Cenozoic/Mesozoic Volcanism of the Eastern Sierra Nevada
Version 2007

David R. Jessey
Cal Poly - Pomona

Note from the Author: This site was created for educational purposes. The images and many of the illustrations are the property of the author.  Those that are not, have been cited.  I receive many requests to utilize images and content from this web site. Unfortunately, time will not allow a response to all of those requests. As such, feel free to utilize the content from this site in any manner you wish (please acknowledge the source when possible).

Also note that some changes have been made in the guide. The location maps have been removed in favor of more accurate GPS Lat/Long locations. Stop numbers have been removed and replaced with place names. This was done to accommodate the ever-expanding nature of this guide without the necessity or renumbering stops. Finally, the table format was abandoned in favor of a "normal" page layout since IE 7.0 now formats print output for most printers with something that is a reasonable facsimile of what you see on screen. For those of you still using IE 6 or some other browser, I have created .pdf files for Day 1, Day2 and References. Click on the link below to retrieve those files. Be aware, however, these pdf files will no longer be updated and they may not represent the most recent iteration of this field guide.

Note: most images are hyperlinked, clicking on them will bring up a full screen version (suitable for framing!) with an explanation and credits.

Day 1 PDF

Day 2 PDF



0.0 STOP - Forest Service - Mammoth Lakes Visitor Center (3738'49"N, 11857'35"). (Reset Odometer) View the restored raised relief map of the Mammoth/Mono Lakes area and note the Long Valley caldera. Members of geology tour groups should inquire about information on the Long Valley caldera. The staff can provide 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 (1.3) - Bear (no pun intended) right on 203 at intersection with Lake Mary Road.

3.2 (1.9) - Turn right at the Earthquake Fault Road.

3.4 (0.2) STOP - "The Earthquake Fault" (3739'10"N, 11900'00"). 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 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).

5.5 (2.1) - 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.)

10.6 (5.1) STOP - Horseshoe Lake Tree Kill (3736'49"N, 11901'15"). From May to November 1989 a swarm of small earthquakes struck the area near Mammoth Mountain. Scientists from the USGS, collecting data 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 largely 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! Interestingly, in 2005 the fault that cuts diagonally through Horseshoe Lake appears to have been reactivated causing the lake to drain as if someone removed the stopper from a bathtub. As of 2007 the lake had begun to refill, but its level is remains well below normal.

A preliminary estimate of the 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 fumarole 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.

15.6 (10.1) - Intersection with Highway 203. Go straight through the intersection and proceed east on 203 to a pullout just east of the US 395 onramp.

19.6 (4.0) STOP - Mammoth Mountain Vista/Casa Diablo (3738'33"N, 11854'44"). 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 into 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 340F. If you look just to the north you will see the well heads for two of those wells. Note the smell of SO2 gas and the heat coming from the active fumaroles. On the hillside to the east of the plant you can see altered, opalized basalt from three fumarolic vents that appear largely inactive. 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. This is perhaps, a good place to talk about the geologic setting of the Long Valley caldera.

The oldest volcanic rocks 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 18 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 were 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 lies 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 for 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 re-adjustments in the upper crust and subsequent 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 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.

19.8 (0.2) - (Turn left, southbound on US 395.

22.7 (2.9) - Turn left at Airport Road.

23.2 (0.5) - Turn right on Fish Hatchery Road (aka Hot Creek Road).

23.6 (0.4) - Turn left into Fish Hatchery Complex.

23.7 (0.1) STOP - Mammoth Fish Hatchery (3738'22.5"N, 11851'21"W). While not the most geologic of stops, my son really liked it! Near the hatchery, geothermal springs mix with the water from Mammoth Creek. The result is a series of ponds that maintain a year-round temperature of 52 to 62F providing an ideal environment for incubating trout eggs and raising fish. The hatchery, run by California Department of Fish and Game has been in operation since the early 1930s. You may feed the fish if you wish, (it costs a quarter) but stick your hand in the water at your own risk!

