Geology for Teachers



Many Questions Arise
Craters of the Moon National Monument provides one of the best examples of basaltic volcanism in the world. Features typical of basalt eruptions are abundant and easily accessible, such as, pahoehoe and aa lava flows, cinder cones, lava tubes, and spatter cones. 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?

The Snake River Plain
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 to Yellowstone. The age of the Snake River Plain can be traced from the most recent eruptions 2,000 years ago to the very oldest activity about 17 million years ago. Although both are volcanic in origin, the Eastern and Western Snake River Plain are considered geologically distinct.

Western Snake River Plain
Geologists believe the Western Snake River Plain was produced through the formation of a rift valley. A rift valley is a down-dropped area bounded by faults and fractures that are created when the earth's crust is pulled apart. This rifting on the Western Snake River Plain caused the crust to subside. The resulting depression then began to fill as upwelling lavas moved onto the surface through the crustal fractures. Wind and water also carried sedimentary deposits from the surrounding uplands into the subsiding area.

This rift valley originated about 17 million years ago and the last eruptions along the rift occurred between 1 million and 300,000 years ago. Because this landform continues to shrink as it cools, and because lava and sediments are periodically deposited on it, the area is in a state of continual subsidence. Although the Western Snake River Plain is still linked geographically to the Eastern Snake River Plain, geologists now find their origins to be distinctly different.

Eastern Snake River Plain
Volcanic eruptions vary according to how much silica the magma contains. Magma rich in silica is quite viscous and erupts very explosively. Magma with very high silica levels produces a rock called rhyolite. Magma with lower silica content generates much gentler eruptions of more fluid basaltic lava. There is evidence of both types of eruption on the Eastern Snake River Plain.

Calderas are massive craters up to several hundred square miles in area formed when explosive eruptions emptied magma chambers beneath the earth's crust, causing the surface to collapse. Calderas formed by rhyolitic eruptions are present on the Eastern Snake River Plain. Vestiges of numerous rhyolitic lava flows composed of small pieces of ash (ash flow tuffs) are also apparent along the margins of the Eastern Snake River Plain. They are all that is visible of a thick rhyolitic lava flow overlain by more recent basaltic lavas. These basaltic and rhyolitic lava flows have been measured by drilling to a depth of 2 1/2 miles. One half mile of basalt rests on more than two miles of rhyolite.

The rhyolitic volcanic deposits range in age from 16 million years on the western edge of the Plain, to 600,000 years at Yellowstone. Any theory about the origin of this area must explain the progressively younger ages of the rhyolitic deposits as you travel west to east, as well as the fact that later basaltic eruptions buried the rhyolite. Most geologists consider the Mantle Plume Theory the best explanation for the formation of the Eastern Snake River Plain.

Mantle Plume Theory
According to this theory, uneven heating caused by radioactive decay deep within the earth causes some material to become hotter than that surrounding it. As this material heats up, it becomes less dense and rises through the cooler material of the earth's interior. These plumes, or "hot spots", produce many successive batches of rising magma. The rising magma eventually reaches the earth's crust and erupts 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 eruptions creating volcanic features which grow older the further they are from the hot spot. This relationship was first identified in the Hawaiian Island chain, and many geologists see the same pattern expressed on the Snake River Plain. Eruptions occurred 10 million years ago at Twin Falls, 5 million years ago at Arco, and finally, 600,000 years ago in Yellowstone.

The events that occur during an eruption associated with a mantle plume can be broken into two distinct stages.

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

Since gases within the thickened rhyolitic magma chamber cannot easily escape, the eruptions tend to be devastating. These eruptions are so violent that they sometimes spew hundreds of cubic miles of ejected material into the atmosphere. By contrast Mount St. Helens produced less than 1/4 cubic mile of ejected material. As the magma chamber empties, there is nothing to support the crust above it. It collapses, forming a caldera up to several 100 square miles in area.

Stage II
The intense volcanic activity associated with the mantle plume ebbs as the North American plate continues its movement southwestward, only to begin again at a new spot farther up the chain. Eventually the explosive rhyolitic activity is replaced by gentler basaltic eruptions. These basaltic eruptions arise from the original deep crustal basaltic magma chamber that formed during Stage I. Magma continues to rise and enlarge the chamber and pressure within it gradually increases. This pressure, coupled with the fractures in the earth's crust caused by regional crustal extension, generate both the force and the pathways necessary for basaltic magma to move to the surface. The magma remains basaltic because there is little silica left in the crust to melt into it.

Upon eruption basaltic lava is very fluid and basaltic lava flows from these relatively calm eruptions spread out to cover the older rhyolite lavas. With each new eruption, less of the rhyolite is left exposed and, after millions of years, almost all of the rhyolite has been covered by basalt.

The Great Rift
Several volcanic rift zones traverse the Snake River Plain. Volcanic rift zones are weak areas where the earth's crust has been stretched and thinned and fissures have developed. Magma under pressure follows these fissures to the surface.

The most extensive system of fissures on the Snake River Plain is called the Great Rift, which passes through Craters of the Moon. This volcanic rift zone is 60 miles long and from 1.5 to five miles wide. At Craters of the Moon it is characterized by short surface cracks, more than 25 cinder cones, and is the point of origin of over 60 lava flows. Geologists believe that the formation of the Great Rift is related to Basin and Range type faulting.

Basin and Range
Basin and Range faulting is responsible for most of the topography in Nevada, Utah, and southern Idaho. It 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 earth's crust suddenly fractures. Large blocks of earth slip or rotate up and down, creating valleys separated by long mountain ranges.

The release of this tension has resulted in about 150 mountain ranges and valleys in the Basin and Range province, all aligned approximately in a north/south direction. They are spaced approximately at 16 mile intervals.

There are Basin and Range mountain ranges to the north and south of the Snake River Plain. The faults that occur at the edge of each the mountain ranges are known as "border" or "range-front" faults, and extend beyond the base of the mountains and out beneath the lavas. The extension of the range-front faults onto the Eastern Snake River Plain is marked by zones of parallel cracks that may run for tens of miles, and are known collectively as volcanic rift zones.

The Great Rift is not a typical Snake River Plain volcanic rift zone, because it 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 Mountains is slightly more than the 16 miles normally found between Basin and Range structures. Furthermore, gravity and seismic information indicates 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 typical Basin and Range fault system.

The Great Rift is the conduit through which a tremendous amount of lava reached the surface to form the Craters of the Moon lava field.

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

During a typical eruption at Craters of the Moon, the force of rising magma causes a section of the Great Rift to pull apart. As magma rises through the crack, 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 frothy cinders.

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 more localized, at this point the cinders are often thrown even higher into the air and as they rain back down around the vent(s) build up in piles forming cinder cones.

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 are the source of most of the rock produced during an eruption. The flows gradually subside and all activity stops.

What Does the Future Hold?
If what geologists tell us proves to be true, it is likely that 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 been able to determine an eruptive pattern that indicates the area is merely in a stage of dormancy. They believe that past eruptions conform to a predictable time schedule and that the eruptive cycle will begin again within the next 1,000 years.

Last updated: February 28, 2015

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Craters of the Moon National Monument and Preserve
P.O. Box 29

Arco, ID 83213


(208) 527-1300

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