Monitoring and Forecasting

How do scientists know when a volcano will erupt, or if it is still active? Much like weather forecasting, there is no set schedule of events. Instead, volcanologists rely on monitoring different aspects of a volcano to determine when it is most likely to erupt.

Learn about the different aspects they study, and the various techniques they use to study them.

Emissions Monitoring

The gases that are emitted from a volcano can provide useful information. Changes in the emission composition from a particular vent or fumarole can indicate a change in the supply of magma, or its pathway.

There are several ways to measure the emissions of a volcano. A large eruption will inject large volumes of gases, like sulfur dioxide and carbon dioxide, into the atmosphere. These gases can be measured by instruments mounted on aircraft. Scientists can also sample gas as it is emitted from a vent or fumarole. The risk of eruption or bad weather can make this very dangerous. Automated monitors can be set up to take timed, continuous readings of the emissions. The data collected by these monitors are radioed to an observatory. Even measuring the amount of gas in local soils can yield important information about emissions.

Aircraft Data

Typically, a plume of gas emitted from a volcanic vent will rise in the air above the volcano. At a certain point in the atmosphere, the plume reaches equilibrium with the surrounding atmosphere. At this point, the plume stops rising and is carried by the wind away from the volcano. Scientists can measure the amounts of specific gases in the plume, as well as the wind speed. With these measurements, they can determine the rate at which the volcano is emitting gases. There are several methods to accomplish this.

To measure the concentration of sulfur dioxide gas in the plume, an optical correlation spectrometer (COSPEC) is used. The instrument is usually mounted on an aircraft, which flies beneath the plume.

An infrared analyzer (LI-COR) is used to measure the carbon dioxide levels in the gas plume. This instrument is flown through the plume several times, so that it can gather data.

Fourier Transform infrared spectrometer systems (FTIR) are also used to measure gas concentration in volcanic plumes. The FTIR has been successful in measuring sulfur dioxide emissions.

Automated Monitors

Automated monitors can be set up on the volcano to take timed, continuous readings of the emissions. The data collected by these monitors are transmitted to an observatory station.

This gas monitoring station is set up on the east rift zone of Kilauea Volcano in Hawaii Volcanoes National Park, Hawaii. Sensors measure SO2 and CO2 emissions from the Pu'u 'O'o vent every 10 minutes to provide scientists at the Hawaii Volcano Observatory with real-time data about the emissions of the vent.

Continuous gas monitoring stations are constantly collecting data. They can gather information on short de-gassing events that can last from minutes to hours, or detect long-term changes that occur over months or years. A station can be set to monitor fumarole gases, air quality, or soil gases.

An air quality monitoring station at the summit of Kilauea Volcano in Hawaii Volcanoes National Park is maintained by the National Park Service and the USGS to measure the level of sulfur dioxide in the air. Equipment inside the station records the data used to determine the effect of volcanic emissions on local air quality.

Laboratory Analysis

Laboratory analysis of volcanic gas samples of carbon dioxide and sulfur dioxide can be done at the Hawaiian Volcano Observatory (HVO) and the Cascades Volcano Observatory (CVO), both USGS facilities.

This detailed chemical analysis provides data that can be used in thermodynamic calculations and computerized modeling programs, giving scientists valuable information about the condition of the magma and its depth below Earth's surface. Increased emissions often precede eruptions.

Local Soil Measurements

Measurements can be taken in the soils around a volcanic vent to determine the concentration of gases such as carbon dioxide that are escaping from the ground near the volcano. The scientists in the photos are working around Kilauea Volcano in Hawaii Volcanoes National Park.

An accumulation chamber can be placed on the soil surface. As carbon dioxide is released from the soil, the chamber fills. It is connected to an infrared analyzer. The analyzer uses a timed rate of carbon dioxide increase in the chamber, the air temperature, and other environmental parameters to calculate the carbon dioxide soil efflux (amount of carbon dioxide leaving the soil) for that area.

Many measurements are taken in an area to determine where carbon dioxide concentrations are highest.

Measurements from a Vent

The volcanologists in the photos below are collecting gas samples from active fumaroles on volcanoes. Direct sampling of gases can provide detailed chemical information for a specific vent, but laboratory analysis of the collected gases can take days or weeks.

Obtaining these measurements can be very dangerous for scientists. Many volcanic gases are hazardous to human health, so it is essential in most cases to wear a gas mask. Also, there is added danger when the volcano becomes restless.


