Volcano Monitoring Techniques

GEOL 205: Lecture Notes

The following summary was written by Dr. Steven Mattox as part of a project for the Hawaii Natural History Association. I've taken the online version of this fromVolcano World and reproduced it here with some minor modifications.

Measuring the temperature of lava is one method used to monitor volcanic eruptions. Photograph by R.L. Christiansen, U.S. Geological Survey, January 9, 1973.


Geologists have developed several methods to monitor changes in active volcanoes. These methods allow geologists to forecast and, in some cases, predict, the onset of an eruption. A forecast indicates that the volcano is "ready" to erupt. A prediction states that a volcano will erupt within a specified number of hours or days (Wright and Pierson, 1992). Several methods developed by and currently used at the Hawaiian Volcano Observatory are introduced in the following paragraphs.

Ground Movements
Tilt is a measure of the slope angle of the flank of the volcano.

Prior to any change in the volcano, a balance is reached between the outward (mostly upward) pressure of the magma in the reservoir beneath the summit and the downward weight of the rocks above the magma reservoir. Tilt measurements will remain constant.

As magma accumulates in the shallow reservoir beneath Kilauea volcano, it exerts pressure on the overlying and surrounding rocks. The pressure causes the summit of the volcano to move upward and outward to accommodate the greater volume of magma. As magma accumulates in the summit reservoir, it causes the slope (i.e., tilt) of the volcano's flanks to increase. Geologists can use precise measurements at specific locations over a period of time to detect movements caused by magma. The number and distribution of earthquakes also indicates changes in the summit area.

If magma leaves the summit reservoir and moves into a rift zone, the summit will deflate and the tilt decreases. Rapid summit deflation is often a precursor to a flank eruption and, therefore, a useful monitoring tool.

These graphs show how the number of earthquakes and the amount of tilt changed over the course of events outlined above. Modified from Unger (1974).

In simplest terms, tiltmeters work like a carpenter's level by measuring changes in slope.

During stable conditions, tilt at the summit changes very slowly or not at all.

As magma fills the reservoir beneath the summit, the volcano inflates and the tilt of the flanks increases, sometimes very rapidly.

As magma leaves the reservoir beneath the summit, the volcano deflates and the tilt of the flanks decreases. All slopes are exaggerated. Modified from Bullard (1984).

The tiltmeters used by the Hawaiian Volcano Observatory are very sensitive because they must measure changes in slope as small as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer (1 million millimeters) long only one millimeter. That's roughly equivalent to lifting a board six city-blocks long only the height of a dime at one end! This unit of measure is called a microradian and most of the tilt changes at Kilauea are reported as microradians (urad) Photograph of a tiltmeter courtesy of U.S. Geological Survey.

This photo shows the tilt record for the onset of a flank eruption in November of 1979. Tilt near the summit changed 5 microradians in about 12 hours. Photograph by Robert Decker, U.S. Geological Survey.

Standard leveling surveys are used to determine changes in elevation or vertical distances. Geologists use permanent markers, called benchmarks, as reference points. As magma intrudes beneath an area, the benchmarks move upward. Elevation changes of less than a millimeter can be measured with careful leveling. Photograph courtesy of U.S. Geological Survey.

Electronic distance measurement (EDM) uses a laser light source to measure the horizontal distance between two locations. For example, if a new batch of magma arrives at the summit of Kilauea, the volcano expands, and the distance between two points increases. EDM is also used across rift zones or in areas with active faults. In the photo EDM is being used to measure the Puu Oo cone. Photograph by J.D. Griggs, U.S. Geological Survey, May 5, 1986.

In many cases, tilt is used in conjunction with EDM and leveling surveys to constrain the location, and in some cases size, of an intrusion.

A new tool, called Global Position System or GPS, is being used to measure the changes in a volcano prior to or during eruptions. GPS uses a system of orbiting satellites, receivers on the volcano, and computers. The position of the satellites are known to within a few meters. They send signals which include the time the signal left the satellite. The receivers note the time that the signal arrived. The time it takes for the signal to travel from satellite to receiver can then be determined. Knowing the travel time and the velocity of the signal (the speed of light) the distance between the satellite and receiver can be determined. GPS can measure horizontal changes between different GPS receivers down to a couple of millimeters and vertical changes of about 10 millimeters (1 cm). By visiting the same locations every few months volcanologists can determine where and how much the volcano is changing shape. Even better, some GPS stations are left permanently on specific locations and data is telemetered back to the HVO, allowing changes in position to be tracked constantly. This is a big advance in ground deformation monitoring. This photo shows a GPS receiver on the south flank of Kilauea. Note the benchmark below the receiver. Photo by Steve Mattox, March 1992.


