Post by Brindley on May 5, 2016 17:43:03 GMT 2
Attempt at a "model answer" for an 8 mark sea floor spreading question. Contains additional detail and diagram in case the question is reframed as a 10mark.
AS_rocksweathering_seafloorSpreadModel.docx (40.88 KB)
"Using diagrams, explain how sea floor spreading occurs (8).
SEA FLOOR SPREADING
Sea floor spreading is theorised to be the lateral extension of ocean floors as the oceanic plates move apart from a zone of separation. The reasons for and nature of this spreading will be detailed below.
Sea floor spreading occurs during the divergence of two plates, forming what is known as a constructive plate margins; so named on account of new crust being created at this boundary. This is most notable in the mid-Atlantic where the North and South American plates are being pushed or pulled apart from the Eurasian and African plates.
Hess theorised that this movement was predominantly due to convection currents within the mantle. These currents would force up magma from below, cracking the earth’s crust and forcing the plates to diverge (move away) as the molten rock pushed through to fill gaps between them in a process known as ridge push. When this molten rock cools down due to the low temperatures of the sea it solidifies and creates the new ocean crust.
As the plate is pushed apart by ridge push, it may be further aided in its movement by a process known as slab pull. If there is an area of subduction, the plate will descend at the trenches, and pressure will cause the rocks to become heavier and therefore they are subducted further. However when there is no trench for old crust to subject under (such as the coast of Africa), then it pushes the continent along with it as crust accumulates. This is why the Atlantic ocean, with few tranches, is getting bigger, while the Pacific ocean, with many, is shrinking.
*additional 10 mark info*
The ridges are elevated above the ocean floor because they consist of rock that is hotter and less dense than the older colder plate. When the hot mantle material pushes up beneath the ridges to fill the gaps created by the diverging plates the rising material gets decompressed and melts partially.
The rate of spreading is not constant throughout the mid ocean ridge system. It varies by a few millimetres per year in the North Atlantic near the east pacific rise. The variation in spreading rates effect the ridge topography.
Slow moving ridges have a bigger pronounced rift down the centre. Fast moving ridges lack in central rifts and have a smooth topography.
The main reasons for the differences in floor spreading rates is that slow spreading ridges are fed by small, discontinuous magma chambers allowing the eruption of a range of basalt type.
Fast spreading ridges have large continuous magma chambers that generate similar magmas. In result of the higher rights of magma discharge, sheet lavas are more common.
Although mid ocean ridges look like continuous features within the ocean they are all broken into segments by transverse fractures that displace the ridges by hundreds of kilometres. Fractures are narrow and linear features that are marked by near vertical fault planes.
"
SEA FLOOR SPREADING
Sea floor spreading is theorised to be the lateral extension of ocean floors as the oceanic plates move apart from a zone of separation. The reasons for and nature of this spreading will be detailed below.
Sea floor spreading occurs during the divergence of two plates, forming what is known as a constructive plate margins; so named on account of new crust being created at this boundary. This is most notable in the mid-Atlantic where the North and South American plates are being pushed or pulled apart from the Eurasian and African plates.
Hess theorised that this movement was predominantly due to convection currents within the mantle. These currents would force up magma from below, cracking the earth’s crust and forcing the plates to diverge (move away) as the molten rock pushed through to fill gaps between them in a process known as ridge push. When this molten rock cools down due to the low temperatures of the sea it solidifies and creates the new ocean crust.
As the plate is pushed apart by ridge push, it may be further aided in its movement by a process known as slab pull. If there is an area of subduction, the plate will descend at the trenches, and pressure will cause the rocks to become heavier and therefore they are subducted further. However when there is no trench for old crust to subject under (such as the coast of Africa), then it pushes the continent along with it as crust accumulates. This is why the Atlantic ocean, with few tranches, is getting bigger, while the Pacific ocean, with many, is shrinking.
*additional 10 mark info*
The ridges are elevated above the ocean floor because they consist of rock that is hotter and less dense than the older colder plate. When the hot mantle material pushes up beneath the ridges to fill the gaps created by the diverging plates the rising material gets decompressed and melts partially.
The rate of spreading is not constant throughout the mid ocean ridge system. It varies by a few millimetres per year in the North Atlantic near the east pacific rise. The variation in spreading rates effect the ridge topography.
Slow moving ridges have a bigger pronounced rift down the centre. Fast moving ridges lack in central rifts and have a smooth topography.
The main reasons for the differences in floor spreading rates is that slow spreading ridges are fed by small, discontinuous magma chambers allowing the eruption of a range of basalt type.
Fast spreading ridges have large continuous magma chambers that generate similar magmas. In result of the higher rights of magma discharge, sheet lavas are more common.
Although mid ocean ridges look like continuous features within the ocean they are all broken into segments by transverse fractures that displace the ridges by hundreds of kilometres. Fractures are narrow and linear features that are marked by near vertical fault planes.
"
AS_rocksweathering_seafloorSpreadModel.docx (40.88 KB)