Geysers erupt because superheated water trapped underground suddenly flashes to steam, expanding to roughly 1,500 times its liquid volume and blasting a column of water into the air. This requires three things that rarely occur together: an abundant water supply, intense heat from magma below, and a specific type of underground plumbing. Only about 1,000 active geysers exist on Earth, and roughly half of them are in Yellowstone National Park.
Three Ingredients a Geyser Needs
A geyser can only form where water, heat, and the right rock structure all converge. Remove any one of these, and you get something else entirely: a hot spring, a fumarole, or just warm ground.
Water: The water feeding a geyser starts as rain or snowmelt. It seeps into the ground and travels downward through cracks in the rock, sometimes for hundreds or thousands of years, before reaching superheated zones deep below the surface.
Heat: The heat comes from magma, either an active volcanic system or a slowly cooling body of molten rock sitting relatively close to the surface. At Yellowstone, volcanic gases and heat from a shallow magma chamber warm dense, mineral-rich water occupying fractured rock above it. Without this kind of intense, sustained heat source, groundwater never gets hot enough to erupt.
Plumbing: This is the rarest ingredient. A geyser needs a network of underground fractures, cavities, and narrow constrictions in the rock. These constrictions act like a pressure lid, trapping superheated water below and preventing it from circulating freely to the surface. Hot springs lack these constrictions. Their open plumbing allows hot water to rise, cool at the surface, sink back down, and be replaced in a steady loop called convection. That continuous circulation keeps the water below boiling and prevents any eruption. A geyser’s tight, irregular channels interrupt this process and allow pressure to build.
How the Eruption Cycle Works
A geyser eruption isn’t a random explosion. It follows a repeating cycle with distinct phases: recharge, preplay, and eruption.
During the recharge phase, water slowly fills the geyser’s underground chambers and channels. Heat from below warms this water, but the weight of the water column above keeps it from boiling, much like a pressure cooker raises the boiling point of water inside it. Deep underground, water can reach well above 100°C (212°F) without turning to steam because of the immense pressure pressing down on it.
As heating continues, the water nearest the top of the system reaches boiling first. Small amounts of water begin spilling out at the surface in what’s called the preplay phase, producing brief, sputtering bursts. Each small discharge reduces the pressure on the deeper, superheated water below. That pressure drop allows deeper water to boil, which reduces pressure further still, creating a chain reaction that propagates downward through the entire water column.
Once the whole column reaches or exceeds boiling temperature, the main eruption begins. Water flashes to steam in a rapid, sustained burst that drives the remaining water upward and out of the vent. The eruption continues until the chamber is largely emptied or cooled, then the cycle starts again. For Old Faithful, the eruption itself lasts two to five minutes, while the full recharge cycle currently takes about 102 minutes on average, with intervals ranging from 54 to 118 minutes.
Why Old Faithful’s Timing Keeps Changing
Old Faithful’s name is slightly misleading. Its interval between eruptions has gradually lengthened over the past several decades, increasing by about 30 minutes in 30 years. Before the 1959 Hebgen Lake Earthquake, eruptions came roughly once an hour. That earthquake altered the geyser’s underground plumbing, and it began erupting with two distinct interval patterns, a short one and a long one, with a higher average overall.
The 1983 Borah Peak Earthquake in Idaho pushed the interval up again. A 1998 earthquake near the geyser, followed by subsequent earthquake swarms, lengthened it further. Each seismic event subtly reshapes the fractures and channels underground, changing how quickly water refills the system. Occasionally the average interval decreases, but the long-term trend has been toward longer waits. As of January 2025, the median interval sits at 102 minutes, plus or minus about 10 minutes.
Cold-Water Geysers: No Heat Required
Not all geysers are driven by volcanic heat. Cold-water geysers erupt using carbon dioxide instead of steam. The mechanism is strikingly similar: dissolved CO₂ in groundwater stays in solution under pressure, just as carbonation stays dissolved in a sealed bottle of soda. When pressure in the water column drops enough, the CO₂ comes out of solution and expands rapidly, displacing water and triggering an eruption.
Crystal Geyser near Green River, Utah, is one of the best-known examples. It shoots water 15 to 20 meters high, erupts every 11 to 18 hours, and each eruption lasts 15 to 45 minutes. Other notable cold-water geysers include the Andernach Geyser in Germany and the Herlany Geyser in Slovakia. Crystal Geyser actually erupts from an old drill hole rather than a natural vent, but the CO₂-driven process powering it is the same one that operates in natural cold-water systems.
Why Geysers Are So Rare
The roughly 1,000 active geysers on Earth are concentrated in just a handful of locations. Yellowstone alone accounts for at least 500 actively erupting geysers out of some 700 total geyser features in the park. The rest are scattered across Iceland, New Zealand, Chile, Russia’s Kamchatka Peninsula, and a few other volcanic regions. The combination of active magma at shallow depth, sufficient groundwater recharge, and the precise rock fracture geometry needed to create constricted plumbing simply doesn’t occur in most places.
That number used to be higher. Over recent decades, human activity has permanently quenched about 260 geysers, reducing the global count by roughly 23 percent. Geothermal energy projects are the primary cause. When wells extract large volumes of hot water, steam, or heat from the same underground reservoirs that feed geysers, they lower reservoir pressure enough that geysers stop erupting. In New Zealand, about 100 geysers were lost this way, roughly 75 percent of the country’s total. Iceland lost about 46 geysers, also around 75 percent. In Nevada, two entire geyser fields were wiped out: all 30 geysers at Beowawe and all 26 at Steamboat Springs. Geysers almost never recover once their reservoir pressure has been drawn down by extraction wells.
Other threats include alteration of nearby riverbeds, hydroelectric reservoir construction, and mining exploration. Yellowstone’s geysers survive in large part because of their protection as a national park. Outside of protected areas, roughly 40 percent of all geysers that once existed have been permanently silenced.