The 3 young lava fields that compose Craters of the Moon National Monument and Preserve range in age from 15,000 years to 2000 years. Craters of the Moon lava field, the largest of the three fields found along the Great Rift, is made up of about 60 lava flows and 25 volcanic cones that cover 618 square miles. It provides outstanding examples of how molten basaltic rock can solidify into various types of lava. The basic types include block lava (very dense and formed into angular blocks), aa lava (a rough, jagged, clinkery surface), and pahoehoe lava (a smooth, ropy, or billowy surface). Visitors can also see vents, fissures, cinder cones, lava tubes, lava bombs, spatter cones, tree molds, rafted blocks, and many other volcanic features and structures.

Cave / Karst Systems

Caves at Craters of the Moon represent Pleistocene or Holocene basaltic lava tubes. The number of documented caves currently exceeds 300, and with further exploration and inventory a realistic estimate of the number might exceed 600. Along the Great Rift, the volcanic rift zone in which the lava fields formed, are additional cave features termed "fissure caves". These fissure caves are present in much fewer numbers than the lava tube caves. Some of these fissures are quite sizeable, including one particular fissure that may be passable to a depth of 650 feet (200 meters) from the surface.

Geologic Activity

Yellowstone Hotspot

Between approximately 8 and 10 million years ago, the Yellowstone Hotspot was beneath Craters of the Moon. This time was characterized by violent rhyolite eruptions and caldera formation. Many geologists think the Yellowstone Hotspot formed just 17 to 18 million years ago; a few geologists think that it is much older. Some evidence points to the hotspot having formed in the Earth's upper mantle at a depth of about 125 miles, rather than being a mantle plume from the core/mantle boundary. The hotspot has a plume shape, but the plume is probably not completely molten. It is a column of hot rock, which may have been produced by radioactive decay, in which some of the molten rock flows upward. The column flows upward until it hits the overlying North American Plate, which consists of the crust and the uppermost mantle, and is colder than the upward-flowing magma. Periodically, blobs of iron-rich basaltic magma rise up into the crust from a depth of about 50 miles. In the crust, these molten blobs melt overlying silica-rich rocks and form sponge-like magma chambers of partially-molten rhyolite. Catastrophic eruptions of huge volumes of rhyolitic magma have taken place along the Eastern Snake River Plain over 100 times in the past 16.5 million years. These eruptions often produced huge craters called calderas; some are 10 to 40 miles wide. Many of the approximately 30-50 calderas overlapped and may be associated with 7 to 13 volcanic centers. Although some of the mountain ranges that existed on the Eastern Snake River Plain before the hotspot may have been blown away by the eruptions, it is more likely that they were swallowed up as the floor of the caldera sank during the violent explosions. The Yellowstone hotspot itself is stationary, while the North American Plate has been moving in a southwesterly direction over it. The plate's movement has produced a progressively younger trend of rhyolitic eruptions to the northeast.

Basaltic Eruptions

Between 6 million and 15,000 years ago, numerous basaltic eruptions produced a 4,000-foot-thick sequence of lava flows in the vicinity of Craters of the Moon. Between 15,000 and 2,000 years ago, the Craters of the Moon Lava Field formed during eight major eruptive periods. During this time the Craters of the Moon lava field grew to cover 618 square miles. The Wapi and Kings Bowl lava fields formed contemporaneously about 2,200 years ago. Recent seismic data suggest that the Yellowstone Hotspot left behind a slab of basalt 6 to 10 miles thick. This slab is poised in a mid-crustal position and some of it is thought to contain partial melt. It is believed that this slab represents the slag left in the bottom of the numerous magma chambers spawned by the hotspot.

Present and Future Events

This region is experiencing basin and range type faulting, which is stretching or pulling apart the crust. The Lost River Range north of the town of Arco is good evidence that these forces are still active. In 1983 these forces caused a magnitude 7.3 earthquake, during which Mount Borah rose about 1 foot and the Lost River Valley in that vicinity dropped about 8 feet. On the Eastern Snake River Plain, rather than producing mountain ranges, the tensional forces have caused decompression melting, which results in dike emplacement and periodic eruption of molten rock onto the surface. As long as these forces continue to act, more eruptions will eventually occur. The recurrence interval for eruptive activity in the Craters of the Moon Lava Field averages 2,000 years and it has been more than 2,000 years since the last eruption. The constancy of most recent lava output rates suggest that slightly over one cubic mile of lava will be erupted during the next eruption period. In the past, eruptions in the Craters of the Moon Lava Field have generally shifted to the segment of the Great Rift with the longest repose interval. Therefore, the next eruptive period is expected to begin along the central portion of the Great Rift in the Craters of the Moon Lava Field, but may well propagate to the northern part of the monument in the proximity of the loop road. Initial flows, based on past performance, will probably be relatively non-explosive and produce large-volume pahoehoe flows. Eruptions from potential vents on the northern part of the Great Rift may be comparatively explosive and may produce significant amounts of tephra (airfall material ejected from a volcano), destroy cinder cones by both explosion and collapse, and build new ones.

