Craters of the Moon, ID


In 2007, I toured and explored the Craters of the Moon National Park and vicinity with a group of teachers, while traveling in southeastern Idaho. A science-teacher-friend of mine, Mark Tolman, helped me document the visit.

Public domain road map of southeastern Idaho

Image above, the main entrance to Carters of the Moon National Monument and Preserve lies about 29 km southwest of Arco, Idaho and is easily accessed from combined US20/US26/US93. There is 11 km paved loop road to visit the variety of volcanic geo-sites.

Public domain map of the volcanic sites found within the Craters of the Moon National Monument and Preserve.

Upon entering the Monument the road leaves the sagebrush desert and enters an area of barren black cinders and basalt (lava). Here, the road winds among the smooth cones and across strips of rough, fresh-looking rock. The similarity of the dark craters and the cold basalt nearly destitute of vegetation to the surface of the moon as seen through a telescope gives to these peculiar features their name. The Monument comprises the most interesting and recent part of a vast basalt field that covers hundreds of square kilometers and merges westward into the Columbia Plateau. This Plateau cover about 300,000 square kilometers.

The northern-entrance sign found on US 20/26; (photo taken in 2022).

Smooth cinder cones and rough, uneven spatter cones ranging in age from 15,000 years to about 2,000 years rise above a sea of black basalt (lava) flows whose rope folds and jagged, blocky tops are so new they are not yet mantled in vegetation.

Looking south along the US20/26 across the Craters of the Moon National Monument at a family of small volcanoes, (photo taken in 2022).

Three kinds of small volcanoes and two kinds of lava flows are represented here, all with counterparts in other volcanic regions.

Looking east from US 20/26 through Craters of the Moon National Monument at Big Cinder Butte, (photo taken 2022).

Pictured above, this region was referred to as “the Cinder Buttes”. Towering above the surrounding landscape by more than 200 meters and spread across an area of more than 8 square kilometers the tallest and largest of these features was appropriately named the Big Cinder Butte. This volcano is just one of the more than 25 cinder cones that make up this extraordinary landscape.

Looking north at Becky (my wife) and I into the “North Crater” at Craters of the Moon National Monument, (photo taken 1998).

Pictured above, cinder cones are steep-sided hills of small, loose fragments of bubble-filled volcanic rock called scoria. Thrown into the air during early stages of eruption, when the molten rock (magma) is frothy and full of gas, scoria cinders fall on all sides of the volcanic vent. Most of them solidify before they reach the ground. As a cinder cone grows, loose cinders avalanche down both the outside and the inside of the crater, controlling the degree of slope at about 30 degrees. At times, cinder cones emit fiery fragments that do not solidify in the air but fall still molten to the ground to weld themselves in place. Late in cinder cone development, lava may flow from vents that open on the sides or at the base of the cone.

Looking southeast, across the Craters of the Moon Monument lava field where the “Great Rift” resides in 2007.

Pictured above, a double line of cinder cones stretches southeastward from the monument headquarters along two parallel fissures that cut the Snake River Plateau here in the national monument, and mark the so-called “Great Rift”. In the southeastern part of the monument some fissures are still partly open and visible as linear depressions in the lava surface. They show that the land here is still being pulled apart, (shown later below).

Two spatter cones found in the Craters of the Moon Monument in 2007

Pictured above, interspersed among the cinder cones and helping to mark the two fissures are lines of “spatter cones”. Just as their name implies, these cones build up from dribbles and clots of lava that spatter out of volcanic openings and fall to the ground in still molten condition.

Two spatter cones found in the Craters of the Moon Monument in 2007

Pictured above and below, spatter cone side are steeper than the slopes of cinder cones because the spatter adheres to itself and shows no tendency to slide.

Spatter cones found in the Craters of the Moon Monument in 2007

Pictured above, the spatter cones are smaller than most cinder cones, never over about 15 meters high.

Looking south across the parking lot of Inferno Cone, (a shield volcano), at the Craters of the Moon Monument in 2007.

Pictured above, shield volcanoes, (broad, low lava cones), are less obvious the either cinder or spatter cones, but they occur at the Craters of the Moon also. They are formed by very fluid lava that spills from vents onto an essentially flat surface and flows outward in all directions.

Looking southeasterly across the parking lot of Inferno Cone, (a shield volcano), at the Craters of the Moon Monument in 2007.

