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Map displaying the northeasterly progression of silicic volcanic centers and ages in millions of years ago (Ma)

Calderas

Silicic caldera eruptions are highly explosive and typically erupt hundreds or even thousands of cubic kilometers of material in a single eruption. Because silica-rich magma is very viscous, gases tend to become trapped at high pressure within the magma. As rising magma nears the earth’s surface, the gas expands quickly, causing explosive eruptions that spread volcanic ash and flows over wide areas. As the magma erupts, the evacuated chamber is not able to support the weight of the volcanic edifice, and a caldera forms as the volcano collapses.

A mantle plume is thought to reside beneath the Yellowstone Plateau and is supplying the earth’s crust with magma. The influx of magma-melted crustal rocks beneath Yellowstone produced silicic magma that erupted in three explosive caldera eruptions, the most recent about 600,000 years ago. These eruptions produced more than 6,000 cubic kilometers of tephra fall (ash) and pyroclastic flow deposits.

Between 4.3 and 6.5 million years ago, four similar silicic caldera eruptions occurred in the Snake River Plain near INL. The rocks for these eruptions are now buried beneath basalt lava flows at depths of 0.6 to 1.2 km (1,960 to 3,900 ft). The hazards associated with explosive-silicic volcanism and caldera formation in the INL area are considered negligible because of the following:

  • The hotspot or source of silicic volcanism has migrated more than 100 km (62 miles) northeast of INL to its present location beneath the Yellowstone Plateau
  • Near INL, the last occurrence of hotspot-related caldera eruptions was 4.3 million years ago
  • For the last 4.3 million years, mild effusive basaltic eruptions have occurred near INL and buried the now extinct hotspot-related silicic flows.

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