The Project Gutenberg eBook ofCraters of the Moon

The Project Gutenberg eBook ofCraters of the MoonThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: Craters of the MoonCreator: United States. National Park Service. Division of PublicationsRelease date: August 21, 2020 [eBook #62994]Most recently updated: October 18, 2024Language: EnglishCredits: Produced by Stephen Hutcheson and the Online DistributedProofreading Team at https://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK CRATERS OF THE MOON ***

This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.

Title: Craters of the MoonCreator: United States. National Park Service. Division of PublicationsRelease date: August 21, 2020 [eBook #62994]Most recently updated: October 18, 2024Language: EnglishCredits: Produced by Stephen Hutcheson and the Online DistributedProofreading Team at https://www.pgdp.net

Title: Craters of the Moon

Creator: United States. National Park Service. Division of Publications

Release date: August 21, 2020 [eBook #62994]Most recently updated: October 18, 2024

Language: English

Credits: Produced by Stephen Hutcheson and the Online DistributedProofreading Team at https://www.pgdp.net

*** START OF THE PROJECT GUTENBERG EBOOK CRATERS OF THE MOON ***

Handbook 139Craters of the MoonA Guide to Craters of the MoonNational MonumentIdahoProduced by theDivision of PublicationsNational Park ServiceU.S. Department of the InteriorWashington, D.C. 1991

A Guide to Craters of the MoonNational MonumentIdaho

Produced by theDivision of PublicationsNational Park Service

U.S. Department of the InteriorWashington, D.C. 1991

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Craters of the Moon National Monument protects volcanic features of the Craters of the Moon lava field.Part 1of this handbook introduces the park and recounts its early exploration.Part 2explores how life has adapted to the park’s volcanic landscape—and how people have perceived it.Part 3presents concise travel guide and reference materials for touring the park and for camping.

National Park Handbooks are published to support the National Park Service’s management programs and to promote understanding and enjoyment of the more than 350 National Park System sites, which represent important examples of our country’s natural and cultural inheritance. Each handbook is intended to be informative reading and a useful guide before, during, and after a park visit. More than 100 titles are in print. They are sold at parks and can be purchased by mail from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. This is handbook number 139.

Craters of the Moon: A Guide to Craters of the Moon National Monument, Idaho/produced by the Division of Publications, National Park Service.

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Rock or rope? Folds of lava rock look like coils of irregular rope. Lava flows of this type are known as pahoehoe, a Hawaiian word pronouncedpah-hoy-hoyand meaning ropey.

Light playing on cobalt blue lavas of the Blue Dragon Flows caught the inner eye of explorer Robert Limbert: “It is the play of light at sunset across this lava that charms the spectator. It becomes a twisted, wavy sea. In the moonlight its glazed surface has a silvery sheen. With changing conditions of light and air, it varies also, even while one stands and watches. It is a place of color and silence....”

Limbert explored the Craters of the Moon lava field in Idaho in the 1920s and wrote those words for a 1924 issue ofNational Geographic Magazine. “For several years I had listened to stories told by fur trappers of the strange things they had seen while ranging in this region,” wrote Limbert, a sometime taxidermist, tanner, and furrier from Boise, Idaho. “Some of these accounts seemed beyond belief.” To Limbert it seemed extraordinary “That a region of such size and scenic peculiarity, in the heart of the great Northwest, could have remained practically unknown and unexplored....” On his third and most ambitious trek, in 1924, Limbert and W. C. Cole were at times left speechless by the lava landscape they explored. Limbert recounted his impressions in magazine and newspaper articles whose publication was influential in the area’s being protected under federal ownership. In 1924, part of the lava field was proclaimed as Craters of the Moon National Monument, protected under the Antiquities Act. It was created “to preserve the unusual and weird volcanic formations.” The boundary has been adjusted and the park enlarged since then. In 1970, a large part of the national monument was designated by Congress as the Craters of the Moon Wilderness. It is further protected under the National Wilderness Preservation System.

Until 1986, little was known about Limbert except for those facts recounted above. That year, however, a researcher compiling a history of the national monument located Limbert’s daughter in Boise. Thedaughter still possessed hundreds of items, including early glass plate negatives, photographs, and manuscripts of her father and that shed more light on his life, the early days of Idaho, and Craters of the Moon. Some of these photographs served as blueprints for the National Park Service in the rehabilitation of fragile spatter cone formations that have deteriorated over the years of heavy human traffic. The Limbert collection has been fully cataloged by Boise State University curators and has already proven to be a valuable resource to historians interested in Limbert and this fascinating part of Idaho. Preservation of the area owes much to Limbert’s imaginative advocacy in the true spirit of the West in its earlier days.

