TECTONICS Flashcards
1
Q
Supercontinents
A
- Pangaea
- Gondwanaland - Southern continents (America, Africa, Australia)
- Laurasia - northen continents (America, Europe, Asia)
2
Q
Wilson cycles
A
- cycle of supercontinents forming and breaking up
- takes about 500Ma
3
Q
Plates - contraction theory
A
Geosynline model
- early geologists thought earth had cooled from molten, sinking as it did
- the skin/crust wrinkled and depressions filled with sediment
- weight of sediment made wrinkles sag further
- fold mountains are created (orogeny)
4
Q
Continental drift theory
A
- alfred wegner 1915
- continents move over ocean floor
- easily disproved and discredited
5
Q
Evidence for continental drift
A
Fit of the continents
- present coastline doesn’t fit due to sea level change and erosion/deposition
- better fit if edge of continental shelf is used
Matching Geology
- rock type/age work in continuous bands across continents
- same mineral composition, physical features and ore age
Fold mountain chains
- map of Gondwanaland shows continuous precambrium mountain chain that crosses Africa and South America
- continents must have been joined in precambrium
Fossils
- fossils of land animals are found in multiple continents
- impossible to swim there so they must have been joined
- e.g mesosaurs found in Africa and South America
Glaciation
- striations show route of ice between continents
- eriatics (rocks found where they don’t belong) show ice routes
Palaeomagnetism
- apparent polar wandering curves will the same if on the same continental crust
- the curves will diverge when the continent splits
6
Q
Palaeomagnetism
A
- iron rich minerals in lava face the North Pole
- rocks cool past the curie point and the minerals are frozen in position
- direction of the magnetic pole is recorded (remnant magnetism)
- latitude is also recorded
- angle of dip (inclination) is roughly equal to the latitude
7
Q
Sea floor spreading
A
- MOR
- plates move apart and magic magma rises to fill gap
- forms basaltic pillow lavas on sea floor
8
Q
MOR - magnetic reversals
A
- magnetite crystals in basalt record the direction of the north pole when they cooled
- magnetic reversals produce striped patern on oceanic crust:
- pattern is parallel to MOR
- pattern is symmetrical either side of MOR
9
Q
Earthquakes at MOR
A
- due to rising magma and normal faults
- due to transform faults (90° to MOR) , plates trying to move past eachother
10
Q
Sea floor spreading rate
A
- rate = distance/time
- e.g crust moves 40km in 1Ma
40km= 4,000,000
1Ma = 1,000,000
4,000,000/1,000,000 = 4cm/year
11
Q
Oceanic and continental crust
A
Oceanic
- 7km thick
- basaltic
- 3 main layers: basalt pillow lavas, dolerite dykes, gabbro
- 2.9g/cm³
- formed at MOR
- <200Ma
Continental
- 35km thick (up to 100km under fold mountains)
- granatic
- 2.7g/cm³
- formed by accretion of lighter elements rising through mantle
- up to 3.8Ga
12
Q
Heat sources in earth
A
- from the formation of earth
- from radioactive decay
13
Q
MOR features
A
- long/narrow
- mafic/ultramafic
- New crust forms in centre (axial rift valley)
- older and thicker sediment further from MOR
- small earthquakes along MOR on transform fault
14
Q
Continental rifting
A
- Continental crust breaks up
- Rift valley with step faults
- Magma rises to fill gap
- Step faults indicate rifting happened
diagram in notes 3
- e.g 50Ma between Scotland and Greenland
- rifting opens continents and can leave a failed rift valley (“aulacogen”)
15
Q
Isostasy
A
- state of equilibrium between lithosphere and asthenosphere so lithosphere “floats”
- isostatic rebound of crust that was depressed by ice sheets