Return to Fish Hatchery Road, turn left.

24.4 (0.7) - Driving over 1980 Fault Scarp. This linear, low scarp is one of several surface ruptures that occurred as a result of the 1980 earthquake swarm, this one possibly on a splay of the Hilton Creek fault. Extension cracks appeared on Hot Creek Road trending in a northwest-southeast direction at the base of the southwest-facing fault scarp. Extension varied from 0.5 to 3 inches with individual cracks 15 to 20 feet in length. Vertical displacement ranged from 6 inches to about one foot. Field mapping and fault plane solutions suggest extension, but the actual displacement mechanism is complex, possibly involving magma movement. (See previous discussion at McGee Creek Fault Scarp)

26.5 (2.1) STOP - Hot Creek &(3739'38"N, 11849'40"W). Turn left into the parking area. From the interpretive overlook note the steam rising from fumaroles and hot springs along the creek. There are also hot springs discharging directly into Hot Creek near the remains of the bridge that formerly spanned the creek. The mingling of hot spring water with snow-melt fed stream water produces extreme temperature gradients in the creek. The wide range of temperatures has made this area popular for swimming year-round so the Forest Service constructed change rooms for visitors. Unfortunately in 2006 Hot Creek experienced an increase in geothermal activity. In the name of public safety the Forest Service fenced off the creek to prohibit swimming and bathing. As of 2007 it remained closed to public access.

Note the altered rhyolite in the gorge. Hydrothermal activity has kaolinized and opalized the rock producing the white, bleached appearance (we will see a better example at the next stop). The rhyolite has been dated 300,00 years. The northeast trend of Hot Creek is consistent with that of the Hilton Creek fault, so many geologists have theorized the main fault or a branch are the conduit for hydrothermal waters. This hypothesis became more difficult to defend when recent strong earthquakes on the Hilton Creek fault (1998) had little impact on the thermal regime of Hot Creek. As you walk down the path to the creek also note the hummocky terrain on the northeast side of Hot Creek, reminiscent of the Owens River gorge.

Many of the current hot springs appeared suddenly on the evening of August 25, 1973. At least five hot springs formed, with the two largest starting as geysers that spouted water 10 feet into the air. Within weeks geyser activity had ceased, but the hot springs remain today. The origin of the new hot springs remains unclear, but it has been noted that they appeared within hours of a relatively small (M=3.5) earthquake 25 miles southeast of Hot Creek. Presumably, seismic activity altered the subsurface plumbing system giving rise to the springs. Prior to the small earthquake, heated water was trapped below an impermeable horizon. The seismic event breached the impermeable strata and superheated water and steam rose rapidly initiating geysers at the surface. After the initial pulse of superheated water reached the surface, the heat flux decreased and the geysers became hot springs.

Turn left onto Hot Creek Road from the parking area.

27.6 (1.1) - Bear left at the intersection with Whitmore Tubs Road.

28.2 (0.6) - Turn left on Owen's River Road.

28.9 (0.7) - Turn left on Antelope Springs Road (3S05)

32.3 (3.4) - Five-way intersection, turn right onto access road for the kaolinite mine you just passed.

32.5 (0.2) STOP - Huntley Kaolinite Pit (3741'18.5"N, 11852'03"W). The Huntley Kaolinite Mine is an active mine currently owned by Standard Industrial Minerals of Reno, Nevada. The mine does not operate on weekends, but mining equipment is parked at the site. Due to vandalism that occurred in 2005-2006 the mine property is now posted with No Trespassing signs. However, the tailing piles can be accessed from Antelope Springs Road without entering the mine site. The kaolinite has been formed by hydrothermal alteration of Pleistocene lakebed sediments and the underlying rhyolite. The rhyolite has been dated at 300,000 years making it correlative with that at Hot Creek. The alteration appears to be controlled by a north trending fault system that created a graben. Kaolinization is best developed along the east side of the graben. Relict bedding from sedimentary layers and flow banding of the rhyolite can be seen in large boulders from the stockpiles and on outcrop in the pit. Opal veins are common, while alunite is rare. The mine is also listed as a pyrophyllite producer. The genesis of the kaolinite and alunite will be discussed at the next stop, but both are common alteration products of acid-sulfate geothermal systems.

