Underground "loops" and side chambers that trap steam is what gives geysers like Yellowstone National Park's Old Faithful their periodic eruptions, volcanologists say.

The key, they say, is the way those bends and chambers trap the steam and subsequently slowly release it to heat a column of water above to just below boiling temperature.

Eventually the top area of the column does reach the boiling point, which releases pressure acting on the water below, allowing it to also come to a boil.

As the water column effectively boils from the top down, steam and water are spewed from the geyser into the air in plumes that can reach hundreds of feet, explains Michael Manga, a professor of earth and planetary science at the University of California, Berkeley.

"Most geysers appear to have a bubble trap accumulating the steam injected from below, and the release of the steam from the trap gets the geyser ready to erupt," Manga says. "You can see the water column warming up and warming up until enough water reaches the boiling point that, once the top layer begins to boil, the boiling becomes self-perpetuating."

Manga has studied geysers in Yellowstone and in Chile, and with the help of his students built an experimental artificial geyser in his laboratory.

The lab geyser is constructed of glass tubing and contains a loop that causes it to erupt periodically, Manga and his colleagues report in the Journal of Volcanology and Geothermal Research.

"At many geysers it looks like there is some cavity that is stuck off on the side where steam is accumulating," Manga explains.

Reproducing such a cavity in the glass geyser has allowed Manga and his students to create eruptions both big and small in his lab.

Some geysers in nature can erupt with astonishing regularity, Manga says.

One geyser dubbed El Jefe in Chile's Atacama Desert, during 6 days of observation, erupted every 132 seconds, plus or minus just 2 seconds, he says.

There are fewer than 1,000 geysers in the world -- about half of them are found in and around Yellowstone -- and they are all in volcanically active or formerly active regions.

In such regions, surface water seeps downward and is heated by hot magma, ultimately returning to the surface as geysers, hot springs or bubbling mud pots.

Manga and his students are studying geysers for a better understanding of volcanic eruptions, possessing many similarities to geyser outbursts but obviously much harder -- and more dangerous -- to study.

Still, Manga says, geysers are themselves fascinating subjects to study.

"One of our goals is to figure out why geysers exist -- why don't you just get a hot spring -- and what is it that controls how a geyser erupts, including weather and earthquakes," he says.