Signs of Impending Volcanic Eruption

Volcanoes have been a popular subject of disaster movies, from "Dante's Peak" to "The Devil at 4 O'Clock" but in reality, volcanic activity is a regular part of nature. Recent volcanic activity in the first few days of 2012 is a reminder that the Earth is constantly changing and churning.

While the expectation is that volcanoes will suddenly and without warning erupt in spectacular displays of lava, ash and smoke, the truth is that volcanic eruptions usually offer plenty of warning. Here's a little basic primer on volcano facts and precursors to volcanic eruptions.

Common Signs of Impending Eruption

Scientists look for several "symptoms" when determining that an eruption is imminent. Gas and steam, seismic activity, swelling of the ground, cracking or subsidence, noted changes in hot springs around a volcano -- all raise alerts that a volcano may be stirring to life and warrant further observation and preparation for those living near a volcano.

Seismic Activity as Warning

Seismic activity is one of the most common and noticeable precursor to volcanic eruption. The most common types of seismic activity that occur near a volcano are volcanic tremor, volcanic-type earthquakes and tectonic-type earthquakes.

Volcanic tremor is seismic vibrations that occur due to magmatic or other volcanic activity such as movement of pressurized fluids within the volcano and the escape of gases and steam. Volcanic tremors are longer in duration than normal earthquakes and according to the USGS, can last "from minutes to days."

Tectonic-type earthquakes occur with breaking and cracking of rock along fractures and faults. Volcanic-type quakes are similar to volcanic tremors in that they are caused by magma movement within a volcano, except that they are shorter in duration.

Steam and Gases

Gas and steam or water vapor may escape from volcanic vents prior to eruption. Steam does not necessarily mean an eruption is imminent as it can escape from dormant volcanoes.

Magma gives off carbon dioxide and hydrogen sulfide, more commonly known as rotten egg gas and the escape of those gases is often associated with a pending volcanic eruption. Gas escape by itself does not always mean an eruption is about to happen -- as evidenced by Germany's Laacher See volcano that produces carbon dioxide bubbles in shallow portions of the volcano's crater lake.

Active Volcanoes in the World

According to the Smithsonian Global Volcanism Program, the question "How many active volcanoes are there in the world?" is a little bit deceiving and difficult to answer. At any given time, GVP estimates that there could be 20 eruptions occurring. In the past 10,000 years, at least 1,300 volcanoes have erupted.

Number estimates from the GVP hold that each year there are between 50 and 70 volcanoes erupting and each decade counts about 160 volcanoes actively erupting. Those numbers do not include active sea floor volcanoes that erupt.