Volcanic Ash: The Gritty Snow That Never Melts, and Where It Lies in the National Parks

Volcanic ash is not ash in the ordinary sense. It is not the product of burning at all, but shattered rock and volcanic glass — fragments smaller than 2 millimeters, torn apart by violently expanding gas bubbles. When an explosive eruption throws it high into the atmosphere, sometimes into the stratosphere, it falls back like an abrasive, gritty, corrosive snow that never melts.

An eruption cloud from Redoubt Volcano on March 23, 2009, moving northeast toward Anchorage. Credit: USGS image by John Bailey.

What ash is made of

As magma rises and loses pressure, water, carbon dioxide and other volatiles come out of solution and form bubbles. Their violent expansion shatters the magma into ash, mostly glass fragments, sometimes with crystals that grew in the magma, or with pulverized older rock. The jagged edges of burst bubbles make ash very abrasive, and it can be mildly corrosive.

Ash from Redoubt Volcano, Lake Clark National Park, March 2009, under a scanning electron microscope: jagged edges where gas bubbles popped. Credit: USGS Alaska Volcano Observatory photo by Tom Kircher.

Why it matters

Ash is a hazard far from the volcano and long after the eruption. Wind lifts old ash into blowing clouds; water carries it into streams, rivers and lakes, where it can choke aquatic life; and it damages infrastructure — especially aircraft that fly into an ash cloud.

Map showing extent of ash-fall deposits from the 1980 Mount St. Helens eruption along with some ancient eruptions. The Mount Mazama (Crater Lake) and Yellowstone eruptions formed large calderas. Crater Lake and Yellowstone national parks are units of the National Park System. USGS map.

How fallout deposits look

Any explosive eruption can make ash, but widespread ash falls usually come from Sub-Plinian and Plinian eruptions at composite volcanoes and from Ultra-Plinian eruptions at resurgent calderas.

• Tephra fall (or fallout, or pyroclastic fall) covers ash, pumice and other fragments falling from an eruption cloud.
• Fallout deposits are usually well sorted — made of particles of similar size — and often layered. They are thicker and coarser closer to the vent, from tens of feet near it to thin dustings far away.
• Consolidated or welded ash and debris form tuff. Ash falls are often loose, but can weld if the tephra is still hot when it lands.

In the national parks

Ash-fall and other pyroclastic-fall deposits appear in 22 units of the National Park System, from deposits near their volcanoes to ash carried hundreds or thousands of miles.

• Yellowstone. Huge caldera-forming eruptions over the last 2.1 million years spread ash across much of the central United States. The Huckleberry Ridge ash, erupted 2.1 million years ago from the largest caldera on the Yellowstone Plateau, had a volume of 590 cubic miles (2,450 cubic kilometers). The Lava Creek ash bed covers all or part of 21 states and reaches into southern Canada; Yellowstone ash appears at Bighorn Canyon National Recreation Area and near Lake Meredith National Recreation Area in the Texas Panhandle.
• Sedimentary parks. Ash beds at Agate Fossil Beds, Badlands, Fossil Butte and Petrified Forest may have fallen directly into the basins or been reworked and moved by streams and rivers.
• Alaska. Redoubt Volcano, in Lake Clark National Park, dusted Anchor Point with ash in April 2009; Aniakchak caldera's climactic eruption about 3,660 years ago, one of the largest of the Holocene, left pyroclastic-fall and flow deposits.

Pumice fall deposit from the 2009 eruption of Redoubt Volcano in Lake Clark National Park. USGS AVO photo by K. F. Bull.

Map showing the extent of ash fall from the three caldera-forming eruptions at Yellowstone during the last 2.1 million years. The ash bed from the 1980 Mount St Helens eruption is shown for comparison. Modified from USGS graphic.

Ash on snow in Anchor Point, Alaska, from the April 4, 2009 eruption of Redoubt Volcano. Credit: USGS Alaska Volcano Observatory photo by Lori Blank.

Sources

Based on "Volcanic Ash, Tephra Fall, and Fallout Deposits" from the National Park Service, a work of the United States government in the public domain. Pictures an earlier version of this page left out were restored on 2026-09-26.