Broad area of land that borders the sea. The coastlines of the world's continents measure about 193,000 mi (312,000 km). They have undergone shifts in position and changes in shape over geologic time because of substantial changes in the relative levels of land and sea. Other factors that alter coasts are erosion processes such as wave action and weathering, deposition of rock debris by currents, and tectonic activity. Coastal features result largely from the interaction and relative intensity of these processes, though the type and structure of the underlying rocks also play a part.