Rocks & Weathering Flashcards
Define sea-floor spreading
The formation of fresh areas of oceanic crust which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side
What are the characteristics of oceanic crust?
Thickness: 6-10km
Density = 3g/cm3
Rock Type = Basaltic
Age = 200 million years old (younger)
What are the characteristics of continental crust?
Thickness: 30-70km
Density = 2.7g/cm3
Rock Type = Silica, Aluminium, Oxygen
Age = Over 1500 million
Briefly explain how palaeomagneticism explains plate tectonics
- Magma is rising, and the compass needle of groups of rock depends on the magnetic field, there has to be space for the magma to rise and cool down to form new rock
Explain how a mid-ocean ridge forms (Divergent O-O margin)
1) The Mid-Atlantic ridge forms from convection currents
2) Which cause a divergent plate boundary between the Eurasian plate and North American Plate
3) Sedimentary rock above the crust allow lava to cool and solidify, forming submarine volcanoes
4) A chain of volcanoes form the mid-atlantic ridge
Briefly explain how a transform fault forms at a mid-ocean ridge
1) Plate doesn’t move at the same speed
2) Causing fissures
3) Due to tensile strength
Explain how volcanoes form at a divergent C-C margin
1) Convection currents in mantle drive plates apart
2) Up-warping of the crust takes place as magma rises as plates are driven apart
3) Tensional faults and cracks appear in the surface, allowing out gassing & steam eruptions
4) Plates continue to move apart as a rift valley is formed
5) Upwelling magma creates volcanoes
Explain the formation of fold mountains in a CONVERGENT (O-C) plate boundary [Nazca]
1) Nazca plate is moving eastwards towards the South American plate
2) Denser oceanic lithosphere of the Nazca plate subducts under the continental lithosphere of the SA plate
3) Friction between plates prevents the subducting oceanic plate from sliding smoothly, as convection currents pull the oceanic plate further into the mantle
4) Slab pull occurs - it drags against the overlying plate, causing it to fracture & deform. Defines zone of earrthquake foci known as Benioff zone
5) As subduction of Nazca continues, some sediments are scraped off and accreted onto the SA plate (Accretionary Wedge)
6) Effect of the collision of the 2 plates deforms the leading edge of the SA plate by folding the rocks, increasing vertical thickness and so produces the fold mountains of the Andes.
Explain the formation of island arcs at convergent (O-O) plate boundaries
1) North American plate subducts beneath the less dense Caribbean plate
2) Subduction forms accretionary wedge
3) Continued subduction of NA plate brings sea water, locked in ocean crust deep into mantle.
4) The plate dehydrates as the water evaportes into the steam, which pressuires and exploits weaknesses in the overlying crust. This allows magma a path to the surface
5) Magma erupts due to the trapped gas and steam rising, forming volcanic island arcs
Explain the formation of mountains at Covergent (C-C) boundaries (Himalayas)
- Eurasian plate partly crumpled and buckeled up above the Indian plate, but due to low density, none could be subducted
- Caused crust to thicken due to thrusting and folding of the crust under compressional forces, forming fold mountains
- Metamorphosis of rock also occurs under intense friction
- Thickening has uplifted the Himalayas and Tibetan plateau to the north
- Creates normal faults, resulting in shallow focus earthquakes.
Explain how a fault is formed on a Conservative (C-C) plate boundary (e.g San Andreas Fault)
1) Plates slide past eachother, horizontal movement, Lithosphere neither created nor subducted.
2) Frictional forces lock the blocks of the lithosphere together for years at a time
3) When forces overcome, plates slip suddenly and shallow focus earthquakes are generated
4) Landscape & Manmade features are displaced across the fault
5) San Francisco has historically suffered significant earthquakes, e.g 1906 and 1989
