Continental drift and tectonic plates Flashcards
What are the properties of the continental crust?
Continental crust is on average 35km thick
2.6 kg/m^3
and composed mainly of granite, silicon, aluminium
What are the properties of the oceanic crust?
5-10km thick
3kg/m^3
composed of basalt, silicon and magnesium
What are the properties of the mantle?
To a depth of 2,900km
3.3kg/m^3 at Moho
5.6kg/m^3 at core
Rich in magnesium and iron
What are the properties of the asthenosphere?
Hot, weak capable of flowing.
Convection currents exist within the asthenosphere
What are the properties of the lithosphere?
Cool, rigid brittle
What theory did Alfred Wegener propose?
250 million years ago Pangea existed.
This broke into Laurasia and Gondwanaland
The movement continued to the present day as the continents separated and spread across the globe
What is the geological evidence on Wegeners theory?
This fit of continents e.g. South America and Africa
Evidence from 290 million years ago of the effects of contemporaneous glaciation in South America, India and Antarctica suggesting these land masses were joined at one time close to the South Pole
Maintain chains and some rock sequences on either side of oceans show great similarity e.g. northeast Canada and northern Scotland
What is the biological evidence supporting Wegeners theory?
Similar fossil brachiopods (marine shellfish) found in Australian and Indian Limestones
Similar fossil reptiles found in South America and South Africa (the Mesosaurus) and it would not have been able to swim
Fossils from rocks younger than the Carboniferous period in places such as Australia and India, showing fewer similarities, suggesting that they followed different evolutionary paths
Glossopteris emerged 299 million years ago and is in many different continents, could not have been carried across oceans, it is large tree
Marsupials are only found in Australia because it drifted away from the main supercontinent before the predators that wiped them out elsewhere had migrated there.
What rock is on the sea floor?
Igneous rocks, for the oceanic crust. Originate as lava flows, contain iron particles. As lava erupts, it cools and the magnetic orientation is the iron particles is locked into the rock
How is palaeomagnetism evidence of sea floor spreading?
The Earth’s polarity is not constant, alternates every 400,000-500,000 years .
When paleomagnetic data from the mid-Atlantic was made available, it was found the width of each strip of ocean bed with the same magnetic orientation was found to correspond with the time scale of each magnetic reversal.
The symmetrical pattern of geomagnetic reversals on either side of mid-ocean ridges indicate that fresh molten rock was being “pushed” away from the ridge.
How does the age of sea-floor rocks prove continental drift and sea-floor spreading?
The thickest and oldest sediments were found nearest to the continents
What are transform faults?
Found at diverging plate boundaries, displace mid-ocean ridges sideways can be by hundreds of kms
What features are found at mid-ocean ridges?
transform faults
pillow lavas
fractures in the lithosphere
underwater rift valleys
black smokers
Give an example of rifting away from mid-oceanic ridges
The Red Sea
What features are associated with oceanic-continental destructive boundaries?
ocean trenches
subduction zones
mountain chains
Beinoff zone
Plutons of magma rise to the surface
