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 (steam explosion) 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
thumb|Mono Craters
thumb|alt=Chain of tall hills with sharp peaks. A crater and lake are in the foreground.|The Mono Craters form an arc of overlapping lava domes and flows.
thumb|alt=Two craters in a forested area and one crater on top of a large hill. A hummocky massive but flat hill is in the background.|The Inyo Craters are a set of explosion pits.
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 highest of the Mono Craters domes is Crater Mountain (elevation 9,172 feet or 2,796 m), which rises above Pumice Valley to the west. Farther north of these craters are five lava domes, including Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte.
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 .
Most of the surface of the Mono Craters is barren but its slopes are covered by Jeffrey pine forest and partial greenery. 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
thumb|alt=Ring of gray rock with a dome of gray rock inside it.|Aerial photo of Panum Crater
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 twofold; an outer tuff ring (forming a classic crater) and an inner plug, or dome of rhyolite, pumice and obsidian created from lavas.
upright|left|alt=Map of directional flow from a large oval labeled "Long Valley Caldera." Glass Mountain is on one side of the oval and Mammoth Mountain is on the other. North of the oval are the Inyo Craters, Mono Craters and Mono Lake. Colors are used to distinguish the age and type of the volcanic rock. Fault lines are depicted along the Sierra Nevada.|Geologic map of the area.
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. Volcanic ash from the massive ( of ejecta) 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.
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.
Basaltic andesite lava built the Red Cones, two small cinder cones southwest of Mammoth Lakes, around 8,500 before present. The five Mammoth Mountain Craters are a set of explosion pits that trend west-north-west for near the northern flank of Mammoth Mountain. A vertical sheet-like mass of magma, called a dike, caused groundwater to explosively flash to steam, creating a line of vents long. The ground above the dike was significantly cracked and faulted.
thumb|alt=Forested area with buildings interspersed looking toward a hill.|left|The town of Mammoth Lakes sits on top of ash beds from the Mono–Inyo eruptions.
Explosive eruptions emanated from three separate vents in the summer of 1350 CE. Pumice and ash covered an extensive area downwind, and about of tephra was deposited where the town of Mammoth Lakes, California, now sits. An intrusion of magma below the lake pushed lakebed sediments upward to form Paoha Island. Exposed rhyolite is on the north part of the island, and a group of seven dacite cinder cones and a lava flow are on the northeastern corner. 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.
thumb|alt=Men working next to chords of wood at the bottom of wooden ramps on the side of a hill. A building and set of trees is on top of the hill. A rail line parallels the chords of wood.|Mono Mills processed wood near Mono Domes for use in the boomtown of Bodie.
Gold rush–related boomtowns sprang up near Mono Basin in the 19th century to exploit bonanzas. The largest of these, Bodie (north of Mono Lake), was founded in the late 1870s and grew large enough to need a tree mill, which was located at Mono Mills, immediately northeast of Mono Domes. Timberland to the east of the Mono Craters was clearcut for wood.
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.
thumb|left|alt=Two peaks with steep slopes that have trees on them.|Drawing of the Mono Craters by John Muir
Naturalist John Muir explored the area in 1869. He described the "Mono Desert" as a "... country of wonderful contrasts. Hot deserts bounded by snow-laden mountains,—cinders and ashes scattered on glacier-polished pavements,—frost and fire working together in the making of beauty. In the lake are several volcanic islands, which show that the waters were once mingled with fire." Muir described the Mono Craters as "... heaps of loose ashes that have never been blest by either rain or snow ..." 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 the Mono Craters. Litigation and outreach by the Mono Lake Committee, the National Audubon Society and other conservation groups has helped to slow water diversions from tributaries feeding Mono Lake.
Volcanic hazards
The Long Valley to Mono Lake region is one of three areas in California that are in the United States Geological Survey's volcanic hazards program. Future eruptions in the area will likely be similar in size to the small to moderate events of the past 5,000 years. Information on camping, hiking, guided and self-guided tours can all be obtained at either location.
Multiple paved roads surround the Mono-Inyo craters. U.S. 395 is a scenic route that roughly parallels the Mono–Inyo Craters volcanic chain. California State Route 120 approaches the northern and eastern parts of the Mono Domes, including Panum Crater. Mammoth Scenic Loop approaches the Inyo Craters. Direct access to the Mono—Inyo Craters requires driving on unpaved roads, then walking.
The town of Mammoth Lakes and Mammoth Mountain are located near the southern end of the chain. 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 birdwatching opportunities. The lake is too salty to support any fish, but fishing is possible in streams that feed Mono Lake. Additional activities include hiking around and on the craters and domes, and mountain biking outside of the Scenic Area boundaries.