layers of the earth?
Lithosphere <1300c (solid)
Mantle 2000-4000 (solid)
Outer core 4000* (liquid)
Inner core 5000* (solid)
How is earths magnetic field produced?
By a magnetic dipole that is roughly parallel to earths spin axis (N-S pole)
Where is earths magnetic field generated?
Outer liquid core when rotating round earths spin axis based on geodynamo
What does the geomagnetic field act as?
A shield for the earth against dangerous particles projected from the sun (solar wind)
What is the shape of earths geomagnetic field distorted by?
The solar winds creating a tail behind earth
What is plate tectonics?
Describes the large scale motion of tectonic plates
How many major and minor tectonic plates are there?
7 major a d 8 minor
What state are tectonic plates in?
Solid and rigid
What are tectonic plates composed of?
Crust and upper mantle
What do tectonic plates coincide with?
The lithosphere (30-150km thick)
What do tectonic plates “float” on?
Thin layer (50-100km) zone of rheid mantle called asthenosphere
What happens at the exterior and interior of tectonic plates?
Deformation at exterior where plates interact but interior (CRATON) is undeformed and old
What is the name of the crust mantle boundary?
Moho
What is the depth of moho for oceanic crust?
~6km
What is the depth of moho for continental crust?
30-60km
What is the thickness of oceanic lithosphere like?
0-100km with it increasing further form spreading ridge
What is the thickness of continental lithosphere like?
100-150km
How do tectonic plates move?
High temperature allows solid state flow of mantle and is supported by lubricating zone (asthenosphere)
What is the rate of solid state flow of the mantle?
~1 to 5cm/yr
What is the reason for the different elevations of continental and oceanic crust?
Continental crust is less dense (more buoyant) so floats more on mantle
What is the density of the mantle?
3.3 g/cm*3
What are the characteristics of continental crust?
Thickeness (30-70 km)
Granodioritic mean composition (felsic)
Density of 2.7 g/cm3
What are the characteristics of oceanic crust?
Thickness (5-6 km)
Basaltic mean composition (mafic)
Density of 3.0 g/cm3
What does the Wilson cycle explain?
the reason continents are much older than oceans
What does the Wilson cycle describe?
The periodic opening and closing if oceans
When was continental drift proposed and by whom?
1915 by Alfred Wegener
What did continental drift propose?
All continents were once united into a supercontinent but they drifted apart on gloating ocean beds
What is evidence is there for continental drift?
Same fossils found across different continents (mesosaurus)
Continent shape fit together kike jigsaw
How is the mesosaurus evidence of continental drift?
Freshwater animal which fossil found in south America and southern Africa different climate at present and separated by saline water
What are alternatives to continental drift?
Land bridges
Expanding Earth
What is the land bridges idea?
fossil distribution explained by land bridges now sunken in oceans
What was the one problem with continental drift when it was proposed?
There was no how/ driving process
When was the Wadati-Benioff zone proposed?
1949
What is the Wadati-Benioff zone?
Specific location distribution of earthquakes at depth forming a planar surface dipping ~45* below continents to 670km
What depth and angle is the Wadati-benioff zone?
670km and 45*
When was the Apparent Polar Wander proposed?
1956
Who proposed Apparent Polar Wander?
Warren Carey
What does Apparent Polar wander show?
Continents have drifted in the past
What happens with magnetism and cooling lava?
Magnetite (magnetic mineral) aligns with magnetic field
How is APW (apparent polar wander) shown?
Rocks of different ages on a continent show change in position of earths pole
How does |APW show continents moving?
As the poles are fixed so continents must have moved
When was seafloor spreading proposed?
1960 by hess
What is the idea of seafloor spreading?
Oceans form and continuously grow at cracks (ridges) where magma rises, cools and forms new oceanic crust
What period were Marine Magnetic Anomalies (MMA) proposed?
1959-63
What are marine magnetic anomalies?
Anomalies in strength of the magnetic field which can be observed in the oceans
What is the pattern is how with magnetic fields on the sea floor?
Alternating pattern of stronger (normal polarity) and weaker (reverse polarity) parallel to mid-ocean ridges
Who linked seafloor spreading and marine magnetic anomalies?
Vine et al explained the mechanism combining Hess’ model with marine magnetic anomalies
What happens to the age of rock as you move away form mid-ocean ridges?
Rocks get older
What is the rate of crust consumed to created like?
Equal
What are the 3 types of plate boundary?
Divergent
Convergent
Transform
What happens at divergent boundaries?
Crust formed
Normal faulting
What happens at convergent boundaries?
Crust consumed
Reverse/thrust faults
What happens at transform boundaries?
Plate slides laterally
Strike-slip
What is the structure of oceanic crust? (top to bottom)
Sediment
Pillow Basalt
Basalt dikes
Gabbro
Peridotite (upper mantle)
How is the structure of oceanic curst known?