Local legends, beginning in the late 1800s, held that this area resembled the surface of the moon, on which—it must now be remembered—no one had then walked! Geologist Harold T. Stearns first used the name Craters of the Moon when he suggested to the National Park Service, in 1923, that a national monument be established here. Stearns found “the dark craters and the cold lava, nearly destitute of vegetation” similar to “the surface of the moon as seen through a telescope.” The name Craters of the Moon would stick after Limbert adopted it inNational Geographic Magazinein 1924. Later that year the name became official when the area was set aside by President Calvin Coolidge as a national monument under the Antiquities Act.

Like some other areas in the National Park System, Craters of the Moon has lived to see the name that its early explorers affixed to it proved somewhat erroneous by subsequent events or findings. When Stearns and Limbert called this lava field Craters of the Moon, probably few persons other than science fiction buffs actually thought that human beings might one day walk on the moon and see firsthand what its surface is like. People have now walked on the moon, however, and we know that its surface does not, in fact, closely resemble this part of Idaho. Although there are some volcanic features on the surface of the moon, most of its craters were formed by the impact of meteorites colliding with the moon.

Moonscape or not, early fur trappers avoided the lava flows along the base of the Pioneer Mountains at the north of today’s park. In doing so, they followed Indian trails such as one found by Limbertthat “resembled a light streak winding through the lava. When the sun was directly overhead it could be seen to advantage, but at times was difficult to follow. Think of the years of travel,” Limbert marveled, “necessary to make that mark on rock!” At least one Indian trail was destined to become part of Goodale’s Cutoff, an alternative route on the Oregon Trail that pioneers in wagon trains used in the 1850s and 1860s. Many adjectives early used for this scene—weird, barren, exciting, awe-inspiring, monotonous, astonishing, curious, bleak, mysterious—still apply. It is not difficult today to see why pioneering folk intent on wresting a living from the land did not tackle this volcanic terrain.

Geologists possessed the proper motivation to tackle it, however. Curiosity aroused by this lava field has led several generations of geologists, beginning with Israel C. Russell in 1901 and Harold T. Stearns in the 1920s, into a deeper understanding of its volcanic origins. With ever increasing penetration of its geological history, the apparent otherworldliness of Craters of the Moon has retreated—but not entirely. The National Aeronautics and Space Administration (NASA) brought the second set of astronauts who would walk on the moon to this alien corner of the galaxy before their moonshot. Here they studied the volcanic rock and explored an unusual, harsh, and unforgiving environment before embarking on their own otherworldly adventure.

Most types of volcanic features in the park can be seen quite readily by first stopping at the visitor center and then driving the Loop Road. Far more features can be seen if you also walk the interpretive trails at the stops along the Loop Road. Still more await those who invest the time required to come to feel the mysterious timelessness and raw natural force implicit in this expansive lava field. Many travelers are en route to Yellowstone National Park and spend only a couple of hours visiting Craters of the Moon. This is ironic because here you are on the geological track of Yellowstone. In fact, Craters of the Moon represents what Yellowstone’s landscape will resemble in the future, and both areas can supplement your insight into what happens when the Earth’s unimaginable inner forces erupt to its surface.

Silvery leaves of the buckwheat dot a cinder garden with such regular spacing they almost look planted. Such spacing results from the shortage of available surface water: Each plant controls with its roots the space surrounding it, discouraging competing plants. Rainwater and snowmelt penetrate volcanic cinders so readily that their moisture quickly drops beyond reach of most plants’ root systems. For a close-up view of a buckwheat, seepage 36.

Although Idaho is famous for forests, rivers, and scenic mountain wilderness, its Snake River Plain region boasts little of these attributes. This plain arcs across southern Idaho from the Oregon border to the Yellowstone area at the Montana-Wyoming border. It marks the trail of the passage of the Earth’s crust over an unusual geologic heat source that now brings the Earth’s incendiary inner workings so close to its surface near Yellowstone. This heat source fuels Yellowstone’s bubbling, spewing, spouting geothermal wonders. Craters of the Moon therefore stands as a geologic prelude to Yellowstone, as its precursor and the ancestral stuff of its fiery secrets.

When did all this volcanism at Craters of the Moon happen? Will it happen again? According to Mel Kuntz and other U.S. Geological Survey geologists who have conducted extensive field research at Craters of the Moon, the volcanic activity forming the Craters of the Moon lava field probably startedonly15,000 years ago. The last eruption in the volcanic cycle ended 2,000 years ago, about the time that Julius Caesar ruled the Roman Empire.

