What is the difference between a rift valley and a mid-oceanic ridge?

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As tectonic plates move they sometimes move away from each other at divergent plate boundaries. The space left behind opens a large hole between the plates that is either filled with sediment, magma flowing from the mantle, or water. The major difference between different types is what type of plate the divergent boundary is between. 

If the boundary is found between two continental plates you are left with a rift valley. This leads to steep mountainsides spreading apart from a widening valley. In the center of the valley either large gaps can form or the area will fill with water from water sources or rain creating rift lakes, much like what can be found in the African Rift Valley. Many lakes have sprung up in the valleys left behind as the plates there move slowly away from each other. 

If two oceanic plates begin moving away from each other it creates a mid-oceanic ridge. Continental plates are less dense, and thus float much higher on the mantle. Magma is usually much farther away, so it's rare to find a splitting continental plate seeping magma. On the other hand, oceanic plates are thin and dense, meaning magma is much more likely to flow to the surface at one of these splits. At the Mid-Atlantic Ridge, the boundary between the North American plate and the Eurasian plate, the spreading boundary is constantly seeping hot magma out of the mantle below. This creates a constantly growing mountain chain. 

The main difference between these two seismic structures would be what types of plates are creating the diverging zone and what is being created at the divide: a valley that may fill with water or sediment, or a growing mountain chain as magma fills the opening gap.

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