Annotated satellite image of the chain
|District||Mono County, California|
|City||Mammoth Lakes, California|
|Highest point||Crater Mountain[note 1]|
|- elevation||9,172 ft (2,796 m)|
|Geology||lava domes, cinder cones|
|Period||About 40,000 years|
|Date||last eruption: 1790 ± 75 years
Last activity: 230 years ago
Mono-Inyo Craters is a north–south-trending volcanic chain in Eastern California that stretches 25 miles (40 km) from south of Mammoth Mountain to the northwest shore of Mono Lake. Politically, it is located in Mono County in the U.S. State of California. Mono Lake Volcanic Field forms the northernmost part of the chain and consists of two volcanic islands in the lake and one cinder cone volcano on its northwest shore. Most of the Mono Craters, which make up the bulk of the northern part of the Mono-Inyo chain, are phreatic (steam explosion) volcanoes that have since been either plugged or over-topped by rhyolite domes and lava flows. Inyo Craters form much of the southern part of that line and are either phreatic explosion pits or rhyolitic lava flows and domes. The southernmost part of the chain consists of fumaroles and explosion pits on Mammoth Mountain and of a set of cinder cones south of the mountain; the latter set is called the Red Cones.
The area has been exploited by humans for centuries. Obsidian was collected by Mono Paiutes for making sharp tools and arrow points, and was used as a trade object. Glassy rock continues to be removed from the volcanoes in modern times for use as commercial scour and yard decoration. Mono Mills processed timber felled on or near the volcanoes for the nearby boomtown Bodie in the late 19th to early 20th centuries. Water diversions into the Los Angeles Aqueduct system from their natural outlets in Mono Lake started in 1941 after a water tunnel was cut under Mono Craters. Mono Lake Volcanic Field and a large part of Mono Craters gained some protection under Mono Basin National Forest Scenic Area in 1984. The Los Angeles Department of Water and Power accepted a state decision in 1994 to cut its water diversions from Mono Lake. Resource use along all of the chain is managed by the United States Forest Service as part of Inyo National Forest. Various activities are available along the chain, including hiking, bird watching, canoeing, skiing, and mountain biking.
Eruptions along the narrow fissure system that underlies the chain began in the west moat of Long Valley Caldera 400,000 to 60,000 years ago. Mammoth Mountain was formed during this period. Multiple eruptions from 40,000 to 600 years ago created Mono Craters and eruptions 5,000 to 500 years ago formed Inyo Craters. Lava flows 5,000 years ago built the Red Cones, and explosion pits on Mammoth Mountain were excavated in the last 1,000 years. Uplift of Paoha Island in Mono Lake about 250 years ago is the most recent activity. These eruptions most likely originated from small magma bodies rather than from a single, large magma chamber like the one that produced the massive Long Valley Caldera eruption 760,000 years ago. During the past 3,000 years, eruptions have occurred every 700 to 250 years. In 1980, a series of earthquakes and uplift within and south of Long Valley Caldera indicated renewed activity along the chain.
Mono-Inyo Craters is a volcanic chain in Eastern California that sits along a narrow, north-trending fissure system that extends south of Mammoth Mountain, through the western moat of Long Valley Caldera and to the north shore of Mono Lake. The chain is located in Inyo National Forest and Mono County, California and the nearest incorporated community is Mammoth Lakes, California. Mono Craters are located in Mono Basin, itself part of the Great Basin.
Mono Craters is a 10.5 miles (16.9 km) chain of at least 27 volcanic domes, three large glass flows called coulees and various explosion pits and other associated volcanic features. The domes of the chain lie on a roughly north–south-trending arc that is concave to the west and located south of Mono Lake. The highest of the Mono Domes is Crater Mountain (elevation 9,172 ft or 2,796 m), which rises 2,400 feet (730 m) above Pumice Valley to the west . Associated volcanic features are located in Mono Lake (Paoha and Negit Islands) and on its north shore (Black Point). The coulees cluster north and south of the overlapping chain of domes. Craters exist at the top of most domes and on flat land south of them.
The two southernmost Inyo Craters are open pits in a forested area that are about 600 feet (180 m) across and 100 to 200 feet (30 to 61 m) deep, each with small ponds covering their floors. A quarter mile (half kilometer) north of these is another Inyo explosion pit on top of Deer Mountain. Farther north of these craters are five lava domes, including Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte. These domes are composed of gray rhyolite, frothy pumice, and black obsidian. The Mono-Inyo Craters volcanic chain extends into Long Valley Caldera but is not related to the caldera's volcanism.
