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Summary Rocks and Weathering
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Rocks and Weathering3.1) Plate tectonics
Plate movements
Plates move between 1cm and 10cm per year
Result of convection currents heat is produced by radioactivity. Some areas of the
mantle are hotter than others. In these areas the mantle rocks are more “plastic”
resulting in faster movement
3 mechanisms are involved
Ridge push
Intrusion of magma into the spreading ocean ridge, such as the Mid-Atlantic
Ridge, which propels the plates apart
Conversion drag
Convection currents in the plastic mantle drag the overlying lithosphere. The
heat source is directly beneath the oceanic ridges. Cooling and descending
parts are in subduction zones
Slab pull
Colder, denser crust sinks - due to gravity – into the subduction zones at
places such as the Aleutian trench. This drags the rest of the plate with it
Oceanic plates
Continental crust is ±35-40km thick. Composed of mainly granite and related rocks in
silicon and aluminium. These rocks are relatively light with a density of ±2.7-2.7gkm 3
The oceanic crust is thinner than the continental crust but denser with a density of
±2.4-3.6gkm3. It is composed of basalt
Plate boundaries
The boundaries between the plates are belts of major earthquakes
There are 3 main types of plate margins:
1) convergent plate boundaries
Where material is being destroyed or subducted
The plates are moving together (compression)
3 types of convergent plate boundaries depending on the nature of the plates involved
1) Continental–oceanic convergent plate boundaries (and its important features)
Fold mountains
Form the highest of the world’s mountains ranges. Long relatively thin/narrow
belts of mountains
Valleys and ridges are parallel
Form where compression caused by plate collision has squeezed the layers of
rock
When the anticlines form the ridges and the synclines form the valleys this is
referred to as normal relief
The folding takes place at great depths in the earth where the high temperature
and pressure cause the rock to behave as a plastic solid
Where the mountains are uplifted, material at depth is forced downwards
leading to a thickening of the earths’ crust in the mountain belt
, Sediments deposited in the adjacent ocean and trench are scraped up against
the leading edge of the continental plate and added to it which forms an
accretionary wedge or accretionary prism
Volcanic cones
Active volcanos form high conical mountains
These are usually strato-volcanos made up of alternate layers of lava and ash
produced by explosive volcanos
At these deep plate margins, the denser oceanic plate is forced under the less
dense continental plate subduction
The oceanic plate is absorbed into the mantle and destroyed. The subducted
plate and the overlying mantle are partially melted
The “pockets” of magma gradually merge with each other and begin to rise to
form volcanos. Collectively, the mountain-building process is known as
orogenesis
Oceanic trenches
The topographic expression of convergence in ocean basins is that of a trench
and these form the deepest parts of oceans
Offshore there is no wide continental shelf and the ocean floor drops steeply into
a long, narrow trench which is parallel to the fold mountains
The water depth is ±100km compared to the 2-5km in the rest of the ocean
Trenches are the result of the surface being dragged down by subduction
The sloping zone of earthquake foci is known as the Benioff zone
2) Oceanic-oceanic convergent plate boundaries
The processes that occur at this type of margin are similar to that at the continental-
oceanic plate margins
Strato-volcanos, ocean trenches, accretionary prisms and a Benioff zone occur
No fold mountains are formed
The main features are ocean trenches and island arcs
The trenches are narrow crescents (e.g. which present their convex sides to the
Pacific oceans and their concave sides to the Asian continent)
Oceanic arcs are island chains with the same concave forms
Island arcs are made principally of active strato-volcanos but also by some
sediments in the accretionary prism
As the 2 converging plates are both oceanic, the rocks at the edges of the plates
have the same density (both made of basalt)
It is always the larger section of plate from the ocean side which is subducted
due to its greater mass
Heat flow (heat from depth to the earths’ surface) is less than normal over the
trench with its cold descending slab higher than normal over the volcanic island
arc
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