Pictured above, successive layers build a lava-covered surface with the highest points at the vents. Slopes may be only a few degrees, depending on the runniness of the lava. Since some of the lavas in this area were extremely fluid, the angles of slope are quite low.

Looking southeast from US20/26 at “Silent Volcano” (a broken crater), within the Craters of the Moon Monument, (photo taken in 2022).

Pictured above, several of the small volcanoes within the Craters of the Moon National Monument have been broken, irregular craters. Silent and North Craters are easily accessible examples.

Aerial view of the “Silent Crater” within Craters of the Moon National Monument, (image taken from an interpretive sign on US20/26 in 2022).

Imaged above, as new vents opened up and lava welled forth, moving rivers of molten rock broke away huge pieces of crater wall and rafted them along in sluggish streams. The jagged crater fragments can be seen as craggy turrets projecting well above the rough sea of basalt. [Note that the silent volcano made some noise approximately 6,500 years ago when eruptions ejected cinders and pumped out lava from the crater]. {Also note that the shady north-facing slope of this cinder cone supports a forest of Limber pines and a few larger Douglas fir trees that provide habitat for a variety of wildlife}.

Examples of pahoehoe and a’a lava (basalt) at the Craters of the Moon in 2007

Pictured above, lava flows of 2 types were recognized ad named by Hawaiian natives long before the advent of geologic studies at the Craters of the Moon National Monument. The Hawaiian names: a’a and pahoehoe.

Pahoehoe lava, (basalt) found at the Craters of the Moon in 2007

Pahoehoe lava cools with an undulating, continuous surface that may be wrinkled or ropy. Lavas that form it are very hot; because their outside “skin” cools to plasticity even while the still-molten interior continues to flow, it commonly twists and folds with remarkable pliability.

Mark Tolman is standing above and pointing at a “Lava Tube” within the Craters of the Moon National Monument in 2007.

Pictured above and below, Pahoehoe lava moves rapidly, sometimes faster than a person can run. Often the outer skin thickens and becomes rigid enough to support itself, yet the interior goes on flowing, finally draining out from under the crust and leaving behind an empty lava tunnel called a “Lava Tube”.

Mark and I are exploring the “Indian Tunnel” at the Craters of the Moon National Monument in 2007

Pictured above and below, several such tunnels at Craters of the Moon are ridged inside with high-lava markings, (narrow benches that register the former positions of a succession of flow surfaces).

Mark and I are exploring the “Indian Tunnel” at the Craters of the Moon National Monument in 2007.

Pictured above, tunnel ceilings are prickly with lava stalactites that vividly illustrate the fluid condition of the lava as it drained out of the tunnel. [Note that the ceiling is sometimes not supported enough and it fails by falling in].

The nearby “Shoshone Ice Caves and lava tube” found on private land southwest of the Craters of the Moon National Monument. (Photo taken in 2007 near Shoshone, Idaho).

Due to the porous nature of basalt, water seeping through the ceiling of the lava tube pools on the floor of the tube. Since, basalt has a insulating property, the water is protected from the sun’s radiation, and freezes.

Inside the “Shoshone Ice Cave or Lava Tube” southwest of Craters of the Moon in 2007.

The ice-cold caves were also known to stay cool enough for the ice to remain frozen throughout the summer season. As such, the town of Shoshone was popular among travelers during the days when refrigeration was uncommon as it was the only place in the West where iced cold beer was served.

Inside the “Shoshone Ice Cave or Lava Tube” southwest of Craters of the Moon in 2007.

The Shoshone Ice Caves are lava tubes believed to be formed 24,000 years ago after the eruption of the nearby Black Butte volcano.

The “Shoshone Ice Cave” tourist trap in 2007

Pictured above, the Shoshone Ice Cave is a natural wonder that have grown to become a popular destination among visitors visiting Craters of the Moon National Monument.

A’a lava example found at the Craters of the Moon National Monument in 2007.

Pictured above, a’a lava has a rough, rubbly, quite obviously broken surface nearly impossible to walk across. It results from breakup of a thick, hard surface crust on a slow-moving, very thick and pasty lava flow. An aa flow advances at a crawl, (only a few meters an hour).

Looking north through the “Devil’s Garden” within the Craters of the Moon National Monument in 2007.

