A quiet volcano is considered dormant. Dormant volcanoes are volcanoes that have not actively erupted in the past 10,000 years, but still have the potential to erupt at some point.
The Mauna Kea volcano, like most other volcanoes in Hawaii, is an example of a dormant volcano. Its last eruption was well over 10,000 years ago, but scientists believe the volcano is not completely extinct. Even volcanoes that are classified as extinct do occasionally erupt, such as the Fourpeaked volcano in Alaska that was believed to be extinct but erupted in September of 2007. Once a volcano erupts, it's classified as active.