Craters of the Moon National Monument provides one of the best examples of basaltic volcanism in the world. Features typical of basalt eruptions -- pahoehoe and a'a flows, cinder cones, lava tubes, spatter cones -- are abundant and easily accessible. As one explores this bizarre landscape, many questions arise. Where is the volcano? Where did all this rock come from? How old is this area? Will the volcanoes erupt again?

Craters of the Moon National Monument is located on the Snake River Plain. This crescent-shaped expanse of volcanic rock stretches 400 miles from the Oregon border in the west to Yellowstone National Park in the east. The age of the Snake River Plain can be traced from the most recent eruptions 2,000 years ago to the oldest activity roughly 17 million years ago. Geologists consider the eastern and western sections of the Snake River Plain to have different geologic histories. This discussion will focus on the eastern Snake River Plain

Volcanic eruptions vary according to how much silica (SiO₂) the magma contains. Magma rich in silica is viscous and erupts explosively, producing a rock called rhyolite. Rhyolitic eruptions may create massive craters called calderas, which form when an eruption empties a magma chamber beneath the Earth's crust, causing the surface to collapse. The Island Park Caldera just east of Yellowstone is an example of a rhyolitic caldera. Magma with lower levels of silica generates gentler eruptions of more fluid basaltic lava, such as that seen at Craters of the Moon. There is, therefore, evidence of both types of eruptions on the eastern Snake River Plain.

Vestiges of numerous rhyolitic lava flows and calderas are apparent along the margins of the eastern Snake River Plain. They are all that is visible of a thick layer of rhyolite that is overlain by more recent basalt flows. In a typical section of the Snake River Plain, 2 kilometers of basaltic rock and sediments lie on top of a layer 8 kilometers thick, believed to consist of rhyolitic rocks.

The rhyolitic volcanic deposits on the Snake River Plain vary in age. Eruptions occurred about 10 million years ago at Twin Falls, five million years ago at Arco, and finally, 600,000 years ago in Yellowstone National Park. Any theory about the origin of this area must explain the progressively younger ages of the rhyolitic deposits from southwest to northeast, as well as the fact that later basaltic eruptions buried the rhyolite. Most geologists consider the Mantle Plume Theory as the best explanation for the formation of the eastern Snake River Plain.

Mantle Plume Theory

According to this theory, uneven heating within the core and lower mantle of the Earth causes some material to become hotter than that surrounding it. The hotter material becomes less dense and rises through the cooler rock of the Earth's interior. These plumes, or "hot spots", produce many successive batches of rising magma. The rising magma may eventually reach the Earth's crust and erupt onto the surface as lava.

The plumes remain stationary while the plates that make up the Earth's crust move over them. Thus, volcanic activity above a plume is expressed as a line of volcanic features which grow older the farther they are from the hot spot. The volcanic events associated with a mantle plume occur in two distinct stages.

Stage I:
Rising basaltic magma formed within the Earth's mantle reaches the base of the Earth's crust. The heat of the collecting magma begins to melt the crustal rock, which is rich in silica, forming pasty rhyolitic magma. The rhyolitic magma rises further, forming a second magma chamber within about 6 miles of the Earth's surface.

Since gases within the thick rhyolitic magma cannot easily escape, the eruptions tend to be devastating, sometimes spewing hundreds of cubic miles of material into the atmosphere. By contrast, Mount St. Helens ejected less than _ cubic mile of rock. As the magma chamber empties, there is nothing to support the crust above it. It collapses, forming a caldera that may be 100 square miles in area!

Stage II:
The intense volcanic activity associated with the mantle plume ebbs as the plate continues its movement, only to begin again at a new spot farther up the chain. Eventually, gentler basaltic eruptions replace the explosive rhyolitic activity. These basaltic eruptions arise from the original deep crustal basaltic magma chamber that formed at the base of the Earth's crust during Stage I. Magma continues to rise and enlarge the chamber and pressure within it gradually increases. The pressure forces the magma toward the surface through fractures in the Earth's crust. The magma remains basaltic because there is little silica left in the crust to melt into it.

Upon eruption, basaltic lava is very fluid. Lava flows from these relatively calm eruptions spread out on the surface to cover older rhyolite. With each new eruption, less rhyolite is left exposed and, after millions of years, basalt covers nearly all of the rhyolite, like frosting on a cake.

The Great Rift

A hot spot provided the magma for the eruptions at Craters of the Moon. The Great Rift provided the pathway for the magma to reach the surface. The Great Rift is the most extensive of several volcanic rift zones which traverse the Snake River Plain. Volcanic rift zones are weak areas where the Earth's crust has stretched and thinned and fissures have developed. Magma under pressure follows these fissures to the surface.

The Great Rift, which passes through Craters of the Moon, is 60 miles long and from 1-1/2 to five miles wide. It is characterized by short surface cracks, more than 25 cinder cones, and is the point of origin for over 60 lava flows. Geologists believe that the formation of the Great Rift is related to typical Basin-and-Range faulting.

