Volcanic crater

A volcanic crater is a circular depression in the ground caused by volcanic activity.PhysicalGeography.net, Glossary of Terms: "Crater"; retrieved 2012-6-10. 

It is usually a circular basin. When active, there is a vent or vents inside it. From these magma erupts as gases, lava, and ejecta. A crater can be huge, and sometimes deep. 

During certain types of gigantic eruptions, the volcano's magma chamber may empty enough for an area above it to subside. This dropped surface crater is called a caldera.

 Related pages 
 Caldera
 Volcano

 References 

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