WEST
THUMB GEYSER BASIN
(YELLOWSTONE HOTSPOT) HYDROTHERMAL
FEATURES
West Thumb Caldera (looking ~NE) - Yellowstone Lake has a
large, irregularly circular embayment on its western side called West Thumb. Reported ages for this feature include 140
ka, 161-162 ka, 174 ka, and 198 ka.
Recent studies on the geology of the West Thumb area have concluded that
it formed as a result of collapse and flooding by Yellowstone Lake’s water
following a series of three, relatively large, explosive ash eruptions. These three ash deposits together form an
estimated 50 km3 unit called the Bluff Point Tuff, which
ranges in age from 170 ka to 200 ka. The
collapse event formed the West Thumb Caldera, the original margin of which is
now covered by post-caldera rhyolite lava flows.
The West Thumb Geyser Basin (see
below photos) is located along the southwestern shore of Yellowstone Lake’s
West Thumb. It is relatively small and
cool compared with other hydrothermal areas in Yellowstone. Three groups of geothermal features make up
the West Thumb Geyser Basin, but only one of these is easily accessible - the
Lower Group.
Mimulus Pools (Upper Pools), southwestern Lower Group,
West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in
August 2011 (looking ~ENE).
Painted Pools (Upper Pools) (above & below),
western Lower Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern
Wyoming, USA.
Above: July 2012, looking ~NNE.
Below: August 2011, looking ~ENE.
Notice that the water clarity in the Upper
Pools varies from year to year.
Painted Pools (Upper Pools) (above & below)
Above: August 2011. Below: July 2012.
Painted Pools (Upper Pools) (above & below) in July
2012 (looking ~SE).
Twin Geysers, western Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA in August 2011 (looking
~SE).
Twin Geysers consists of the two
water-filled vents shown above, surrounded by a low, irregularly-mounded
geyserite that is partially stained reddish-brown and yellowish by iron
oxides. Geyser eruptions here have been
rare in recent decades, and occurred sporadically before then, including the
1910s, the 1930s, the late 1940s, the early 1950s, and the early to mid-1970s.
The highest reported Twin Geysers eruption
was about 200 feet high in 1910. Another
significant eruption was about 120 feet high in 1934.
Twin Geysers was most active from 1971 to
1973, with maximum water column heights of about 120 feet. During eruptions, water would spout from both
vents, but essentially converge in the air.
Twin Geysers also erupted on two days in
1998 and 1999.
Abyss Pool, northern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA.
Abyss Pool is a hot, moderately deep,
intensely-colored pool. It has been
blue-colored in the past. A combination
of bubble-brained vandals throwing objects into the pool and almost basin-wide
cooling has lowered the temperature of this feature, resulting in yellowish
extremophile bacterial mats lining most of the pool’s walls. The “blue” water plus the yellowish-colored
bacterial mats results in a deep green color.
Partial blockage of the vent at the bottom
of the pool from tourist garbage has also decreased the amount of overflow in
the runoff channel (see 4th photo below).
Abyss Pool has had rare geyser eruptions,
including in 1904 and 1905, when water columns reached up to 100 feet
high. Surrounding geyserite was
sometimes ripped from the upper walls of Abyss Pool during these powerful
eruptions, resulting angular, up-to-boulder sized geyserite clast
deposition. 1987 had a single geyser
eruption from Abyss Pool. In 1991 and
1992, it was quite active, with forceful eruptions reaching up to 100 feet
high.
Above: July 2012, looking ~NW.
Below: August 2011, looking ~NE.
Abyss Pool (above & below) in August 2011
(looking ~NE)
Abyss Pool in August 2011 (looking ~NW) -
multicolored runoff channel lined by various species of extremophile bacteria
and bordered by whitish-gray geyserite.
Black Pool (above & below), northern Lower
Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA
in August 2011 (looking ~SE).
Black Pool is a very hot, moderately deep,
irregularly-outlined, and intensely blue- to deep turquoise-colored. It was formerly a deep greenish-black color,
from the presence of yellowish extremophile bacterial mats on the pool walls
and floor. Subsequent natural heating of
this feature has knocked out most of the bacterial mats. Abundant, suspended, <0.5µ-sized,
colloidal silica particles are responsible for the blue coloration of the
water.
In 1991 and 1992, Black Pool had atypical
activity, coinciding with activity at nearby Abyss Pool (see above). One explosive eruption here in August 1991
involved the forceful detachment and nearby deposition of geyserite
clasts. Boiling water domes up to 3 feet
high were present in the pool after that eruption, accompanied by significant
overflow.
Small eruptions also occurred at Black
Pool in 1992.