The frequency, magnitude, location, and type of earthquakes associated with active volcanoes are used for monitoring and forecasting eruptions. For example, on a typical day, Kilauea has 200 low-magnitude earthquakes that are too small to be felt. In contrast, just prior to the onset of an eruption, hundreds of earthquakes are recorded and dozens are felt near the epicenter. This photo shows the earthquake record for the onset of a flank eruption in November of 1979. Photograph by Robert Decker, U.S. Geological Survey.

The distribution of earthquakes provides information about magma pathways and the structure of volcanoes. The red dots show earthquakes associated with magma movement. They define the east and southwest rifts of Kilauea. The blue dots show earthquakes associated the sliding of the south flank of Kilauea. Photograph courtesy of U.S. Geological Survey.

In 1974, one year before the eruption of Mauna Loa volcano, the number of earthquakes increased deep beneath the volcano. Throughout the year, the number of earthquakes continued to increase and migrated towards the surface (Koyanagi and others, 1975). Photograph by J.D. Griggs, U.S. Geological Survey, December 10, 1986.



Magma movement and the onset of an eruption produce a distinctive seismic pattern called harmonic tremor. Seismologist must sort through the records of hundreds of earthquakes and determine which are related to the volcano and which were caused by man-induced or natural forces.

Seismometers, the instruments that detect the earthquakes, are set up at numerous locations on the volcano. The information about the earthquakes is sent by radio waves to the Volcano Observatory. This seismometer is on the island of Pagan. Photograph by Robert Koyanagi, U.S. Geological Survey, March 1983.

Gas Geochemistry

Gas samples are collected from fumaroles, like those around and in Halema`uma`u Crater and those near Sulfur Banks, and when possible from active vents. The composition of the gas or a change in the rate of gas emission provides additional information on what is happening inside the volcano. For example, an increase in the ratio of carbon to sulfur can be used to indicate the arrival of a new batch of magma at the summit reservoir. Shortly before the onset of the Puu Oo eruption, the amount of hydrogen gas at the summit of Kilauea Volcano increased significantly (McGee and others, 1987). This photo shows gas geochemists collecting a sample. Photograph by J.D. Griggs, U.S. Geological Survey, March 9, 1990.

The amount of sulfur dioxide (SO2) released by the volcano can be measured indirectly by a correlation spectrometer or COSPEC. The spectrometer compares the light coming through the volcanic plume to a known spectra of sulfur dioxide. The flux of sulfur dioxide (SO2) nearly doubled after the 1983 eruption began (Greenland and others, 1985). About 150 tons of SO2 is released from Halema`uma`u Crater into the air each day. While this seems like a lot (a big coal power plant releases about the same), nearly 10 times this much SO2 is released from Pu`u `O`o each day! The large amount of sulfur dioxide released by Kilauea each day causes numerous problems including adverse health effects( headaches, fatigue, respiratory difficulties), vog (volcanic smog), and acid rain. Photo shows volcanologist measuring sulfur dioxide plume at Puu Oo vent. Photograph by J.B. Stokes, U.S. Geological Survey.


The geologic study of volcanoes provides information about the potential hazards during eruptions and the chemical and physical processes associated with eruptions. This photo shows geologists collecting a lava sample through a skylight. Photograph by T.N. Moulds, U.S. Geological Survey, March 9, 1990.

Geologists from the U.S. Geological Survey's Hawaiian Volcano Observatory have been monitoring the current eruption of Kilauea since 1983. The geologists monitor changes at the active vents and ocean entries, collect lava and tephra samples, map the distribution of vents and lava flows, measure lava temperatures, conduct research, and provide information to the public and the press, and advise local civil defense authorities. In this photo, a volcanologist is preparing a time lapse camera to make observations of the Puu Oo lava pond. Photograph by J.D. Griggs, U.S. Geological Survey, October 11, 1991.

Scott Rowland's Working on Hawaiian Volcanoes at Volcano World provides more information on the methods used to monitor active volcanoes.

The U.S. Geological Survey's Cascade Volcano Observatory also describes the methods used to monitor volcanoes.

Click here for a list of references about Volcano Monitoring Techniques.

If you have comments or suggestions, email me at kenhon@hawaii.edu