1872 congress set aside land to create our first national
park. The land making up this national park is distributed
across three states with 96% in Wyoming, 3% in Montana, and 1%
Yellowstone is a Hot Spot, a volcano produced by a plume
of magma that “punches” through
the Earths’ crust. The current Yellowstone Caldera
was created by a massive eruption 640,000 years ago.
Major eruptions occur at 600,000 to 900,000 year intervals.
National Park is well known for its wildlife and geothermal
features. Today, the forests of Yellowstone help to define
a Subalpine Ecosystem. Lodgepole Pine makes up 80% of
the forested areas.
Engelmann Spruce, Douglas-fir, Subalpine Fir, Whitebark
Pine, and Quaking Aspen are also common. Petrified
wood deposits found in Yellowstone reveal a past environment
and ecosystem very different from the one we enjoy today.
Yellowstone remnants of 27 fossil forests are embedded
within 366m of sediment (Kenrick & Davis, 2004,
p. 61). Clusters of fossilized trees, exposed at Specimen
Ridge, represent a succession of over a dozen of these
forest remnants (Yuretich, 1984, p. 159). Many trees
are preserved in their original growth position (Fritz,
1980, p. 313, Retallack, 1981, p. 52, Yuretich,
1984, p. 161). The hike to Specimen Ridge begins at
a pullout on the south side of the road 5.3 miles east
of Tower Junction in the Lamar Valley. The 1.5-mile
hike to the trees has a gain of 1,200 feet. The round
trip is 3 miles and takes 2 to 3 hours. The hike is
fossil plant deposits were formed during the Eocene
by periodic volcanic activity some 48 million years
ago as indicated by radiometric dating and biostratigraphic
correlation (Fritz, 1984, p. 638). During the Eocene
two northwest trending subparallel volcanic chains
formed the Western and
Eastern Absaroka Belts. The Absaroka eruptive centers
were 25 to 60 km apart with a narrow intermontane
basin in-between them. The Larmar River Formation
petrified wood deposits are found between these two
volcanic chains (Fritz, 1980, p. 312).
volcanic eruptions mud flows and braided streams originating
on surrounding stratovolcanic peaks of the Absaroka
Volcanic Supergroup transported plant parts from higher
to lower elevations as well as buried plants in place.
Transported trees were stripped of bark, branches,
and roots (Fritz, 1980, p. 312). Fossil trees preserved
in situ have bark and intact root systems penetrating
the substrate (Fritz, 1981, p. 54).
at specimen ridge are rooted in fine-grained tuffaceous sandstone
that represents immature soils. Conglomerates that overlie
these root-zone sandstones formed from volcanic sediments that
flowed around and buried the trees where they grew (Yuretich,
1984, p. 161). The burial of these plants by intermittent volcanic
sediment flows occurred in localized areas (Fritz, 1980, p.
312, 1984, p. 638). During quiescent times, new soil layers
formed and a new forest would grow. Growth rings suggest that
some of the forests grew for 500 years.
tree composition of these ancient forests was typical of
warm temperate to subtropical floras. This contrasts with
Subalpine Ecosystem. More than 80 kinds of trees, shrubs,
and herbs are known. Redwoods, maples, oaks, chestnuts,
walnuts, persimmons, dogwoods, laurels, and bays are some
of the more common trees. The flora also contained some exotic
trees whose relatives are now found in East Asia. Fossil
woods reported from Specimen Ridge include: pine, fir, redwood,
cypress, oak, beech, sycamore, willow, and laurels (Beyer,
1954, p. 567). You can take a virtual
hike up Specimen Ridge
by clicking on the picture to the right.
Trees buried in Quaternary fluvial volcaniclastic sediments
and lahars at Mount St. Helens in Washington serve as a
modern analogue for the Eocene aged volcanic deposits found
in Yellowstone. Karowe and Jefferson (1987) examined trees
buried in lahars and fluvial sediments by the 1980 eruption
of Mount St. Helens. Trees buried in lahars or mudflows
dated at 1885, A.D. 1450-1550 and 36,000 years B.P. were
also investigated. Characteristics of trees buried in
situ or in place were compared with those buried during transport
transported by fluvial volcaniclastic sediments at
Mount St. Helens
are poorly preserved and damaged by
abrasion. Most trees transported by fluvial sediments were
in a horizontal or inclined position, with many forming
log jams. Although rare, trees transported upright were
found to have a height no greater than 2 m with broad root
mats measuring 1.5 m wide. These dimensions made them stable
for being transported upright. The roots of trees transported
upright are encased within mudflow matrix, thus they are
not in a layer representing soil (Karowe & Jefferson,
1987, p. 197).
buried in mudflows were found to be well preserved.
encased both transported trees as well as trees
buried in place. The 1980 eruption produced mudflows that
exposed trees buried by an 1885 lahar. The exhumed trees
measured up to 7 m tall and 1 m in diameter. The tops of
these trees are near the surface level of the 1885 mudflow,
exhibit poor preservation and are covered with root systems
from plants that grew after the eruption. Root systems
of the upright trees penetrate a finer-grained matrix below
the mudflow deposit that is laterally continuous, representing
a soil-like layer and indicating in situ burial. Horizontal
trees uprooted or snapped off and subsequently transported
by the 1980 mudflow were deposited alongside the 1885 upright
trees. At another site, root systems embedded in undisturbed
layers provided evidence for trees buried in situ during
a 1482 eruption. Trees buried in a mudflow dated at 36,000
years B.P. were found to have excellent preservation, showing
that wood can be buried in lahar deposits for thousands
of years without substantial decay (Karowe & Jefferson,
1987, pp. 192-196).
buried in situ at Mount St. Helens share similar characteristics
with fossil trees exposed at Specimen Ridge.