The Huntley Mine has been operating since 1952. Standard Minerals has a patent on 180 acres of land. The kaolinite is trucked from the mine site to a processing plant near Bishop, where it is crushed and bagged for shipment. Kaolinite has been used as a filler in paint and paper as well as a whitening agent in cosmetics, ceramics and portland cement.

Turn around and return to five-way intersection.

32.7 (0.2) - Five-way intersection. Go straight through the intersection and continue southeast on Forest Service Road 3S07.

34.0 (1.3) - Turn left on unimproved dirt road (look for the rusted water storage tank.

34.1 (0.1) STOP - Blue Chert Mine (3740'18.25"N, 11851'22.75"W). The term mine is something of a misnomer for this outcrop. This small hill appears to be the top of a fossil hot springs system. Numerous other small knolls occur throughout this area. (Remember those we could see looking to the north from Hot Creek). Each hill represents the locus of a hot springs or fumarole. As hot water rises in the vent conduit and cools it deposits microcrystalline silica along the walls of the fissure. This often seals the vent system which remains dormant for decades until seismic activity reopens the fissures. When that happens, the trapped geothermal waters rise rapidly to the surface and flash to steam. The explosiveness of the erupting geyser often overcomes the tensile strength of the rock shattering it and creating a breccia pipe. The rapid temperature drop deposits a silica sinter blanket around the vent opening.

The chert from this locality has been dated at approximately 275,000 years B.P. This is consistent with ages for the rhyolite at the Huntley Kaolinite Mine and that at Hot Creek. Presumably, the chert formed during a major period of hot springs activity that developed following volcanism associated with western "moat" rhyolite emplacement. One intriguing question is the source of the blue color. It appears restricted to this hill, and has not been explained adequately. The source of all color in cherts, according to many websites, is impurities incorporated during chemical precipitation. The most common impurity is amorphous iron oxide resulting in the red variety of chert know as jasper. The nature of the impurities in other colors of chert is uncertain and the subject of ongoing research.

Vista Gold Inc. of Littleton, Colorado holds a lease on the Blue Chert property. Through a purchase agreement with Standard Minerals, owner of the Huntley Kaolinite Mine, they currently control approximately 1800 acres. An extensive drilling project was completed in 1998 which outlined 68,000,000 tons of ore grading 0.018 oz/ton gold. The company green-lighted property development in 1998, but environmental opposition has delayed plans. During a visit in 2007 no evidence of recent activity was noted.

Vista (2008) reports that gold deposit is underlain by rock units related to the caldera formation and subsequent resurgence. Lithologies include volcaniclastic siltstones and sandstones deposited in a lacustrine setting within the caldera, debris-flows with local intercalated silica sinter and rhyolite flows and dikes. All lithologies have been altered and/or mineralized to variable degrees. The rhyolites have been dated at 200,000 to 300,000 years. The north-south trending Hilton Creek fault defines the eastern limit of the resurgent dome within the central part of the Long Valley Caldera and extends outside the caldera to the south. Vista believes this fault system controls the distribution of gold mineralization at the Blue Chert deposit. Fault activity is episodic and together with active hot springs, earthquakes, and very recent volcanism suggests that the area is still geologically active.

The epithermal (hot springs) gold and silver mineralization falls within the low sulfidation (quartz-adularia) type deposit. Several areas, termed the North, Central, South, Southeast, and Hilton Creek zones, are mineralized with low grades of gold and silver along north-south striking zones up to 8,000 ft in length with widths ranging from 500 ft to 1,500 ft. The tabular bodies are generally flat-lying or have shallow easterly dips. Mineralization is typically from 50 to 200 ft thick and exposed at (or very near) the surface. Mineralized zones are correlated with zones of intense argillic alteration or and/or silicification. The predominant clay mineral is kaolinite, while the silicification varies from chalcedony to amethystine quartz or opal. Multiple periods of brecciation and silification are evidenced by cross-cutting veinlets and silicified breccia zones.

