To understand how the rocks on Leka and surrounding areas are formed, one must know something about the movements in the earth’s crust. The crust is currently divided into 6 large plates and each of those includes both continental and oceanic crust. The plates are moving apart along spreading zones and against each other in collision zones (see figures below). New oceanic crust is formed in the spreading zones of lava that solidifies on the seabed, and old oceanic crust disappears in collision zones between either two oceanic crusts or between oceanic crust and continental crust. The reason for these movements are currents in the mantle below the crust.
The continental drift theory, introduced by Alfred Wegener in 1912, explains how different plates have moved through earth-history, and continental drift is the main force of the shaping of landscape and developement of new rocks. New continents have been created, while other have separated or vanished. The most important geological event in Norwegian geological history is the Caledonian orogeny, where the two continental plates Laurentia (incl. North-America) and Eurasia collided due melting of all oceanic crust between the continents in collision zones.
Figure 1: Shows two oceanic plates move away from each other from a spreading zone where it forms new oceanic crust.
Figure 2: Shows the collision between two oceanic plates with the formation of volcanic islands in the contact zone between them.
Figure 3: Shows the collision between oceanic crust and continental crust, where oceanic crust due to the higher specific gravity pushed the continent. If the process goes far enough one will eventually have a collision between two continents. The second continent is somewhere to the right of the figure.
Here you can read more about plate tectonics.