South of Inyo Craters proper are other features related to the
responsible for creating the craters, volcanoes and lava flows.
These include a north–south trend of fault scarps up to 20 feet
(6.1 m) high and pull-apart cracks or fissures in the
These fissures can not technically be called faults because
little or no vertical or horizontal movement has occurred along
Most notable among these is "Earthquake Fault", a fissure up to
10 feet (3.0 m) wide that cuts 60 to 70 feet (18 to
21 m) into glassy rhyolite lava flows.
Stairs to the bottom of the fissure were removed after being
damaged by earthquakes in 1980.
Several Mono-Inyo-related explosion pits are on Mammoth
The Red Cones, south of Mammoth Mountain, are basaltic cinder cones and are
the southernmost part of the Mono-Inyo Craters volcanic chain.
The desert environment of Mono Basin receives about 14 inches (36 cm) of precipitation a year. Annual precipitation around Mammoth Lakes, which is close to the Inyo Craters, is about 23 inches (58 cm). Moisture travels over the Sierra crest from the Pacific Ocean through the San Joaquin Gap. Temperatures in Mono Basin range from average winter lows of 20 to 28 °F (-7 to -2 °C) to average summer highs of 75 to 84 °F (24 to 29 °C). Temperatures near the Inyo Craters and Mammoth Lakes area range from winter average lows of 16 to 21 °F (-9 to -6 °C) to summer average highs of 70 to 78 °F (21 to 26 °C).
Most of the surface of the Mono Craters is barren but slopes on the sides of that part of the Mono-Inyo range is covered by Jeffrey Pine forest. Pumice Valley, directly to the west, is covered by sagebrush scrubland. The soil consists primarily of deep pumice, which does not hold water well. Mycorrhizae fungi in the soil invade the roots of Jeffrey Pine trees in a symbiotic relationship that helps the pine absorb water and provides nutrients to the fungi. Jeffrey Pine forests also surround the Inyo Craters and Mammoth Mountain. Mule Deer, Coyote, Black Bear, Yellow-bellied Marmot, Raccoon and Mountain Lions all have ranges that are coincident with forests that cover parts of the Mono-Inyo craters.
Panum Crater is the northernmost volcano in the sequence and is a good example of both a tuff ring and a rhyolite dome. Its structure is two-fold; an outer tuff ring (forming a classic crater) and an inner plug, or dome of rhyolite, pumice and obsidian created from lavas. In this case, the magma feeding Panum reached the surface, as lava after its heat had already created a steam explosion crater. Other Mono Craters also were formed in this manner, but their plug domes grew larger than their tuff ring craters. The domes have steep sides and are flanked by slopes of scree consisting of large angular and glass-rich rocks. Devil's Punch Bowl, located south of the main dome complex, stopped forming at an earlier stage of development. It is a 1,200-foot (370 m) wide and 140-foot (43 m) deep explosion pit with a much smaller glass dome on its floor.
The large North and South Coulee and the smaller Northwest
Coulee consist of obsidian-rich rhyolite. They were formed from
slow-moving pasty lava that had a thin and brittle crust. Once the
flow stopped, it formed steep sided tongues of sharp and angular
rock that are typically 200 to 300 feet (61 to 91 m) thick and
have scree piles along their
South Coulee is 2.25 miles (3.62 km) long,
0.75 miles (1.21 km) wide and has a volume of
0.1 cubic miles (0.4 km3); making it the
largest Mono coulee in volume. South
Coulee originates from the crest of Mono Domes, about 3 miles
(4.8 km) from the southern end, flows down its east and west
flanks and terminates at its foot.
North Coulee is nearly as large, flows mostly to the east and
terminates in a divided pair of lobes. Northwest Coulee is located
northwest of North Coulee and was intruded by Upper Dome after the
Permanent pockets of ice from snowmelt have been found 75 to 147
feet (23 to 45 m) inside the coulees and domes.
Obsidian from the Mono-Inyo Craters was collected by native Mono Paiutes for making sharp tools and arrow points, and was used as a trade object. Unworked obsidian was carried by the Mono Paiutes over passes in the Sierra Nevada to trade with other Native American groups. Chips of Mono-Inyo obsidian can still be found at many ancient mountain campsites.
