Mono–Inyo Craters

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.[7] 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.

The Mono Craters are a 10.5-mile (17 km) chain of at least 27 volcanic domes, three large glass flows called coulees and various explosion pits and other associated volcanic features.[8]^: 289 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.[9] The highest of the Mono Craters domes is Crater Mountain (elevation 9,172 feet or 2,796 m), which rises 2,400 feet (730 m) above Pumice Valley to the west.[8]^: 290 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.[8]^: 290

Photo from the edge of southernmost of the Inyo craters.

The Inyo volcanic chain stretches 6 miles (10 km) from Wilson Butte to the Inyo Craters, proper.[10] The Inyo Craters are open pits in a forested area that are about 600 feet (180 m) across and 100 to 200 feet (30 to 60 m) deep, each with small ponds covering their floors.[8]^: 252 A quarter mile (half kilometer) north of these is another explosion pit on top of Deer Mountain.[11] Farther north of these craters are five lava domes, including Deadman Creek Dome, Glass Creek Dome, Obsidian Dome, and Wilson Butte.[11] 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.[8]^: 290

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 20 feet (6 m) high and pull-apart cracks or fissures in the earth.[8]^: 253 These fissures are not technically faults because little or no vertical or horizontal movement has occurred along them.[8]^: 254 Most notable among these is "Earthquake Fault", a fissure up to 10 feet (3 m) wide that cuts 60 to 70 feet (18 to 21 m) into glassy rhyolite lava flows. The fissure was formed by stretching induced by the intrusion of the Inyo dike.[8]^: 253 Stairs to the bottom of the fissure were removed after being damaged by earthquakes in 1980.[8]^: 253 Several Mono–Inyo-related explosion pits are on Mammoth Mountain.[2] The Red Cones, south of Mammoth Mountain, are basaltic cinder cones and are the southernmost part of the Mono–Inyo Craters volcanic chain.[2][12]

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 14 inches (36 cm) of precipitation a year.[13] Annual precipitation around Mammoth Lakes, which is close to the Inyo volcanic chain, is about 23 inches (58 cm).[14] Moisture travels over the Sierra crest from the Pacific Ocean through the San Joaquin Gap.[15]^: 30 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).[13] Temperatures near the Inyo volcanic chain 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).[14]

Most of the surface of the Mono Craters is barren but its slopes are covered by Jeffrey pine forest and partial greenery.[15]^: 4 Pumice Valley, directly to the west, is covered by sagebrush scrubland.[15]^: 4 The soil consists primarily of deep pumice, which does not hold water well.[15]^: 30 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.[15]^: 30 Jeffrey pine forests also surround the Inyo volcanic chain[16] and Mammoth Mountain.[17] 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.[18]

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.[19]^: 137 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.[8]^: 291 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.[8]^: 291

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.[8]^: 294 Once the flow stopped, it formed steep sided tongues of sharp and angular rock that are typically 200 to 300 feet (60 to 90 m) thick and have scree piles along their base.[8]^: 294 South Coulee is 2.25 miles (3.6 km) long, 0.75 miles (1.2 km) wide and has a volume of 0.1 cubic miles (0.4 km³); making it the largest Mono Craters coulee in volume.[8]^{: 294–295} South Coulee originates from the crest of the Mono Domes, about 3 miles (5 km) from the southern end, flows down its east and west flanks and terminates at its foot.[8]^: 294 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.[8]^: 295 Permanent pockets of ice from snowmelt have been found 75 to 147 feet (23 to 45 m) inside the coulees and domes.[8]^: 295

Hand samples of the Bishop Tuff, normal with pumice on left, compressed with fiamme on right.

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.[20]^: 21–22 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.[21]

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.[8]^: 290 Volcanism occurred north of the chain, in the Bodie Hills, as far back as 28 million years.[22]^: 44 Nearly all the rock east of the Sierra Nevada in the Mono Basin area is volcanic in origin.[23]^: 371

Volcanoes erupted from 3.6 to 2.3 million years ago near what is now Long Valley.[24]^: 270 Rhyolitic eruptions occurred in and around Glass Mountain in the same area from 2.1 to 0.8 million years ago.[25] Volcanic ash from the massive (600 cubic kilometres or 140 cubic miles 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.[7] Eruptions around 300,000 years ago filled the west moat with 800 feet (240 m) of basaltic lava.[24]^: 276 Basaltic and andesitic eruptive activity then moved to Mono Basin and lasted from 40,000 to 13,000 years ago.[7]

