Inside Old Faithful
Scientists look down the throat of a geyser
The story below was the cover story
of the Oct. 11, 1997,
It appeared on page 232 of that issue.
By Sid Perkins
When the world's most famous geyser opened up and said "Aaaah" for
geologists, there were surprises in store.
Under the supervision of National Park Service personnel, researchers
lowered several instruments, including a video camera, into Yellowstone's
Old Faithful to observe conditions there. Their findings, published in the
October GEOLOGY, provide the first detailed look inside a geyser that is
unmatched in its combination of size, frequency and regularity of
James A. Westphal, a geologist at the California Institute of Technology
in Pasadena, says many of the team's discoveries confirmed previous
suspicions, but there were also a few unexpected findings - for example,
the eruption cycle behaves more simply than the scientists had
In April and October 1984, during several eruption cycles, the researchers
measured temperature and pressure every 5 seconds at eight different
depths along the uppermost 21.7 meters of the geyser, its only accessible
portion. Susan W. Kieffer, a coauthor of the report, says these first data
were so complicated that they did not match any established theory of
Says Westphal, "That's when she [Kieffer] said to me, 'You know, I'd sure
like to see what it looks like in there.'"
In 1992, when miniature video cameras became widely available, the
scientists returned to Yellowstone. They insulated a 2-inch video
camera and lowered it into Old Faithful to map the vent and see what
happens there between eruptions. "The vent had a much more complicated
shape than we expected," Westphal says.
Kieffer had assumed that the vent would be a smooth tube, but the camera
showed that it is actually a crack running in an east-west direction
that extends downward at least 14.3 m. In some portions of this
fissure, the wall lie beyond the 1.8-m field of view of the camera, but
other regions are extremely narrow. In one spot, at a depth of about 6.8
m, the walls close to within 15 centimeters of each other - a
feature critical to explaining the geyser's behavior.
The camera also revealed that the wall of the vent is riddled with
cracks and that water enters continuously at several different depths.
Cool water from the local water table flows into the fissure 5.5 m and 7.5
m underground, and superheated water and steam shoot into the vent 14.3 m
Spikes in temperature readings at the beginning of an eruption - some of
them reaching nearly 130 degrees C - suggest that water and steam also
surge into the fissure from geothermal sources at depths below 21.7
m, says Westphal.
Old Faithful blows its top every 79 minutes, on average, with intervals
between eruptions ranging from 45 to 105 minutes, depending on the amount
of boiling water left in the fissure when the geyser runs out of steam.
"There's no real pattern, except that a short eruption is always followed
by a long one," Westphal says. "The geyser pretty much keeps its own
Temperature and pressure measurements show that for the first 20 or 30
seconds of each 3- to 5-minute eruption, steam and boiling water rocket
through the narrowest part of the fissure at the speed of sound. During
that time, the 15-cm-wide slot behaves just like the throat of a wind
tunnel, limiting the geyser's discharge rate. When the pressure driving
the eruption drops below a critical value, the outflow slows and the
geyser's plume, which can reach a height of 55 m, begins to shrink.
"I was assuming a simple geometry for the fissure and expected a
complicated physical explanation for the geyser's behavior," Kieffer says.
"The video camera showed us it was just the reverse."
The researchers observed the water as it boiled, churned and surged
through the fissure. The behavior of the bubbles in the water confirmed
their suspicion that carbon dioxide is present in the geyser's water,
although the power behind the eruption comes from steam.
Moreover, Westphal was surprised to find that even though groundwater
constantly replenishes the pool of liquid in the fissure, the water would
sometimes recede to the unobservable lower levels of the fissure for
minutes at a time. What's happening in the deeper recesses of Old Faithful
remain a mystery, he says.
The forces of nature that give life to the geyser also contain the agents
of its ultimate demise. The superheated water that constantly recharges
the fissure is saturated with minerals, which precipitate onto the walls
of the vent as silica. In a geological equivalent of hardening of the
arteries, Old Faithful will eventually become clogged and die, a fate
Westphal says has already befallen several nearby geysers.
Copyright 1997 by Science Service.
All rights reserved.