Craters of the Moon is a dormant, but not extinct, volcanic area. Its sleeping volcanoes could become active again in the near future. The largest earthquake of the last quarter century in the contiguous United States shook Idaho’s tallest mountain, Borah Peak, just north of here in 1983. When it did, some geologists wondered if it might initiate volcanic activity at Craters of the Moon. It did not. According to Kuntz, however, this is no reason not to expect another volcanic eruption heresoon—probably “within the next 1,000 years.” Part Two of this handbook explores the still young and rapidly evolving understanding of the fascinating geologic story of Craters of the Moon.

Today’s Craters of the Moon National Monument encompasses 83 square miles of the much larger Craters of the Moon lava field. Reaching southeastward from the Pioneer Mountains, the park boundary encloses a series of fissure vents, volcanic cones, and lava flows known as the Great Rift volcanic zone. This volcanic rift zone is a line of weakness in the Earth’s crust that can be traced for some 60 miles across the Snake River Plain. Recent volcanism marks much of its length. You can explore the Great Rift and some of its volcanic features via the park’s 7-mile Loop Drive, as described in Part Three of this handbook. In the park’s northern part you will findspatter cones, cinder cones, lava flows, lava caves, and an unexpected variety of wildflowers, shrubs, trees, and wild animals. The much larger southern part of the park, designated by Congress in 1970 as the Craters of the Moon Wilderness Area, is a vast and largely untraveled region of stark volcanic features flanking the Great Rift. It offers a challenge to serious hikers and explorers—latter day Robert Limberts—who are prepared for rugged wilderness travel.

Despite its seeming barrenness, Craters of the Moon is indeed home to a surprising diversity of plant and animal life. As Limbert noted in 1924: “In the West the term ‘Lava Beds of Idaho’ has always signified a region to be shunned by even the most venturesome travelers—a land supposedly barren of vegetation, destitute of water, devoid of animal life, and lacking in scenic interest.

“In reality the region has slight resemblance to its imagined aspect. Its vegetation is mostly hidden in pockets, but when found consists of pines, cedars, junipers, and sagebrush: its water is hidden deep in tanks or holes at the bottom of large ‘blow-outs’ and is found only by following old Indian or mountain sheep trails or by watching the flight of birds as they drop into these places to quench their thirst. The animal life consists principally of migrant birds, rock rabbits, woodchucks, black and grizzly bears: its scenery is impressive in its grandeur.”

Years of patient record-keeping by scientists have fit numbers to Limbert’s perceptive observations. The number of species identified includes more than 300 plants, 2,000 insects, 8 reptiles, 140 birds, 30 mammals—and one amphibian, the western toad. We now call Limbert’s “rock rabbit” the pika. The grizzly is long gone here. With few exceptions, the park’s denizens live mostly under conditions of great environmental stress.

Near constant winds, breeze-to-gale in strength, sweep across the park to rob moisture from all living things. Scant soils, low levels of precipitation, the inability of cinder cones to hold rainwater near the surface, and the heat of the summer sun—intensified by heat-absorbing black lavas—only aggravate such moisture theft. Cinder surfaces register summer soil temperatures of over 150°F and show a lack of plant cover. Plants cover generally less than 5 percent of the total surface of the cinder cones. A recent study found that when the area is looked at on a parkwide basis, most of the land is very sparsely vegetated (less than 15 percent vegetative cover). On a scale of sand trap to putting green, this would approach the sand trap end of the scale.

Winter snow transforms these landscapes, smoothing out both contours and the jagged edges of lavas. Less lunar in appearance now, the park nonetheless maintains an otherworldly aura.

The park was named in 1924, 45 years before humans walked on the Moon. Although we now know more about the Moon’s actual surface, the park’s name still rings true. Only a few trees immediately suggest that the large photo was taken on Earth. In the inset photo, astronaut Edwin E. “Buzz” Aldrin walks on the Moon near the lunar module.

Into this difficult environment wildlife researcher Brad Griffith ventured to count, mark, and scrutinize the mule deer of Craters of the Moon in May 1980. Griffith, of the University of Idaho, conducted a three-year study of the park’s mule deer population because the National Park Service was concerned that this protected and productive herd might multiply so much that it would eventually damage its habitat. Among other things, he would find that the herd has developed a drought evasion strategy that makes it behave unlike any mule deer population known anywhere else.

“By late summer,” Griffith explains, “plants have matured and dried so that they no longer provide adequate moisture to sustain the deer in this landscape that offers them no free water. Following about 12 days of warm nights and hot days in late July, the deer migrate from 5 to 10 miles north to the Pioneer Mountains. There they find free-flowing creeks and the cool, moist shade of aspen and Douglas-fir groves and wait out summer’s worst heat and dryness. Early fall rains trigger the deer’s return to the park’s wilderness from this oasis in late September to feed on the nutritious bitterbrush until November snowfalls usher them back to their winter range.”

