CALDERAS VS. CRATERS
Calderas and craters, while seeming similar, are two different geological features. The simplest way to distinguish the two is to determine the size of the feature. Craters generally are not larger than 1.00 mile in diameter while calderas can exceed 5-10 miles or even more in diameter.
Not only is size a difference, but also the process by which they form is as well. Calderas normally form from collapse and the process of subsidence. Volcanoes with magma chambers underneath them are the main setting for the formation of calderas. Before the volcano erupts, the magma chamber beneath it is filled with magma and gasses. The pressure of these keep the chamber intact while a volcanic cone or a layer of the earth’s crust sit on top of it. The equilibrium of the weight of the crust and the pressure within the chamber keep it collapsing in on itself. Caldera collapse happens when the lithostatic pressure on the roof of the magma chamber exceeds the chamber pressure by the compressive strength of the overlying rock. For this reason a crater from the result of an explosion is related to cone building and is thus a positive volcanic form, while a caldera is considered a negative passive form.
To form a caldera, there has to be a point where some of that gas or magma is released, usually in the form of a volcanic eruption. The trigger for this eruption could be an earthquake, or it could be as simple as the heat and pressure growing so large that there needs to be a release of energy. The resulting eruption could be a release of magma, gasses, or both. Pyroclastic material can come out in the form of a pyroclastic flow or be spewed into the air. When there is significant empty space gained in the magma chamber, the upper part of the volcanic cone collapses downward filling the empty space beneath.
Below is a picture of Lake Pinatubo at the Mt. Pinatubo caldera in the Philippines. Often mistakenly called “The Crater Lake,” the lake actually sits in a caldera which was formed by the collapse of the volcano’s summit in its 1991 eruption. The Pinatubo caldera is considered a relatively small caldera being only 1.6 mi (2.5 km) in diameter.