An essential ingredient for thermal activity is heat. A body of buried
molten rock, such as the one that produced volcanic eruptions in
Yellowstone as late as 60,000 to 75,000 years ago, takes a long time to
cool. During cooling, tremendous quantities of heat are transmitted by
conduction into the solid rocks surrounding the magma chamber (fig. 45).
Eventually the whole region becomes much hotter than non-volcanic areas
(fig. 46). Normally, rock temperatures increase about 1°F per 100 feet
of depth in the earth’s crust, but in the thermally active areas of
Yellowstone the rate of temperature increase is much greater. The amount
of heat given off by the Upper Geyser Basin, for example, is 800 times
the amount given off by normal (nonthermal) areas of the same size. This
excess heat is enough to melt 1½ tons of ice per second! And, contrary
to popular opinion, the underground temperatures have not cooled
measurably in the 100 years that records have been kept on the thermal
activity in the Park. In fact, geologic studies indicate that very high
heat flows have continued for at least the past 40,000 years.
[Illustration: NORRIS GEYSER BASIN, as viewed northward from the
Norris Museum. This is one of the most active thermal areas in
Yellowstone, but the photograph was taken on a warm dry summer day
when little hot-water and steam activity was visible from a
distance. Clouds of water droplets (the visible “steam” in thermal
areas) normally form only when the air is cool and (or) moist. The
floor of the basin is covered by a nearly solid layer of hot-spring
deposits. (Fig. 44)]
[Illustration: HEAT FLOW AND SURFACE WATER. Diagram showing a
thermal system, according to the explanation that water of surface
origin circulates and is heated at great depths. (Based on
information supplied by D. E. White, L. J. P. Muffler, R. O.
Fournier, and A. H. Truesdell.) (Fig. 45)]
Water enters at ground surface and sinks in conduit formed by fault or
fracture
Surface (meteoric) water sinks to levels perhaps as much as 10,000
feet below ground. Heated far above its normal boiling point,
it begins to rise toward the surface
Descending cool surface water
Permeable zone allows water to flow through it
Cooling magma chamber
Water begins to boil near ground surface because of greatly reduced
pressures
Rising hot water
Hot spring or geyser
[Illustration: INFRARED IMAGE of a part of Upper Geyser Basin.
Infrared instruments, sensitive to heat, are able to detect “hot”
spots in the landscape. Note especially the sharp “image” of Old
Faithful. (Image courtesy of National Aeronautics and Space
Administration.) (Fig. 46)]
A second, equally essential ingredient for thermal activity is water.
Many thousands of gallons are discharged by the hot springs and geysers
in Yellowstone every minute—where does all this water come from? Studies
show that nearly all the water originates above ground as rain or snow
(meteoric water; fig. 45), and that very little comes from the
underlying magma (magmatic water).
The mechanism for heating the water, on the other hand, is a matter of
some uncertainty. Until a few years ago the heating was assumed to occur
near the ground surface and to be caused by hot magmatic gases (mostly
steam) rising from the underlying magma chamber. Deep wells drilled
recently in many thermal areas throughout the world (including research
drill holes in Yellowstone), however, suggest a better explanation.
According to this explanation, the surface water enters underground
passages (fractures and faults) and circulates to great depths—as much
as 5,000-10,000 feet in some areas (fig. 45)—there to become heated far
above its surface boiling point. Research drill holes in Yellowstone,
for example, have demonstrated that water of surface origin exists at
all depths at least to the maximum drilled (1,088 feet), and that the
water reaches temperatures up to at least 465°F. The increase in
temperature with depth causes a corresponding decrease in the weight
(density) of the water. Because of this, the hot, “lighter,” water
begins to rise again toward the ground surface, pushed upward by the
colder, “heavier,” near-surface water which sinks to keep the water
channels filled. Thus is set into motion a giant _convection current_
which operates continuously to supply very hot water to the thermal
areas (fig. 45). Just how deep the waters circulate in Yellowstone no
one really knows; as a guess, the depth probably is at least 1 or 2
miles.
The effect of pressure on the boiling temperature of water also plays a
vital role in thermal activity. In a body of water, the pressure at the
surface is that exerted by the weight of air above it (_atmospheric
pressure_). Water under these conditions boils at 212°F at sea level and
at about 199°F at the elevation of most of the geyser basins in
Yellowstone. However, water at depth not only is subjected to
atmospheric pressure but also bears the added weight of the overlying
water. Under such additional pressures, water boils only when the
temperature is raised above its surface boiling point. In a well 100
feet deep at sea level, for example, the water at the bottom would have
to be heated to 288°F before it will boil. Thus it follows that in the
underground “workings” of hot springs or geysers, (1) The deepest water
is subjected to the greatest pressures, and (2) these deeper waters (in
Yellowstone) must be heated well above 199°F before they can actually
begin to boil. By this same reasoning but in reverse, if the pressure is
released, which happens as the water rises toward the ground surface,
the “hotter-than-boiling” water will then begin to boil. The boiling
will be rather quiet if the pressure is released gradually, as in most
hot springs. But if the pressure is released suddenly, boiling may
become so violent that much of the water flashes explosively into steam,
expanding to several hundred times its normal volume. This expansion
provides the necessary energy for geyser eruptions.