DESCRIPTION:
Idaho Volcanoes and Volcanics

The Columbia River Basalt Group (CRBG) is the youngest and most studied flood basalt. The province underlain by the basalt is loosely termed the Columbia Plateau. Such an overall designation is a misnomer, however, for the basalt has been sharply folded and broadly warped, so that its top varies in elevation from slightly below sea level in the Pasco Basin to more than 2.5 kilometers above sea level in the Wallowa Mountains of northeast Oregon. ... The group is formally divided into five formations, which in turn are broken into formal and informal members. ... The group has a volume of about 174,000 cubic kilometers and covers about 164,000 square kilometers. These figures have been revised downward from previous estimates. It was erupted between 17.5 and 6 million years ago, as measured by K-Ar and 40Ar-39Ar ages. ... Linear vent systems occur only in the eastern half of the province, except for feeders of the Picture Gorge (the Monument dike swarm) near the southern limit of the province. Some vent systems are longer than 150 kilometers, and all trend within a few degrees of due north, mostly north-northwest. The systems are correlated with specific stratigraphic units chiefly by the presence of dikes of appropriate chemical composition, petrography, magnetic polarity, and stratigraphic position. Hundreds of dikes have been identified ... Most dikes are known from Chief Joseph dike swarm in the tri-state area of Washington, Oregon, and Idaho ... The dikes typically are a few meters wide, but some are wider than 20 meters ...

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Craters of the Moon is a monogenetic volcanic field, 1,630 to 1,810 meters in elevation, of olivine basalt composition. Craters of the Moon had 8 eruptive episodes from 15,000 to approximately 2,000 years ago. ... Craters of the Moon lava field lies along the northern border of the Snake River Plain, midway between Arco and Carey, Idaho. It consists of Holocene to Pleistocene lava flows, cinder cones, spatter cones, lava tubes, and other features typical of basaltic volcanism. Much of the field lies within the Crater of the Moon National Monument, administered by the National Park Service. ... Craters of the Moon is 29 kilometers southwest of Arco, Idaho, on U. S. Alternate Route 93 in Butte and Blaine counties. Paved roads are found within the National Monument open to vehicular traffic; the southern part of the Monument is a wilderness area closed to vehicles.

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The largest tributary of the Columbia River is the Snake River, flowing from its headwaters in Yellowstone National Park for over 1,670 kilometers through Idaho, Oregon, and Washington. Here, on the Oregon and Idaho border, the Snake River carved the 2,370 meters Hell's Canyon, the deepest gorge in North America

Yellowstone lies at the intersection of the Basin and Range tectonic province, dominated by E-W extension, and the eastern Snake River Plain, a linear downwarp or graben that has been a locus for basaltic volcanism since middle Miocene time. According to one popular model, the rhyolitic Yellowstone Plateau marks the current location of a "hotspot" or melting anomaly in the upper mantle, and the basaltic Snake River Plain records the hotspot's northeastward track across the mobile North American Plate.

An 80-kilometer-wide swath of basaltic and rhyolitic volcanism cuts across southeast Idaho for 450 kilometers. This Snake River Plain-Yellowstone (SRPY) volcanic province is the most dynamic area of volcanism in North America. This is not because of abundant historic eruptions -- there have been none -- but rather because of its rapid motion. SRPY is propagating to the northeast at 3.5 centimeters per year (Armstrong, et.al., 1975); it will slice through Montana and be at the Canadian border in approximately 20 million years. If past activity is a guide, SRPY doesn't simply cover terrain with volcanic rocks, but rather the pre-existing ground subsides up to 6 kilometers (Braile, et.al., 1982) between major faults (Sparlin, et.al., 1982) and is further churned up by the transit of magma and the formation of magma chambers. SRPY is a geologic roto-tiller.

According to the radiometric dating of Armstrong, et.al. (1975), SRPY activity began approximately 15 million years ago with silicic volcanism in southern Idaho. A series of now buried rhyolitic calderas formed in a northeast progression, with abundant basaltic volcanism lagging behind by 2-5 million years. Island Park and the two Yellowstone calderas are the most recent manifestations of the silicic volcanism, and Island Park is now being colonized by the basaltic wave of magma. ...

