Land
Tectonic plates move over time, which is called continental drift. When two continental plates push into each other, they fold upwards to make mountains. When they move apart, they form a rift valley: the East African rift valley is forming now. Islands can be formed by volcanoes: the Hawaiian islands are a good example. Over time, land can wear away through erosion and sink below sea level. This can be seen with the Hawaiian islands, because the newer islands are taller than the older islands.