Pyroclastic Flows, Surges and Ignimbrites: The Eruption Rock in National Parks

Pyroclastic flows and surges are among the most awesome and most destructive of all volcanic phenomena. They travel faster than lava flows and farther than many of them, and some can overcome topographic barriers. Pyroclastic flow deposits, called ignimbrites, are found in at least 21 units of the National Park System.

The leading edge of a pyroclastic flow on March 27, 2009 at Redoubt Volcano in Lake Clark National Park, Alaska, channeled down the Drift Glacier into the Drift River valley. The photo was taken one minute after an explosive event (USGS time-lapse camera).

Flows and surges

Pyroclastic density currents are ground-hugging flows of hot volcanic gas and particles, such as ash, pumice, crystals and small rock fragments. Gravity drives them, and they move at more than 200 miles per hour (320 km per hour). There are two kinds, and they sometimes occur together, with surges coming just before flows.

One special kind, the base surge, comes from phreatomagmatic (hydrovolcanic) eruptions, where hot magma meets shallow groundwater, surface water or permafrost. Base surges are common at maars and tuff rings, which may be basaltic or of intermediate composition.

A pyroclastic flow on Augustine Volcano during a 1986 eruption. Augustine lies south of Lake Clark National Park, outside it (USGS Alaska Volcano Observatory photo by M. E. Yount).

The rock they leave

Tuff is any rock made of consolidated or welded volcanic ash, tephra and other debris. It forms by ash fall or by pyroclastic flows and surges. A flow deposit is also called an ash-flow tuff or ignimbrite.

Ignimbrites are typically massive, with no layering, and are a poorly sorted mix of pumice, rock fragments and crystals in a matrix of ash. They compact under their own weight. If still hot when laid down, the ash particles weld together and the pumice collapses into flattened shapes called fiamme. Often only the interior is densely welded because it holds the heat. Sheets can be very thick and cover wide areas: the Valley of Ten Thousand Smokes ignimbrite in Katmai National Park is up to 670 feet (200 m) thick.

Surge deposits are usually better sorted than flow deposits and have more crystals and rock fragments. They may show bedding and low-angle crossbedding, and are generally much smaller in volume and thinner than ash-flow tuffs.

Welded Lava Cliff Tuff, Yellowstone National Park, with flattened pumice, rock fragments and crystals in fused ash (NPS photo by John Good).

Flow deposits in the parks

• Aniakchak National Monument. The eruption that formed the caldera about 3,660 years ago was one of the largest of the Holocene. Its pyroclastic flow ran north into Bristol Bay and caused a tsunami, and to the south climbed over passes as high as 850 feet (260 m). The ignimbrite covers 965 square miles (2,500 square km).
• Chiricahua National Monument. The hoodoos are welded ignimbrite. The Rhyolite Canyon Tuff erupted from the Turkey Creek Caldera about 26.9 million years ago, and weathering along vertical joints carved the spires.
• Crater Lake National Park. The climactic eruption of Mount Mazama about 7,700 years ago formed the caldera. Thick pyroclastic flows from the collapsing eruption column made the Wineglass Welded Tuff.
• Katmai National Park. The 1912 eruption of Novarupta-Mount Katmai was the largest of the 20th century. Its thick flow gave the Valley of Ten Thousand Smokes its name, from the many fumaroles the cooling ignimbrite produced for decades. Today the surface is mostly barren and cut by gorges where streams have eroded it.
• Lake Clark National Park. Redoubt and Iliamna Volcanoes commonly produce pyroclastic flows, from collapsing lava domes or eruption columns. Redoubt made small ones in 2009 and in 1989-1990.
• Valles Caldera National Preserve and Bandelier National Monument. The Tshirege Member of the Bandelier Tuff erupted 1.26 million years ago in the eruption that formed the Valles Caldera. It weathers to a swiss-cheese surface of pockets called tafoni. Its soft, nonwelded lower section let the Ancestral Puebloans enlarge the pockets and carve hollowed chambers into the cliff, the interior rooms of their cliff-side pueblos.
• Yellowstone National Park. Three caldera-forming eruptions, 2.1, 1.3 and 0.64 million years ago, each produced large silicic ash-flow tuffs. The Huckleberry Ridge Tuff, from the first caldera about 2.1 million years ago, is three ignimbrite members and two pyroclastic-fall members. Each flow member was laid down in one event that may have lasted days, with gaps between them. Perhaps as long as a few decades separate members B and C, and a chilled margin at the base of C shows that B had at least partly cooled.

The stratigraphy of the Huckleberry Ridge Tuff (USGS).

Surge deposits in the parks

At least four park units hold surge deposits.

• Aniakchak National Monument. Half Cone, a small composite volcano inside the caldera, erupted about 400 years ago and produced pumice fall, a pyroclastic flow and surges. The surge deposits are about 42 feet (13 m) thick, with low-angle crossbedding.
• Bering Land Bridge National Preserve. The Espenberg maars formed from the explosive meeting of magma and permafrost. Base surges left the wedge-shaped deposits typical of maars and tuff rings.
• Death Valley National Park. The Ubehebe Craters are maars formed when magma met shallow groundwater, and their surge deposits show bedding and crossbedding.

Pyroclastic surge deposits on the rim of Devil Mountain maar (NPS photo by Chad Hults).

Sources

• National Park Service, Pyroclastic Flows and Ignimbrites, and Pyroclastic Surges, Geology; rewritten in hubnx's own words.