The Snake River Plain forms a broad arch across the southern part of Idaho extending 600 kilometers eastward from the Oregon border to the Yellowstone Plateau. Its width ranges from 65 to 100 kilometers. The western part is a complex graben bounded by a system of normal faults. Structure in the eastern part is less certain, but may involve both downwarping and faulting. to the north the plain cuts Mesozoic-Tertiary plutonic rocks and folded Paleozoic-Mesozoic rocks. To the south, the plain is bounded by basin-and-range fault-block mountains and Tertiary rhyolitic and basaltic rocks. There is no evidence that the Snake River Plain existed as a structural feature prior to the Miocene.

Eruptions of high-volume, bimodal rhyolite and basalt began in the middle Miocene in the southwest region, and activity shifted east-northeast where it now focused on the Yellowstone Plateau. The present surface of the Snake River Plain is dominated by basaltic lava flows as recent a approximately 2,000 years. Although a thin veneer of loess and windblown sand covers parts of the plain, most of the primary surface features are preserved.

The Snake River Plain represents a style of volcanism between flood basalt eruptions and Hawaiian volcanism. Like Hawaiian volcanism, plains volcanism involves multiple, thin (3-5 meters) flow units erupted from central vents, and minor fountaining to produce cinder cones. However, like flood basalt eruptions, the vents are often aligned on rift zones, and some of the flows are fissure fed. The surface of flow accumulation is planar, because the vents are spread over a wide area, not focused in a central zone. Typical of plains volcanism, most flows on the Snake River Plain accumulate as (1) small, low shields, (2) fissure flows, and (3) or large tube-fed flows. All were probably emplaced relatively slowly, often advancing only a few meters per hour, forming "toey" lava flows with hummocky surfaces of several meters relief. Pressure ridges and collapse craters are common. ...

Several types of craters occur in the Snake River Plain, including calderas, pit craters, maar craters, and small collapse depressions. Calderas are rare and tend to be small, except for the Island Park Caldera on the northeastern boundary of the plain. Pit craters on the Snake River Plain are generally less than one kilometer in diameter and are commonly found at the summits of low shields. Maar craters and tuff rings occur in several parts of the plain. The largest, Menan Buttes, may have formed by interaction of magma with surface water, possibly the ancestral Snake River. Others, such as Sand Crater and Split Butte, are young maar craters that apparently have involved groundwater. They consist of tephra deposits around central craters 500-800 meters in diameter that developed in lava flows. They appear to have formed by an early phreatomagmatic phase followed by an effusive phase that emplaced a lava lake, which then subsided to form the craters.

Rhyolitic hills occur in several places in the eastern Snake River Plain. One of the most prominent is Big Southern Butte. It rises 760 meters above the plain, is 6.5 kilometers across, and comprises 2 coalesced cumulo domes of 0.3 million-year-old rhyolite and an elevated section of older basalt flows.

The Snake River Plain is traversed by Interstate Highways 15, 86, and 84, plus a relatively good network of US, State, and County roads.

The primary fissure system, called the Great Rift, passes through the Monument as a set of en echelon fissures which strikes N35degreesW and occurs in a zone up to 3 kilometers wide. Many of the flows and cinder cones have been described in detail.

Much of the volcanism of the Snake River Plain was confined to volcanic rift zones. A volcanic rift zone is a concentration of volcanic landforms and structures along a linear zone of cracks in the earth's crust. The Great Rift volcanic rift zone is a zone of cracks running approximately northwest to southeast across almost the entire eastern part of the Snake River Plain. The entire Great Rift is 62 miles long.

The Great Rift is an example of basaltic fissure eruption. This type of volcanic activity is characterized by extrusion of lavas from fissures or vents that is relatively quiet in comparison with highly explosive eruptions such as the 1980 Mount Saint Helens eruption. Where the Great Rift intersects the earth's surface, there is an array of cinder cones, lava cones, eruptive fissures, fresh-appearing lava flows, noneruptive fissures, and shield volcanoes.