The pristine and high-quality forage of the Craters of the Moon Wilderness Area, historically nearly untouched by domestic livestock grazing, has inspired this migratory strategy for evading drought. In effect, the mule deer make use of a dual summer range, a behavioral modification unknown elsewhere for their species.

“Their late summer and fall adaptations simply complete the mule deer’s yearlong strategy for coping with the limits that this volcanic landscape imposes on them,” Griffith explains.

Taking a walk in the park on a mid-summer afternoon gives you a good opportunity to experience the influence of wind, heat, and lack of moisture. The park’s winds are particularly striking. The lava that has flowed out of the Great Rift has built upand raised the land surface in the park to a higher elevation than its surroundings so that it intercepts the prevailing southwesterly winds. Afternoon winds usually die down in the evening. As part of the dynamics of temperature and moisture that determine mule deer behavior, this daily wind cycle helps explain why they are more active at night than are mule deer elsewhere. These deer do not move around as much as mule deer in less ecologically trying areas. They have adapted behaviors to conserve energy and moisture in this environmentally stressful landscape.

Early mornings may find park rangers climbing up a cinder cone to count the deer, continuing the collection of data that Brad Griffith set in motion with his three-year study. The rangers still conduct spring and late summer censuses: over a recent three-year period the deer populations averaged about 420 animals. Another several years of collecting will give the National Park Service a body of data on the mule deer that is available nowhere else.

The uniqueness of this data about the park’s mule deer population would surely please the booster aspect of Robert Limbert’s personality. Likewise, the research challenges involved in obtaining it would appeal to his explorer self. History has justified Limbert on both counts. Publicity arising from his explorations led to creation of the national monument. Furthermore, that publicity put forth a rather heady claim that history has also unequivocally borne out: “Although almost totally unknown at present,” Limbert prophesied in 1924, “this section is destined some day to attract tourists from all America....”

Every year tens of thousands of travelers fulfill Robert Limbert’s prophecy of more than a half-century ago.

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What last happened here about 2,000 years ago looked much like this photograph of a volcanic eruption in Hawaii. Bubbling, pooling, and flowing lava blanketed the landscape as molten materials poured or gushed out of the Earth. Most volcanic phenomena preserved at Craters of the Moon have been seen in action in Hawaii.

A 400-mile-long arc known as the Snake River Plain cuts a swath from 30 to 125 miles wide across southern Idaho. Idaho’s official state highway map, which depicts mountains with shades of green, shows this arc as white because there is comparatively little variation here compared to most of the state. Upon this plain, immense amounts of lava from within the Earth have been deposited by volcanic activity dating back more than 14 million years. However, some of these lavas, notably those at Craters of the Moon National Monument, emerged from the Earth as recently as 2,000 years ago. Craters of the Moon contains some of the best examples of basaltic volcanism in the world. To understand what happened here, you must understand the Snake River Plain.

Basaltic and Rhyolitic Lavas.The lavas deposited on the Snake River Plain were mainly of two types classified as basaltic and rhyolitic. Magma, the molten rock material beneath the surface of the Earth, issues from a volcano as lava. The composition of this fluid rock material varies. Basaltic lavas are composed of magma originating at the boundary of the Earth’s mantle and its crustal layer. Rhyolitic lavas originate from crustal material. To explain its past, geologists now divide the Snake River Plain into eastern and western units. The following geologic story relates to the eastern Snake River Plain, on which Craters of the Moon lies.

On the eastern Snake River Plain, basaltic and rhyolitic lavas formed in two different stages of volcanic activity. Younger basaltic lavas mostly lie atop older rhyolitic lavas. This portion of the plain runs from north of Twin Falls eastward to the Yellowstone area on the Wyoming-Montana border. Drilling to depths of almost 2 miles near the plain’s midline, geologists found ½ mile of basaltic lava flows lying atop more than 1½ miles of rhyolitic lava flows. How much deeper the rhyolitic lavas may extend is not known. No one has drilled deeper here.

Crossing Idaho in an arc, the Snake River Plain marks the path of the Earth’s crustal plate as it migrates over a heat source unusually close to the surface. It is believed that the heat source fueling Yellowstone’s thermal features today is essentially the same one that produced volcanic episodes at Craters of the Moon ending about 2,000 years ago.

