How Scientists Study Volcanoes

• Research directed toward understanding volcanic processes and products.
• Evaluation of the ongoing hazards posed by the active volcanoes.
• Delivery of warnings to public officials regarding these hazards.

To realize these goals, it is necessary to conduct visual and instrumental monitoring of volcanic activity. Monitored changes common to each volcano include the following:

Seismicity

Earthquakes commonly provide the earliest warning of volcanic unrest, and earthquake swarms immediately precede most volcanic eruptions.

Ground Movements

Geodetic networks are set up to measure the changing shape of the volcano surface caused by the pressure of magma moving underground. Techniques commonly used include electronic distance measurement using a laser light source (EDM); measurement of tilt, both electronically and by repeated leveling of triangular arrays; and standard leveling surveys to obtain elevation changes. Additionally, very simple and inexpensive techniques, such as measuring crack openings using a steep tape, or noting changes in water level around a crater lake, have proven useful in certain situations. Upward and outward movement of the ground above a magma storage area commonly occurs before eruption. Localized ground displacement on steep volcanoes may indicate slope instability precursory to mass failure.

Geophysical Properties

Changes in electrical conductivity, magnetic field strength, and the force of gravity also trace magma movement. These measurements may respond to magma movement even when no earthquakes or measurable ground deformation occurs.

Gas Geochemistry

Changes in fumarole gas composition, or in the emission rate of SO2 and other gases, may be related to variation in magma supply rate, change in magma type, or modifications in the pathways of gas escape induced by magma movement.

Hydrologic Regime

Changes in ground water temperature or level, rates of streamflow and transport of stream sediment, lake levels, and snow and ice accumulation are recorded to evaluate (1) the role of ground water in generating eruptions, (2) the potential hazards when hot, energetic volcanic products interact with snow, ice, and surface streams, and (3) the long-term hazard of infilling of river channels leading to increased flood potential.

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Geologic Mapping

Geologic mapping places layered and more irregular deposits in the proper stratigraphic order and establishes their thickness and areal extent (and thus volume). Field descriptions of stratigraphic units are used to classify deposits and interpret the type of eruption that produced them. Mapping of ash deposits is used to correlate widely separated stratigraphic sections associated with a given volcano. Dating of ash layers is especially valuable to bracket ages of other, less extensive, deposits in individual stratigraphic sections.

Dating

Dating of deposits establishes the time intervals in which eruptions or hydrologic events occurred. Techniques commonly used for young deposits are:

• Carbon-14

  This technique is used where eruptions overlie or incorporate vegetation or organic-rich soil and the carbon-bearing material is preserved.

• Tree Rings

  Traumatic injuries to trees are represented by interruption or distortion of growth rings. In some cases, the season in which the event occurred can be specified based on knowledge of the yearly cycles of tree-ring growth.

• Paleomagnetism

  In some areas, it has been possible to calibrate yearly changes in the position of the Earth's magnetic pole over the past several hundreds or thousands of years. In such cases the magnetic directions preserved in a series of eruptive deposits may be used to specify their approximate age.

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Direct observation of volcanic and hydrologic events gives important but incomplete insights into the nature of volcano hazards. The following topics represent some of the avenues pursued to gain a fuller understanding of volcanic processes that control hazardous events.

Numerical Modeling

Numerical modeling is used to test our understanding of physical processes, and hazard predictions can eventually be made on the basis of modeled events. Volcano-related processes amenable to modeling include (1) the gravity-driven flow of lava, hot pyroclastic debris, landslide debris, water-saturated mixtures of mud and rock, and water floods; (2) the dispersal of volcanic ash plumes and thickness of ash accumulation on the ground; (3) the development of eruption- or landslide-induced waves; (4) the time of occurrence and magnitude of outbreak floods from lakes dammed by volcanic debris; and (5) the flow of groundwater and the dynamics of hydrothermal systems.

Experimental Research

Experimental research is necessary to model volcanic processes that cannot be studied directly or safely in the field or are too complicated to model numerically. Experiments can be designed to simulate volcanic conditions and infer possible consequences of volcanic activity. For example, a gelatin mold injected with a colored fluid mimics patterns of subsurface magma movement. Specially designed flumes simulate the properties of dense slurries and help scientists to better understand the development and movement of debris flows. Other topics, such as the origin of magmas by melting in the Earth's mantle, and their subsequent crystallization, can be studied by a combination of laboratory experiments, numerical modeling, and interpretation of chemical variation in erupted lavas.

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When the average repose period and other information regarding a particular volcano's eruptions, for example, when the amount of inflation preceding the previous eruption is matched by current conditions at that volcano, a general forecast can be made that the volcano is "ready" to erupt. The start of microearthquakes or other common eruption precursors would lead to an updated forecast -- that the volcano may erupt soon.

In the past, forecasts of eruptions were based solely on recurring patterns of unrest before eruptions. The occurrence of one particularly diagnostic type of unrest, for example, volcanic tremor, might be the basis for a prediction that the volcano would erupt within a specified number of hours or days. The appearance of other known eruption precursors helped narrow the time window and lent certainty to the prediction.

We now recognize the need to understand why particular patterns and events occur before some eruptions, and this need requires a thorough physical understanding of the volcano's internal plumbing and the processes associated with the generation, transport, storage, and ultimately, eruption of magma. For example, a combination of seismic and geodetic data demonstrates the existence of a complex magma reservoir 2 to 6 kilometers beneath Kilauea's summit from which all eruptions on the volcano ultimately originate. Earthquake foci outline the area of magma storage, whereas horizontal, vertical, and tilt changes above the reservoir define the depth to "centers" of inflation (swelling) or deflation. Understanding of this storage system has greatly improved the ability to determine when Kilauea is fully inflated and ready to erupt. Accurate short-term (within days to weeks) prediction of Hawaiian eruptions remains elusive, as both Kilauea and Mauna Loa may reach a highly inflated state, and wait with no further ground deformation or increase in seismicity until eruptions occurs. some Kilauea rift eruptions are preceded within hours by a strong earthquake swarm whose foci migrate toward the point of outbreak, giving a short but accurate prediction of this type of activity. Volcano monitoring, combined with study of Kilauea's volcanic history, yields the information necessary for long-term eruptions forecasts.

As with Hawaiian eruptions, the dome-building eruptions of Mount St. Helens are not predictable many months ahead. Prediction of dome-building eruptions were made, however, within days or weeks, using very simple methods, with relatively little prior knowledge or understanding of the volcano's plumbing system.

Accurate predictions are still rare in volcanology, and probabilities associated with eruption from a given volcanic system may change after an eruption takes place. Often volcanic systems are in delicate balance and may be considered "ready" to erupt; this determination of readiness allows a medium-range forecast of increased likelihood of eruption. For many currently dormant but potentially active volcanoes, we may only be able to give factual information regarding past activity without specifying what the future holds. For well-studied, historically active volcanoes we can make more specific forecasts of future activity. The most accurate predictions are in the short-term where either rapid ground movements or an earthquake swarm directly precedes eruption at the surface.