Mark Twain visited Mono Basin in the 1860s and wrote about Mono Lake, but did not mention any of the Mono-Inyo Craters except for the lake's two volcanic islands. He wrote in Roughing It that the lake was in a "lifeless, hideous desert..." that was the "loneliest spot on earth... little graced with the picturesque." This book was published in 1872 and helped shape attitudes about the area.
Gold rush-related boomtowns sprang up near and in Mono Basin in the 19th century to exploit local bonanzas. The largest of these, Bodie (north of Mono Lake), was founded in the late 1870s and was successful enough to need a tree mill. The tree mill was located at Mono Mills, immediately northeast of Mono Domes. Trees directly around the domes and on their slopes were felled to provide timber for the mill.
As part of the California Water Wars, the Los Angeles Department of Water and Power purchased large tracts of land in the 1930s within Mono Basin and Owens Valley in order to control water rights. Excavation of an 11.5-mile (18.5 km) tunnel under the southern part of the Mono dome complex started in 1934 and was completed in 1941. Tunnel workers had to deal with loose and often water-charged gravels, pockets of carbon dioxide gas and flooding. About one man was lost for each mile excavated.
The purpose of the tunnel was to divert water from Rush, Lee Vinning, Parker and Walker creeks from their natural outlet in Mono Lake into the Owens River, which in turn was diverted farther downstream into the Los Angeles Aqueduct system. Once the tunnel was in use, the water level of Mono Lake dropped 1 to 1.6 feet (0.30 to 0.49 m) a year. The aqueduct system was expanded in 1970 and diversion from streams feeding Mono Lake intensified. Lake level was at 6,417 feet (1,956 m) above sea level before diversions began through the Mono Craters Tunnel and reached a low of 6,372.3 feet (1,942.3 m) in 1981.
The United States Pumice Company, based in Chatsworth, California, has mined the area for pumice since 1941. The company markets the pumice in slabs for use in commercial scouring and in large irregular chunks sold as yard decoration.
The creation of Mono Basin National Forest Scenic Area in 1984 stopped any further geothermal prospecting. Mono Basin was the first National Forest Scenic Area in the United States. It offers more protection than other United States Forest Service lands, surrounds Mono Lake and its two volcanic islands, Black Point, Panum Crater and much of the northern half of Mono Craters.
Litigation and outreach by the Mono Lake Committee, the National Audubon Society and other conservation groups has helped to slow and, more recently, reverse some water diversions from tributaries feeding Mono Lake through the Mono Craters Tunnel. Starting in 1989, no diversions were allowed if the lake level fell below 6,377 feet (1,944 m) above see level, and in 1994 the Los Angeles Department of Water and Power accepted a decision by the state government to cut back its water diversions until the lake level reaches 6,392 feet (1,948 m). In 1994, lake level was 6,374.6 feet (1,943.0 m) above sea level and by 2008 it had risen 7.7 feet (2.3 m).
A series of earthquakes inside Long Valley Caldera, coincidently starting two weeks after the May 1980 eruption of Mount St. Helens in Washington, alerted geologists to the possibility of renewed volcanic activity in the region.[note 2] Four magnitude 6 earthquakes struck the southern margin of Long Valley Caldera in an area that was close to the Mono-Inyo fissure system. The caldera floor had also uplifted by 10 inches (30 cm) in five years. Upward movement of magma under the caldera was thought to be the cause of the earthquakes and uplift.[note 3]
Persistent earthquake swarms in 1982 prompted the U.S. Geological Survey (USGS) to issue a "potential volcanic hazard" notice for Long Valley. That same year, permanent monitoring of the area by the Long Valley Observatory started. The hazard notice was lifted in 1984 after USGS scientists concluded that magma had been injected into fissures below Mammoth Mountain but had congealed underground. From 1990 to 1996, 150 acres (61 ha) of trees were killed on Mammoth Mountain by 20% to 95% concentrations of carbon dioxide (CO2) in the soil (less than 1% is normal). Chemical analysis of the CO2 indicated it was derived from magma.
The basement rock under the Mono-Inyo chain consists of the same granitic and metamorphic rock that the Sierra Nevada is made of. Above that are basaltic to rhyolitic volcanic rocks that are 3.5 million to less than 760,000 years old. Volcanism occurred north of the chain, in the Bodie Hills, as far back as 28 million years. In fact, nearly all the rock east of the Sierra Nevada is volcanic in origin.
Near what is now Long Valley, volcanoes erupted from 3.6 to 2.3 million years ago. Eruptions occurred in and around Glass Mountain in the same area from 2.1 to 0.8 million years ago. The massive eruption of Long Valley Caldera some 760,000 years ago laid down a thick sequence of Bishop Tuff over the region.