This combination—a thinner layer of younger basaltic lavas lying atop an older and thicker layer of rhyolitic lavas—is typical of volcanic activity associated with an unusual heat phenomenon inside the Earth that some geologists have described as a mantle plume. The mantle plume theory was developed in the early 1970s as an explanation for the creation of the Hawaiian Islands. According to the theory, uneven heating within the Earth’s core allows some material in the overlying mantle to become slightly hotter than surrounding material. As its temperature increases, its density decreases. Thus it becomes relatively buoyant and rises through the cooler materials—like a tennis ball released underwater—toward the Earth’s crust. When this molten material reaches the crust it eventually melts and pushes itself through the crust and it erupts onto the Earth’s surface as molten lava.

The Earth’s crust is made up of numerous plates that float upon an underlying mantle layer. Therefore, over time, the presence of an unusual heat source created by a mantle plume will be expressed at the Earth’s surface—floating in a constant direction above it—as a line of volcanic eruptions. The Snake River Plain records the progress of the North American crustal plate—350 miles in 15 million years—over a heat source now located below Yellowstone. The Hawaiian chain of islands marks a similar line. Because the mechanisms that cause this geologic action are not well understood, many geologists refer to this simply as a heat source rather than a mantle plume.

Two Stages of Volcanism.As described above, volcanic eruptions associated with this heat source occur in two stages, rhyolitic and basaltic. As the upwelling magma from the mantle collects in a chamber as it enters the Earth’s lower crust, its heat begins to melt the surrounding crustal rock. Since this rock contains a large amount of silica, it forms a thick and pasty rhyolitic magma. Rhyolitic magma is lighter than the overlying crustal rocks, therefore, it begins to rise and form a second magma chamber very close to the Earth’s surface. As more and more of this gas-charged rhyolitic magma collects in this upper crustal chamber, the gas pressure builds to a point at which the magma explodes through the Earth’s crust.

Explosive Rhyolitic Volcanism.Rhyolitic explosions tend to be devastating. When the gas-charged molten material reaches the surface of the Earth, the gas expands rapidly, perhaps as much as 25 to 75 times by volume. The reaction is similar to the bubbles that form in a bottle of soda pop that has been shaken. You can shake the container and the pressure-bottled liquid will retain its volume as long as the cap is tightly sealed. Release the pressure by removing the bottle cap, however, and the soft drink will spray all over the room and occupy a volume of space far larger than the bottle from which it issued. This initial vast spray is then followed by a foaming action as the less gas-charged liquid now bubbles out of the bottle.

Collectively, the numerous rhyolitic explosions that occurred on the Snake River Plain ejected hundreds of cubic miles of material into the atmosphere and onto the Earth’s surface. In contrast, the eruption of Mount Saint Helens in 1980, which killed 65 people and devastated 150 square miles of forest, produced less than 1 cubic mile of ejected material. So much material was ejected in the massive rhyolitic explosions in the Snake River Plain that the Earth’s surface collapsed to form huge depressions known as calderas. (Likecaldron, whose root meaning it shares, this name implies both bowl-shaped and warmed.) Most evidence of these gigantic explosive volcanoes in the Snake River Plain has been covered by subsequent flows of basaltic lava. However, traces of rhyolitic eruptions are found along the margins of the plain and in the Yellowstone area.

Quiet Outpourings of Basaltic Lava.As this area of the Earth’s crust passed over and then beyond the sub-surface heat source, the explosive volcanism of the rhyolitic stage ceased. The heat contained in the Earth’s upper mantle and crust, however, remained and continued to produce upwelling magma. This was basaltic magma that, because it contained less silica than rhyolite, was very fluid.

The basalt, like the rhyolite, collected in isolated magma chambers within the crust until pressures built up to force it to the surface through various cracks and fissures. These weak spots in the Earth’s crust were the results of earlier geologic activity, expansion of the magma chamber, or the formation of a rift zone.

Microscopic cross section of basaltic rock.

Microscopic cross section of rhyolitic rock. Cross sections show vastly different textures. Rhyolitic magma contains more silica; it is very thick and does not allow trapped gas to escape easily. Its volcanic eruptions blast large craters in the Earth’s crust. Basaltic magma is more fluid and allows gas to escape readily. It erupts more gently. Here in the eastern Snake River Plain, basaltic lava flows almost completely cover earlier rhyolitic deposits.

