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View looking west at the silicic dome called East Butte

Volcanic Domes

Volcanic domes are steep-sided mounds of lava, commonly of silicic (rhyolitic) composition, for which the magma is too viscous to flow more than a few kilometers (1 to 2 miles) from the vent. The growth of domes is predominately an effusive process, and blocks of the surrounding terrain can be uplifted and tilted as the viscous magma approaches the surface. Growing domes are steep-sided with loose rubble making them gravitationally unstable, and thus are prone to slope failure. In addition, dome lavas commonly contain sufficient dissolved gas to generate small explosions. As a result, small-volume tephra-fall (ash) deposits and blocky pyroclastic flows are frequently associated with dome growth.

Several small volume (less than 7 cubic kilometers) rhyolite domes were emplaced near the southern boundary of INL over the last 1.2 million years. The rhyolitic domes postdate the earlier Yellowstone hotspot-related caldera volcanism by about 3 million years. They also are compositionally dissimilar to the silicic caldera rocks, suggesting they are separate and distinct volcanic eruptions.

Volcanic Domes

Age (years)

Big Southern

300,000

Cedar

400,000

East

600,000

Middle

1,100,000

Unnamed

1,200,000

Volcanic hazards at INL because of dome eruptions are minimal due to their long repeat times and localized effects. The recurrence interval of domes near INL is one every 240,000 years (5 domes in 1.2 million years). The most likely area of future dome emplacement is along the southern boundary of INL. Several centimeters of tephra could accumulate 10 km (6.2 miles) or more downwind of a growing volcanic dome. Any fumes and tephra associated with dome growth would be carried northeastward along the southern INL boundary, and eventually offsite, by prevailing southwesterly winds.

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