El Boqueron caldera in El Salvador. Image credit Wikispaces.com

El Boqueron caldera in El Salvador. Image credit Wikispaces.com

ZIMSEC O Level Geography Notes: Calderas

  • Calderas are very large craters often two or more kilometers in diameter that form at the top of volcanoes.
  • They are often deep extending downwards for a few hundred meters and sometimes, in cases of inactive volcanoes, contain lakes e.g. Lake Bosumtwi in Ghana.
  • There are several theories regarding how calderas are formed although it is more likely that each of these theories is applicable in certain instances.
Formation of a caldera. Image by ElateAfrica.

Formation of a caldera. Image by ElateAfrica.

Violent eruption

  • A composite volcano may sometimes explode violently that its top is blown off and disintegrates into a mass of rocks and ashes
  • Leaving the crater at the top of volcanoes greatly enlarged thus forming a caldera.


  • A caldera may also be formed as a result of block subsidence (downward displacement of the central block).
  • After an eruption the supply of magma is depleted ┬ácausing a huge chasm to form beneath the volcano.
  • The weight of the cone sometimes causes faults to develop and after some time the whole cone collapses into the chasm beneath.
  • Longonot in Kenya may have been formed this way.

Meteor theory

  • This theory suggests that solid objects from the space (meteors) fall by gravity and on impact with the earth formed calderas.
  • While meteors falling on earth have been known to form craters this theory even if proved true cannot account for all the calderas.

Mountain collapse

  • A mountain may collapse if it has a large mass floating on a wetter surface resulting in a caldera.

To access more topics go to the Geography Notes page.

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