The Earths crust is split into over 30 different plates which swim on the Mantle, abit like a raft on water
2 Plate Boundaries
Convection currents (see plate tectonics) causes 2 plates to move away from each other causing magma to rise - creating new land. This is known as a contructive plate boundry.
Key tip to remember:
To "construct" land use a "constructive" plate boundry