12.2: Northern Basin - Mono-inyo Craters

The Mono–Inyo Craters are a volcanic chain of craters, domes and lava flows in Mono County, Eastern California. The chain stretches 25 miles (40 km) 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.

In hydrothermal systems, the circulation of ground water is driven by a combination of topography and geothermal heat sources. The system in the Long Valley Caldera is recharged primarily from snowmelt in the highlands around the western and southern rims of the caldera. The meltwater infiltrates to depths of a few kilometers (or miles), where some is heated to at least 430 °F (220 °C) by hot rock near the Inyo craters. The heated water, kept from boiling by high pressure, still has lower density than cold water, and it rises along steeply inclined fractures to depths of 0.3–1.25 miles (0.48–2.01 km). It then flows eastward through rock layers to hydrothermal vent discharge points at the surface along Hot Creek, around Crowley Lake and in Mammoth. The water temperature declines eastward because of heat loss and mixing with cold water, and in the springs near Crowley Lake temperatures are at only about 125 °F (50 °C).

Figure 12.3: Geothermal Springs at Hot Creek. California. Image by Jeremy Patrich is licensed under a CC-BY 4.0.

Volcanic 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.

Figure 12.4: Satellite Photo of the Mono-Inyo Craters& Mammoth Mountain, California. Image is in the public domain.

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.

Physical Geography

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 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. 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 (elevation 9,172 feet or 2,796 m), which rises 2,400 feet (730 m) above Pumice Valley to the west.  Associated volcanic features are 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.

Figure 12.5: Map of Mono & Long Valley Calderas. Image is in the public domain.

Inyo Volcanic Chain

The Inyo volcanic chain stretches 6 miles (10 km) from Wilson Butte to the Inyo Craters, proper. 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. A quarter mile (half kilometer) 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.

Figure 12.6: An Aerial Photo of the Mono Craters—With Panum Crater in the Middle. Image is in the public domain.

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 20 feet (6 m) 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 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.  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.

Case Study - Panum Crater

Panum Crater is a volcanic cone that is part of the Mono–Inyo Craters. Panum Crater is between 600 and 700 years old, and it exhibits all the characteristics of the textbook rhyolitic lava dome. Rhyolitic volcanoes are characterized by having large amounts of silica (quartz) in their lava. The content of silica at Panum is about 76 percent. It makes the lava very viscous, or thick, and very glassy. Products of this rhyolitic eruption are pumice and obsidian, the volcanic glass that Native Americans used to make arrow points and scrapers.

Panum Crater formed in a sequence of events. The first event was caused by magma rising from deep within the Earth's crust. When this extremely hot, liquid rock contacted water just below the surface, the water expanded into steam and a large, violent eruption occurred. The material that was thrown into the air by the steam, mainly old lake bottom sediments, was deposited around the new vent in little mounds. So much debris was blown out that a gaping crater was left behind.

Once this debris was blown out, a fountain of cinders shot up a great distance into the sky. As this huge amount of ash and pumice began to fall back towards the earth, it formed a pumice ring, or cinder cone, about the original vent. This cinder cone is still visible today.

Figure 12.7: Birdseye View of Panum Crater, Near Mono Lake. Image is in the public domain.

Following the violent eruptions of the first two phases, the remainder of the thick magma slowly rose to the surface in a series of domes. Each dome began with an outpouring of the viscous, rhyolitic lava which hardened and formed a cap over the vent. As magma continued to push up, the cap (or dome) shattered and fell to the outside of the newly formed dome. This happened so many times that a new mountain was created out of these broken pieces, called crumble breccia. The mountain continued to build in this manner until the force within the volcano weakened and no more new domes formed. The final one still stands today.

As the final dome hardened, a period of spire building began. Thick lava pushed up through cracks of the hardening dome and formed castle-like spires. The formation of the spires was analogous to toothpaste squeezing through the opening of a tube and forming a small tower before it topples over. Most of the spires at Panum Crater fell over and broke because of their rapid cooling and because of many small explosions at their bases. Most of the rocky debris at the top of the dome is the remains of spires that have crumbled.

The central lava dome was erupted from degassed material and is made up of pumice and obsidian of the same composition. The difference between the two has to do with gas escaping as the magma cooled. The magma that created the dome had dissolved gas in it, like a bottle of seltzer water. As the magma rose towards the surface where there was less pressure on it than at depth, the gas expanded producing the holes or bubbles in the pumice. The magma that remained pressurized while it cooled quickly or that had already lost its gas, formed the obsidian.

