How were the continents of the earth formed?
The basic mass and composition of the earth was fixed by the accretion of matter into the planet at its formation. It is really the existence of tremendous amounts of liquid water that define our “continents”, as planet like Mars and Venus do not have such boundaries. It is the exposed solid earth crust that we call continents.
The specific shape and location of the continents appears to be defined by pieces of the earth's crust that have geologic mobility on the liquid under-layers. Island chains (see also the “Pacific Ring of Fire”) are also formed by this movement, as rifts between the pieces (tectonic plates) cause magma to come up from the lower earth and form volcanoes, which become the islands if the extend above the ocean surface.