|
Note: most images are hyperlinked,
clicking on them will bring up a full screen version (suitable for
framing!) with an explanation and credits. |
0.0 STOP 11 (Reset Odometer) Forest Service/Mammoth
Lakes Visitor Center. View the restored raised relief map of the
Mammoth/Mono
Lakes area and note the Long Valley caldera. Members of
geology tour groups should ask the ranger on duty for information on the
Long Valley caldera. They can provide a several USGS Fact Sheets
with information on the local volcanic hazards. And remember, this
may be your last chance to purchase Smokey the Bear memorabilia!
Turn right on Highway 203.
1.3 - Bear (no pun intended) right on 203 at intersection with
Lake Mary Road.
3.2 - Turn right at Earthquake Fault Road. (Note: there used to be a
sign, but it blew down during a winter blizzard and all that remains is
the stone signpost.)
3.4 - STOP 12 "The Earthquake Fault". The
Earthquake Fault is a popular stop for many skiers on the way to Mammoth
Mountain.
 Although much has been written about this "fault" its
origin remains the subject of much debate. The fissure trends north-south like
many of the known faults in the Mammoth area. Some geologists have
suggested that it represents the southern extension of the Hartley Springs
fault. In fact, as early as 1936 it was proposed that the west side
of the fault had dropped as much as three feet. Nonetheless, no slickensides are
apparent on outcrop and the postulated offset is problematic at best. Even
the suggestion of west-side down motion is inconsistent with the typical
east-side down pattern of range front faulting. This
has led to a suggestion that the fault is merely a fissure formed
during cooling of the black, glassy, rhyolitic lava flows. This,
however, is difficult to rationalize as the Rhyodacites of Mammoth
Mountain are thought to be at least 50,000 years old and a crack of this
nature is almost certainly Holocene in age. A more intriguing
explanation is that the crack represents the head scarp of a large
landslide block or that it is a tension crack formed during warping,
similar to those those at the Big Pumice Cut. Reportedly the "fault" was reactivated during the
1980 Mammoth earthquake and the trail down into the fissure was closed.
It is now thought the apparent motion at the bottom of the fissure
resulted from settling of unconsolidated material. Some researchers
have also suggested an Indian legend about a massive earthquake that
struck the area 200 years ago causing a large rift to open is evidence
that this is an earthquake fault.
Others state that there are numerous Indian legends and it is hard
to separate fact from myth. The consensus among geologists is that
this is an earthquake fault, but we see no strong evidence to support that
view. You be the judge. Is this a true fault or merely a fissure?
Return to Highway 203 and turn left (east).
5.4 - Intersection with Lake Mary Road, turn right. (In winter the road
to Horseshoe Lake is closed, inquire at the Mammoth Ranger Station.
Sometimes it is possible to park within a mile or less of the lake and
hike in.)
10.5 - STOP 13 Horseshoe Lake Tree Kill. From Ma y
to November 1989 a swarm of small earthquakes struck the area near Mammoth
Mountain. Scientists from the USGS, collecting data from from a
seismograph array suggested that a small body of magma was rising beneath
the mountain. In 1990, U.S. Forest Service rangers noticed areas of dead
and dying trees on the mountain. Since the rangers are generally
biologists, they assumed that either the prolonged drought or a beetle
infestations was responsible. After complaints of nausea from several
field assistants and the near death of a ranger who entered a snow covered
cabin near Horseshoe Lake, the Forest Service decided they were dealing
with a more serious problem. USGS scientists were brought in, their
measurements indicated that the roots of the trees were being killed by
extremely high concentrations of CO2 gas in the soil. Surveys have
identified more than 100 acres of dead and dying trees on and near Mammoth
Mountain. Fortunately, the town of Mammoth Lakes has been spared, Recently
USGS scientists have conducted a series of CO2 flow measurements. CO2
flow is measured in units of grams/day/square meter.