Pictured above, coarse, broken blocks of a’a lava tumble from its steep advancing edge, to be followed by more broken blocks, so the flow seems to roll forward on itself like the tread of an armored tank.

Looking northwest through the “Devil’s Garden” within the Craters of the Moon National Monument in 2007.

Pictured above, the difference between the 2 types of lava seems to be a matter of temperature, silica content, and dissolved gases. Pahoehoe flows are hotter, less siliceous, and very gaseous; a’a flows are cooler, more siliceous, and less gaseous. Both types contain small bubbles of volcanic gas that leave little rounded chambers as the flows solidify. These bubbles, called vesicles, may be pulled out and elongated by lava movement. They are in places so numerous that the rock looks like a sponge. Such rock, called scoria, is quite light in weight.

A public domain image of Scoria, (volcanic basalt).

Almost all the volcanic rocks in the national monument are made of basalt. Basalt magma originates 40 km or more below the earth’s surface, where heat from the earth’s interior is passed upward toward the crust. By and large, the pressure exerted by overlying rocks is great enough to keep material below the crust in a almost solid state, even at temperatures of 2200 degrees Farhenheit.

Hell’s Half Acre

North America’s slow passage over the Yellowstone hot spot had completely formed the broad eastern Snake River Plain by about 2 million years ago, but localized volcanism linger long after the main activity moved into northwest Wyoming, (imaged below).

Yellowstone “Hotspot” starting near the Nevada/Idaho boarder, (image is taken from “Earth: An Introduction to Physical Geology” Lutgens and Tarback 2004).
Public domain image of the formation of the Snake River Plain by the northeastern movement of the North American Continent, (image is taken from “Earth: An Introduction to Physical Geology” Lutgens and Tarback 2004)).

Image above and below, smaller northwest-trending faults related to Basin and Range uplift of the major mountain ranges north and south of the plain.

Public domain image of the “Great Rift” of the Snake River Plain (image is taken from “Earth: An Introduction to Physical Geology” Lutgens and Tarback 2004).

Imaged above and below, the monument’s “Great Rift”, (a zone of fractures), is the best developed example of such fault systems. Basaltic lava with its low silica content tends to flow fluidly from these fissure sources, spreading out to build low structure such as cones and shields.

The “Great Rift” at the Craters of the Moon National Monument; (image taken from the southeastern corner of the monument, near the “Kings Bowl” at American Falls, Idaho).

Eight eruptive periods have occurred during the past 15,000 years along the Great Rift at Craters of he Moon.

Aerial photo taken of the “Great Rift” over the Kings Bowl at the southeast portion of Craters of the Moon near American Falls, Idaho.

Flow surfaces are well preserved because of the dry climate and lack of soil and plant life. Pictured below, basalts lose heat rather rapidly when quickly traveling down hell from a source fissure, causing the flow surface to stiffen and fragment into a rough and jagged texture of a’a vesicles, (fossil bubbles formed by escaping gases), are ubiquitous in the rock.

Looking south of the Kings Bowl along the “Great Rift” in the southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

Past eruptions have been spaced as much as 3,000 years apart over the past 15,000 year, and the most recent volcanism occurred on 2,000 years ago, near the Kings Bowl. Pictured above and below, the Kings Bowl lies along the Great Rift, (a narrow 80 km zone of young volcanic vents cutting northwest across the Snake River Plain).

Looking south of the Kings Bowl along the “Great Rift” in the southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

The eruption at the Kings Bowl location began about 2,200 years ago when magma ascended through a narrow, linear fracture and poured out as lava. This type of eruption, (known as a fissure eruption), differs markedly from other eruptions that emanated from a centralized vent.

Looking south of the Kings Bowl along the “Great Rift” in the southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

Pictured above and below, depending on the amount of gas present and the width of the fissure, these eruptions may blast clots of lava, or spatter, tens or even hundreds of meters into the air, but ultimately they tend to be benign, producing expansive lava flows that inundate the surrounding landscape.

Looking south of the Kings Bowl, within the “Great Rift” in the southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

Based on the relatively small area, (about 3.4 square kilometers), of the kings Bowl lava field, we can surmise the eruption in this area was a relatively short-lived, (perhaps lasting only a few hours or so).