Mineralization consists of poorly crystalline pyrite often with framboidal texture and lesser euhedral pyrite. Investigations have shown the occurrence of submicroscopic gold with the framboidal varieties of pyrite. This would explain the general lack of visible gold even when using an electron microprobe. Where gold grains have been observed, the grains are small, (1 to 6 microns), and have low amounts of contained silver. A significant portion of the gold resource is present in material which has been at least partly oxidized. The pyrite is altered to iron oxides (goethite) releasing the gold as submicroscopic grains.

The association of gold with fossil hot springs systems has been recognized for over half a century.  What was missing was a model to explain this association.  To the neophyte this may not seem difficult, but remember gold is one of the most insoluble and inert of commodities. To get enough gold into solution to make an ore deposit and then concentrate it is a perplexing problem. In the 1970s academics came up with an explanation for the solubility problem, complexes. It seems that complexes increase the solubility of gold dramatically, cyanide is the best known example. Of course natural cyanide solutions are unknown. However, bisulfide (HS-) can also increase the solubility of gold significantly and this is a common constituent of reducing environments. So we now had the mechanism to dissolve gold, all that was missing was a model for deposition. That was provided in the early 1980s by exploration geologists. This model, called the acid-sulfate gold model, was based largely on research conducted in the Nevada gold belt. The illustration to the right is from a model proposed for the Round Valley Mine.

This model envisions the gold being carried as a bisulfide complex. Bisulfide is only stable under reducing conditions.  As the hydrothermal solutions rise along fracture systems they are oxidized and the bisulfide becomes sulfate (SO4).  The gold can no longer remain in solution since the complex has broken down and it is deposited. The sulfate combines with the free hydrogen released when HS- breaks down generating H2SO4 (sulfuric acid) a powerful acid. This explains the fact that many hot springs are highly acidic. The newly formed acidic water cools as it flows laterally, and convectively percolates downward into the subsurface. The powerful acid leaches the rock leaving only the most insoluble of compounds, kaolinite and the characteristic alunite. The water heats as it circulates downward and is reduced. It recharges the aquifer system and is recycled upward through the breccia pipe conduit. So the characteristic elements of this model are a fossil hot springs system i.e., chert/silica sinter, brecciation, argillic alteration (kaolinite), and alunite. It seems the Blue Chert property was a potential "gold mine" waiting to be found by anyone working with this model. Remember the Blue Chert Mine is a fossil hot springs system. Think about what might be happening in the subsurface along Hot Creek today?

Return to Antelope Spring Road (3S07).

35.5 (1.4) - Turn left on Antelope Springs Road.

41.0 (5.5) - Turn southeast (left) at the dead-end; continue on the paved road running parallel to Highway 395.

41.7 (0.7) - Turn right on Substation Road (Highway 203).

41.8 (0.1) - Turn right onto the northbound onramp for U.S. 395.

45.2 (3.5) - Smokey Bear Flat on right. Smokey Bear Flat was once the estate and residence of Smokey the Bear. Smokey, celebrity spokesperson for U.S. Forest Service, amassed a sizeable personal fortune from commercial endorsements and the sale of "Smokey the Bear" memorabilia. He acquired this large parcel of land and built a 27-room, 30,000 square foot palatial mansion. Life was good and Smokey became a legend among bears. Alas, his good fortune was not to last. Smokey was advancing in years and the Forest Service made a decision that a younger, more vigorous bear was needed as spokesperson. Smokey was let go. A bitter Smokey sued the Forest Service claiming creative control of the Smokey Bear likeness and "Only You Can Prevent Forest Fires" logo. Sadly, the courts ruled in favor of the Forest Service. Smokey grew increasingly despondent. Mammoth residents report that in his last days Smokey was often seen wandering aimlessly about the town muttering things like, "Only You Can Prevent Forest Gump". Faced with dwindling financial resources and declining health Smokey took his own life. His heirs squandered what remained of the Smokey fortune and in a cruel irony, a forest fire destroyed his mansion. All that survives of this tragic chapter in American history is Smokey Bear Flat.