Upright trees at Specimen Ridge are tall, with their sheared
off tops coinciding with the upper surface of the mudflow
units in which they were encased. The upper portions of
the trees were poorly preserved. The root systems of the
fossil trees penetrate a finer-grained matrix that is laterally
continuous for many meters and positioned below the volcanic
sediments (Karowe & Jefferson, 1987, pp. 201 & 202).
The fine-grained matrix in which the roots are found is
interpreted as a palaeosol. Thus, evidence at Mount St.
Helens and at Specimen Ridge supports an in situ burial
model for the upright trees of Yellowstone.
The cyclic nature of mudflow deposits through time at
Mount St. Helens illustrates how successive lahars
can bury and preserve portions of forests representing
different ages. Species of fossil wood that, today, exhibit
climatic tolerances are found at Yellowstone. Climatic
changes occurring between mudflows of different ages
could help to explain how tree species exhibiting different
climatic tolerances can be found in the same location.
trees buried in these lahar deposits eventually become
Karowe & Jefferson (1987) examined the wood
found at Mount St. Helens for the initial stages of silicification.
Wood buried in the 1980 eruptions showed no significant
silica deposition. Wood buried in 1885 and A.D. 1450-1550
exhibited silica deposition on cell walls along with some
decomposition. In wood dated at 36,000 years B.P. silica
had penetrated cell walls. Break down of cell walls had
also occurred. Karowe and Jefferson (1987) concluded that
the increase in silica deposition as well as decay associated
with trees buried in mudflows at Mount St. Helens supported
the currently accepted theory of silicification (p. 203). The
apparent increase in silica deposition over time was an
exciting find. However, researchers at the University of
Bonn were unable to reproduce the results of Karowe and
Jefferson even when using specimens prepared from the same
trees (Hellawell, Gee, Ballhaus, Clynne, and Sander (2011).
Perhaps Karowe and Jefferson were looking at an instrumental
Scientists at the University of Bonn are currently
working on a paper that will explore the results of their
study in more depth.
is clear that many petrified wood deposits, such
as those found in Yellowstone, are associated
with ancient volcanic mudflows. George Mustoe from Western
Washington University has examined many Holocene wood
specimens from mudflows and has found no evidence
(Mustoe, written personal communication, 2012). Read
our article Petrified
Wood: The Silicification of Wood by Permineralization to
explore possible pathways and the time needed to
form petrified wood. Click
Ridge for a printable version of this
R. & Anderson, L. (2000). A Ranger's Guide to Yellowstone
Day Hikes [Updated Edition]. Canada: Farcountry Press.
A.F. (1954). Some Petrified Wood from the Specimen Ridge
Area of Yellowstone National Park. American Midland Naturalist,
Vol 51, No 2. pp 553-576.
J., Gee, C. T., Ballhaus, C., Clynne, M. A. and Sander,
P. M. (2011). Silicification of wood: identifying ancient
and present-day processes. Abstract for poster presentation,
The Palaeontological Association 55th Annual Meeting, Plymouth
W.J. (1980). Reinterpretation of the depositional environment
of the Yellowstone “fossil forests”: Geology,
v. 8, p. 309-313.
Fritz, W.J. (1981).
Reply to Comment on “Reinterpretation of the
depositional environment of the Yellowstone “fossil
forests”: Geology, v. 9, p. 53-54.
Fritz, W.J. (1984).
Comment and Reply on “Yellowstone fossil forests:
New evidence for burial in place”: Geology,
v. 12, p. 638-639).
A.L. & Jefferson, T.H. (1987). Burial of Trees
by Eruptions of Mount St. Helens, Washington: Implications
for the Interpretation of Fossil Forests. Geological
Magazine, vol 124, no. 3, pp. 191-302.
Kenrick, P. and
Davis, P. (2004). Fossil Plants. Smithsonian
F.H. (1899) The Fossil Forest of the Yellowstone National
Park. USGS Monograph 32, pp. 651-791.
or see online:
Mustoe, G. E., (2012) Written Personal Communication.
(1981). Comment and Reply on “Reinterpretation
of the depositional environment of Yellowstone fossil
forests”: Geology, v. 9, p. 52-53.
(1984). Yellowstone fossil forests: New evidence for
burial in place: Geology, v. 12, p. 159-162.
R.F. (1984). Reply to Comment on “Yellowstone
fossil forests: New evidence for burial in place”: Geology,
v. 12, p. 638-639).