Fossils

The earliest report of bone material found within the lava tubes at Craters of the Moon dates back to the 1880s. A variety of bovine-like bones were collected from the lava tubes, but are too fragmentary or poorly preserved for definitive identification. The remains of bighorn sheep (Ovis canadensis), including a horn, have also been found in the monument. Packrat middens within the lava tubes contain bone material including the remains of microtine rodents. An grizzly bear (Ursus arctos) skull and femur were found just south of the old monument boundary. In a rare example of fossils preserved in igneous rocks, dozens of tree mold impressions are preserved in the basaltic lava flows and in lava tubes. These impressions were formed as lava flowed around a fallen tree. The molds typically show shrinkage cracks. Moisture in the wood may have prevented incineration of the trees. Similar tree molds are known from other NPS areas including El Malpais National Monument, Hawaii Volcanoes National Monument, Lava Beds National Monument, and Pu'uhonua o Honaunau National Historic Park.

Volcanoes / Lava Flows

Of the 60 lava flows visible on the surface of the Craters of the Moon Lava Field today, 20 have been dated. The oldest is about 15,000 years old and the youngest about 2,000.

Some lava flows were very dense and have a surface of angular blocks referred to as block lava. Others have a rough, jagged, or clinkery surface called áa lava. Still others have a smooth, ropy, or billowy surface called pahoehoe lava. Three special kinds of pahoehoe may be observed in the Craters of the Moon Lava Field: (1) slabby pahoehoe is made up of jumbled plates or slabs of broken pahoehoe crust; (2) shelly pahoehoe, which forms from gas-charged lava, contains small open tubes, blisters, and thin crusts; and (3) spiny pahoehoe, which is very thick and pasty, contains elongated gas bubbles on the surface that form spines. The slabby and spiny varieties of pahoehoe are transition phases to áa.

Bombs
Four kinds of volcanic bombs are found at Craters of the Moon, all of which began as a volume of molten rock that is ejected into the air. If the lava gets twisted during its flight, it is called a spindle bomb and typically measures from a few inches to several feet in length. If it is very tiny and twisted, it is called a ribbon bomb. When the volume of lava forms a crust that is cracked by expanding gases as it flies through the air, it is called a breadcrust bomb, which exhibits a surface texture that resembles bread rising in the oven. If the lava mass does not completely solidify during flight, so that it flattens and spreads on landing, it is called a cow-pie bomb. Some cow-pie bombs are over 10 feet long.

Lava Tube Caves
Lava tubes are hollow spaces beneath the surface of solidified lava flows. They are formed by the withdrawal of molten lava after the formation of the surface crusts. Indian Tunnel, in the northern area of the park, has a 40-foot high ceiling and is 800 feet long. Bear Trap Cave, which lies between the Craters of the Moon and the Kings Bowl Lava Fields, is about 15 miles long, but is not continuously passable. Most of the Craters of the Moon lava flows are composed of pahoehoe and were fed through tubes and tube systems, although there are some sheet flows.

Other lava features
At Craters of the Moon, structures representing both inflation and deflation of the lava surface can be seen along with hot and cold collapses of the roofs of lava tubes. Inside lava tubes, one can see lava stalactites, remelt features, and lava curbs. In other places lava flows formed ponds, built levees, and produced lava cascades. Some lava flows produced small mounds (tumuli) or elongated ridges (pressure ridges) on their crusts. In some places, squeeze-ups formed when pressure was sufficient to force molten lava up through tension fractures in the top of pressure ridges or cracks in the solidified crust of lava ponds. Pressure plateaus were produced by the sill-like injection of new lava beneath the crust of an earlier flow that had not completely solidified.

When magma emerges at the surface along a segment of a rift, it often begins by producing a curtain of fire and a line of low eruptions. As portions of the segment become clogged, the fountains jet higher. If magma emerges at the surface highly charged with gas it sprays high in the air; the fire fountains that produced many of the Craters of the Moon cinder cones were probably over 1,000 feet high. Big Cinder Butte, the tallest cinder cone at Craters of the Moon, is over 700 feet high. The highly gas-charged molten rock cools and solidifies during flight and rains down to form cinder cones. If you look closely at cinders you will see that they are laced with gas holes and resemble a sponge.

Some vents along the rift ejected very fluid particles (spatter) that accumulated to form steep-sided spatter cones. Along eruptive fissures where a whole segment erupted, spatter accumulated to produce low ridges called spatter ramparts. Hornitos, also known as rootless vents, are similar in appearance to spatter cones. Hornitos form from spatter ejected from holes in the crust of a lava tube instead of directly from a feeding fissure. Craters of the Moon also has collapse features known as sinks or pit craters. During some eruptions, pieces of crater walls were carried off like icebergs by lava flows. These wall chunks are known as rafted blocks. The monoliths on the North Crater Flow Trail are excellent examples of these volcanic formations.