See the context of this sign.

Old Faithful's Plumbing

Old Faithful's steaming and spouting
merely hint at the thermal action
below ground. After an eruption,
the partially emptied geyser
chambers again fill with hot water.

As steam bubbles rise, they clog
narrow sections of the geyser tube.
Pressure and temperature then
increase until steam abruptly forces
its way upward, discharing some
water in preliminary splashing.

This splashing apparently unloads
enough water to start a chain
reaction deep within the system.
As larger quantities of water flash
into steam, the geyser surges into
full eruption.

When the geyser tubes are nearly
empty, eruption ceases. The system
then begins to refill, and the entire
cycle starts anew.

Predicting Old Faithful

Every geyser has its own unique eruption
indicators. Between spoutings, watch Old
Faithful for signs of impending thermal

For most of the interval, steam gently
billows from Old Faithful's cone. A few
minutes before an eruption, intermittent
jets of spray spurt a few feet above the
surface. When the spurts become
sustained and surge upward, a full
eruption has begun.

The average interval between Old
Faithful's eruptions is growing longer.
Frequent earthquakes in and near
Yellowstone may alter the underground
plumbing, changing the routes of
circulating water. Vandalism - people
throwing objects into Old Faithful's
vent - may also contribute to the

No two eruptions are the
same; the height and
duration vary. Because
of the duration of Old
Faithful's last eruption
determines the time
between eruptions, only
the next discharge can be
predicted. Subsequent
eruptions are impossible
to predict.

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