The volcano is then classified again as an active volcano. An extinct volcano is one that has not erupted for thousands of years. Sometimes it is hard to tell if a volcano is dormant or extinct. They will be listed as dormant until volcanologists are sure there will no more eruptions. Most volcanoes were formed many, many years ago.
New volcanoes form every century, but usually in the same area as existing volcanoes.
In 1943 a volcano growing in the middle of a flat cornfield in Mexico was discovered. Paricutin grew to 1,353 feet and erupted for 9 years. Today it is 9,213 feet high. In 1973 a new volcano covered the island of Heimaey in Iceland with ash and lava.