The background rate in this area is approximately
25 g/d/m2. In the CO2 flow map made at the
Horseshoe Lake tree kill in January, 1999 (when the lake was frozen),
the area of dead trees is outlined in red. The pattern shows several small
areas of very high flow (red patches) surrounded by a broader region of
lower gas flow rates. Note the arm of high flow extending into the lake. The
maximum measured CO2 flux at the time of this survey was in excess off
7000 g/d.
 Although
plants produce oxygen from carbon dioxide during photosynthesis, their
roots need to absorb oxygen directly. Trees are dying because the high
carbon dioxide concentrations are denying this oxygen to the root systems.
In the areas of tree kill, carbon dioxide makes up 20 to 95% of the gas
content of the soil (Sorey, et. al., 1996) (<1% is normal). Fortunately,
when carbon dioxide leaves the soil, it mixes with the air and rapidly
dissipates. However, carbon dioxide is heavier than air and can collect at
high concentrations in depressions and enclosures.
Poorly ventilated areas above and below ground can be dangerous in areas
of CO2 seepage. Where thick snow packs accumulate in winter, the CO2 can
be trapped within and beneath the snow. Breathing air with concentrations
of carbon dioxide above 30% quickly causes unconsciousness and death.
Concentrations in utility vaults have been measured at 85% and snow caves
at 30%. Snow camping is not advised here Horseshoe Lake!
A preliminary estimate of the current rate of CO2 gas emission at
Mammoth Mountain is 1,300 tons per day (Sorey, et. al., 1996). Similar rates of emission have
been measured from craters at Mt. St. Helens (Washington) and Kilauea
(Hawaii) during periods of low-level eruptive activity.
Measurements of concentrations of sulfur dioxide, another common
constituent of volcanic gasses, have not revealed any similar anomalies.
Past
eruptions at Mammoth Mountain, such as the phreatic (steam-blast)
eruptions that occurred about 600 years ago on the volcano's north flank,
may have been accompanied by CO2 emissions.
Based on age dates for the oldest trees in the active tree-kill areas, scientists
think that the current episode of high CO2 emission is the first
large-scale release of the gas on the mountain for at least 250 years.
Recent measurements of helium isotopic ratios in carbon dioxide gas
emissions from a fum arole on the flank of Mammoth Mountain by a team of
USGS scientists has demonstrated conclusively that a
magmatic source is responsible for the CO2 gas. They suggest that large amounts of gas have probably been
trapped under Mammoth Mountain for decades/centuries. A 1989 earthquake
caused a fissure to be reactivated. This allowed the magma to breach an
impermeable cap and the gas to rise upward with subsequent degassing of
the magma chamber. Note: during our 2003 visit to Horseshoe Lake it
had drained. A fissure, thought to be the source of the previously
high CO2 concentrations measured in lake water was clearly visible.
Turn around and follow Lake Mary Road back to Highway 203 in Mammoth Lakes.
15.5 - Intersection with Highway 203. Go straight through the
intersection and proceed east on 203 to a pullout just east of the US 395
onramp.
19.5 - STOP 14 Mammoth Mountain Vista/Casa
Diablo.
To the east of your present location you can see (and hear) the Casa
Diablo Geothermal Power Plant built in 1984.
Casa Diablo was once a favorite "tourist" stop for prospectors and
settlers who would "bathe" in the bubbling mud pots and hot springs. In
1937 and 1959 geysers spouted water 60 to 80 feet in the air. However,
with the onset of geothermal drilling in the early 1960's most of the hot
spring activity has ceased. A series of wells to depths
of 400 to 1000 feet tap a geothermal field at temperatures of 340°F
If you look just to the north you will see the well heads for two of those
wells. You can walk over to look at them if you don't mind picking fox
tails out of your socks. Note the smell of SO2 gas and the heat coming
from the active fum aroles. On the hillside to the east of the plant you
can see altered, opalized basalt from three fumarolic vents that appear
largely inactive (2003). The plant
pumps hot water from the wells through heat exchangers and converts it to electricity. The facility
produces 8,000 kilowatts of electricity, an amount that meets about
one-fourth to one-third of Mammoth Lakes' demand.