Black Pool in August 2011 (above & below)
Black Pool in July 2012 (above & below) - the
whitish and grayish areas are geyserite (siliceous sinter). The orangish and yellowish areas are
extremophile bacterial mats.
Black Pool (above & below)
Above: July 2012
Below: August 2011 - nicely formed, loose and
coalesced nodules of grayish-colored geyserite are present in the shallowest
portions of the Black Pool. The
brownish-black areas are bacterial mats.
Black Pool (August 2011) - this is a runoff channel
heading ~ENE from the pool toward Yellowstone Lake’s West Thumb. The yellowish, orangish, and brownish areas
are extremophile bacterial mats. The
dry, whitish-gray areas are geyserite.
Black Pool has multiple runoff channels.
King Geyser, northern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA in August 2011 (looking
~NNE).
King Geyser’s eruption activity has been
sporadic during the 20th century.
Up to 60 feet high eruptions occurred in 1905 and up to 8 feet high eruptions
occurred in the 1930s. Some post-1930s
eruptions have also occurred, including 1997.
Big Cone, northeastern Lower Group, West Thumb
Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA.
Big Cone is one of several geyserite cones
along the shoreline of Yellowstone Lake’s West Thumb that have been flooded or
partially flooded by lake water.
Geyserite is a subaerial, opalline siliceous chemical sedimentary rock,
so underwater occurrences of this rock type indicate post-formation submergence
has occurred.
Most of Big Cone’s geyserite mound is
still above water. The vent is
water-filled (see 3rd photo below), with bubbling action and slight
overflow (see wet drainage channels on the southern side of Big Cone).
Geyser eruptions here are rare, reaching
up to about 1 foot high.
Above: August 2011 (looking ~NNW)
Below: July 2012 (looking ~NE)
Big Cone (above & below) (looking ~NE).
Above: August 2011. Below: July 2012
Fishing Cone (above & below), northeastern Lower
Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA
(looking ~NE).
Fishing Cone occurs a little southeast of
Big Cone, along the shores of Yellowstone Lake’s West Thumb. Unlike Big Cone, Fishing Cone is partially to
completely submerged by lake water. It
was formerly a geyser, and its surrounding geyserite mound originally formed
above lake level. Water levels in
Yellowstone Lake have been changing due to intrusion-related uplift in the
Yellowstone Caldera. This has resulted
in the northern shores of the lake experiencing lower water levels and the
southern shores of the lake experiencing higher water levels. The lakeshore portions of West Thumb Geyser
Basin have thus been altered, and Fishing Cone no longer erupts.
The feature’s name refers to the old-time
tradition of fishing in the lake while standing upon this cone and cooking a
fish in the vent’s boiling hot water before unhooking it.
Fishing Cone had geyser eruptions in the
1910s up to 40’ feet high and in the 1920s-1930s up to 4 feet high. Small splashing has occurred in the Fishing
Cone vent in modern times.
Above: August 2011. Below: July 2012.
Fishing Cone (above & below), northeastern Lower
Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA
(looking ~NE).
Above: August 2011. Below: July 2012.
Fishing Cone (above & below), northeastern Lower
Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA
(looking ~NE).
The hole on the northwestern flanks of
Fishing Cone’s geyserite mound (see above) was a steam vent in the early
1990s. This activity was accompanied by
a lack of water in the central vent (see below). By mid-1994, the steam vent ceased activity
and the central vent filled with water.
Above: July 2012. Below: August 2011.
Vandalized Pool, eastern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA. Notice the pine needles floating on the
pool’s surface and along the pool’s edge, plus the geyserite-encrusted and
geyserite-cemented pine needles (= whitish spike-shaped structures).
Lakeshore Geyser, southeastern Lower Group, West Thumb
Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in July 2012
(looking ~NE).
Lakeshore Geyser is similar to Big Cone and
Fishing Cone in that it is located along the shores of Yellowstone Lake’s West
Thumb and is partially to completely submerged by lake water. Low lake levels are required for geyser
eruptions to occur here - the upper parts of the geyserite mound have to be
exposed.
Lakeshore Geyser eruptions have been
recorded to reach up to 30 feet high.
Occasionally during the 2000s, regular
small eruptions occurred at Lakeshore Geyser, up to 6 feet high. Other recent activity here has included
episodes of small perpetual spouter-like splashing.
Lakeside Spring, southeastern Lower Group, West Thumb
Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in August 2011
(looking ~WSW).
Lakeshore geyserite sediments (above & below), southwestern shores
of Yellowstone Lake’s West Thumb at West Thumb Geyser Basin, Yellowstone
Hotspot, northwestern Wyoming, USA.
