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Volcanic activity
Active
• Steam or ash visible.
• Eruption recorded in last
15 years.
• Frequent earthquakes.
• Lava flows common.
• Deposits of recent lava and
ash.
• Smoke may be coming from
the vent.
• There may be earthquakes
focused below it.
Dormant
• Vent blocked.
• No eruption in last 15 years. • Evidence of eruption in
historical time.
• May erupt if magma can blow
out the plug of rock in the vent.
All volcanoes have a “life cycle.” They may be active for many thousands of years, but eventually the source of lava and ash gives out, and they first become dormant and then extinct.
In dormant volcanoes there is often a long pause between eruptions, since it can take a long time for the magma chamber to refill and come back to pressure. As a result, volcanoes may only erupt after several hundred years. This
is why it is extremely difficult
to tell the difference between a volcano that is active, dormant, or extinct. Many dormant volcanoes have snow and even glaciers on their summits.
In general, the longer the period between eruptions, the more violent the next eruption will be. As a result, nobody wants to make a mistake and declare a dormant volcano extinct when it might erupt
 Volcanic activity—Agathla Peak, Arizona, is an example of an extinct volcano. The cone has been stripped away by the weather, and all that remains is the plug that was once the vent of the volcano.
Extinct
• Crater collapsed or vanished. • No evidence of eruption in
historical time.
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