How can we tell when a volcano will erupt?

Most volcanoes provide warnings before an eruption. Magmatic eruptions involve the rise of magma toward the surface, which normally generates detectable earthquakes. It can also deform the ground surface and cause anomalous heat flow or changes in the temperature and chemistry of the groundwater and spring waters. Steam-blast eruptions, however, can occur with little or no warning as superheated water flashes to steam.

Notable precursors to an eruption might include:

  • An increase in the frequency and intensity of felt earthquakes
  • Noticeable steaming or fumarolic activity and new or enlarged areas of hot ground
  • Subtle swelling of the ground surface
  • Small changes in heat flow
  • Changes in the composition or relative abundances of fumarolic gases

These precursors do not indicate the type or scale of an expected eruption (that information is best obtained by mapping previous eruptions). Precursors can continue for weeks, months, or even years before eruptive activity begins, or they can subside at any time and not be followed by an eruption. Italy’s Campi Flegrei volcano has been showing signs of unrest for over 60 years.

Learn more:

Related Content

Filter Total Items: 11

Can earthquakes trigger volcanic eruptions?

Sometimes, yes. A few large regional earthquakes (greater than magnitude 6) are considered to be related to a subsequent eruption or to some type of unrest at a nearby volcano. However, volcanoes can only be triggered into eruption by nearby tectonic earthquakes if they are already poised to erupt . This requires two conditions to be met: Enough "...

What is the largest sinkhole in the United States?

There are some very large, ancient, ‘inactive’ sinkholes in some areas of the U.S. that are thousands of years old. Alabama claims to have the largest recent collapse sinkhole. It is called the “Golly Hole” and is located in Shelby County in the central part of the state. It collapsed suddenly in 1972. The sinkhole is about 325 feet long, 300 feet...

Do earthquakes large enough to collapse buildings and roads accompany volcanic eruptions?

Not usually. Earthquakes associated with eruptions rarely exceed magnitude 5, and these moderate earthquakes are not big enough to destroy buildings and roads. The largest earthquakes at Mount St. Helens in 1980 were magnitude 5, large enough to sway trees and damage buildings, but not destroy them. During the huge eruption of Mount Pinatubo in...

Lava sampling: Why do we do it?

Hot lava samples provide important information about what's going on in a volcano's magma chambers. We know from laboratory experiments that the more magnesium there is in magma, the hotter it is. Chemical analysis, therefore, provides the means not only to determine the crystallization history of lava but also to establish the temperature at...

Why is it important to monitor volcanoes?

The United States and its territories contain 169 geologically active volcanoes, of which 54 volcanoes are a high threat or very high threat to public safety. Many of these volcanoes have erupted in the recent past and will erupt again in the foreseeable future. As populations increase, areas near volcanoes are being developed and aviation routes...

How are volcanic gases measured?

Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode. An instrument that detects carbon dioxide can be installed on a volcano and configured to send data continuously via radio to an observatory...

Is it dangerous to work on volcanoes? What precautions do scientists take?

Volcanoes are inherently beautiful places where forces of nature combine to produce awesome events and spectacular landscapes. For volcanologists, they're FUN to work on! Safety is, however, always the primary concern because volcanoes can be dangerous places. USGS scientists try hard to understand the risk inherent in any situation, then train...

Which volcanoes in the conterminous United States have erupted since the Nation was founded?

Excluding steam eruptions, these volcanoes have shown activity: Mount St. Helens, Washington - Eruptions and/or lava dome growth occurred in the late 1700s, 1800-1857, 1980-1986, and 2004-2007. Lassen Peak, California - A series of steam blasts began on May 30, 1914. An eruption occurred 12 months later on May 21, 1915. Minor activity continued...

What are some benefits of volcanic eruptions?

Over geologic time, volcanic eruptions and related processes have directly and indirectly benefited mankind: Volcanic materials ultimately break down and weather to form some of the most fertile soils on Earth, cultivation of which has produced abundant food and fostered civilizations. The internal heat associated with young volcanic systems has...

Can an eruption at one volcano trigger an eruption at another volcano?

There is no definitive evidence that an eruption at one volcano can trigger an eruption at a volcano that’s hundreds of kilometers/miles away or on a different continent. There are a few historic examples of simultaneous eruptions from volcanoes (or volcanic vents ) located within about 10 kilometers (6 miles) of each other, but it's difficult to...

How Do Volcanoes Erupt?

Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava. Some volcanic...
Filter Total Items: 6
Date published: May 6, 2019

The National Volcano Early Warning System (NVEWS) will help USGS better monitor nation’s most dangerous volcanoes

In September 2004, USGS scientists detected sudden, but unmistakable, signs that Mount St. Helens was waking up. Volcano monitors had picked up the occurrence of hundreds of small earthquakes and other signals that the volcano’s crater floor had begun to rise. Within a week, several eruptions blasted clouds of ash into the atmosphere, and soon after, a new lava dome emerged in the crater.

