Pyroclastic Flows and Ignimbrites, and Pyroclastic Surges (U.S. National Park Service)

photo of a dust and ash cloud moving down a treeless u-shaped valley — The leading edge of a pyroclastic flow on March 27, 2009 at Redoubt Volcano in Lake Clark National Park in Alaska. The flow was channeled down the Drift Glacier into the Drift River valley. Photo was taken one minute after an explosive event at the volcano.
USGS image from a time-lapse camera.

Introduction

Explosive volcanic eruptions can produce pyroclastic flows and surges, both among the most awesome and most destructive of all volcanic phenomena. They travel faster than lava flows and also greater distances than many lava flows. Some can even overcome topographic barriers.

Pyroclastic density currents are ground-hugging flows of hot volcanic gases and particles (volcanic ash, pumice, crystals, and small rock fragments) that are propelled by gravity and move extremely rapidly, travelling at speeds more than 200 miles per hour (320 km per hour).

There are two types of pyroclastic density currents: pyroclastic flows and pyroclastic surges.

Pyroclastic flows are high-density currents of pumice, ash, blocks, and volcanic gas that rapidly move down the slopes of a volcano.
Pyroclastic surges are low-density currents of ash, pumice, crystals, and volcanic gases that are more dilute than pyroclastic flows.

Pyroclastic surges and flows sometimes occur together, with surges occurring immediately before flows.

Pyroclastic Flows

Pyroclastic flow may be initiated by collapse of Plinian eruptive columns due to loss of buoyancy, by explosive eruptions that boil over instead of forming a high eruption column, or by the collapse of lava domes.

Pyroclastic flows usually occur during highly explosive eruptions of silicic magmas at composite volcanoes and resurgent calderas. Pyroclastic flow deposits are important components of most composite volcanoes. Caldera-forming eruptions, at both resurgent and summit calderas, are accompanied by the production of voluminous pyroclastic flows.

photo of a cloud of dust and ash moving down a mountainside. — A pyroclastic flow on Augustine Volcano during an eruption in 1986. Augustine is located south (outside) of Lake Clark National Park where Iliama and Redoubt Volcanoes are located. This classic image of an active pyroclastic flow shows the density current traveling down the volcano’s slopes and an ash cloud rising from it.
USGS AVO photo by M. E. Yount.

Pyroclastic Surges

Pyroclastic surges are less dense and travel faster than pyroclastic flows. They sometimes occur close to the start of an explosive eruption and may precede pyroclastic flows.

A specific type of pyroclastic surge (base surge) is associated with phreatomagmatic (hydrovolcanic) eruptions where hot magma encounters shallow groundwater, surface water, or permafrost.

Like for pyroclastic flows, most surges occur during the eruption of silicic magmas, except for base surges that are common at maars and tuff rings that may be of basaltic or intermediate compositions.

Pyroclastic Density Current Deposits and Ash-Flow Tuffs (Ignimbrites)

photo of a person's hands holding a rock and pointing to the fine particles — A sample of the Lava Creek Tuff.
USGS photo by Ray Salazar (Montana State University), public domain.

Any rock made of consolidated or welded deposits of volcanic ash, tephra, and other volcanic debris is known as a tuff. Tuffs form either by fallout (ash-fall) or by pyroclastic flows or surges.

Pyroclastic flow deposits are also known as ash-flow tuffs or ignimbrites.

Ignimbrites are typically massive (e.g., without bedding or layering), and are composed of a poorly-sorted mixture of pumice fragments, rock fragments, and crystals in a groundmass of volcanic ash. They compact under their own weight. If they are still hot enough when they are emplaced, the ash particles fuse or weld together and pumice fragments collapse into flattened structures known as fiamme.

Frequently only the interior or central part of an ash-flow tuff is densely welded due to heat retention in that part of a deposit. Ignimbrite sheets can be very thick. For example, the Valley of Ten Thousand Smokes ignimbrite in Katmai National Park is up to 670 feet (200 m) thick. They also may have extremely large volumes and cover large geographic areas.

Pyroclastic surge deposits are usually better sorted than flow deposits and usually have a greater proportion of crystals and lithics (rock fragments). They may be bedded with low-angle crossbedding. They usually have a much smaller volume than ash-flow tuffs, are relatively thin, and are not as geographically extensive as large ash-flow tuffs.

Photo of volcanic rock close-up showing fine- and course-grained materials "welded" together. — Welded ash-flow tuffs contain pieces of pumice that have been compressed and flattened due to welding, small lithic fragments (pieces of preexisting rock that were transported by the ash flow), and crystals all set within a matrix of volcanic ash that has been fused together due to the heat that was present after deposition. Lava Cliff Tuff, Yellowstone National Park.
NPS photo byJohn Good.

copyrighted photo showing close-up rock texture of welded tuff — Fiamme (the thin black streaks) in a densely welded portion of the Bandelier Tuff.
Photo by / © Marli Miller, geologypics.com.

National Park Pyroclastic Flow Deposits

Pyroclastic flow deposits are found in at least 21 units of the National Park System including:

Aniakchak National Monument

photo of a cliff face showing 3 different rock layers

The eruption that formed Aniakchak caldera about 3,660 years ago was one of the largest eruptions of the Holocene. It was accompanied by the eruption of a voluminous pyroclastic flow that flowed north into Bristol Bay and caused a tsunami. To the south, it surmounted passes as high as 850 feet (260 m). The ignimbrite left by this pyroclastic flow covers an area of 965 square miles (2,500 square km).