Quartz (SiO2 - ~pure silica) is the
most common sediment type. Sediments
composed of geyserite are extremely rare. West Thumb Geyser Basin is a great example of
a locality having an abundance of lacustrine geyserite sediments - they are
easily examined along the lakeshore adjacent to tourist boardwalks. Geyserite is composed of opal (SiO2·nH2O
- hydrous silica). The lakeshore
sediments here are angular to subangular and appear to be dominated by small
pebbles, granules, and coarse sand (I don’t have a sample for study - that
requires a research permit from the park service). The 2nd photo below shows a
shoreline-parallel ridge of geyserite sediments, formed when lake levels were
higher than on the day the picture was taken.
Geyserite breccia in very shallow water, West Thumb of
Yellowstone Lake, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern
Wyoming, USA in July 2012.
This rock appears to have formed by
shallow burial and cementation of lakeshore geyserite sediments (dominated by
angular to subangular pebbles and granules).
The lithified geyserite sediments have since been eroded and reworked,
resulting in geyserite breccia blocks sitting on the shallow lake floor. This is probably the rarest variety of
breccia that I’ve ever seen.
Seismograph Pool, southern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA in July 2012 (looking
~SSW).
Seismograph Pool and Bluebell Pool, southern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA in July 2012 (looking
~SSW).
Seismograph Pool is the large, somewhat
deep pool in the center foreground.
Bluebell Pool is the smaller, bluish-colored, steaming pool at upper
left.
Thumb Paint Pots, southern Lower Group, West Thumb Geyser
Basin, Yellowstone Hotspot, northwestern Wyoming, USA.
Thumb Paint Pots consists of several
partially foam-covered, irregularly shaped pools, small mud volcanoes, small
mud geysers, and small, churning mud pools.
This area is dynamic and considerable changes have been observed through
the 20th century. In the
earlier 1900s, the Thumb Paint Pots area was fairly extensive, possessed multiple
pastel colors, and was quite active.
Along with the rest of West Thumb Geyser Basin, activity here has
lessened in modern times.
Above: August 2011. Below: July 2012.
Thumb Paint Pots (above & below), southern Lower
Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA.
Above: August 2011. Below: July 2012.
These photos show mud cones that have
formed by small-scale mud geyser eruptions.
The mud cone below has had seeping mud flows, as evidenced by the
narrow, elongated, striated flow lobes.
Mud geyser (above & below), Thumb Paint Pots,
southern Lower Group, West Thumb Geyser Basin, Yellowstone Hotspot,
northwestern Wyoming, USA in July 2012.
The mud geyser shown above and in the
three photos below had a short-duration, spurting eruption on 6 July 2012. Fresh, low-viscosity mud quickly spread out
over the surrounding dry, mudcracked surface.
Mud geyser (above & below), Thumb Paint Pots,
southern Lower Group, West Thumb Geyser Basin, Yellowstone Hotspot, northwestern
Wyoming, USA on 6 July 2012.
WTLGNN031, far-southwestern Lower Group, West Thumb
Geyser Basin, Yellowstone Hotspot, northwestern Wyoming, USA in July 2012
(looking ~WSW).
This unnamed muddy pool is located in the
parking lot for West Thumb Geyser Basin.
The presence of two surface boils indicates that at least two active
vents are present.
Info. mostly synthesized from:
Bryan, T.S. 2008. The Geysers of Yellowstone, Fourth Edition. Boulder, Colorado. University Press of Colorado. 462 pp.
Fritz, W.J. 1985. Roadside Geology of the Yellowstone Country. Missoula, Montana. Mountain Press Publishing Company. 149 pp.
Morgan, L.A. & W.C. Shanks. 2005.
Influences of rhyolitic lava flows on hydrothermal processes in
Yellowstone Lake and on the Yellowstone Plateau. pp. 31-52 in Geothermal biology and geochemistry in
Yellowstone National Park. Proceedings of the Thermal Biology Institute
Workshop, Yellowstone National Park, Wyoming, October 2003.
Morgan, L.A. & W.C. Shanks. 2010.
Post-200 ka pyroclastic eruptions of the Yellowstone Plateau. American
Geophysical Union Fall Meeting 2010 Abstracts V11D-2320.
Ohsawa et al. 2000.
Geothermal blue water colored by colloidal silica. Proceedings
of the World Geothermal Congress, 2000, Kyushu-Tohoku, Japan, May 28-June 10,
2000: 663-668.
Schreier, C. 1987. A Field Guide to Yellowstone’s Geysers, Hot
Springs and Fumaroles. Moose,
Wyoming. Homestead Publishing. 96 pp.