Calderas are very large depressions that form by collapse. Many, like this 10-km-wide caldera that truncates Alaska's Aniakchak volcano, are created by very powerful explosive eruptions that empty a magma chamber beneath a volcano, causing it to collapse inward. Other calderas, such as those on Hawaiian volcanoes, are produced by collapse following major lava extrusion. Calderas often form incrementally, during widely spaced eruptions. Later activity can cover their floors with a wide variety of volcanic landforms.