One explanation for the existence of the Snake River Plain and the Craters of the Moon lava field is called the mantle plume theory. This theory states that beneath the crust of the Snake River Plain lies a "hot spot" or localized heat source. Periodically, this hot spot consists of a "plume" of molten rock (magma) which rises buoyantly to the surface of the earth. The hot spot does not move but rather remains in a fixed position. What does move is the crust of the earth; as the North American plate slides southwestward over the hot spot. As the plate moves over the hot spot volcanic eruptions occur on the surface.

Initially these eruptions are very violent and produce a lava known as rhyolite. Huge calderas of up to 30 miles in diameter are formed when these devastating eruptions take place. Later a more fluid lava known as basalt flows onto the surface and covers the rhyolitic flows. Yellowstone National Park, the area where the hot spot is believed to be located at this time, is the place where catastrophic rhyolitic eruptions last occurred 600,000 years ago. Craters of the Moon represents the second stage of the eruptions where fluid basaltic lava covered the landscape as recently as 2,000 years ago.

As you drop into the Snake River Canyon, you can trace some of the geologic history of the area. The canyon cliffs show layer upon layer of lava flows interspersed with sedimentary layers. As you reach the bottom, the canyon floor is scattered with hundreds of house-size boulders left behind from the Bonneville flood. This flood raced through the canyon 15,000 years ago with more than 100 meters (350 feet) of water rushing at 110 kilometers (70 miles) per hour.

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Rhyolitic hills occur in several places in the eastern Snake River Plain.

One of the most prominent is Big Southern Butte. It rises 760 meters above the plain, is 6.5 kilometers across, and comprises 2 coalesced cumulo domes of 0.3 million-year-old rhyolite and an elevated section of older basalt flows.

Craters of the Moon is a monogenetic volcanic field, 1,630 to 1,810 meters in elevation, of olivine basalt composition. Craters of the Moon had 8 eruptive episodes from 15,000 to approximately 2,000 years ago. ... Craters of the Moon lava field lies along the northern border of the Snake River Plain, midway between Arco and Carey, Idaho. It consists of Holocene to Pleistocene lava flows, cinder cones, spatter cones, lava tubes, and other features typical of basaltic volcanism. Much of the field lies within the Crater of the Moon National Monument, administered by the National Park Service. ...

Craters of the Moon is 29 kilometers southwest of Arco, Idaho, on U.S. Alternate Route 93 in Butte and Blaine counties. Paved roads are found within the National Monument open to vehicular traffic; the southern part of the Monument is a wilderness area closed to vehicles.

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King's Bowl is a composite flow field of 1,500 meters elevation, and basalt composition. This field erupted 2,130 years ago. ...

The King's Bowl Field is small and covers less than 2.6 square kilometers. It is situated on the King's Bowl Rift Set,, one of several such sets which collectively make up Idaho's Great Rift, a series of tension fractures that cross cut the eastern Snake River Plain.

The King's Bowl field is a composite feature made up of flows from several point sources along the Rift as well as a larger, apparently dike-fed sheet flow, which for a time was held in a lava lake. These flows locally overlap, indicating that the eruptive sequence was complex and issued from different vents at different times. ...

King's Bowl itself is a large ovoid crater (85 meters x 30 meters x 30 meters deep) apparently generated by an explosive event coupled with collapse. ...

King's Bowl is located near American Falls, Idaho, on State Route 39. Crystal Ice Cave - a U. S. National Landmark, is located on the Rift at the King's Bowl Flow.

Menan Buttes are olivine tholeiitic basalt tuff cones which erupted Mid- to Late-Pleistocene. ... North and South Menan Buttes are the two most prominent phreatomagmatic cones of the Menan Complex, a group of six cones roughly aligned along a north-northwest trend, 16 kilometers west-southwest of Rexburg, Idaho. North Menan Butte is the larger, standing 250 meters above the surrounding Snake River Plain. It is elliptical in plan with axes 3.5 and 2.5 kilometers in length. South Menan Butte measures 3 kilometers x 2 kilometers and has 145 meters of relief. The buttes are asymmetrical with a greater accumulation of material on their northeast flanks, presumably due to strong southwest winds during eruption.

