1 - Plate tectonics Flashcards
Who came up with the idea of continental drift?
Alfred wegner
What are the three main pieces of evidence for continental drift?
- Glaciation: Evidence of glaciation can show that continents were once closer to the poles where glaciation may have occured.
- Fossil Evidence: Fossils of a species found in two locations not connected by land, but with identical evolution can show the two locations used to be connected. An example of this is the Mesosaurus.
- Jigsaw fit: If pieced together the worlds continents would fit well with very little overlap suggesting they were once connceted in a supercontient called Pangea millions of years ago.
Who initially discovered the idea of sea floor spreading? What was his theory?
- Henry Hess when he discovered ocean ridges and trenches.
- He believed that oceans grew from their centres, with molten material (basalt) oozing up from the Earth’s mantle along the mid ocean ridges. This created new seafloor which then spread away from the ridge in both directions.
- Ridges were higher due to thermal expansion
- As spreading continued, the older ocean floor cooled and subsided to the level of the abyssal plain which is approximately 4 km deep.
- Hess believed that ocean trenches were the locations where ocean floor was destroyed and recycled.
What evidence supported sea floor spreading and exlpain why?
Paleomagnetism:
- Cambridge University team discovered that there was stripes of magnetic anomolies in the sea bed related to the reversal of the earths poles every few hunderd thousand years
- In 1963 it was deduced that lavas erupting at mid ocean ridges recorded the earths polarity at the time.
- They saw a symmetrical pattern in the magnetic fields of the rock each side of the mid Atlantic ridge which means the rock is moving away from the mid Atlantic ridge proving the theory of tectonics.
Carbon dating:
- Rock was found to be older further away from ocean ridges
What is the benioff zone and how did this support the theory of plate tectonics
- The area where one plate subducts beneath another and rub together at a convergent (destructive) plate boundary, causing earthquakes. When the crust subducts into the asthenosphere it is destroyed.
- This supported plate tectonics because crust must be destroyed at the same rate it is created.
What is a hotspot and why does it support plate tectonics.
Areas not at plate boundaries where mantle plumes rise from deep within the earth beneath the crust causing volcanic activity.
Plates move over a hotspot causing chains of volcanoes to form, such as the Hawaii island chain. Once a volcano has moved fullt over the hotspot it becomes decreasingly active.
How do seismic waves help us understand the structure of the earth
- P waves travel through solids and diffract in liquids
- S waves travel through solids but not liquids
- Outer core is liquid as S waves do not pass through
- Diffraction patterns of P waves show innercore is solid
- P wave shadows on far side of earth due to diffraction allow scientists to calculate size of outer core and innercore
What is the basic structure of the earth?
- Crust: Solid, Oceanic crust is 5-10km thick, while continental is up to 70km thick
- Lithosphere: Crust and solid upper part of the mantle, usually around 100km to 200km thick but very thin at hospots and plate boundaries, what plates are made of.
- Asthenosphere: Very viscous (putty tecture), allows convection currents, up to 660km deep
- Mesosphere: Rigid due to extreme pressure, no convection, up to 2900km deep
- Outer core: dense liquid metalic elements, up to 5100km deep
- Inner core: solid metalic elements, up to 6400km deep
What is the lithosphere
The rigid outer part of the earth made up of the crust and solid upper mantle
What is the asthenosphere
Part of the upper mantle directly below the lithosphere, in which the rock is soft enough to allow convection but it is not fully liquid.
What are the similarities and differences between the inner and outer core
Similarities:
- Made of similar metalic elements
Differences:
- Outer core is liquid while inner core is solid due to lower pressure in the outer core
- Temperatures are as high as 6000C in the centre, while the outer core is 3000C
What is the difference between the crust and the lithosphere
- The crust is the thin layer on top of the mantle and takes two forms. Continental crust is thicker at around 10-70km thick, while oceanic crust is only around 5-10km thick.
- The lithosphere includes the crust as well as the upper layer of the mantle which is still rigid and can stretch to 200km deep especially under continental crust.
- The lithosphere is mainly made up of mantle rocks as the upper layer of mantle is far thicker than the crust, rather than crustal rocks.
What are the causes of movement of tectonic plates
- Convection currents: Due to internal heat from the breakdown of radioactive isotopes in the earths core, there is convection in the mantle. This happens very slowly as the viscous mantle only moves around 2cm a year. This is much slower than actual plates (8cm a year) move so other things are mainly driving movement.
- Slab pull: When a plate at destructive plate boundaries subducts it sinks into the mantle as it is more dense, pulling the rest of the plate along with it as GPE is converted into KE
- Ridge push: Upwelling of the mantle at divergent plate boundaries causes a ridge at higher elevation, and the constant upwelling pushes the plates apart.
- Slab suction: A subducting section of ocean crust forces convection currents downwards. When this crust breaks off, it sinks faster leaving a vacum for the the rest of the plate to fall into with less resistance.
What are the three main types of plate boundaries
- Divergent
- Convergent
- Conservative
Explain the processes and features of a divergent plate boundary.
- Plates are moving apart and new crust is created by magma pushing up from the mantle by ridge push.
- As this magma cools it forms shield and fissure volcanoes.
- Some sections of the plate move back faster than others in transform faults.
- Crust is stretched thin by plates moving apart
- Form mid ocean ridges.
- EG: Mid Atlantic Ridge