Date published: June 21, 2018

Kīlauea Volcano Erupts

Today's update for June 21st, 2018 will be the last of the daily updates on this USGS feature story.  We encourage you to keep checking the USGS Hawaiian Volcano Observatory (HVO) Kīlauea status website for daily activity updates. You can also visit the USGS Facebook page and the USGS Twitter feed as updates become available. For press inquiries, please email

Date published: May 18, 2017

Ever Vigilant: USGS Marks the 37th Anniversary of Mount St. Helen's Eruption and the 35th Anniversary of the Cascades Volcano Observatory

Today, in 1980, Mount St. Helens unleashed the most devastating eruption in U.S. history. Two years later, USGS founded the Cascades Volcano Observatory to monitor Mount St. Helens and all the Cascades Volcanoes.

Date published: August 2, 2016

30 Years Saving Lives from Volcanoes

There are approximately 1,550 potentially active volcanoes around the world. VDAP works to reduce loss of life and property, limit economic impact and prevent volcanic crises from becoming disasters.

Date published: April 1, 2013

25 Years Monitoring Alaska Volcanoes

Twenty-five years of monitoring and studying Alaska's volcanoes by the Alaska Volcano Observatory have improved global understanding of how volcanoes work and how to live safely with volcanic eruptions. Timely warnings from AVO throughout its 25-year history have helped reduce the impact of erupting volcanoes, protecting lives, property, and economic well-being.

Attribution: Region 11: Alaska
Filter Total Items: 15
March 22, 2016

Monitoring Volcanoes Using ASTER Satellite Imagery

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor is one of five sensors on board NASA's Terra satellite. ASTER data and imagery are crucial tools for monitoring volcanoes for any clues of imminent eruptions, for studying volcanoes during an eruption, and for analyzing impacts after an eruption. Scientists use ASTER imagery to study the

video thumbnail: Volcano Hazards
July 30, 2012

Volcano Hazards

The United States has 169 active volcanoes. More than half of them could erupt explosively, sending ash up to 20,000 or 30,000 feet where commercial air traffic flies. USGS scientists are working to improve our understanding of volcano hazards to help protect communities and reduce the risks.

Video Sections:

  • Volcanoes: Monitoring Volcanoes
May 9, 2012

Volcano Web Shorts 5 - Volcanic Ash Impacts

Volcanic ash is geographically the most widespread of all volcanic hazards. USGS geologist Larry Mastin describes how volcanic ash can disrupt lives many thousands of miles from an erupting volcano. The development of ash cloud models and ash cloud disruption to air traffic is highlighted.

May 9, 2012

Volcano Web Shorts 4 - Instruments

USGS technologist Rick LaHusen describes how the development and deployment of instruments plays a crucial role in mitigating volcanic hazards.

May 9, 2012

Volcano Web Shorts 3: Seismology

USGS volcano seismologist, Seth Moran, describes how seismology and seismic networks are used to mitigate volcanic hazards.

May 9, 2012

Volcano Web Shorts 2: Debris Flows

Debris flows are hazardous flows of rock, sediment and water that surge down mountain slopes and into adjacent valleys. Hydrologist Richard Iverson describes the nature of debris-flow research and explains how debris flow experiments are conducted at the USGS Debris Flow Flume, west of Eugene, Oregon. Spectacular debris flow footage, recorded by Franck Lavigne of the

May 9, 2012

Volcano Web Shorts 1: Photogrammetry

Photogrammetry is the science of making precise measurements by the use of photography. USGS geologist Angie Diefenbach describes how she uses a digital camera and computer software to understand the growth rate of lava domes during a volcanic eruption.

December 8, 2011

PubTalk 12/2011 — Tracking Ongoing Kilauea Eruptions

--fissures...fountains...and flows

by Matthew Patrick, USGS, Hawaiian Volcano Observatory


  • Spectacular Kilauea eruptions have produced a summit lava lake, roiling for several years, and a flank eruption recently sending lava flows downslope to threaten residential areas
  • How do USGS scientists monitor and track
video thumbnail: Mount St. Helens: May 18, 1980
May 10, 2010

Mount St. Helens: May 18, 1980

USGS scientists recount their experiences before, during and after the May 18, 1980 eruption of Mount St. Helens. Loss of their colleague David A. Johnston and 56 others in the eruption cast a pall over one of the most dramatic geologic moments in American history.

video thumbnail: Mount St. Helens: A Catalyst for Change
May 10, 2010

Mount St. Helens: A Catalyst for Change

The May 18, 1980 eruption of Mount St. Helens triggered a growth in volcano science and volcano monitoring. Five USGS volcano observatories have been established since the eruption. With new technologies and improved awareness of volcanic hazards USGS scientists are helping save lives and property across the planet.

Image: Alaska Volcano Observatory Operations
February 8, 2009

Alaska Volcano Observatory Operations

USGS scientists Tina Neal (left) and Melissa Pfeffer (right) analyze data as it returns from several volcanos around Alaska.

Attribution: Region 11: Alaska
video thumbnail: Yellowstone Volcano Observatory (Part 2 of 3)
January 29, 2009

Yellowstone Volcano Observatory (Part 2 of 3)

USGS Scientist-in-Charge of Yellowstone Volcano Observatory, Jake Lowenstern, answers
the following questions to provide a tour of the Yellowstone Volcano Observatory: "What is YVO?", "How
do you monitor volcanic activity at Yellowstone?", "How are satellites used to study deformation?", "Do
you monitor geysers or any other aspect of the Park?", "Are

Attribution: Yellowstone