When magma and volcanic gas rise toward the surface, they create pressure, and often force overlying rocks out of the way. As a result of this movement, the surface of the volcano can change shape. The ground can rise, subside, tilt, bulge, or develop cracks of all sizes and depths.

Electronic distance measurements (EDM) are taken by measuring the distance between benchmarks on a volcano tens to thousands of meters apart. Lasers and reflectors provide precise information on horizontal movement of parts of the volcano. Tiltmeters are installed underground in active areas to measure the slope of the ground, which changes as magma moves. New innovations in satellite technology now allow for the use of Global Positioning Systems (GPS) to measure the horizontal changes in the volcano. Also, the satellite-based technique of radar interferometry allows scientists to view the complete picture of deformation.

Electronic Distance Measurements

When magma rises towards the surface beneath a volcano, it creates great pressure that can actually move overlying rocks by pushing them upward or to the side. When this happens, part of the volcano can actually move a distance between a few millimeters or tens of meters!

Geologists use electronic distance measurements (EDM) to measure these deformations and pinpoint the location where magma is rising. An EDM is an instrument that sends and receives an electromagnetic signal. The signal is send from a benchmark somewhere on the volcano, and reflected back by a mirror thousands of meters away. The difference between the initial and returned signals measures changes in distance between the two points.

Global Positioning Systems

Global Positioning Systems (GPS) allow scientists to measure and estimate horizontal and vertical movements of the ground around a volcano more conveniently than EDMs or tiltmeters. This recent advance in technology is full of advantages.

Unlike EDMs, GPS does not require line-of-sight between benchmarks, only a clear view of the sky. Also, GPS measurements can be made in most any weather conditions. Vertical and horizontal positions can be measures with accuracy of a few millimeters horizontal distance to several millimeters on the horizontal.

GPS receivers are portable and easy to set up compared to some labs. Once set up, this equipment can be manned or left to take automated readings.

Radio Interferometry

The use of satellite based radar interferometry is allowing scientists to see deformation at higher resolution and at more volcanoes. With this technology, scientists can see the whole picture of the volcano, in three dimensions.

Since the use of this technology began about 10 years ago, geologists have learned that many dormant volcanoes have episodes of inflation caused by rising magma between eruptions without much seismic activity. Radar interferometry has great predictive potential.

This diagram is a model of uplift measured by radar interferometry. This measurement is done from space by satellites sending radar waves to Earth, and timing the wave as it returns to space after having been reflected off of the ground. A picture is drawn by the computer collecting the data, and a map of any deformation around the volcano is generated. Visit the USGS web site to learn more.


These geologists are using tiltmeters to determine changes in the surface on the volcano. These yield precise measurements for changes in the slope of the crater floor, caused by moving magma. Tiltmeters are in operation 24 hours a day, and the data they collect is relayed to the monitoring station by telemetry.

Hazard Assessment

In populated areas around active volcanoes, it is important to know what to expect in the event of an eruption. Scientists work hard to develop modeling techniques that can predict the hazards resulting from volcanic activity.

From monitoring data, it is possible to make predictions of what type of eruption will occur and where on the volcano it will occur. Also, the local geology allows models to predict where lava flows and lahars could be directed. There is no absolute certainty in predictions, but they can save lives if enough time is given for people to evacuate or take proper protective measures.

Maps are extremely useful to scientists who wish to predict the impacts and hazards of volcanic activity. Geographic Information Systems (GIS) can help create models that map the areas of greatest danger or impact for hazards such as ashfall, pyroclastic flows, blow-down, debris avalanches, and lahars. Click on a subject to view hazard maps for Mount Rainier National Park in Washington state. All maps were created by the United States Geologic Survey, and obtained from the Cascades Volcano Observatory.


This USGS volcanic hazard map shows the probability of areas in Washington to be affected by ashfall from Mount Rainier. The first map shows the probably of each area to get 1/3 inch of ash deposited in a one-year time frame. This amount of volcanic ash can disrupt ground and air transportation systems, as well as create problems for electronics and machinery.

The areas in white have a very small probability of being affected by ash. The yellow areas indicate only a one in 10,000 chance of being affected. The pink areas will experience that amount of ashfall only one time out of 1000. The green areas will see 1/3 inch of ash one time in 500, and the red area (right around Mount Rainier itself) will experience ashfall of this thickness one in a hundred times.

This map follows the same pattern for areas that would receive 4 inches of volcanic ash in a year. The affected areas are much smaller in this case.


This USGS volcanic hazard map shows the areas that would be affected by the occurrence of lahars from Mount Rainier. Mount Rainier National Park is located in the center of the map, and is outlined in green.