47.1 (1.9) - Turn right on gravel road opposite the Mammoth Scenic Loop Road.

48.1 (0.9) - Bear right and follow the signs to the summit of Lookout Mountain.

50.2 (2.2) STOP - Lookout Mountain (3743'45"N, 11856'51"W). From the summit of Lookout Mountain (elevation 8352 feet) we have an eagle's eye view of the Long Valley caldera. Since we are all geologists, let's get out the Brunton Compass and orient ourselves. We will give all azimuths with respect to True North so make sure you have the proper declination set! Bald Mountain (most likely hidden by the trees) lies 4.5 miles away at 35. The summit of Glass Mountain is 13 miles away at 75. Note the fire tower on the north end of the ridge. In the distance, the White Mountains from 95 to 115 are 30 miles away. Crowley Lake at 120 lies 14 miles distant. Along the Sierra Crest flat toped McGee Mountain (13 miles) is at 145; Laurel Mountain is 11 miles away at 165; and Bloody Mountain 12 miles distant at 170. Mammoth Mountain is situated 8 miles to the southwest at 215. Deer Mountain (4 miles), one of the Inyo Craters, is at 240 , and the Minarets are on the skyline at 250 (13 miles). North of Deer Mountain, also at 250 is an unforested dome of Holocene rhyolite along the south side of Deadman's Creek. The prominent nonforested dome at 310 is Wilson Butte, six miles distant. Between Wilson Butte and Deer Mountain are low, nonforested Holocene rhyolite domes north and south of Glass Creek. The flow north of Glass Creek is the source for Obsidian Dome. To put these landmarks in perspective, most lie along the edge of the Long Valley caldera. The caldera measures roughly 18 miles in diameter along the east-west axis and 10 miles in diameter along the north-south axis. Hopefully, this gives you a feel for the immense size of this caldera. For the sake of further orientation, Yosemite Village lies 36 miles due west of our present location.

Lookout Mountain itself is a rhyolite, obsidian plug with a crater about 0.6 miles in diameter. We are on the rim of the crater, which lies to the east and is difficult to see since it is heavily forested. Obsidian from Lookout Mountain has been age dated at 690,000 years, suggesting it is a resurgent dome formed after the massive Bishop Tuff extrusion caused caldera subsidence. You will note numerous small fragments of obsidian lying on the ground (no this is NOT broken glass from partiers as one student thought). The source of this obsidian is uncertain, some feel it is from the vent near the summit of Lookout Mountain while others suggest it may be tephra from a more distant explosive eruption.

Return to the intersection with US 395.

53.1 (3.1) - Go straight through intersection and take the Mammoth Scenic Road west.

56.3 (3.2) - Turn right on unimproved road leading to the Inyo Craters. Follow the signs Carefully!

57.5 (1.2) STOP - Inyo Craters (3741'20"N, 11900'20"W).. There are three large craters comprising the Inyo Craters; the two lake-filled craters (North and South Inyo Craters) and another at the summit of Deer Mountain (Summit Crater), the peak to the north. The Inyo Craters chain also includes nine much smaller craters and at least five additional volcanic domes. The craters and domes of the Inyo Craters extend northward from Long Valley to the south end of the Mono Craters The linear distribution of the Inyo craters and domes has been attributed to a north-striking fault system along the eastern front of the Sierra Nevada range. The longest segment of this fault system, the Hartley Springs fault, cuts the southeast rim of the southernmost Inyo Crater raising the west part of the crater rim 20 feet. The fault offsets explosion debris from the crater and thus postdates crater formation. To examine the feeder system for the craters an angled drill hole was bored beneath the southern carter. It intersected three breccia zones 2000 feet beneath the crater. One of the zones was interpreted as the main feeder dike, but there was no definitive evidence for fault control.

