Imagine thousands of gallons of boiling water rushing forth from the earth. Shooting
toward the sky, the water forms a tall, steamy column – sometimes 180 feet high! When
the water falls, it’s only a matter of time before another surge will appear. Old Faithful
geyser in Yellowstone National Park erupts regularly and has attracted sightseers since at
least 1870.

In 1870, a team of surveyors explored the area of northwestern Wyoming that would
become Yellowstone National Park. The team (known as the Washburn Expedition)
explored lakes, mountains, plants, and wildlife. They also observed many geothermal
features. While camping, they noticed a geyser that erupted about every hour. Since this
geyser was nearly as reliable as a wristwatch, the men named it Old Faithful.

A geyser is a hot spring that occasionally erupts; the term is derived from an Icelandic
word meaning “to gush”. Such gushers are rare. A geyser can become blocked by mineral
deposits, and tectonic activity (earthquakes) or human intervention can alter their
behavior. Only about 1,000 geysers are known to exist on Earth, and about half of those
are located in Yellowstone.

How does a geyser like Old Faithful work? A geyser, like any natural hot spring, has its
water heated by magma, or melted rock deep within the earth. The force of heat
(convection) pushes the water up through porous rocks. After steam and boiling water are
expelled, the cycle starts anew. Geysers’ “schedules” widely differ; for example, some
erupt every ten minutes, and some erupt just twice a day.

In geological terms, Old Faithful is a cone geyser. The name refers to a cone-shaped
formation of minerals that has formed at the geyser’s mouth. This cone shapes the narrow
spray that bursts forth. In contrast, a fountain geyser has eruptions burst from an open

Observers have documented more than 137,000 Old Faithful eruptions, and people have
noticed changes in the eruption schedule since 1870. This might be a result of a 1998
earthquake changing underground water levels, or the cumulative effect of many tiny
quakes. The geyser has also been altered by vandalism; e.g. visitors have thrown items
into Old Faithful. At the start of the 21st century, a handy formula involves measuring the
duration of an eruption. If it lasts for 2.5 minutes or less, the next eruption will follow
about 65 minutes later. If it lasts for longer than 2.5 minutes, the geyser may be
“exhausted” until 92 minutes later.

How hot is Old Faithful’s water? In the 1980s and 1990s, scientists lowered
thermometers about 70 feet into the geyser. They measured a temperature of 244 degrees
Fahrenheit. Apparently, the temperature remained constant since a 1942 recording. Steam
temperatures reached 265 degrees. Right before eruption, water at the opening is about
204 degrees.

How much water is expelled? With each eruption, Old Faithful puts forth between 3,700
and 8,400 gallons. This forms a column that’s between 106 and 184 feet high. An average
eruption is about 130 feet tall.

Old Faithful is not Yellowstone’s largest geyser; that distinction belongs to Steamboat
Geyser. However, the landmark attracts the attention of most who visit Yellowstone.
Eruption times are posted at the park’s Visitor Center, and growing crowds are also a clue
that the time is near. Walkways and benches are provided for viewers.

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