^{Photo (right): Pyroclastic-flow deposits from the climatic eruption of Aniakchak caldera. The thick pyroclastic-flow deposits overlie a thin ash-fall deposit.}

^{USGS AVO photo by Christina Neal.}

Chiricahua National Monument

The hoodoos that make up Chiricahua NM’s wonderland of rocks are made of welded ignimbrite. The Rhyolite Canyon Tuff was erupted from the Turkey Creek Caldera approximately 26.9 million years ago. The rocks spires formed as a result of weathering and erosion along intersecting systems of vertical joints.

photo of a rolling landscape covered with trees and tall rock spires — Scenic vista of pinnacles, columns, and balanced rocks in Chiricahua National Monument, Arizona.
NPS photo by Ron Kerbo.

Crater Lake National Park

The climactic eruption of Mount Mazama approximately 7,700 years ago formed the Crater Lake caldera. The collapse of the eruption column sent thick pyroclastic flows that traveled down valleys on the flanks of the volcano producing the Wineglass Welded Tuff.

Photo of layered rock outcrop with unusual erosional features surrounded by forested hills — The Wineglass Welded Tuff in Crater Lake National Park. The pinnacles are the result of fumarole pipes that were cemented by hot gases so that they more resistant to erosion than other parts of the deposit.
Photo by / © James St. John on Flickr.

Katmai National Park

The eruption of Novarupta-Mount Katmai in 1912 was the largest eruption of the 20th century (VEI 3, Catastrophic). The eruption deposited a thick pyroclastic flow in the area to be named the Valley of Ten Thousand Smokes for the numerous fumaroles produced by the cooling ignimbrite in the decades after the eruption. Today, the Valley of Ten Thousand Smokes is a stark landscape because the surface of the ignimbrite sheet is mostly still barren and is cut by narrow gorges where streams have incised into it.

photo of a stream flowing past a steep bank of fine-grained rock — Streams and rivers have cut gorges and gullies in the Valley of Ten Thousand Smokes ignimbrite like at Ukak Falls, Katmai National Park, Alaska.
NPS photo.

photo of a volcanic landscape with large flow deposit and snow covered peaks in the distance. — The Valley of Ten Thousand Smokes, Katmai National Park, Alaska.
NPS photo.

Lake Clark National Park

Composite volcanoes such as Redoubt Volcano and Iliamna Volcano in Lake Clark National Park commonly produce pyroclastic flows during their eruptions. Pyroclastic flows may be initiated when a lava dome growing in the summit crater collapses or when eruption columns collapse.

Recent eruptive episodes at Redoubt Volcano in 2009 and 1989-1990 have produced small pyroclastic flows.

Photo of an erupting volcanic mountain with a steam cloud above the summit crater and snow on its lower slopes — Pyroclastic flow and surge deposit on the flanks of Redoubt Volcano following the April 4, 2009 eruption. Lake Clark National Park, Alaska.
USGS AVO photo by R. G. McGimsey.

Valles Caldera National Preserve and Bandelier National Monument

The Bandelier Tuff consists of a series of thick ash flow tuffs that were erupted from the Valles and Toledo Calderas. The Tshirege Member of the Bandelier Tuff was erupted 1.26 million years ago during the eruption that formed the Valles Caldera, the younger of the two resurgent calderas in the area. This tuff weathers to form a swiss-cheese type surface (with pockets in the cliff face called tafoni). The nonwelded lower section of this ignimbrite is soft and the Ancestral Puebloans who lived in the area enlarged tafoni and carved the cliff face to excavate cavates (Human-made hollowed out chambers cut in soft rock) to be the interior rooms of their cliff-side pueblos.

Photo of a tall cliff with windows and doors carved in to the rock at the base. — Long House in the Frijoles Canyon and the Tshirege Member of the Bandelier Tuff in Bandelier National Monument.
NPS photo by Sally King.

Photo of cliff with carved windows and doors. — Archeological site in Frijoles Canyon and the Tshirege Member of the Bandelier Tuff in Bandelier National Monument.
NPS photo by Katie KellerLynn (Colorado State University).

Yellowstone National Park

Photo of a rock outcrop showing two different rock layers.

Yellowstone’s three caldera-forming eruptions at 2.1, 1.3, and 0.64 million years ago each produced large-volume silicic ash-flow tuffs.

Large ignimbrites such at the Huckleberry Ridge Tuff were formed in multiple eruptive episodes. The Huckleberry Ridge Tuff was erupted from the First Caldera (Island Park Caldera or Huckleberry Ridge Tuﬀ Caldera) about 2.1 million years ago. It actually consists of three different ignimbrite members and two pyroclastic-fall members.

Each of the three pyroclastic flow members was emplaced in a single event that may have lasted a number of days. But there were time gaps between eruptions that produced the different members. For example, a period of time that perhaps lasted as long as a few decades separates the B and C ignimbrites. The B ignimbrite had had time to at least partially cool as is seen in a chilled margin at the base of ignimbrite C.

^{Photo (right): The arrow marks the contact between two different ignimbrite sheets (members B and C) in the Huckleberry Ridge Tuff. The base of member C shows shows that ignimbrite B has at least partially cooled by the time it was emplaced.}

^{USGS photo by Colin Wilson (Victoria University of Wellington), public domain.}

Summary diagram of the geological record and timing of the Huckleberry Ridge Tuff eruption. See Swallow et al. (2019) for more details. — The stratigraphy of the Huckleberry Ridge Tuff.
USGS image, Public Domain.

Photo of a rocky bluff with two distinct rock layers and an unconformity noted. — Photo of the Huckleberry Ridge Tuff lying unconformably above a much older marine sediment sequence. Hot spring pools and terraces are seen in the foreground. Yellowstone National Park.
USGS photo illustration.