Plate Boundary Types + Splitting Up Of Plates

Question 1

Continental crust thickness

Answer 1

. Thicker than oceanic

. 30 - 50km thick

Question 2

Oceanic crust thickness

Answer 2

. Thinner than continental

. 5-10km thick

Question 3

Continental crust age

Answer 3

. Older - about 4 billion years

Question 4

Oceanic crust age

Answer 4

. Younger/newer - 200 million years old

Question 5

Continental crust density

Answer 5

. Less dense

. Around 2.7g per cubic cm

Question 6

Oceanic crust density

Answer 6

. Denser + can be sub ducted and is constantly being destroyed and replaced at plate boundaries
. 3G per cubic centimetre

Question 7

Continental crust mineral content

Answer 7

Silicon and aluminium

Question 8

Oceanic crust mineral content

Answer 8

Iron, magnesium + sigma

Question 9

Continental crust rock type

Answer 9

Granite

Question 10

Oceanic crust rock type

Answer 10

Basalt

Question 11

Jigsaw fit

Answer 11

. 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)

Question 12

Geological fit

Answer 12

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.

Question 13

Tectonic fit

Answer 13

. 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.

Question 14

Fossil evidence

Answer 14

. 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.

Question 15

Conservative boundary

Answer 15

. 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

Question 16

Constructive boundary

Answer 16

. 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

Question 17

Destructive boundary

Answer 17

. 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

Question 18

Collision boundary

Answer 18

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

Question 19

Why can oceanic plate be destroyed and why is it important for plate tectonics

Answer 19

. Oceanic more dense so sinks at destructive boundary
. If this did not happen the crust would keep growing
. Allows destruction of plates