In divergent boundaries, plates move away from one another, often causing volcanic activity. The cooled magma is able to form islands such as the ones on the Aleutian archipelago.
*I don't think any of these are actual types of mountains. Arc contient and Andean aren't even real things.
As for your second question:
There are two main types of rocks in the Earth's crust.
Granite is less dense and rises higher, usually above land in continental crust.
Basalt is denser and so it will sink lower and form oceanic crust.
However, there is a layer of sediment on top of that crust. (sand, gravel, etc.)
