perhaps this will answer your question?
http://www.geyserstudy.org/ov_quick_guide.htm
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Landscape & Geology
The Yellowstone and Grand Teton region is one of the most dynamic
seismic areas in the world -- racked by earthquakes, cracked by water
boiling to the surface, and littered with the detritus of previous
volcanic eruptions. Today the bowels of the Yellowstone caldera are
again filling with magma. Geologic studies show that, for the last two
million years, the plateau has blown its top every 600,000 years or so
-- and the last explosion was about 600,000 years ago. That means that
a titanic blow -- bigger than anything seen in recorded history --
could happen, well, any century now. The geological time frame is a
long one, by human standards.
As you'll learn when you visit the exhibits on the park's geology at
Moose, Mammoth, and the various geothermal areas, what you see on the
surface -- great layers of ash and the core of volcanic vents, such as
Mount Washburn and Bunsen Peak -- is only a fraction of the story of
Yellowstone and Grand Teton.
Situated on 2.2 million acres, Yellowstone is significantly larger
than its sister to the south. Encompassing 3,472 square miles,
Yellowstone boasts 370 miles of paved roads and 1,200 miles of trails,
and is home to more geysers and hot springs than the combined total in
the rest of the world.
Geysers, Hot Springs, Mud Pots & Fumaroles: What's the Difference?
Here's a way to identify the four most common types of thermal
attractions in Yellowstone.
Geysers, the most prominent and spectacular of the thermal features in
the park, are formed by the marriage of three elements: heat in the
center (mantle) of the Earth, water, and pressure-resistant rock.
Here's how they work together: The water source comes in the form of
snowmelt that sinks into the Earth, reaching depths of 10,000 feet --
that's 2 miles, but, remember, thermal areas in Yellowstone are that
close to the surface. The second element is heat, which is provided by
the cauldrons of magma. When water moves toward the center of the
Earth, the heat brings it to a boil and sends it back toward the
surface, often with traces of numerous minerals. The third element,
pressure-resistant rock, acts like a geologic traffic cop by guiding
the water toward the surface. Because the geyser's plumbing system
does not have a way to diffuse energy, pressure builds, the boiling
water is converted to steam, and boom! Plumes of steam are spewed
forth from vents in the Earth's surface. Following the eruptions,
silica that once was a component of the magma returns down the pathway
to form a lining through which more snowmelt will travel, and the
process repeats itself.
Hot springs are closely related to geysers but don't display the same
eruptions because they don't develop the same subsurface pressures.
Their colorful appearance is a result of different minerals, algae,
the absorption of light by colloidal particles -- tiny, suspended
particles of liquid -- and reflections.
Mud pots are hot springs on the Earth's surface that are formed as
heated water mixes with clay and congealed minerals. Some mud pots are
very colorful; others are unsightly and smelly.
Fumaroles are steam vents known as "dry geysers," from which gases
rush into the air; they are considered hot springs that lack a liquid
component.
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