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At Craters of the Moon the black rocks are lava flows. The surface lava rocks, basaltic in composition, formed from magma originating deep in the Earth. They are named for their appearances:Pahoehoe(pronounced “pah-hoy-hoy” and meaning “ropey”),Aa(pronounced “ah-ah” and meaning “rough”), orBlocky. Geologists have seen how these flows behave in modern volcanic episodes in Hawaii and elsewhere.Pahoehoe lavaPahoehoeMore than half the park is covered by pahoehoe lava flows. Rivers of molten rock, they harden quickly to a relatively smooth surface, billowly, hummocky, or flat. Other pahoehoe formations resemble coiled, heavy rope or ice jams.Aa lavaAaAa flows are far more rugged than pahoehoe flows. Most occur when a pahoehoe flow cools, thickens, and then turns into aa. Often impassable to those traveling afoot, aa flows quickly chew up hiking boots. Blocky lava is a variety of aa lava whose relatively large silica content makes it thick and often dense, glassy, and smooth.Blocky lavaBombsLava pieces blown out of craters may solidify in flight. They are classed by shape: spindle, ribbon, and breadcrust. Bombs range from ½ inch to more than 3 feet long.Tree MoldsWhen molten lava advances on a living forest, resulting tree molds may record impressions of charred surfaces of trees in the lava.Blue Dragon Flows lavaBreadcrust bombSpindle bombWood-like lavaTree moldLava river

Pahoehoe lava

PahoehoeMore than half the park is covered by pahoehoe lava flows. Rivers of molten rock, they harden quickly to a relatively smooth surface, billowly, hummocky, or flat. Other pahoehoe formations resemble coiled, heavy rope or ice jams.

Aa lava

AaAa flows are far more rugged than pahoehoe flows. Most occur when a pahoehoe flow cools, thickens, and then turns into aa. Often impassable to those traveling afoot, aa flows quickly chew up hiking boots. Blocky lava is a variety of aa lava whose relatively large silica content makes it thick and often dense, glassy, and smooth.

Blocky lava

BombsLava pieces blown out of craters may solidify in flight. They are classed by shape: spindle, ribbon, and breadcrust. Bombs range from ½ inch to more than 3 feet long.

Tree MoldsWhen molten lava advances on a living forest, resulting tree molds may record impressions of charred surfaces of trees in the lava.

Blue Dragon Flows lava

Breadcrust bomb

Spindle bomb

Wood-like lava

Tree mold

Lava river

Mt. St. Helens erupts in 1980. Because the lava contained a large amount of silica, its explosive eruption contrasts sharply with recent basaltic flows in volcanic activity in Hawaii.

Basaltic flows in Hawaii.

Upon reaching the surface, the gases contained within the lava easily escaped and produced rather mild eruptions. Instead of exploding into the air like earlier rhyolitic activity, the more fluid basaltic lava flooded out onto the surrounding landscape. These flows were fairly extensive and often covered many square miles. After millions of years, most of the older rhyolitic deposits have been covered by these basaltic lava flows.

The Great Rift and Craters of the Moon.Craters of the Moon National Monument lies along a volcanic rift zone. Rift zones occur where the Earth’s crust is being pulled in opposite directions. Geologists believe that the interactions of the Earth’s crustal plates in the vicinity of the Snake River Plain have stretched, thinned, and weakened the Earth’s crust so that cracks have formed both on and below the surface here. Magma under pressure can follow these cracks and fissures to the surface. While there are many volcanic rift zones throughout the Snake River Plain, the most extensive is the Great Rift that runs through Craters of the Moon. The Great Rift is approximately 60 miles long and it ranges in width from 1½ to 5 miles. It is marked by short cracks—less than 1 mile in length—and the alignment of more than 25 volcanic cinder cones. It is the site of origin for more than 60 different lava flows that make up the Craters of the Moon Lava Field.

Eight Major Eruptive Periods.Most of the lavas exposed at Craters of the Moon formed between 2,000 and 15,000 years ago in basaltic eruptions that comprise the second stage of volcanism associated with the mantle plume theory. These eight eruptive periods each lasted about 1,000 years or less and were separated by periods of relative calm that lasted for a few hundred to more than 2,000 years. These sequences of eruptions and calm periods are caused by the alternating build up and release of magmatic pressure inside the Earth. Once an eruption releases this pressure, time is required for it to build up again.

Eruptions have been dated by two methods: paleomagnetic and radiocarbon dating. Paleomagnetic dating compares the alignment of magnetic minerals within the rock of flows with past orientations of the Earth’s magnetic fields. Radiocarbon dating makes use of radioactive carbon-14 in charcoal createdfrom vegetation that is overrun by lava flows. Dates obtained by both methods are considered to be accurate to within about 100 years.