The Mono-Inyo Craters system began with the eruption of basalt and andesite in the west moat of Long Valley Caldera 400,000 to 60,000 years ago. Eruptions around 300,000 years ago filled the west moat with 800 feet (240 m) of basaltic lava. Basaltic and andesitic eruptive activity then moved to Mono Basin and lasted from 40,000 to 13,000 years ago.
Seismic data indicates that a magma chamber with an estimated volume of 50 to 140 cubic miles (200 to 600 km3) exists 5 to 6 miles (8 to 10 km) directly below Mono Craters. About 600 feet (183 m) of subsidence has occurred along a ring fracture system above the chamber in the last 700,000 years. Mono Craters sit atop a 7.5-mile (12 km)-long arc on the 11-mile (18 km)-wide ring-fracture system. Magma feeding the domes may have exploited arc-shaped fissures around an intrusion of granitic rock deep below the chain. This magma chamber is separate from the magma chamber under Long Valley Caldera. The Mono Arc and Mono Craters may represent an early stage of caldera development.
Repeated eruption of dacite and rhyodacite from vents on the southwest rim of the caldera from 220,000 to 50,000 years ago formed Mammoth Mountain, a volcano composed of overlapping lava domes. Dacite and rhyodacite also were erupted in the Mono Basin from 100,000 to 6,000 years ago.
The Mono Craters were formed by multiple eruptions of silica-rich rhyolite from 40,000 to 600 years ago. Black Point, today on the north shore of Mono Lake, is a flattened volcanic cone of basaltic tephra that formed under the surface of a much deeper Mono Lake about 13,300 years ago, during the most-recent glacial period. Negit Island, in Mono Lake, was formed by several eruptive episodes from 1600 to 270 years ago (as of 2000).
None of the Mono Craters near the lake show the effects of wave erosion but a hill at the southern end does show incision by a beach line. This indicates that a high stand of Mono Lake reached this area before the formation of the northern Mono Craters. Glaciers did not reach as far as the Mono Craters but stream-rounded stones are found under and on the volcanoes. Stones on the volcanoes were lifted from the ground as the volcanoes grew.
The most recent eruptive episode on the Mono Craters occurred sometime between 1325 and 1365. A vertical sheet-like mass of magma, called a dike, caused groundwater to explosively flash to steam, creating a line of vents 4 miles (6.4 km) long. A mix of ash and pulverized rock, called tephra, covered about 3,000 square miles (7,770 km2) of the Mono Lake region to a depth of 8 inches (20 cm) 20 miles (32 km) downwind to 2 inches (5.1 cm) 50 miles (80 km) downwind. (Wind direction varied during that time).
Pyroclastic flows of glowing-hot clouds of gas, ash and pulverized lava erupted from the vents in narrow tongues that extended up to 5 miles (8.0 km) away and covered 38 square miles (98 km2) in the second phase. Second-phase magma had been largely depleted of gas and steam. The rhyolite magma was rich in the mineral silica and thus oozed out of the vents to form several steep-sided domes, including Panum Dome and the much larger North Coulee flow. The youngest domes and coulees are 600 to 700 years old.
The Inyo Craters were formed by eruptions of silica-poor rhyolite 5,000 to 500 years ago. The most recent activity occurred just a few years after the 1325 and 1365 Mono Crater eruptions described above and was caused by a dike of similar composition. The dike spread both horizontally and upward, eventually extending 6.8 miles (11 km) long and up to 33 feet (10 m) wide. Ground above the dike was significantly cracked and faulted.
A series of explosive eruptions later occurred at three separate vents. Pieces of molten and solid rock were ejected, small craters were formed, and a tall eruption column rose above the vents. Pumice and ash covered an extensive area downwind and about 1 inch (2.5 cm) of tephra was deposited where the town of Mammoth Lakes, California, now sits. A pyroclastic flow traveled about 3.7 miles (6.0 km) from the South Deadman vent. Some of the open pits filled with pasty lava to form South Deadman Creek, Glass Creek and Obsidian Flow domes. Others, such as the Inyo Crater Lakes near Deer Mountain, remained open and were later partially filled with water. Smaller explosion pits on the north side of Mammoth Mountain were also formed at this time. In the last 6,000 years, approximately 0.19 cubic miles (0.79 km3) of magma has been erupted from the Inyo part of the chain.
