Craters:
A crater is a funnel-shaped pit at the top of a volcanic vent. It is almost circular in plan and flanked by in facing scarp. A crater may form either from explosive activity or from subsidence.
The volcanic ash, lapilli and other ejected materials by the volcano may build a ring around a volcanic vent and produce a crater.
There are four types of craters such as: (i) explosion craters, (ii) pit crater, (iii) eruption craters, and (iv) Nested craters. Explosion craters are circular depressions with low rims of ejected materials around them.
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They are generally occupied by lake. Pit craters found on Hawaii Islands have lava lakes. Eruption craters are the pits at the top of volcanic cones.
When the volcano stops its activity, such craters are partially filled with debris. There are many volcanoes which have small craters within their larger craters. Mt. Vesuvius and Etna have such craters. These are nested craters.
Calderas:
A caldera is defined as a volcanic crater that has been enlarged by subsidence. In the opinion of Daly, the term caldera is used to designate any large volcanic crater formed due to subsidence.
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However, the only distinguishing feature of a caldera is that the diameter of its floor is much larger than that of the volcanic vent. Calderas are of three kinds: (i) Kraktoa type, (ii) Katmai type, and (iii) Valles type.
Kraktoa type Caldera is formed due to the eruptions of pumice. Katmai type is formed due to the drainage of magma through adjacent conduits rather than through the main vent.
Vales type is formed due to collapse following the emission of large volume of ash and pumice from fissures unrelated to pre-existing volcanoes.
Calderas are huge basins:
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Their diameter varies from 1.6 km to about 75 km. The larger calderas are formed on the summits of the composite cones. The site of Crater Lake, Oregon, is a caldera 8-10 km in diameter and 1100 m deep that was formed by the Mount Mazama tephra eruption 6700 years ago.
It may be pointed out that in humid regions; caladeras are frequently the sites of deep lakes. Besides, there may be some minor volcanic landforms on the floor of a caldera.
The Valles Caldera in the Jemez Mountains of New Mexico, the Timber Mountain Caldera in Nevada are some of the typical examples of very large calderas.
One fine examples of a Katmai type caldera is the one that was formed during the multiple Yellowstone eruptions. Later on, this huge caldera was filled by several thousands meters of tephra and rhyolite flows. Certain other features associated with volcanic activity.