A small trail inside the “Kings Bowl” (closed), southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

Pictured above, the eruption at Kings Bowl began about 2,200 years ago when magma ascended through a narrow, linear fracture and poured out as lava. This type of eruption, (known as a fissure eruption), differs markedly from other eruptions that emanated from a centralized vent. {Note that the layers of lava from earlier eruptions are visible in the crater walls. In some places, a combination of heat and water oxidized the lava, turning it red}.

Kings Bowl gas eruption, image taken from “Geology Underfoot in Southern Idaho, Willsey 2017”.

Throughout much of the fissure eruption, the pressure of rising magma kept groundwater from entering the fissure and mixing with the magma, resulting in the typical basalt lake. However, as the eruption waned, groundwater came in contact with hot magma and flashed to steam, creating tremendous gas pressure, (imaged above). Because this occurred relatively close to the surface, the steam pressure exceeded the weight of the overlying rock, and the resulting explosion broke chunks off the fissure wall and scattered them around leaving a massive hole in the fissure.

Inside the Kings Bowl, looking north along the “Great Rift” is the entrance to a old Ice Cave.

Pictured above, the tunnel once led to Crystal Ice Cave where, as recently as the 1980s, massive columns of ice attracted thousands of visitors. Ice seems unusual in a formerly fiery volcanic fissure. In the heat of midsummer, the idea of ice here seems even more unlikely.

Inside the Kings Bowl, looking north along the “Great Rift” is the entrance to Crystal Ice Cave.

The key dynamics that lead to ice formation are precipitation, climate, and air flow. Here in the high desert, snow accumulates all winter. In spring, as meltwater drips into underground cavities that have reached below-freezing temperature, it is cooled and crystallized into stalagmites and pillars of ice.

Cross-section image of Crystal Ice Cave at Kings Bowl, (image taken from an interpretive sign).

Imaged above, changes in the temperature and quantity of air flowing in and out of the caverns determine the amount of ice present. The historic tour entrance in now closed for safety reasons. Falling rocks, unstable walls, and cracks hundreds of meters deep are all hazards found in the underground area.

Peculiar mushroom-shaped mounds found a few meters west of Kings Bowl, southeastern part of the Craters of the Moon National Monument, (photo taken in 2022).

Pictured above, is a group of peculiar mushroom-shaped mounds littering the lava flow, just a few meters west of the Kings Bowl. These odd features have a markedly different texture than the angular blocks; they tend to be smooth and ropy, indicating that flowing lava was likely involved in their origin.

Kings Bowl gas eruption, image taken from “Geology Underfoot in Southern Idaho, Willsey 2017”.

During the explosive phase of the eruption, a lava lake formed on the west side of the fissure where it was hemmed in by elevated terrain. As the surface of this lava lake began to solidify, a thin crust formed above it’s molten interior. Angular basalt blocks thrown from Kings Bowl struck the surface, penetrating the hardened crust. Through these openings, lava oozed out and cooled, forming the strange features called squeeze-ups.

A closer look at a “squeeze-up” formation near the Kings Bowl.

Pictured above, most of the squeeze-ups are somewhat conical in shape, resembling candy kisses or cow patties. A few angular blocks lie in holes with no accompanying squeeze-up.

Looking east across the Snake River Basaltic Plain, from US 20/26 on the northside of Craters of the Moon National Monument in June of 2022.

Most of the eastern Snake River Plain is virtually flat, thanks to the horizontal basaltic lavas that flowed out across older rhyolite over the past few million years. Subtle volcanic vents are hard to spot from the ground, particularly the inconspicuous low domes of shield volcanoes typically constructed by fluid basalt eruptions. Yet here and there we see a distinct butte rising from the plain whose origin is clearly volcanic. Examples include East, Middle, and Big Southern Buttes west of Idaho Falls, and Menan Buttes west of Rexburg, Idaho. {Note the purple Dwarf Monkeyflower in the foreground}.

Looking south at “Big Southern Butte” southwest of Craters of the Moon National Monument & just west of the old “Atomic City” ghost town, (picture taken in 2022)

Pictured above, Big Southern Butte is a partial rhyolitic dome. Rhyolite lava is more viscous (thicker) than basalt lava because of the abundance of silica and a lower temperature. This difference is much like the difference in viscosity between sticky bread dough and runny pancake batter. When erupted from a relatively small vent area of extruding lava, rhyolite can build up an obvious edifice, or dome.