The last recorded volcanic activity in the chain was at Mono Lake between the years 1720 and 1850. An intrusion of magma below the lake pushed lakebed sediments up to form Paoha Island. Columns of rhyolite stuck through the sediment on the north part of the island, and lava erupted on the northeastern corner to form a group of seven dacite cinder cones and a lava flow. Steam rose in columns hundreds of feet high (tens of meters) from Hot Spring Cove on the island and the spring water was 150 °F (66 °C) when geologist Israel Russell visited the island in the early 1880s.
The Long Valley to Mono Lake region is one of three areas in California that are studied under the United States Geological Survey's (USGS) volcanic hazards program (the other two areas are the Mount Lassen area and Mount Shasta). These areas were chosen to be in the program because they have been active in the last 2000 years and have the ability to produce explosive eruptions in the future.
The Mono-Inyo Craters have erupted at intervals of 250 to 700 years during the past 3,000 years. The most recent eruptions are from Panum Crater and the Inyo Craters 500 to 600 years ago, and Paoha Island about 250 years ago. Seismic soundings of the crust and the composition of lava indicate that these eruptions most likely originated from small, discrete magma bodies rather than from a single, large magma chamber like the one that produced the Long Valley Caldera eruption 760,000 years ago. The rate of eruption over the last 1,000 years has increased, with at least 12 eruptions occurring.
Each episode of volcanic activity in the past 5,000 years from the Mono-Inyo Craters has expelled less than 0.25 cubic miles (1.0 km3) of magma. The next eruption in the area will most likely be similar in size to the small to moderate eruptions in the past 5,000 years.
Tephra may accumulate to a thickness of 33 feet (10 m) near an erupting Mono-Inyo vent. Areas downwind could be covered by more than 8 inches (20 cm) of tephra at a distance of 22 miles (35 km) and by 2 inches (5 cm) at 53 miles (85 km). Winds in the area tend to blow toward an east or northeasterly direction more than 50 percent of the time, and toward any easterly direction more than 80 percent of the time. In general, the grain size and thickness of ash gradually decrease with increasing distance from a vent. Air routes east of the vent are likely to get contaminated with volcanic ash.
Areas as far as 9.3 miles (15 km) from an explosive eruption could be covered by pyroclastic flows and surges; this coverage depends on the location of the vent or vents, topography, and volume of magma erupted. Pyroclastic flows generated from vents on Mammoth Mountain could travel farther than 9.3 miles (15 km) because of the extra momentum gained from descending thousands of feet (hundreds of meters).
Basalt lava typically forms thin flows that may reach more than
30 miles (50 km) from their vent.
Dacite and rhyolite lavas typically produce short, thick flows that
seldom move as far as 3 miles (5 km) from their vent.
These short flows often build mound-shaped features called lava
domes. Rock fragments thrown from a growing lava dome may reach 3
to 6 miles (5 to 10 km) from the dome.
A partial collapse of the steep-sided growing dome can send
pyroclastic flows outward at least 3 miles (5 km).
Taller domes tend to form larger pyroclastic flows that travel
The Mono Basin National Scenic Area visitor center is located near Mono Lake just off U.S. Route 395. A bookstore, an information desk staffed by U.S. Forest Service Rangers and museum exhibits help to orient visitors. The Mono Lake Committee has a staffed office and visitor information center in Lee Vining on the corner of U.S. Route 395 and 3rd Street. Information on camping, hiking, guided and self-guided tours can all be obtained at either location.
U.S. 395 is a scenic route that roughly parallels the Mono-Inyo Craters volcanic chain. California State Route 120 provides access to the northern and eastern parts of the Mono Domes, including Panum Crater. Mammoth Scenic Loop (also called Dry Creek Road), provides access to the Inyo Crater lakes and some of Inyo domes.
The town of Mammoth Lakes, located near the southern end of the chain and Mammoth Mountain, is the largest populated area nearby. Mammoth Mountain Ski Area is located nearby and gondola rides can be taken year-round (weather permitting) to the mountain's summit. The summit of Mammoth Mountain provides panoramic views of the craters and domes of the Mono-Inyo volcanic chain, Mono Lake, the Sierra Nevada and Long Valley Caldera.
Mono Lake itself has its own set of activities, including walking tours among towers of tufa, boat tours of the lake, and birdwatcher opportunities. The lake is too salty to support any fish but fishing is possible in streams that feed Mono Lake. Hiking around and on the craters and domes and mountain biking outside of the Scenic Area boundaries are also available activities.