Plate Boundary Types + Splitting Up Of Plates Flashcards
Continental crust thickness
. Thicker than oceanic
. 30 - 50km thick
Oceanic crust thickness
. Thinner than continental
. 5-10km thick
Continental crust age
. Older - about 4 billion years
Oceanic crust age
. Younger/newer - 200 million years old
Continental crust density
. Less dense
. Around 2.7g per cubic cm
Oceanic crust density
. Denser + can be sub ducted and is constantly being destroyed and replaced at plate boundaries
. 3G per cubic centimetre
Continental crust mineral content
Silicon and aluminium
Oceanic crust mineral content
Iron, magnesium + sigma
Continental crust rock type
Granite
Oceanic crust rock type
Basalt
Jigsaw fit
. Similarity in outline of the coastlines of eastern South America and West Africa have been noted
. Best fit if coastlines are matched at depths of 1000m below current sea level
. Gaps or overlaps can be explained by erosion, deposition, rises in sea level (eustatic change) + changes in land level (isostatic change)
Geological fit
When the geology of eastern South America and West Africa was mapped it revealed that ancient rock outcrops (cratons) over 2,000 million years old were continuous from one continent to the other.
Tectonic fit
. Fragments of an old fold mountain belt between 450 and 400 million years ago are found on widely separated continents today.
. Pieces of the Caledonian fold mountain belt are found in Greenland, Canada, Ireland, England, Scotland and Scandinavia.
. When these land masses are re-assembled the mountain belt forms a continuous linear feature.
Fossil evidence
. There are many examples of fossils found on separate continents and nowhere else, suggesting the continents were once joined
. The alternative explanations would be:
species evolved independently on separate continents contradicting Darwin’s theory of evolution
They swam to the other continent/s in breeding pairs to establish a second population
. For instance fossils of Lystrosaurus are only found in Antarctica, India and South Africa
. When the continents of the southern hemisphere are re-assembled into the single land mass of Gondwanaland, the distribution of these four fossil (Lystrosaurus, Cynognathus, Mesosaurus, Glossopteris) types form linear and continuous patterns of distribution across continental boundaries.
Conservative boundary
. Exist where two plates do not directly collide but slide past each other along a fault (weakness)
. No volcanoes are found along these plate boundaries, but earthquakes do occur
. E.g. San Andreas Fault in California
Constructive boundary
. 2 plates are moving apart leaving a gap in the middle for magma to rise up through
. Volcanoes form but don’t erupt with force as the magma can escape easily
. Earthquakes occur but with minor tremors
. Mid ocean ridges form
. Example is the mid-Atlantic ridge
. Rift valleys can form when plates move apart and land between them falls down into the space left behind
. Common if 3 plates meet at a junction
Destructive boundary
. Dense Oceanic plate descends below less dense continental plate
. As the plate descends it is melted due to friction with the continental plate, farming hot, liquid magma.
. This magma can then rise through cracks in the continental plate and form volcanoes if it reaches the surface.
. Powerful earthquakes, fold mountains, ocean trenches
Collision boundary
Collision boundaries occur when two plates of similar densities move together (i.e. a continental plate and a continental plate). This causes the material between them to buckle and rise up, forming fold mountains.
The Himalayas are an example of a chain of fold mountains. They have been formed by the African plate colliding into the Eurasian plate
Why can oceanic plate be destroyed and why is it important for plate tectonics
. Oceanic more dense so sinks at destructive boundary
. If this did not happen the crust would keep growing
. Allows destruction of plates