 At
this stop we get an excellent view of Mammoth Mountain to our west.
Mammoth Mountain is one of the "rim domes" that marks the western margin
of the Long Valley caldera. (We will get a better view of the entire
caldera later today at the summit of Lookout Mountain). This is perhaps, a
good place to talk about the geologic setting of the Long Valley caldera.
The oldest volcanic rocks clearly associated with the Long Valley
caldera are the Glass Mountain rhyolite
flows. They were extruded along a ring fracture system bordering the northeast margin of the
caldera. Extrusion began about 2.1 ma and continued for
approximately 1.3 million years.
The next major event in the formation of the caldera was the extrusion
of the Bishop Tuff 760,000 years ago. About 150 cubic miles of ash flows
were emplaced over a span of hours or days, making it one of the most
catastrophic events in recent geologic history. Eruption of the Bishop
Tuff partially emptied the magma chamber and its roof collapsed forming
the present caldera, an elliptical depression measuring 10 miles
north-south by 20 miles east-west. Caldera subsidence totaled 2 miles of
which 3500 feet is reflected in the present topographic relief, the
remainder is buried by post-caldera basin fill.
 Following caldera subsidence, the central part of the basin underwent
resurgent doming. Rhyolite and rhyodacite was emplaced from at least 12
vents during an interval of 100,000+ years, beginning 680,000 years ago.
Evidence suggests that rhyolite extrusion and doming had ceased by 510,000
years B.P. producing a 'resurgent caldera' surrounded by a 'moat'.
The next phase of volcanism involved the emplacement of moat rhyolites
in three separate events; 500,000, 300,000 and 100,000 years B.P. The moat
rhyolites, emplaced at the periphery of the central dome, are related to
ring fractures around the resurgent dome. A later stage of volcanism
produced rim rhyodacites from at least ten vents in the western part of
the caldera. The main mass of these hornblende-biotite rhyodacite flows is
Mammoth Mountain. The Mammoth flows range in age from 180,000 to 50,000
years. Overlapping the final stage moat rhyolites, basalt and andesite were extruded in the west
moat of the Long Valley caldera. The ages for these flows range from
220,000 to 60,000 years B .P.
The most recent eruptive activity in the caldera has been Holocene rhyolite
volcanism that formed the Inyo Craters and domes in the northwest quadrant
of the Long Valley. These volcanic features appear to be aligned on a
north-trending fissure extending from the Long Valley caldera to Mono
Craters. Activity within this zone has occurred sporadically for at least
the last 40,000 years; the last eruption about 200 years ago.
The past two decades have seen a dramatic increase in seismic activity
within the Long Valley caldera. In May, 1980 an earthquake with a
magnitude 6.0 struck the southern rim of the caldera, followed by three others within 48 hours, and thousands
of smaller quakes over the next two decades. A large cluster of epicenters lie
near the junction of US 395 and Highway 203, our present location. Tiltmeter measurements by the USGS
indicate two feet of ground uplift has
occurred in this portion of the caldera since 1980. Initially the
culprit in these seismic events was thought to be range front faulting.
Early seismic data established a north-south lineation to earthquake
epicenters. However, as more data has been collected the pattern has
become more diffuse (note the almost elliptical shape to the cluster of
epicenters along the south rim of the caldera).
Modeling of
the seismic data by USGS seismologists led to the cross section shown
below.
It is now thought that the recent increase in seismic activity was
triggered by a small apophyse of magma that may have risen to depths as
shallow as 1 mile below the southwest rim of the caldera. The rise
of this small body, caused readjustments in the upper crust and sequent
movement along existing faults. The recorded seismicity would appear
to be a function of both fault movement and magma movement. In the
past few years levels of seismicity have dropped causing some scientists
to believe the
magma body has become buoyantly stable. Earthquakes will continue as
stress is relieved along range front faults, but magma movement will be
limited unless a tectonically-induced earthquake opens a fissure into the
existing magma chamber.
Turn around and head west on 203.
|
