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Mount Merapi in Indonesia is expected to erupt at any moment, and Indonesian government officials have begun to evacuate the people living in villages near the volcano. The volatile mountain's last eruption was in 2006, and two people were killed when a plume of scorching gas, rocks and volcanic ash spewed down its slopes, according to the NASA Earth Observatory.

Officials from the island of Java, where Mount Merapi is located, told news sources they predict lava to spill over the southern side of the mountain. Volcanic earthquakes, an increase in seismic activity and swelling of the ground around the volcano have caused Indonesian officials to raise the alert on Merapi to level three, with four being the maximum alert level.

What are the signs that a volcano is about to erupt?

While volcanoes may give off several warning signs they are about to blow, some are taken more seriously by volcanologists, who alert and advise government officials regarding when evacuations need to take place.

"These signs may include very small earthquakes beneath the volcano, slight inflation, or swelling, of the volcano and increased emission of heat and gas from vents on the volcano," said U.S. Geological Survey (USGS) Volcano Hazards Program coordinator John Eichelberger.

"Rising magma causes solid rock to break, sending earthquake signals," Eichelberger told Life's Little Mysteries. "This pushes the ground surface upward, and boils off hot gas that travels ahead of the magma."

The USGS looks for these changes using seismic and GPS networks that surround volcanoes. Radar satellites are also used to detect swelling of the ground by comparing images taken at different times, according to Eichelberger.

How far in advance do the warning signs occur?

"Most volcanoes give warnings signs beginning weeks or months before they erupt," Eichelberger said.

During field observations of a potentially active volcano, volcanologists use sophisticated equipment and techniques to predict and monitor an eruption. These include measuring water temperature and pH (acidity), analyzing ground cracking patterns and searching for new areas of avalanche-felled rocks, according to the USGS.

Ground-based, airborne and satellite detectors are used to measure gas and heat emission. However, some eruption warning signs are harder to read than others.

"Increases in the flow rate of sulfur dioxide and carbon dioxide gases indicate that magma is coming," Eichelberger said. "Water is actually the most abundant gas in magma, but because there is already so much of it in the atmosphere, measuring it is not useful in forecasting eruptions."

Remy Melina is a staff writer for Life's Little Mysteries, a sister site to LiveScience.