Seismic data indicate that a magma chamber with an estimated volume of 48 to 144 cubic miles (200 to 600 km³) exists 5.0 to 6.2 miles (8 to 10 km) directly below the Mono Craters.[26][27]^: 231 About 660 feet (200 m) of subsidence has occurred within a ring fracture system centered on Pumice Valley west of the chamber in the last 700,000 years.[27]^: 231 The Mono Craters sit atop a 7.5-mile (12 km)-long arc on the eastern side of the 11-mile (18 km)-wide ring-fracture system.[27]^: 231 Magma feeding the domes may have exploited arc-shaped fissures around an intrusion of granitic rock deep below the chain.[8]^: 289 This magma chamber is separate from the magma chamber under Long Valley Caldera.[21] The recent eruptions of the Mono Craters have been similar in volume and nearly identical in composition ("crystal-poor high-silica rhyolite") to those of Glass Mountain that preceded the Long Valley Caldera-forming eruption.[10] It has been suggested that the Mono Craters volcanism may represent an early stage in the development of a future caldera.[27]^: 231,[28]^: 55,[29]

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.[24]^: 277 Eruptions of dacite and rhyodacite occurred in Mono Basin from 100,000 to 6,000 years ago.[7]

Eruptions in the last 5,000 years along the Mono–Inyo chain

Multiple eruptions of silica-rich rhyolite from 40,000 to 600 years ago built the Mono Craters.[20]^: 24 Black Point, today on the north shore of Mono Lake, is a flattened volcanic cone of basaltic debris that formed under the surface of a much deeper Mono Lake about 13,300 years ago, during the most-recent glacial period.[22]^: 53 Several eruptive episodes from 1,600 to 270 years before present in Mono Lake formed Negit Island.[22]^: 54 The magma reservoir feeding the Mono Lake Volcanic Field is unrelated to the Mono Craters magma reservoir.[10]

Basaltic andesite lava built the Red Cones, two small cinder cones 6.2 miles (10 km) southwest of Mammoth Lakes, around 8,500 before present.[30] The five Mammoth Mountain Craters are a set of explosion pits that trend west-north-west for 1.6 miles (2.5 km) near the northern flank of Mammoth Mountain.[30]

None of the Mono Craters near the lake show the effects of wave erosion, but a hill at the southern end of the field shows what Israel Russell called a "beach line".[23]^: 384 The present elevation of this beach line is the level of the Mono Lake high stand before the formation of the northern Mono Craters,[23]^: 384 plus any surface deformation that has happened since that time. Stream-rounded stones are found on the volcanoes, and were lifted up as the volcanoes grew.[23]^: 388 Although glaciers were present throughout the Sierra Nevada, they did not reach as far down as the Mono Craters.

The most recent eruptive episode on the Mono Craters occurred sometime between the years 1325 and 1365.[31][32] 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 km) long.[33]^: 67 A mix of ash and pulverized rock, called tephra, covered about 3,000 square miles (8,000 km²) of the Mono Lake region. The tephra were carried by the wind and deposited in a layer 8 inches (20 cm) deep 20 miles (32 km) from the vents and 2 inches (5 cm) deep 50 miles (80 km) away.[33]^: 67

Pyroclastic flows of hot clouds of gas, ash and pulverized lava erupted from these vents in narrow tongues that extended up to 5 miles (8 km) away and covered 38 square miles (100 km²).[31][33]^: 67–68 Rhyolite lava oozed out of the vents to form several steep-sided domes, including Panum Dome and the much larger North Coulee flow.[33]^: 68 The youngest domes and coulees are 600 to 700 years old and are, therefore, the youngest mountains in North America.[8]^: 290

The Inyo volcanic chain formed approximately 600 years ago.[34] This activity occurred just a few years after the Mono Crater eruptions, and was caused by a dike of similar composition.[33]^: 69 The dike eventually became 6.8 miles (11 km) long and up to 33 feet (10 m) wide.[34] The ground above the dike was significantly cracked and faulted.[35]