The two craters with lakes (North and South Inyo Craters) are over 600 feet in diameter. The northern crater is about 150 feet deep and the southern one over 200 feet deep. The craters originated as phreatic explosion pits, rather than typical pyroclastic eruptions. The formation of the craters was probably triggered when circulating groundwater encountered heat from an underlying magma body. When hydrostatic pressure exceeded lithostatic pressure the superheated water vaporized expanding explosively. The explosion blasted the overlying material upward and outward blanketing the area around the craters with a debris layer up to 50 feet thick.

The southern crater is the best vantage point to observe the local stratigraphy, talus in the northern crater obscures much of the crater walls. The lowermost unit is a massive flow of dark gray andesite. Although age dates are unavailable, the andesite is believed to be late Pleistocene in age and to postdate the dacites of Mammoth Mountain. The strata overlying the andesite are best exposed in the north crater wall. They consist of a zone of tephra layers about 30 feet thick. Two distinct tephra types occur within the zone; a lower, fine-grained, reddish ash and lapilli deposit and an upper, light-colored ash and lapilli layer about 8 to 10 feet thick. On top of the upper layer is a light gray to white pumice-lapilli layer four to six feet thick that is widespread throughout the region. This layer has been dated at 720 years B.P. Overlying these layers is the mixed layer of explosion debris ejected when the craters formed. The debris contains clasts of a variety of rock types up to several feet in diameter in a poorly sorted matrix of finer-grained material. Radiocarbon dating suggests an age of 500 years B.P. for the explosive event.

Return to US 395 and turn left (north).

68.0 (10.5) - Turn left (west) on Glass Flow Road.

69.0 (1.0) - Bear to the left to Obsidian Dome.

69.4 (0.4) STOP - Obsidian Dome (3745'48"N, 11901'32"W). Obsidian Dome is one of several domes in the Inyo-Mono Craters chain. It is comprised of flow-banded obsidian and weakly porphyritic rhyolite extruded as extremely viscous lavas creating domes that locally exceed 300 feet in height. The non-vesiculated rhyolite is black and glassy whereas the vesiculated blocks are gray. Contorted flow banding and tension cracks can be seen in many blocks Ropy textures with iron-staining may also be seen. The obsidian lacks the brilliant glassy texture, rather it is resinous to dull. Samples were analyzed with XRF and compared to other local outcrops of obsidian (see figure). In general, Obsidian Dome was lower in silica (75%) and contained about 1% more iron than other local obsidians. Some publications have suggested that obsidian from this stop was used by Native Americans to make tools, but there is no evidence of any activity and the poor quality of the obsidian argues otherwise.

The age of this "dome" is uncertain, but tree ring and tephra dating indicate that it is less that 1500 years old. Miller (1985) suggested the dome may be as little as 500 years old. Drilling was undertaken in 1984 to explore the feeder system for Obsidian Dome (Eichelberger, et. al, 1985). Three drill holes intersected a 125 foot thick rhyolite dike that is thought to have been the feeder structure. The Obsidian Dome lavas were extruded as highly viscous flows along the feeder dike much like toothpaste might to squeezed from the tube. The viscosity caused the flows to mound up rather than spread laterally. As the exterior cooled and cracked large blocks of autobreccia rolled down the steep side of the flow generating the blocky talus apron.

Return to US 395 and turn left (north).

73.8 (4.4) - Turn right on Pumice Mine Road (paved).

74.1 (0.3) - Left on graded dirt road (Pumice Mine Road).