The Wapi lava field is one of several Holocene to Pleistocene volcanic fields on the Snake River Plain. ... The slope of the field is typical of the lows shields of the Snake River Plain; over distances of 10-20 kilometers, slopes are typically less than one degree. This flat slope is a consequence of the very fluid pahoehoe lavas and relatively high rates of effusion that typify the Snake River Plain. The only area of the field having a steeper slope is in the vicinity of Pillar Butte, the summit region of the field, where the slopes range from 5 to 7 degrees. ...

The steep profile of Pillar Butte is at least partly attributed to the relatively low-volume flows that did not travel very far from the central vent, and the higher proportion of viscous aa flows. The butte is a prominent mass of agglutinate and layered flows, possibly injected with dikes; the mass rises approximately 18 meters above the general summit region on the south side. This prominent structure served as a landmark for early travelers along the Oregon Trail, and still serves as a general reference point in the south-central Snake River Plain. ...

The Pillar Butte summit region contains at least 11 eruptive centers, identified by pit craters and former lava lakes. Flows from these centers are typically short and often contain small lava tubes and channels. Many of the tubes and channels were used repeatedly by subsequent flows, sometimes forming roofs over previous channels, or draining into older lava tubes through skylights.

As you drop into the Snake River Canyon, you can trace some of the geologic history of the area. The canyon cliffs show layer upon layer of lava flows interspersed with sedimentary layers. As you reach the bottom, the canyon floor is scattered with hundreds of house-size boulders left behind from the Bonneville flood. This flood raced through the canyon 15,000 years ago with more than 100 meters (350 feet) of water rushing at 110 kilometers (70 miles) per hour.

MORE about Lake Bonneville and the Bonneville Flood

Split Butte is an olivine basalt tuff cone/pit crater, 1,410 meters in elevation, which erupted more than 2,270 years ago. ...

Split Butte on the south-central Snake River Plain overlies basalt flows of the Snake River Group and was encroached from the southeast by a lobe of the Wapi lava flow, which has been dated at 2,270 years B.P. The butte consists of vitric ash forming a ring 600 meters across. The ring is asymmetrical, having a greater accumulation on the east, the result of prevailing westerly winds during the eruption. Although the eroded ring stands 50 meters above the surrounding plain, an original ash thickness of 80 meters on the east is estimated from the dip of the beds and the ring diameter. A topographic notch or erosional "split", approximately 150 meters wide occurs in the thick eastern ash accumulation. ...

A central lava lake was retained by the tephra ring, and basaltic lavas are in disconformable contact with the tephra. Minor lake overflow occurred on the low southwest section of the ring. The lava lake margin is preserved as a narrow circular shelf of basalt, but the central portion has subsided to form a pit crater 20 meters deep and 420 meters across. Two low mounds of spatter occur along the pit crater scarp. The spatter consists of highly oxidized scoria and may represent a degassing outlet for post-subsidence liquids. ...

Split Butte lies 40 kilometers west-northwest of American Falls, Idaho.

Wapi is a monogenetic low-shield volcano which erupted 2,270 years ago. Wapi is 1,635 meters in elevation with basalt composition. ...

The Wapi lava field is one of several Holocene to Pleistocene volcanic fields on the Snake River Plain. In many respects, it is typical of the older fields of low shields that make up the present surface of the plain. It covers a large (300 square kilometers) area that is elongate in the north-south direction and has three prominent lobes extending east, west, and northwest from the main mass of the field. ...

The slope of the field is typical of the lows shields of the Snake River Plain; over distances of 10-20 kilometers, slopes are typically less than one degree. This flat slope is a consequence of the very fluid pahoehoe lavas and relatively high rates of effusion that typify the Snake River Plain. The only area of the field having a steeper slope is in the vicinity of Pillar Butte, the summit region of the field, where the slopes range from 5 to 7 degrees. ...

Wapi lava field lies 40 kilometers west-northwest of American Falls, Idaho.

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