There is a nearly circular area right around the volcano that makes up the pyroclastic flow zone (light pink.) This area would be in danger if a large eruption triggered a pyroclastic flow. See the next map for more on pyroclastic flows. The orange areas on the map indicate the locations that would be in danger if a large lahar, or volcanic mudslide, occurred. These large events occur at Mount Rainier around every 500-1000 years. The yellow zones would be impacted by moderate lahars, while the purple would experience only the smaller events.

Notice that the regional topography plays an important role in lahar hazards. This is because lahars flow downhill, often through large river channels.

Pyroclastic and Lava Flows

During an eruption, tephra can be ejected high into the atmosphere. The simple statement ”What goes up must come down" expresses how a pyroclastic flow begins. This hot material that is ejected into the atmosphere above the volcano comes roaring back to the ground, and flows downslope from the volcano. Pyroclastic flows are extremely hot, and can travel very fast, so they are a particularly dangerous hazard.

Lava flows also are part of volcanic eruptions. As magma reaches the surface of Earth, it pours out of the volcanic vent and flows downslope like a thick river. Depending on its composition, it will either be sticky and flow very slowly or be more runny and flow faster.

Remote Sensing

Satellite images can aid scientists in monitoring volcanic activity, learning about volcanic features, and predicting future activity. Also, satellite images can show the extent and path of the volcanic clouds during or after an eruption. This is essential information for pilots who need to be aware of these clouds to avoid flying into them. It is also possible to measure the amounts of some gases discharged in eruption clouds. Satellites sent into the atmosphere to measure ozone concentrations can also be used to measure sulfur dioxide concentrations in eruption clouds. Heat sensing imagery can be used to trigger warnings when the temperature rises on or around a remote volcano. Rises in temperature can be the signature of a new thermal feature, pyroclastic flow, or pending eruption.

Radar imagery can identify deformations around the summit or flanks of a volcano.


Earthquakes have been the most effective predictors of volcanic activity, since they usually occur prior to volcanic eruptions. The gases and magma below a volcano cause severe stress to the local rocks, causing them to crack and/or vibrate. The cracks cause high-frequency earthquakes, while the vibrations trigger low-frequency quakes or volcanic tremors. Volcanic quakes have magnitudes usually below 3 on the Richter scale, and are shallow quakes, occurring at 10km or less below the volcano.

To measure the frequency and strength of these earthquakes, a network of seismometers are installed around the volcano's vent at different locations, so scientists can tell where the activity is occurring. Each type of activity or volcanic event has its own "seismic signature" that scientists use to identify volcanic hazards. Click on the questions below to learn how seismic monitoring helps us learn about volcanoes.

Why do earthquakes happen around volcanoes?

An inactive volcano has an empty magma conduit. This volcano is not seismically active. As the volcano becomes active, the empty magma conduit fills with molten magma from deep below the surface. The magma fills the conduit and begins to expand, exerting great pressure on the rocks around the filling magma chamber. The pressure causes the rocks to crack and vibrate, releasing pressure in the form of earthquakes and volcanic tremors.

Inactive volcano5
Generally, there is no seismic activity associated with a dormant volcano.

Volcano Becomes active
When a volcano becomes active, the magma conduit is filled with magma from deep below the surface, creating pressure.

Great pressure
The magma filling the chamber and conduits exerts great pressure on the rocks that form the volcano.

Crack and vibrate
Under the pressure of the magma, the rocks vibrate and crack. Thus pressure release causes earthquakes known as volcanic tremors.

How do scientists monitor seismic activity?

Scientists set up seismometers around active volcanoes, so that they can record volcanic tremors and earthquakes. Typically, seismometers are installed in a network of about 4 to 8 in a radius of 20km from the vent. This ensures that all of the small movements will be recorded.

A seismograph records a unique seismic "signature" for each volcanic quake. Although computers have helped to map and locate volcanic activity, seismographs remain one of the most valuable tools for predicting volcanic activity. Computers help scientists store and analyze the data collected by seismometers.

Stations and Observatories

The United States Geological Survey (USGS) operates several volcano observatories throughout the world. The ones listed below provide a wealth of data and information about volcanic activity in the United States. Visit the web sites below to learn how volcanoes are monitored in each area.

Alaska Volcano Observatory
Cascade Volcano Observatory
Hawaii Volcanoes Observatory
Long Valley Observatory
Yellowstone Volcano Observatory



Introduction to Volcanism

Eruptions and Hazards

Landforms and Features

Monitoring and Forecasting

Volcanisim in National Parks

Challenge Your Understanding


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