78.3 (4.2) STOP - U.S. Pumice Mine (3750'35"N, 11900'12"W). Park at the locked gate. U.S. Pumice Corporation of Lee Vining has operated a pumice mine on 160 acres of Forest Service land at the southeast end of Mono Craters for decades. The mine operates only intermittently and appears to be inactive at the present time. Nonetheless, the mine site is private property. The pumice is mined for the manufacture of "stone-washed" jeans and as abrasive pumice blocks. Much of this portion of the Mono Craters consists of pumice with sporadic outcrops of obsidian. This is a collecting stop for those of you who don't have your own pumice blocks. If you search carefully, there are also outcrops of glassy, rainbow obsidian on some of the hill slopes.  Banging on, or handling obsidian without eye protection or gloves is not advisable (numerous cuts attest to this statement). Also the glassy slopes can be slippery and treacherous.

Return to Highway 395.

82.8 (4.5) - Turn right (north) on US 395.

85.6 (2.8) STOP - Mono Lake Vista (3750'32"N, 11904'04"W). If the gate is open (sadly is often locked for inexplicable reasons), turn right onto CalTrans Mono Basin Overlook Rd. From the parking area there is a short walk east to the interpretive sign and overlook . Lets start by orienting ourselves, we will use the azimuth method again. Beginning with those features nearest to us and within the Mono Basin; the smaller, darker island in Mono Lake is Negit. It lies 12 miles away at 10 Israel Russell named the island for a blue-winged goose (although there are no geese around Mono Lake) apparently because in the proper light the dark gray dacite cinder cone comprising the island looks bluish-gray. Just to the south and east (10 miles at 15), the larger white island is Paoha; an Indian term meaning "diminutive spirit with long, wavy, white hair rising from the vapors".  (No we don't make this up!). Most of Paoha is comprised of lake bed sediments uplifted above lake level about 200 years ago during intrusion of a magma body. The north end of the island, however, has a small dacitic lava flow and a very recent rhyolite dome. Panum Crater lies 7 miles away at 15. The Aeolian Buttes lie 1.5 miles to our northeast at about 40. The Aeolian Buttes consist mostly of Bishop Tuff in unconformable contact with Cretaceous Quartz Monzonite of Aeolian Buttes. Although the Buttes are the product of aeolian weathering and glaciation, their present topographic expression probably closely resembles the appearance of this area immediately after extrusion of the pyroclastic flows of Bishop Tuff, 760,000 years ago. The Mono Craters chain can be seen from 25 to 70, three to six miles in the distance. The highest dome in the chain (to the northeast) is Crater Mountain (9172'). On the north shore of Mono Lake at 355 are the subaqueous lava flows of Black Point 12 miles distant. They were emplaced about 14,000 years B.P. Black Point is a flat-topped pile of horizontally bedded cinders and tuff. It has been proposed that Black Point erupted under water during the late Pleistocene when Mono Lake was 200 feet higher than its present level. Looking to the distant horizon, outside the Mono basin, due north at a distance of 27 miles is Bodie Peak and just a few degrees to its west the round, distinctive outline of Potato Mountain. These two landmarks form a portion of the Bodie Hills, a ridge of silicic, Miocene age volcanics. The Anchorite Hills lie 30 miles away at 40 and the low ridge of the Cowtrack Mountains 17 miles distant at 55-70 (best seen from Highway 120). The high peaks to our west are a part of the Ritter Range. The town of Lee Vining is 9 miles distant at 340 and Conway Summit on the northwest horizon (25 miles) at 345.

The Mono Craters form a spectacular 8 mile-long arc of young silicic domes and explosion craters between the Long Valley caldera and Mono Lake (see photo). The arc marks the eastern margin of a 12 mile-wide ring-fracture system that has subsided over 600 feet during the past 800,000 years.  Mono Craters includes 3 large rhyolitic (obsidian) flows, 6 to 8 steep-sided rhyolite domes, a number of explosion craters, as well as the islands in Mono Lake, and Black Point, the basaltic lava flow on the north shore of the lake. All of these were formed within the last 35,000 years, with the majority less than 10,000 years old. The volcanic islands in Mono Lake are less than 2000 years old, and lavas and ash on the eastern edge of Paoha Island are reported to be only about 200 years old. The Mono domes and flows are steep, blocky, and barren, reflecting their glassy texture and youthful age. The domes are large and often composed of multiple flows.

