A caldera is a volcanic structure that is caused by the collapse of the ground around a volcano. At first glance, it often looks a great deal like a volcanic crater, but the process of its formation is different from that of traditional craters, which is why a distinction is made between the two. One of the most famous calderas in the world lies beneath Crater Lake, a stunning volcanic lake in the United States. The Greek island of Santorini also boasts a notable one that forms a natural harbor.
These structures are usually associated with stratovolcanoes and basaltic shield volcanoes. They can form in several different ways. In many cases, a volcanic eruption empties magma chambers beneath the volcano, and these chambers collapse, creating a deep depression in the ground. In other instances, the caldera is created through a gradual process of subsidence triggered by slowly weakened ground. In the case of erosion calderas, the formation is caused by the weather in the region.
The largest type are known as resurgent calderas. This is caused by the widespread collapse of magma chambers that are not associated with a particular volcano or point of eruption, and they are the largest volcanic structures on Earth. The level of volcanic activity needed to produce a resurgent caldera is quite sizable, and would cause a great deal of damage.
Some vulcanologists prefer “volcanic collapse crater” to “caldera,” while others refer to these structures as “cauldrons.” In fact, the term comes from the Latin word for “cauldron,” reflecting the shape of a classical caldera, so both terms are entirely appropriate. Superficially, it can be difficult to tell the difference between a caldera and a crater, especially in the case of a dormant volcano. The key thing to know about craters is that they occur around volcanic vents, which makes standing in one a much more perilous proposition.
Some calderas are so large that they cannot be identified from the ground, because people do not realize that they are walking or living in them. In these instances, satellite photographs revealed the existence of the caldera, much to the surprise of people who had settled in these regions. Others on the ocean floor have been identified with sonar technology which has been used to determine the depth of the ocean for the purpose of making relief maps.