  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. (Like caldron, 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. (continued on page 28) Identifying the Lava Flows
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 created 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. (continued on page 34) Indian Tunnel{uncaptioned}
Cinder Cones and Spatter Cones
Lichens often pioneer new life on Earth. Two plants in one, lichens are composed of an alga and a fungus growing together to their mutual benefit, usually on rock. Hardy and slow-growing, lichens help break down rock to soil-building mineral matter. Eventually their vegetable matter decays, helping to form the first soils that other plants can then use. Tough in the extreme, some lichens can be heated to high temperatures and still be capable of resuming normal growth when returned to viable conditions. |
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