Basin-and-Range

Basin-and-range faulting is responsible for the topography of eastern California, Nevada, Utah, and southern Idaho and is typified by alternating uplifted mountain ranges and down-dropped valleys. Forces in these areas are pulling apart and thinning the Earth's crust, producing a tremendous buildup of tension. When this tension becomes extreme, the crust suddenly fractures. Large blocks of earth slip or rotate up and down, creating valleys separated by long mountain ranges. There are about 150 mountain ranges and valleys in the basin-and-range province, all aligned in a north/south direction and spaced at approximately 16 mile intervals.

There are basin-and-range mountain ranges to the north and south of the Snake River Plain. The block faults that occur at the edge of each are known as "border" or "range-front" faults. These faults may extend a short distance beyond the base of the mountains and out beneath the lavas. They are marked by zones of parallel cracks in the basalt lava flows that may run for tens of miles.

The Great Rift cannot be readily identified as a continuation of a basin-and-range border fault. However, the distance from the Great Rift to the Lost River Range is slightly more than the 16 miles normally found between basin-and-range structures. Furthermore, gravity and seismic information indicate a fracture extending from the Great Rift into the Pioneer Mountains to the north. This leads some geologists to conclude that the Great Rift is an extension of a basin-and-range fault system.

Geologists have suggested that the release of strain, which occurs during basin-and-range faulting, may result in magma production along connected volcanic rift zones as the decompression causes melting. If so, then periods of major faulting near the margins of the eastern Snake River Plain may correlate with periods of volcanism on collinear rift zones. The volcanic rift zones associated with basin-and-range faults would be long-lived and self-perpetuating. Techniques for dating faults and volcanic deposits are not yet accurate enough to fully investigate this idea.

Eruptions at Craters of the Moon

The lava flows exposed at Craters of the Moon erupted between 2,000 and 15,000 years ago during eight eruptive periods, each separated by periods of relative calm. This cycle of eruptions interspersed with dormancy is associated with the buildup of pressure as magma accumulates beneath the surface. Strain increases until the magma overcomes the confining resistance of the earth's crust and an eruption takes place. As soon as the magmatic pressure dissipates, the eruptions cease until the pressure can build once more.

During a typical eruption at Craters of the Moon, rising magma forces a section of the Great Rift to pull apart. As magma rises, gases contained within the magma expand. The frothy magma is very fluid and charged with gas. Eruptions begin as a long line of tall fountains along a crack that may extend more than a mile. These are called "curtain of fire" eruptions and produce downwind blankets of bubbly cinders and ash.

After hours or days, the initial expansion of gases decreases and the eruption becomes less violent. Some sections of the fissure seal off and the eruption becomes smaller and more localized. Cinders are still thrown into the air, but they build up in piles around individual vents and form cinder cones such as North Crater, Big Craters, and Inferno Cone.

As the amount of gas contained in the magma continues to drop, the volcanic activity again changes. Huge outpourings of lava flow from various fissures and vents. These lava flows typically continue for days to a few months, but may continue for years. They flows are the source of most of the rock produced during an eruption. The flows gradually subside and all activity stops. Hot, thin lava containing relatively little silica forms smooth-surfaced pahoehoe flows. Cooler lava, or lava containing slightly more silica, forms coarse, uneven a'a flows.

If what geologists tell us proves to be true, there will be another eruption at Craters of the Moon. By studying the flows that make up the Craters of the Moon lava field, geologists have discovered an eruptive pattern that indicates the area is merely in a stage of temporary dormancy. They believe that past eruptions conform to a predictable time schedule and that the eruptive cycle will begin again sometime within the next 1,000 years.

The landscape at Craters of the Moon National Monument is deceptively fragile. A first glance convinces you that it would be virtually impossible to do damage to all that sharp, angular lava. The truth, of course, is just the opposite. The lava breaks down quickly under feet, bikes and vehicles. Because the interior of many lava flows is red in color, human-caused erosion of the darker lava surface is immediately obvious. Look, for example, at the trail to the top of Inferno Cone or the pahoehoe surface along the North Crater Flow trail. This damage is irreversible.

Of course, natural forces are constantly at work breaking down the rock. The contraction and expansion of water in cracks as it freezes and thaws pushes the rock apart. So do the roots of trees and shrubs gradually working their way into crevices. The lava rock will break down eventually on its own. The point is whether the National Park Service should permit humans to accelerate the process.

Not only the rocks themselves, but also the plants growing on them, are subject to destruction. There is little soil and limited plant growth on the lava flows. Nothing grows on the surface of the rock except lichens. Lichens hundreds of years old may be only inches wide. If destroyed, it may take decades for them to reestablish themselves. Trampling may also kill wildflowers and other plants.

The National Park Service strives to keep as much of Craters of the Moon open to the public as possible. However, sometimes the level of human impact is too great, and protective action becomes necessary. What is an acceptable level of human impact? There is no clear answer to the question. Preservation of irreplaceable resources has to balance visitor use. At present, there are two areas in the monument which are closed to off-trail travel due to the exceptionally fragile nature of the rocks and heavy human use: the Spatter Cones and the North Crater Flow. How would you handle this complex problem?