An erupting geyser dazzles visitors like few other natural events. The ground rumbles and shakes and the geyser releases a warning burp before thousands of gallons of scalding water and steam rocket from the Earth. This geo-phenomenon can last as long as 45 minutes and ascend to heights of 400 feet.
Scientific experts call geysers "the rarest fountains of all." They are hot springs that erupt, holes in the ground that emit violent bursts of steam and boiling water. The eruptions are rare, however. There are about 1,000 active geysers on Earth, and half of them are within Yellowstone National Park. The park's Norris Geyser Basin, at 7,000 feet, has an exceptional blend of the three ingredients that are key to forming geysers: abundant water, intense heat, and water and pressure-tight underground plumbing.
Although the elements are simple, many visitors wonder: How do the geysers really work? "That is a frequently asked question," says Hank Heasler, park geologist at Yellowstone. "I usually am asked by some very curious school-age children. In the past, I explained the operation of a geyser through the analogy of a pressure cooker, but school-age children today have no concept of what that is. They're used to microwaves."
So how does it work? Basically, the geyser's underground plumbing has volcanic rock that conducts heat, radiating into surrounding rock. Water from rain and snow works its way underground through several narrow passageways. As water hits the rock, the heated water rises to the surface, passing through volcanic ash or lava. The water dissolves solid matter and carries it up through the rock crevices, forming constrictions. Those constrictions impede the mounting pressure, and expanding steam bubbles from the rising hot water built up in and behind them.
The steam squeezes through passageways and causes water to overflow from the geyser. That spray of water at the surface sparks a rapid decline in pressure for the hotter water deeper in the geyser, triggering a chain reaction of steam explosions that cause the volume of (now-boiling) water to grow 1,500 times or more. This water then erupts into the sky-a spectacular geyser.
Still confused? Here's another way to think about it.
"The key principle," says Heasler, "is the relationship of boiling temperature to pressure. At sea level, water boils at about 212 degrees. Up in Norris Geyser basin, water boils at about 200 degrees. As you get less and less pressure, the water boils at a cooler and cooler temperature. As you get more and more pressure, the water boils at a much higher temperature. In the geyser, the water deep within it gets well above the boiling temperature due to overlying water pressure.
"Keep putting heat in, and eventually the water will get so hot that the overlying water column's pressure can't constrain the rapidly heating water anymore." Then, Heasler explains, the initial burp of water indicates that some pressure has been forced out by a big steam bubble.
"Once the pressure has been slightly relieved by this burp of water, the entire plumbing system flashes to steam, because the super-heated water is no longer constrained by the overlying water pressure. That forces out the rest of the water in a spectacular, usually high, eruption," wowing visitors.
"The steam phase is where the geyser is sort of clearing its throat," says Heasler. "After the water pressure has been reduced, steam forces its way out of the geyser and the re-filling process begins."
Once that extraordinary pressure has been released, the warm-to-cool water flows back into the geyser's plumbing system through cracks in rocks. The somewhat cooler water begins to refill the geyser but is reheated by very hot surrounding rock. The water becomes so hot that it's above boiling but it's still water.
When the water gets so hot that it can no longer be constrained, the eruption begins anew. At the Old Faithful geyser in Yellowstone, this can happen every 45 minutes or so-making it among the most predictable of geysers, thus the name.
