Conservative plate margins are found where two plates slide past each other. There is no subduction or collision. The plates can move in different directions or in the same direction but at different speeds. They can cause extremely violent earthquakes.
Why do the plates move?
Convection currents in the mantle (asthenosphere) move the plates laterally. These currents are caused by radioactive decay of elements generating very high temperatures. This causes the rising and falling of magma which drags the plates above. They can move either in the same direction or in opposite directions. They do not necessarily move at the same speed.
How are conservative margins formed?
Conservative margins are caused by very complex movements of crust. Plates curve across the ocean floor and to avoid ridges twisting into ‘S’ shapes they crack perpendicular to it. This creates a series of conservative (transform) margins bisecting the ridge. This forms a zig-zag pattern along its length.
As the huge plates slide past one another, they become stuck. Tension and stress builds. This is eventually released as seismic waves in an earthquake event. Crust is not created or destroyed at conservative margins and so there is no volcanic activity.
The San Andreas Fault
The San Andreas Fault in California marks a conservative plate boundary. The Pacific and North American Plates are both sliding north-west but at different relative speeds.The Pacific Plate moves faster at 6cm per year whereas the North American Plate moves at just 2cm per year.
There is a large region along this margin that is prone to earthquakes. Los Angeles and San Diego are on the Pacific Plate while San Fransisco and Sacramento are on the North American Plate. There are up to 10,000 earthquakes at this boundary every year! Most of them are never felt though as they are weak on the Richter Scale.
In 1906 there was an extremely violent earthquake which was caused by the crust being displaced by 7m when tension was released. This generated a 7.8 magnitude earthquake.
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