Mono–Inyo Craters
The Mono–Inyo Craters are a volcanic chain of craters, domes and lava flows in Mono County, Eastern California, United States. The chain stretches from the northwest shore of Mono Lake to the south of Mammoth Mountain. The 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 volcanoes that have since been either plugged or over-topped by rhyolite domes and lava flows. The Inyo volcanic chain form much of the southern part of the chain and consist of phreatic explosion pits, and rhyolitic lava flows and domes. The southernmost part of the chain consists of fumaroles and explosion pits on Mammoth Mountain and a set of cinder cones south of the mountain; the latter are called the Red Cones.
Eruptions along the narrow fissure system under 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 the Mono Craters and eruptions 5,000 to 500 years ago formed the Inyo volcanic chain. 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 250 to 700 years. In 1980, a series of earthquakes and uplift within and south of Long Valley Caldera indicated renewed activity in the area.
The region has been used by humans for centuries. Obsidian was collected by Mono Paiutes for making sharp tools and arrow points. Glassy rock continues to be removed 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 the Mono Craters. Mono Lake Volcanic Field and a large part of the Mono Craters gained some protection under Mono Basin National Forest Scenic Area in 1984. Resource use along all of the chain is managed by the United States Forest Service as part of Inyo National Forest. Various activities are possible along the chain, including hiking, bird watching, canoeing, skiing, and mountain biking.
Geography and description
Setting
The Mono–Inyo Craters form a volcanic chain in Eastern California that sits along a narrow north–south-trending fissure system extending from the north shore of Mono Lake through the western Long Valley Caldera, south of Mammoth Mountain. The chain is within the Inyo National Forest and Mono County; the nearest incorporated community is Mammoth Lakes. The craters are in the Great Basin geographic area.Mono Craters
The Mono Craters are a 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 Craters domes is Crater Mountain, which rises above Pumice Valley to the west. Associated volcanic features are located in Mono Lake and on its north shore. The coulees cluster north and south of the overlapping chain of domes.Inyo volcanic chain
The Inyo volcanic chain stretches from Wilson Butte to the Inyo Craters, proper. The Inyo Craters are open pits in a forested area that are about across and deep, each with small ponds covering their floors. A quarter mile north of these is another 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 Inyo volcanic chain extends into Long Valley Caldera but is not related to the caldera's volcanism.Red Cones
South of the Inyo volcanic chain are other features related to the dike system responsible for creating the craters, volcanoes and lava flows. These include a north–south trend of fault scarps up to high and pull-apart cracks or fissures in the earth. These fissures are not technically faults because little or no vertical or horizontal movement has occurred along them. Most notable among these is "Earthquake Fault", a fissure up to wide that cuts into glassy rhyolite lava flows. The fissure was formed by stretching induced by the intrusion of the Inyo dike. 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 Mountain. The Red Cones, south of Mammoth Mountain, are basaltic cinder cones and are the southernmost part of the Mono–Inyo Craters volcanic chain.
Climate and ecology
The Mono–Inyo Craters are in the Central Basin and Range ecoregion of the North American Desert. The desert environment of Mono Basin receives about of precipitation a year. Annual precipitation around Mammoth Lakes, which is close to the Inyo volcanic chain, is about. 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 to average summer highs of. Temperatures near the Inyo volcanic chain and Mammoth Lakes area range from winter average lows of to summer average highs of.Most of the surface of the Mono Craters is barren but its slopes are covered by Jeffrey pine forest and partial greenery. Pumice Valley, directly to the west, is covered by sagebrush scrubland. The soil consists primarily of deep pumice, which does not hold water well. Mycorrhizal 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 volcanic chain and Mammoth Mountain. Mule deer, coyotes, black bears, yellow-bellied marmots, raccoons and mountain lions all have ranges that are coincident with forests that cover parts of the Mono–Inyo craters.
Typical evolution
is the northernmost volcano in the sequence and is a good example of both a tuff ring and a rhyolite dome. Its structure is twofold; an outer tuff ring and an inner plug, or dome of rhyolite, pumice and obsidian created from lavas. In this case, heat from the magma feeding Panum flashed groundwater to steam to create the tuff ring before lava reached the surface. 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 wide and 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 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 thick and have scree piles along their base. South Coulee is long, wide and has a volume of ; making it the largest Mono Craters coulee in volume. South Coulee originates from the crest of the Mono Domes, about 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 coulee solidified. Permanent pockets of ice from snowmelt have been found inside the coulees and domes.
Geology
Background
The Mono–Inyo chain of craters lies in east-central California, roughly parallel to the eastern escarpment of the Sierra Nevada mountain range. Volcanism and seismic activity in eastern California are a result of two major geologic processes: northwest movement of the Pacific Plate with respect to the North American Plate along the San Andreas Fault system near the coast, and east–west extension of the crust that formed the Basin and Range Province. In the Long Valley region, where the craters are located, basin and range extension encroaches onto the thick and stable crust of the Sierra Nevada.Basement rock under the Mono–Inyo chain consists of the same granitic and metamorphic rock that make up the Sierra Nevada. Above that layer are basaltic grading 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. Nearly all the rock east of the Sierra Nevada in the Mono Basin area is volcanic in origin.
Volcanoes erupted from 3.6 to 2.3 million years ago near what is now Long Valley. Rhyolitic eruptions occurred in and around Glass Mountain in the same area from 2.1 to 0.8 million years ago. Volcanic ash from the massive eruption of Long Valley Caldera some 760,000 years ago is preserved in the thick Bishop Tuff that covers much of the region.
Eruptions of basalt and andesite 400,000 to 60,000 years ago in the west moat of Long Valley Caldera were the first activity associated with the Mono–Inyo Craters system. Eruptions around 300,000 years ago filled the west moat with 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 indicate that a magma chamber with an estimated volume of exists directly below the Mono Craters. About of subsidence has occurred within a ring fracture system centered on Pumice Valley west of the chamber in the last 700,000 years. The Mono Craters sit atop a -long arc on the eastern side of the -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 recent eruptions of the Mono Craters have been similar in volume and nearly identical in composition to those of Glass Mountain that preceded the Long Valley Caldera-forming eruption. It has been suggested that the Mono Craters volcanism may represent an early stage in the development of a future caldera.
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. Eruptions of dacite and rhyodacite occurred in Mono Basin from 100,000 to 6,000 years ago.