A crater is a depression found on the surface of a planet or a moon. Craters can be either natural or manmade. When people use the word “crater,” they are generally referring to impact craters.
An impact crater is caused by the high-velocity collision of a smaller projectile with the larger body of a planet or a planetary object. Because impact craters are usually caused by meteorites, they are often called “meteorite craters.” Impact craters generally have a roughly circular outline and a raised rim. The size of an impact crater can range anywhere from a small, simple, shallow depression in the ground to an extremely large, multi-ringed basin.
The impact craters on Earth are not usually easy to recognize due to many years of erosion and weathering. Famous impact craters include Meteor Crater in Arizona and Chicxulub on the Yucatan coast in Mexico. Most scientists believe the meteor that hit Chicxulub is the one that led to the extinction of the dinosaurs at the end of the Cretaceous period. It is interesting to note that the Chicxulub crater is far below the surface of the earth and cannot be seen by the naked eye.
Although craters are usually formed by meteor impacts, they can also result from volcanic processes. A volcanic crater is a roughly circular, basin-like, rimmed structure usually found at the summit of a volcanic cone. Volcanic craters are typically caused by the gradual build-up of erupted volcanic deposits such as lava flows, but they can also form when part of the volcano collapses. Some volcanic craters fill with rain or melted snow and form a crater lake. Volcanic craters have been found on Earth, the Earth’s moon, Venus, Mars, and Jupiter’s moon, Io.
One special kind of volcanic crater is called a maar crater. Maar craters form when molten lava rises towards the surface and encounters a lot of groundwater or water-saturated rocks. This causes all kinds of havoc, resulting in an impressive blowout of steam, smoke, and ash. This explosion causes a circular depression to form. Maar craters usually fill with water to create shallow crater lakes.
Man-made craters can also be formed from underground nuclear explosions. The Nevada Test Site, used for many years as a center for nuclear testing, is one of the most crater-pocked sites on Earth. These man-made craters are often called “subsidence craters,” and typically form when the underground explosion causes a cavity to form in the earth and the roof of the cavity caves in. This causes the surface of the ground to depress into a "sink," so this type of crater is also called a “sink crater.”