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Geysers like Old Faithful in Yellowstone National Park have baffled researchers for decades.

Now,  a team has uncovered exactly what causes them to blow - and say it if the geological equivalent of a u-bend,

They erupt periodically because of loops or side-chambers in their underground plumbing.

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Geysers like Old Faithful in Yellowstone National Park (pictured) erupt periodically because of loops or side-chambers in their underground plumbing,researchers say.

The key to geysers, said Michael Manga, a UC Berkeley professor of earth and planetary science, is an underground bend or loop that traps steam and then bubbles it out slowly to heat the water column above until it is just short of boiling.

Eventually, the steam bubbles trigger sudden boiling from the top of the column, releasing pressure on the water below and allowing it to boil as well. 

The column essentially boils from the top downward, spewing water and steam hundreds of feet into the air.

'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 said. 

'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.'

The new understanding of geyser mechanics comes from Manga's studies over the past few years of geysers in Chile and Yellowstone, as well as from an experimental geyser he and his students built in their lab. 

Made of glass with a bend or loop, it erupts periodically, though, surprisingly, not as regularly as a real geyser they studied in the Atacama desert of Chile, dubbed El Jefe. 

Over six days of observation, El Jefe erupted every 132 seconds, plus or minus two seconds.

The team built an artificial geyser in their lab to test theirtheories. Made of glass with a bend or loop, it erupts periodically, though, surprisingly, not as regularly as a real geyser they studied in the Atacama desert of Chile, dubbed El Jefe.

A geyser spouts water in Southern Iceland. An underground bend or loop traps steam and then bubbles it out slowly to heat the water column above until it is just short of boiling.

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

'So we said, 'Let's put in a cavity and watch how the bubble trap generates eruptions.' It allows us to get both small eruptions and big eruptions in the lab.'

Manga and his colleagues, including first author Carolina Munoz-Saez, a UC Berkeley graduate student from Chile, report their findings on the Chilean geysers in the February 2015 issue of the Journal of Volcanology and Geothermal Research.  

Fewer than 1,000 geysers exist around the world – half of them in Yellowstone – and all are located in active or formerly active volcanic areas. 

Why do geysers erupt? Secrets of natural phenomenon explained

Water from the surface trickles downward and gets heated by hot magma, eventually, perhaps decades later, rising back to the surface in the form of hot springs, mud pots and geysers.

Why geysers erupt periodically, some with a regularity you can set a clock by, has piqued the interest of many scientists, but German chemist Robert Bunsen was the first to make pressure and temperature measurements inside a geyser – the Great Geysir in Iceland, after which geysers are named – in 1846.  

HOW THEY DID IT

Manga and his students feed temperature and pressure sensors as deep as 30 feet into geysers – something impossible to do with a volcano – and correlate these with above-ground measurements from seismic sensors and tiltmeters to deduce the sequence of underground events leading to an eruption. 

They have also been able to submerge video cameras as deep as six feet into geysers to view the submerged conduits and chambers below.

Here, the team are threading temperature and pressure sensors down a geyser hole in Chile. 

The new understanding of geyser mechanics comes from Manga's studies over the past few years of geysers in Chile and Yellowstone, as well as from an experimental geyser he and his students built in their lab. 

He hopes to be able to extrapolate his findings to volcanoes, deducing the internal mechanics from exterior seismic and gravity measurements.

But geysers are fascinating in themselves, he said.

Fewer than 1,000 geysers exist around the world – half of them in Yellowstone – and all are located in active or formerly active volcanic areas. Here, a minor eruption from Steamboat Geyser at Yellowstone National Park's Norris Geyser Basin in Wyoming.

'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 said. 

In places like Yellowstone, the bubbles that slowly escape from the underground loop cause mini-eruptions called preplay leading up to the major eruption. 

Eruptions stop when the water column in the geyser cools below the boiling point, and the process repeats. 

All these underground processes seem to be affected only by the heat source deep below the geyser, because they could find no evidence that the surface temperature affected eruptions.