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.[34][36] Pieces of molten and solid rock were ejected, small craters were formed, and an eruption column rose above the vents.[34][36] 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.[33]^: 69 A pyroclastic flow from South Deadman vent traveled about 3.7 miles (6 km).[34]

Some of the open pits were filled with thick and slow-moving lava to form the South Deadman Creek, Glass Creek and Obsidian Flow domes.[36] 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.[28]^: 55 In the past 6,000 years, approximately 0.19 cubic miles (0.8 km³) of magma has been erupted from the Inyo part of the chain.[27]^: 233

The last recorded volcanic activity in the chain was at Mono Lake between the years 1720 and 1850.[37] 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.[33]^: 69 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.[23]^: 372

People have used resources on and around the Mono–Inyo Craters for centuries. Mono Paiutes gathered obsidian from the Mono–Inyo Craters to make sharp tools and arrow points.[38] 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.[39]

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.[40] Timberland to the east of the Mono Craters was clearcut for wood.[41]

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.[8]^: 296 Excavation of an 11.5-mile (18.5 km) water tunnel under the southern part of the Mono Craters dome complex started in 1934 and was completed in 1941.[22]^: 51 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.[22]^: 51 Water diverted from its natural outlet in Mono Lake passes through the tunnel on its way to the Los Angeles Aqueduct system.

The chain of craters has been the subject of several writers and naturalists. 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 (1872) that the lake was in a "lifeless, hideous desert ..." that was the "loneliest spot on earth ... little graced with the picturesque."[42]

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."[43] Muir described the Mono Craters as "... heaps of loose ashes that have never been blest by either rain or snow ..."[43]

In the spring of 1881 and the fall of 1882, geologist Israel Russell studied the area as a side-trip during his field research of Lake Lahontan, a now dry lake that covered much of nearby Nevada during the last glacial period.[23]^: 267 His Quaternary History of the Mono Valley (1889), which included a topographic survey by Willard D. Johnson, was the first thorough scientific description of Mono Lake and its volcanic features.

Russell named the Mono Craters and wrote: "The attention of every one who enters Mono Valley is at once attracted by the soft, pleasing colors of these craters as well as by the symmetry and beauty of their forms. They are exceptional features in the scenery of the region, and are rendered all the more striking by their proximity to the angular peaks and rugged outlines of the High Sierra."[23]^: 378

The Mono Basin National Forest Scenic Area, created in 1984, was the first National Scenic Area in the United States.[44] 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.[45] 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.[46]

Potential thickness of tephra on the ground from eruptions of less than 0.25 cubic miles (1 km³)

A wide range of effects are expected from future eruptions along the Mono–Inyo Craters. Ash and rock fragments (tephra) may accumulate to a thickness of 33 feet (10 m) near an erupting Mono–Inyo vent.[47] Downwind accumulations of tephra may exceed 7.9 inches (20 cm) at a distance of 22 miles (35 km) and 2.0 inches (5 cm) at 53 miles (85 km).[48]^: 8 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.[48]^: 8 Grain size and thickness of tephra generally decreases gradually with distance from a vent. Volcanic ash will likely contaminate air routes east of the vent.[47]

Severe damage from super-heated flows of gas, ash and pulverized rock (pyroclastic flows and surges) may occur at least 9.3 miles (15 km) from an explosive eruption.[48]^: 8 The amount of damage depends on vent location, topography, and volume of magma erupted. Pyroclastic flows from vents on Mammoth Mountain or other high vent could travel farther by gaining extra momentum from their descent. Valleys along the route will be more impacted than ridges but flows and surges could overtop some ridges. Eruptions near snowpacks may produce lahars of mud and ash that devastate valleys and watersheds. Steam blast eruptions under a lake could form large waves capable of flooding nearby areas and starting mudflows.[48]^: 7

Basalt lava flows may extend more than 31 miles (50 km) from their vent.[48]^: 5 Dacite and rhyolite lavas produce short, thick flows that rarely extend more than 3.1 miles (5 km) from their vent.[48]^: 5 Mound-shaped features called lava domes are often created from these flows. Rock fragments thrown from a growing lava dome may reach 3.1 to 6.2 miles (5 to 10 km) from the dome.[47] A partial collapse of the steep-sided growing dome can send pyroclastic flows outward at least 3.1 miles (5 km).[47] Taller domes tend to form larger pyroclastic flows that travel farther.