Skip to main content
U.S. flag

An official website of the United States government

How long does it take to know and how do you know a volcano is becoming active?

Right-click and save to download

Detailed Description

Listen to hear the answer.




Public Domain.


Welcome to CoreFacts. I'm Kaylee from Page Middle School in Arizona.

Today's question is...

How long does it take to know and how do you know a volcano is becoming active?

I'm Rosalind Helz. I am a scientist at the U.S. Geological Survey and I have worked on volcanoes for much of my career. Well I'll start by saying I think we can all tell when a volcano is erupting because it is putting out lava or ash and hot gases. In order for a volcano that has been dormant for a while to be able to erupt, its magma chamber has to be resupplied with new magma. Dormant means the volcano has been asleep, not doing anything. The magma is what we call lava before it gets to the surface. So the warning signals that a volcano is becoming active are caused by the arrival of new magma. 

There are four kinds of signals that we look for. The earliest warning is deformation, specifically uplift or inflation of the volcano. Sometimes there may be new cracks in the ground. As magma moves into the upper part of the volcano's plumbing system, it pushes out and cracks the rocks around it. Small earthquakes are produced. So a very important part of volcanic unrest is an increase in small quakes inside the volcano, fairly near the surface of the earth.

Volcanic gases come out ahead of the magma as it moves up and they can make their way to the surface, somewhat ahead of the magma itself. So, the third indicator of volcanic unrest is an increase in the amount of gases like SO2 or carbon dioxide or both coming out of the volcano. The fourth signal that magma is approaching the surface is an increase in the amount of heat coming out of the volcano. Hot springs (if there are any) will get hotter. Ground cracks may start to steam and get hotter than they have been. So we can measure the temperatures of these features as well.

When we put all these things together, we can tell that a volcano is becoming restless and may erupt. However, it is not possible to know exactly when an eruption will start. Most of the time, we can anticipate that an eruption will start within days or perhaps within weeks. But occasionally Nature still surprises us.

Thank you Rosalind. And now you know. Join us every weekday for a new CoreFact. For other CoreFacts, or for CoreCast, the USGS in-depth science podcast, go to If you'd like to have a question featured on this show, send an email to or a phone call to 703-648-5600, that is 703-648-5600. Remember, long distance fees do apply.

CoreFacts is a product of the U.S. Geological Survey, Department of the Interior.

Show Transcript