A volcanic eruption is the penetration of volcanic material above the ground through an opening called a volcano. Volcanoes that emit steam, lava or pyroclastic material are considered to be active or erupting. A volcano that does not erupt from the crater at the moment, but did so a few hours or days ago, is also considered to be currently operating. The end time of a volcanic eruption can only be fixed afterwards. About ten percent of volcanic eruptions last less than a day, but there are also volcanoes that have been permanently active for thousands of years. The median duration of volcanic eruptions is seven weeks.