A Typical Eruption at Craters of the Moon.Research at the monument and observations of similar eruptions in Hawaii and Iceland suggest the following scenario for a typical eruption at Craters of the Moon. Various forces combine to cause a section of the Great Rift to pull apart. When the forces that tend to pull the Earth’s crust apart are combined with the forces created as magma accumulates, the crust becomes weakened and cracks form. As the magma rises buoyantly within these cracks, the pressure exerted on it is reduced and the gases within the magma begin to expand. As gas continues to expand, the magma becomes frothy.

At first the lava is very fluid and charged with gas. Eruptions begin as a long line of fountains that reach heights of 1,000 feet or less and are up to a mile in length. This “curtain of fire eruption” mainly produces cinders and frothy, fluid lava. After hours or days, the expansion of gases decreases and eruptions become less violent. Segments of the fissure seal off and eruptions become smaller and more localized. Cinders thrown up in the air now build piles around individual vents and form cinder cones.

With further reductions in the gas content of the magma, the volcanic activity again changes. Huge outpourings of lava are pumped out of the various fissures or the vents of cinder cones and form lava flows. Lava flows may form over periods of months or possibly a few years. Long-term eruptions of lava flows from a single vent become the source of most of the material produced during a sustained eruption. As gas pressure falls and magma is depleted, flows subside. Finally, all activity stops.

When Will the Next Eruption Occur?Craters of the Moon is not an extinct volcanic area. It is merely in a dormant stage of its eruptive sequence. By dating the lava flow, geologists have shown that the volcanic activity along the Great Rift has been persistent over the last 15,000 years, occurring approximately every 2,000 years. Because the last eruptions took place about 2,000 years ago, geologists believe that eruptions are due here again—probably within the next 1,000 years.

From the air the Great Rift looks like an irregularly dashed line punctuated by tell-tale cones and craters.

Chainlike, the Hawaiian group of islands traces the migration of Earth’s crustal plate over an unusual undersea heat source. The Hawaiian chain of islands and the Snake River Plain map similar happenings.

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Indian Tunnel looks like a cave, but it is a lava tube. When a pahoehoe lava flow is exposed to the air, its surface begins to cool and harden. A crust or skin develops. As the flow moves away from its source, the crust thickens and forms an insulating barrier between cool air and molten material in the flow’s interior. A rigid roof now exists over the stream of lava whose molten core moves forward at a steady pace. As the flow of lava from the source vent is depleted, the level of lava within the molten core gradually begins to drop. The flowing interior then pulls away from the hardening roof above and slowly drains away and out. The roof and last remnants of the lava river inside it cool and harden, leaving a tube.Lava tubeGreat horned owlMany lava tubes make up the Indian Tunnel Lava Tube System. These tubes formed during the same eruption within a single lava flow whose source was a fissure or crack in the Big Craters/Spatter Cones area. A tremendous amount of lava was pumped out here, forming the Blue Dragon Flows. (Hundreds of tiny crystals on its surface produce the color blue when light strikes them.) Lava forced through the roof of the tube system formed huge ponds whose surfacescooled and began to harden. Later these ponds collapsed as lava drained back into the lava tubes. Big Sink is the largest of these collapses. Blue Dragon Flows cover an area of more than 100 square miles. Hidden beneath are miles of lava tubes, but collapsed roof sections called skylights provide entry to only a small part of the system. Only time, with the collapse of more roofs, will reveal the total extent of the system.Icicles (ice stalactites)Lava stalactitesStalactitesDripped from hot ceilings, lava forms stalactites that hang from above.Mineral depositsSulfate compounds formed on many lava tube ceilings from volcanic gases or by evaporation of matter leached from rocks above.IceIn spring, ice stalactites form on cave ceilings and walls. Ice stalagmites form on the cave floor. Summer heat destroys these features.WildlifeLava tube beetles, bushy-tailed woodrats (packrats), and bats live in some dark caves. Violet-green swallows, great horned owls, and ravens may use wall cracks and shelves of well-lit caves for nesting sites.

Lava tube

Great horned owl

Many lava tubes make up the Indian Tunnel Lava Tube System. These tubes formed during the same eruption within a single lava flow whose source was a fissure or crack in the Big Craters/Spatter Cones area. A tremendous amount of lava was pumped out here, forming the Blue Dragon Flows. (Hundreds of tiny crystals on its surface produce the color blue when light strikes them.) Lava forced through the roof of the tube system formed huge ponds whose surfacescooled and began to harden. Later these ponds collapsed as lava drained back into the lava tubes. Big Sink is the largest of these collapses. Blue Dragon Flows cover an area of more than 100 square miles. Hidden beneath are miles of lava tubes, but collapsed roof sections called skylights provide entry to only a small part of the system. Only time, with the collapse of more roofs, will reveal the total extent of the system.