Looking south at the “Big Southern Butte” 15 km west of Atomic City, Idaho on 4WD roads.

Picture above, an elevated island amid a vast sea of basalt, “Big Southern Butte abruptly rises 730 m above the desolate plain and has a diameter of about 6.4 km. This makes it one of the largest lava domes on Earth. Its steep slopes exceed 30 degrees in places.

On top of the western side of the Big Southern Butte, looking over onto the northeastern side. Note the white rhyolite cliffs. The photo was taken in 2022.

Pictured above, Big Southern Butte’s steep slopes reflect the magma composition of lava domes. The rhyolitic tacky lava does not flow far, accumulating and stacking up upon itself to form a steep, massive pile. Occasionally, the slow and steady extrusion is interrupted or preceded by minor explosive outbursts of ash, caused by the release of gases trapped in the sticky magma.

White Rhyolitic rock found on the western and southern side of the “Big Southern Butte”

Pictured above and below, half of the “Big Southern Butte” dome consists of a white rhyolite (the western and southern sides) and the other half consists of basalt (the eastern and northern sides). Rhyolite is the product of high-silica magma, erupting at a relatively low temperature (around 1,600 degrees Fahrenheit) and possessing a viscous or sticky consistency. In contrast, basalt originates from low-silica, high-temperature magma (around 2,200 degrees Fahrenheit) and is much les viscous, flowing easily.

Tilted basalt lava flows on the northern and eastern side of the “Big Southern Butte”. Picture taken near the top of the southern end of the dome in 2022.

Pictured above, basalt is confined solely to the northern slope, where it forms an impressive, continuous 4.7 square kilometer block. This immense slab is made of a series of stacked lava flows, the whold package dipping steeply to the northeast.

The evolution of the “Big Southern Butte”, (image taken from “Geology Underfoot in Southern Idaho”, Willsey 2017).

Pictured above, the eruption of “Big Southern Butte” begun about 300,000 years ago as an intrusion of rhyolitic magma pushed its way upward through basalt flows. (A) Rhyolitic magma rises and accumulates at the base of older basalt. (B) The continued intrusion, domes the basalt upward. (C) Rhyolitic lava oozes onto the surface on south side of Big Southern Butte. A steeply dipping basalt slab remains on the north side, while others sink, due to their density, into the buoyant rhyolite.

Looking east towards Idaho Falls on US20 are the Middle and East Butte, (photo take in 2022).

Pictured above, looking eastward from Atomic City and the “Big Southern Butte”, are two prominent buttes rising abruptly from the basaltic plains about 27 km away. The closer and rightmost of the two is Middle Butte, and the pointy butte topped with communication towers is East Butte. Along with Big Southern Butte, these form the three most prominent landmarks in the region. Like their big neighbor, both are lava domes, but Middle Butte lacks surface exposures of rhyolite and is cover by older basalt. Subsurface data suggests it is underlain by a shallow mass of rhyolite, which domed the basalt as it rose, creating the steep profile characteristic of lava domes. The three buttes are roughly aligned in a northeast direction, parallel to the Snake River Plain’s axis but at odds with the dominant northwesterly tren of other linear volcanic features and faults in the region.

Looking north at South Menan Butte, west of Rexburg, Idaho; (photo take in 2022).
Looking north from atop of the “South Menan Butte” at the rim and its composition of tuff flows. (Photo taken in 2022)

Pictured above, in contrast to the rhyolite domes, Menan Buttes, are tuff cones formed explosively between 140,000 and 10,000 years ago when the ascending basaltic magma hit near-surface, water-saturated alluvial sediment and basaltic lava flows.

Map showing Menan Buttes southwest of Rexburg, Idaho and northeast of the Big Southern Butte; (Image taken from “Idaho Rocks! A Guide to Geologic Sites in the Gem State”, Lewis, McFaddan, Burch, & Feeney 2020)
Looking north at North Menan Butte from atop of South Menan Butte, (photo take in 2022)

Pictured above, the Menan Buttes asymmetrical shapes were caused by prevailing southwest winds that blew the granules-small pebbles sized fragments produced by the steam powered detonations.

Looking south across the crater of the “South Menan Butte” from the rim. Photo taken in 2022.

I was introduced to the Menan Buttes when studying topographic maps and stereo aerial-images in 3-D at the University of Utah Geology Department in the 1980s.

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