Plate tectonic theory Flashcards
Who made the plate tectonic theory and in what year?
Alfred Wegener - 1912
What did Wegener propose?
A single supercontinent existed about 300 million years ago called Pangaea
What happened to Pangaea?
It split into two continents: Laurasia in the North and Gondwanaland in the south
How did today’s continents form?
By Laurasia and Gondwanaland splitting off as a result of continental drift
What is the main geological evidence for continental drift?
The bulge of south America fits into the indent below West Africa
What is geological evidence for continental drift - glaciation
Deposits from a late-Carboniferous glaciation, 290 million years ago, are found in South America, Antarctica and India. The formation of these deposits cannot be explained by their present position; they must have been formed together and then moved. There are all striations of rocks in Brazil and West Africa which point to a similar situation.
What is geological evidence for continental drift - rock sequences
Rock sequences in northern Scotland closely agree with those found in eastern Canada, indicating that they were laid down under the same conditions in one location
What is biological evidence for continental drift - fossil brachiopods
Fossil brachiopods found in Indian limestone are comparable with similar fossils in Australia
What is biological evidence of continental drift - reptile
fossil remains fo the reptile mesosaurus are found in both South America and southern Africa. It is unlikely that the same reptile could have developed in both areas or that it could have migrated across the Atlantic
What is biological evidence for continental drift - plant
the fossilised remains of a plant which existed when coal was being formed have been located only in India and Antarctica
Why did people not take Wegener seriously?
His theories were unable to explain how continental movement could have taken place and his ideas gained little ground
When did evidence start to accumulate for Wegener’s theory?
1940s onwards
What is sea floor spreading?
The mid-Atlantic ridge was studied and they found that the ocean crust on either side of it suggested sea-floor spreading was occurring. This is because the alternating polarity of the rocks that form the oceanic crust. Iron particles in lava erupted on the ocean floor are aligned with the Earth’s magnetic field. As the lavas solidify, these particles provide a permanent record of the Earth’s polarity at the time of eruption, known as palaeomagnetism. However, the Earth’s polarity reverses every 400,000 years resulting in a series of magnetic ‘stripes’ with rocks aligned alternatively towards the north and south poles. This stripped pattern, which is mirrored exactly on either side of a mid-oceanic ridge suggests that the oceanic crust is slowly spreading away from this boundary. Moreover, the oceanic crust gets older with distance from the mid-oceanic ridge.
What are the implications of sea floor spreading?
The earth must be getting bigger, as this is not the case then plates must be being destroyed somewhere to accommodate the increase in their size at mid-oceanic ridges. Evidence of this was found in the discovery of huge oceanic trenches where large areas of ocean floor were being pulled downward in a process known as subduction.
Describe the role of hot spots
Hot spots around the core of the earth generate thermal convection currents within the asthenosphere, which cause magma to rise towards the crust and then spread before cooling and sinking. This circulation of magma is the vehicle upon which the crustal plates move. The crust can be thought of as ‘floating’ on the denser material of the asthenosphere. This is a continuous process with new crust being formed along the line of constructive boundaries between plates and older crust being destroyed at destructive boundaries. Where two crustal plates slide past each other and the movement of the plates is parallel to the plate margin, there is no creation or destruction of crust. At these conservative margins, there is no subduction and therefore no volcanic activity.