When volcanoes erupt, a large amount of material from the Earth’s interior, including extraordinary amounts of noxious gases like sulfur dioxide, are released into the atmosphere. The ‘smoke’ you see isn’t from combustive ash, however; volcanic ash is fundamentally different. (European Geosciences Union)

‘Volcanic Ash’ Isn’t Actually Ash

Whatever you do, don’t try to wash it away with water.


Every few months, a volcanic eruption occurs on Earth, with lava flows and enormous plumes of volcanic ash.

In 2015, the Chilean volcano Calbuco erupted for the first time in 42 years. Although the sight of volcanic lightning may be beautiful, the eruption itself causes significant damage and widespread devastation. (Jose Mancilla/LatinContent/Getty Images)

These small eruptions might produce only ~0.01 cubic kilometers of ash, while large, rare ones can produce thousands.

Scientists studying the ash from a recent eruption of Ol Doinyo Lengai volcano in Tanzania. This ash will not wash away during the next rainfall like combustive ash would. (public domain)

Unlike the result of combustion, however, what we call volcanic ash isn’t ash at all.

An ash particle imaged with an electron microscope from the Mt. St. Helens eruption of 1980. Volcanic ash has fundamentally different properties from normal, combustion-driven ash. (USGS / A.M. Sarna-Wojcicki)

Combustive ash is what’s left when carbon-based material burns in the presence of oxygen: calcium carbonate, potash, nitrogen, and minerals and oxides.

A wildfire as seen from near Stevenson Wash., across the Columbia River, burning in the Columbia River Gorge above the Bonneville Dam near Cascade Locks, Oregon. Although this type of disaster can have devastating effects, the ash produced is easily cleaned up and washed away, unlike volcanic ash. (Tristan Fortsch/KATU-TV via AP)

It has many practical and survival uses, and is easily washed away with water.

The three-member crew of the Expedition Five mission onboard the International Space Station was able to observe Mt. Etna’s spectacular 2002 eruption, and photograph the details of the eruption plume and smoke from fires triggered by the lava as it flowed down the 11,000 ft mountain. Note the visual difference between the fire-caused ash (in white) and the volcanic ash (much darker) even as viewed from space. (NASA / ISS)

Instead, volcanic ash is made of rock, mineral, and glass fragments as small as 4 microns (μm) each.

A scanning electron microscope (SEM) image of an ash particle, collected following the volcanic eruptioin of Mount Redoubt in Alaska on March 22, 2009. (Pavel Izbekov and Jill Shipman, University of Alaska Fairbanks)

As magma rises from beneath the Earth, the gases dissolved within it expand and escape, shattering solid rock and shredding magma fragments into the air.

This photo taken on April 6, 2018 shows Mount Sinabung volcano spewing thick volcanic ash, seen from the town of Karo. (Anto Sembiring / AFP / Getty Images)

These tiny, airborne rock and glass fragments then solidify, where they can be blown tens or even thousands of kilometers away.

Volcanic ash — pulverized rock ejected from a volcano — consists of tiny jagged pieces of rock and glass. Unlike wood ash, newly ejected volcanic ash is sharp and abrasive. It can damage car finishes, clog machinery, vents, and pipes, and can cause respiratory discomfort. In large enough quantities, its weight can be enough to collapse roofs, especially if it gets wet. (Pavel Izbekov and Jill Shipman, University of Alaska Fairbanks)

Macroscopically, volcanic ash is hard, abrasive, corrosive, and does not dissolve in water.

Volcanic ash cannot be washed off, but must be gathered and swept up, as it is made of insoluble minerals, rocks, and glasses formed at the magma/atmosphere interface. (USGS)

Large eruptions can block the sun, cause acid rain and thunder/lighting, and even suffocate nearby residents.

Heavy ashfall around Mount Pinatubo in the Philippines in 1991 not only blanketed the surrounding areas in a thick layer of volcanic ash particles, but caused midday darkness due to the thickness of the ash clouds. (US Geological Survey)

Ashfall then poses its own hazards to buildings and the environment.

Street ash cleanup in Bariloche, Argentina following the 2011 Puyehue eruption of the volcano in Chile. Ash cleanup can be time and resource intensive, and ideally involves sweeping and gathering the dried ash and moving it only once. (USGS / Xinhua / Telam)

Proper clean-up and disposal is difficult, but essential.


Mostly Mute Monday tells the scientific story of an astronomical or physical phenomenon, object, or image in visuals and no more than 200 words.

Starts With A Bang is now on Forbes, and republished on Medium thanks to our Patreon supporters. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.