Icicles (ice stalactites)

Lava stalactites

StalactitesDripped from hot ceilings, lava forms stalactites that hang from above.Mineral depositsSulfate compounds formed on many lava tube ceilings from volcanic gases or by evaporation of matter leached from rocks above.IceIn spring, ice stalactites form on cave ceilings and walls. Ice stalagmites form on the cave floor. Summer heat destroys these features.WildlifeLava tube beetles, bushy-tailed woodrats (packrats), and bats live in some dark caves. Violet-green swallows, great horned owls, and ravens may use wall cracks and shelves of well-lit caves for nesting sites.

{uncaptioned}Cinder coneSpatter coneCinder ConesWhen volcanic eruptions of fairly moderate strength throw cinders into the air, cinder cones may be built up. These cone-shaped hills are usually truncated, looking as though their tops were sliced off. Usually, a bowl- or funnel-shaped crater will form inside the cone. Cinders, which cooled rapidly while falling through the air, are highly porous with gas vesicles, like bubbles. Cinder cones hundreds of feet high may be built in a few days. Big Cinder Butte is a cinder cone. At 700 feet high it is the tallest cone in the park. The shape develops because the largest fragments, and in fact most of the fragments, fall closest to the vent. The angle of slope is usually about 30 degrees. Some cinder cones, such as North Crater, the Watchman, and Sheep Trail Butte, were built by more than one eruptive episode. Younger lava was added to them as a vent was rejuvenated. If strong winds prevailed during a cinder cone’s formation, the cone may be elongated—in the direction the wind was blowing—rather than circular. Grassy, Paisley, Sunset, and Inferno Cones are elongated to the east because the dominant winds in this area come from the west. The northernmost section of the Great Rift contains the mostcinder cones for three reasons:1.There were more eruptions at that end of the rift.2.The lavas erupted there were thicker, resulting in more explosive eruptions. (They are more viscous because they contain more silica.)3.Large amounts of groundwater may have been present at the northern boundary of the lavas and when it came in contact with magma it generated huge amounts of steam. All of these conditions lead to more extensive and more explosive eruptions that tend to create cinder cones rather than lava flows.Spatter ConesWhen most of its gas content has dissipated, lava becomes less frothy and more tacky. Then it is tossed out of the vent as globs or clots of lava paste called spatter. The clots partially weld together to build up spatter cones. Spatter cones are typically much smaller than cinder cones, but they may have steeper sides. The Spatter Cones area of the park (Stop 5 on themap of the Loop Drive) contains one of the most perfect spatter-cone chains in the world. These cones are all less than 50 feet high and less than 100 feet in diameter.

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Cinder cone

Spatter cone

Cinder ConesWhen volcanic eruptions of fairly moderate strength throw cinders into the air, cinder cones may be built up. These cone-shaped hills are usually truncated, looking as though their tops were sliced off. Usually, a bowl- or funnel-shaped crater will form inside the cone. Cinders, which cooled rapidly while falling through the air, are highly porous with gas vesicles, like bubbles. Cinder cones hundreds of feet high may be built in a few days. Big Cinder Butte is a cinder cone. At 700 feet high it is the tallest cone in the park. The shape develops because the largest fragments, and in fact most of the fragments, fall closest to the vent. The angle of slope is usually about 30 degrees. Some cinder cones, such as North Crater, the Watchman, and Sheep Trail Butte, were built by more than one eruptive episode. Younger lava was added to them as a vent was rejuvenated. If strong winds prevailed during a cinder cone’s formation, the cone may be elongated—in the direction the wind was blowing—rather than circular. Grassy, Paisley, Sunset, and Inferno Cones are elongated to the east because the dominant winds in this area come from the west. The northernmost section of the Great Rift contains the mostcinder cones for three reasons:1.There were more eruptions at that end of the rift.2.The lavas erupted there were thicker, resulting in more explosive eruptions. (They are more viscous because they contain more silica.)3.Large amounts of groundwater may have been present at the northern boundary of the lavas and when it came in contact with magma it generated huge amounts of steam. All of these conditions lead to more extensive and more explosive eruptions that tend to create cinder cones rather than lava flows.

Spatter ConesWhen most of its gas content has dissipated, lava becomes less frothy and more tacky. Then it is tossed out of the vent as globs or clots of lava paste called spatter. The clots partially weld together to build up spatter cones. Spatter cones are typically much smaller than cinder cones, but they may have steeper sides. The Spatter Cones area of the park (Stop 5 on themap of the Loop Drive) contains one of the most perfect spatter-cone chains in the world. These cones are all less than 50 feet high and less than 100 feet in diameter.

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