Field Trip: Panum Cratere

Let’s head on a field trip to explore a Panum Crater, near Mono Lake, California. Either scan the QR code or visit this link to join Professor Patrich for a lecture at the 700-year-old volcanic crater. (Video length: 5 min).

Weather & Climate

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. Annual precipitation around Mammoth Lakes, which is close to the Inyo volcanic chain, 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 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).

Flora & Fauna

Most of the surface of the Mono Craters is barren but its slopes are covered by the world’s largest 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.

As a remnant of Pleistocene Lake Russel, Mono Lake has hypersalinity and high alkalinity (pH=10 or equivalent to 4 milligrams of NaOH per liter of water). This means that no fish are native to the lake. However, the whole food chain of the lake is based on the high population of single-celled planktonic algae present in the photic zone of the lake. These algae reproduce rapidly during winter and early spring after winter runoff brings nutrients to the surface layer of water. By March the lake is "as green as pea soup" with photosynthesizing algae.

The lake is famous for the Mono Lake brine shrimp, Artemia monica, a tiny species of brine shrimp, no bigger than a thumbnail, that are endemic to the lake. During the warmer summer months, an estimated 4–6 trillion brine shrimp inhabit the lake. Brine shrimp have no food value for humans but are a staple for birds of the region. The brine shrimp feed on microscopic algae.

Figure 12.8: A Brine Shrimp from Mono Lake. Image by Wikimedia is licensed under CC BY 2.0.

Mono Lake is a vital resting and eating stop for migratory shorebirds and has been recognized as a site of international importance by the Western Hemisphere Shorebird Reserve Network. Nearly 2,000,000 waterbirds, including 35 species of shorebirds, use Mono Lake to rest and eat for at least part of the year. Some shorebirds that depend on the resources of Mono Lake include American avocets, killdeer, and sandpipers. One to two million eared grebes and phalaropes use Mono Lake during their long migrations.

Late every summer tens of thousands of Wilson's phalaropes and red-necked phalaropes arrive from their nesting grounds, and feed until they continue their migration to South America or the tropical oceans respectively.

Physical Geology

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 three 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, and 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.

About an hour south it the Owens Valley is a graben—a down-dropped block of land between two vertical faults—the westernmost in the Basin and Range Province. It is also part of a trough which extends from Oregon to Death Valley called the Walker Lane.

Figure 12.9: Map of Volcanoes Between Mono Lake & the Long Valley Caldera. Image is in the public domain.

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 (600 cubic kilometers 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. 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.

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.

Case Study - Mono Lake Tufa Towers

Many columns of limestone rise above the surface of Mono Lake, yet were, and only can be formed underwater. These limestone towers consist primarily of calcium carbonate minerals such as calcite (\(\ce{CaCO3}\)). This type of limestone rock is referred to as tufa, which is a term used for limestone that forms in low to moderate temperatures.

Mono Lake is a highly alkaline lake, or soda lake. Alkalinity is a measure of how many bases are in a solution, and how well the solution can neutralize acids. Carbonate and bicarbonate are both bases. Hence, Mono Lake has a very high content of dissolved inorganic carbon. Through supply of calcium ions, the water will precipitate carbonate-minerals such as calcite. Subsurface waters enter the bottom of Mono Lake through small springs. High concentrations of dissolved calcium ions in these subsurface waters cause huge amounts of calcite to precipitate around the spring orifices. As mentioned earlier, the tufa only forms at the bottom of the lake. It took many decades or even centuries to form the well-recognized tufa towers. When lake levels fell, the tufa towers came to rise above the water surface and stand as the pillars seen today.

Figure 12.10: Tufa Towers at the Southern Edge of Mono Lake. Image by Jeremy Patrich is licensed under CC-BY 4.0.

Field Trip: Mono Lake

Let’s head on a field trip to explore the tufa towers at Mono Lake, California. Either scan the QR code or visit this link to join Professor Patrich for a lecture at the southern edge of the lake. (Video length: 7min)

Cultural Geography

The current tribal name "Mono" is a Yokutsan loanword from the tribe's western neighbors, the Yokuts, who however designated the Owens Valley Paiutes as the southernmost Northern Paiute band living around Owens and Mono Lakes as monachie/monoache ("fly people") because fly larvae was their chief food staple and trading article.This "Kucadikadi Northern Paiute Band", whose autonym Kutsavidökadö/Kutzadika'a means "eaters of the brine fly pupae", are also known as Mono Lake Paiute or Owens Valley Paiute, a holdover from early anthropological literature, and are often confused with the non-Northern Paiute ethnic group of the Western mono "Mono".

Figure 12.11: An Early Photo of Mono People with Their Baskets of Acorns, Circa 1920. Image is in the public domain.