Geophysics and fragments of oceanic crust on land - OPHIOLITES
What is the rate if spread at fast spreading ridges?
> 4cm/yr
What are the characteristics of fast spreading ridges?
Sufficient melt production
Steady-state magma chamber
All 4 layers of oceanic crust are found
Not many faults present
What is the rate of spreading at slow spreading ridges?
2-4 cm/yr
What are the characteristics of slow spreading ridges?
Lower melt production
Small magma chamber
All layers present but thinner
Occurrence of normal faults
What are slow spreading ridges generally?
Rift valleys with intermittently present magma chamber
What is the rate of spreading for ultra-slow spreading ridges?
<2cm/yr
What forms at ultra-slow spreading ridges?
Oceanic detachments
What are ultra-slow ridges like?
Low angle normal faults with curved shape rooted below spreading ridge
What is east Africa experiencing as a result of continental rifting?
tectonic extension, lithospheric break up and volcanism
What happens to ridges with transform boundaries?
Will be offset
What are fracture zones?
lateral continuation of transform faults, but unlike those are inactive
How do fracture zones grow?
Laterally as long as the ridge remains active
What can fracture zones indicate?
direction of plates in the past
What crust boundaries will subduction occur at?
Ocean-Ocean (Tonga)
Ocean-Continent (Andes)
What crust boundary will under-thrusting occur at?
Continent-Continent (Tibet-Himalaya)
What is the slab with subduction?
portion of the down-going plate sank into the mantle
What is the trench when subducting?
Depression where plate bends into subduction zone
What is a volcanic arc?
100-150 km above slab where fluids are expelled causing overlying mantle to melt
What is the forearc?
region located between the trench and the volcanic arc
What is the accretionary wedge?
pile of sediments scraped off (bulldozed) subducting plate
What is the back arc?
region located behind the volcanic arc where spreading/extension may occur
What is slab “roll back”?
When the slab sinks faster then convergence between the plates causing the trench to retreat back
What might slab roll back lead to?
Extension of overlying plate opening a back-arc basin
Where doe volcanoes mainly form?
At convergent margins (subduction)
What is the average distance of a volcanic arc trench?
150-200km
Where do earthquakes occur?
At plate boundaries (most large at subduction zones)
What process usually occurs at Continent-Continent boundaries?
Orogeny- mountain building
What will indicate a diffuse plate boundary?
A broad seismicity distribution
What are the main drivers of tectonic plates?
Slab pull
Ridge push
What is slab pull?
weight of cool, dense subducting slab pulls plate along
What is ridge push?
bit like the plate sliding downhill from the high ridge
What triggers ridge push?
Elevation difference between the ridge and adjacent deeper seafloor
What causes slab pull?
negative buoyancy of dense slab (vertical down) applied along inclined surface (subduction zone) drag the plate down producing horizontal motion towards trench
What is the habitable zone Earth is found in called?
Goldilocks zone
How can earths activity be described?
Alive due to heat/ temperature and processes
What metals are in the outer core?
Nickle and iron
Why does the outer core produce the magnetic field?
Motion of boiling material and planet spin generates magnetic field
What would happen with no magnetic field?
The atmosphere would be blown away
What does convection not do?
Produce cells which drive spreading of ridges
What is a hypsometic curve?
% of topography above and below sea level
What % area is below and above sea level?
65% below
35% above
What is the oldest continental crust?
Gneiss 4000Ma
What is the oldest oceanic crust?
270 Ma
Why are oceanic rocks younger than continental?
As they subduct and are destroyed
How often does earths magnetic field switch?
300,000 years
Is crust created or destroyed at strike slip?
Conserved none created or destroyed
What is the sediment from an ancient ocean like?
Sediment on top layer of ocean veyr defomred?
What are sheeted dikes like?
Sheeted dikes intruded each other and split one another and are thus parallel to each other
2km thick
What might gabbro’s like for oceanic structure?
Isotropic or layered around 2.5km
What occurs to perodotitie?
Serpentinasation
Why are normal faults present at constructive margins?
gravitational collapse
Are there faults at fast spreading ridges?
No faults in fast spread as new material just produced
Why are there faults at slow spreading ridges?
Two plates moving apart so not controlled by ridge upwell this creates faulting
What is the rock structure like at ultra-slow spreading ridges?
Gabbro predominanlty
Thinner lithosphere
What are oceanic detachments like?
Oceanic detachment like spoon shaped which dip to spreading axis
What might be created as a result of the east African rift valley?
A new ocean within a few million years
Is a fracture zone a fault?
No just a damaged area
What happens to crust at a continent-continent boundary?
Vast thickening
What controls distance of volcanic arc?
Inclination of subduction - shallower is further
What leads to the creation of volcanoes at subduction?
Fluids lower melting point of material producing volcanoes to reduce pressure
