Frequently Asked Questions About Eruption Precursors

Before a volcano erupts, magma must force its way upward through solid rock beneath a restless volcano. This process causes the ground above to heave and shake as rock is shoved aside or broken. At the same time, gases are released from the magma as it rises to shallower levels where the pressure is lower and the temperature of the ground increases. These phenomena--ground movements, earthquakes, and changes in heat output and volcanic gases--provide the clues that scientists use to recognize a restless volcano and anticipate what might happen next.

Q: What kinds of unusual activity might be noticed before an eruption?

A: Common symptoms of volcanic unrest include an increase in the frequency or intensity of earthquakes beneath a volcano; the occurrence of volcanic tremor; swelling, subsidence, or cracking of the ground; increased steam emission or small steam explosions; temperature changes as indicated by melting snow or ice; changes in existing fumaroles or hot springs, or the appearance of new ones; and increased discharge of magmatic gases. Volcanologists assess the significance of volcanic unrest partly by monitoring the pace and intensity of such activity. It is important to remember that volcanic unrest is common and most times unrest does not lead to eruptions. By studying past unrest and eruptions, scientists can better figure out what might happen next at that volcano.

Q: What is volcanic tremor, and how does it differ from earthquakes?

A: Tremor is a seismic vibration, similar to a volcanic earthquake, but of longer duration and more continuous than earthquakes of the same amplitude. Volcanic tremor can last from minutes to days. It may be caused by magma moving through narrow cracks, boiling and pulsation of pressurized fluids within the volcano, or escape of pressurized steam and gases from fumaroles.

Q: Do volcanoes produce different kinds of earthquakes?

A: Yes. A variety of earthquake types can occur at a volcano that is getting ready to erupt. These include earthquakes caused by rocks breaking along faults or fractures, termed tectonic-type earthquakes. Another common type a long-period or volcanic earthquake. These can occur when bubble-filled magma is on the move beneath a volcano. The differences between tectonic- type and volcanic-type earthquakes are so subtle that they can be distinguished only by using seismometers.

Q: What kind of gases escape from volcanoes?

A: The fumes escaping from a volcano consist mostly of water vapor (steam). Steam may be emitted from the hot interiors of volcanoes even when they are dormant. But steaming usually increases dramatically as magma intrudes and heats groundwater beneath a volcano. Magma gives off carbon dioxide (CO2) and hydrogen sulfide (H2S, rotten egg gas) that do not totally dissolve in groundwater and can therefore show up at the surface. As water inside the volcano boils away, other more water-soluble volcanic gases can reach the surface, signaling an increasingly grave situation. These gases include sulfur dioxide (SO2) and common halogen gases such as hydrogen chloride (HCl), and hydrogen fluoride (HF). For more information, please see Monitoring Volcanic Gases in our Activity Section.