The arcuate shape of the Mono crater chain has lead scientists to suggest that the Mono Craters may be analogous to the Glass Mountain rhyolite and hence a precursor to caldera formation (the dashed line on the map to the left). Recent seismic surveys have failed to establish the existence of a large magma body beneath the Mono Craters, but data resolution limited the investigation to a depth of five miles. By all accounts the "Mono Caldera" would be in an early stage of conception and thus a magma body could easily lie at depths greater than five miles.

89.2 (3.6) - Turn right on Highway 120.

92.3 (3.1) - Turn left on dirt road.

93.2 (0.9) STOP - Panum Crater (3755'32"N, 11902'56"W).. Panum Crater is an example of an phreatic explosion pit in which the subsequent lava plug was not large enough to completely engulf the initial tephra ring. The tephra ring, with a diameter of 3500 feet is comprised of pumice ash and lapilli, obsidian fragments and granite pebble ejecta. The central rhyolite dome consists of light gray, pumiceous rhyolite and rhyolite breccia containing angular, pebble and cobble-sized fragments. Most of the rhyolite is flow banded, the bands dipping steeply away from the dome summit.

The formation of Panum Crater required a sequence of events. The first event occurred as magma rising from depth made contact with water just below the surface. The water expanded into steam and a violent eruption followed (see Inyo Craters discussion). The blast opened a huge crater. After the initial explosion, a fountain of ash and cinders shot into the air, falling back to earth and forming a pumice ring, around the original crater. When the violent eruptions ceased, the remainder of the thick magma rose slowly to the surface to form a series of domes. Each dome began with an extrusion of viscous, rhyolitic lava that hardened and formed a cap over the vent. As magma continued to push up, the cap shattered and fell away from the newly formed dome. This created a mountain of broken rock, a crumble breccia. As the final dome hardened, a period of spire building began. Thick lava squeezed up through cracks in the dome and formed needle-like spires. Imagine toothpaste squeezing through the opening of a tube and forming a small tower before it topples over under gravity. Most of the spires at Panum collapsed and broke. The debris you see at the top of the dome is the remains of crumbled spires.

Return to Highway 120 and turn left.

95.7 (2.5) - Turn left on South Tufa Road and follow the signs to the South Tufa area.

96.7 (1.0) STOP Mono Lake Tufa Columns (3756'19.5"N, 11901'37"W).. This is the final stop! The pinnacles of calcareous tufa along the south shore of Mono Lake were first described by Israel Russell over 100 years ago when he visited the area. Those interested should read his report on the Quaternary geology of the Mono Valley (Russell, 1889). It is truly a "classic" with an elegant prose style rarely seen in modern scientific papers. Most of the tufa deposits lie along the south side of the Mono Lake, but a few isolated knobs and towers lie north of Lee Vining, near the Visitor's Center.

The knobby calcium carbonate columns of tufa form as fresh water springs discharge subaqueously into Mono Lake. The tufa masses that surround the saline lake record former high stands of the present Mono Lake and its predecessor, Lake Russell. Some of the tufa columns have delicate fluting (see photo), adding to their unusual appearance and the fairyland nature of the tufa outcrops.

Tufa forms by chemical precipitation of calcium carbonate in the alkaline lake waters (pH=9.7). Remember, the solubility of calcium carbonate decreases dramatically in alkaline solutions. CO3 saturated fresh water (pH=7) would immediately precipitate calcium carbonate when encountering alkaline waters like those of Mono Lake. A recent study suggests that algae play a role in the tufa formation. The algae extract carbon dioxide during photosynthesis reducing calcium carbonate solubility and causing precipitation of tufa.

The End. Hope you enjoyed the trip.


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