Volcanic Ash
Volcano Terms and Definition


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Volcanic ash collected in Randle, Washington, located about 40 km NNE of Mount St. Helens. The north edge of the eruption cloud of May 18, 1980, passed over Randle and deposited between 1 and 2 cm of ash on the community. At the same distance along the axis of the eruption cloud, however, about 7 cm of ash and larger-sized tepra fell to the ground.



Close view of a single ash particle from the eruption of Mount St. Helens; image is from a scanning electron microscope (SEM). The tiny voids or "holes" are called vesicles and were created by expanding gas bubbles during the eruption of magma.
Volcanic ash

Volcanic ash consists of rock, mineral, and volcanic glass fragments smaller than 2 mm (0.1 inch) in diameter, which is slightly larger than the size of a pinhead. Volcanic ash is not the same as the soft fluffy ash that results from burning wood, leaves, or paper. It is hard, does not dissolve in water, and can be extremely small--ash particles less than 0.025 mm (1/1,000th of an inch) in diameter are common.

Ash is extremely abrasive, similar to finely crushed window glass, mildly corrosive, and electrically conductive, especially when wet.

Volcanic ash is created during explosive eruptions by the shattering of solid rocks and violent separation of magma (molten rock) into tiny pieces. Explosive eruptions are generated when ground water is heated by magma and abruptly converted to steam and also when magma reaches the surface so that volcanic gases dissolved in the molten rock expand and escape (explode) into the air extremely rapidly. After being blasted into the air by expanding steam and other volcanic gases, the hot ash and gas rise quickly to form a towering eruption column directly above the volcano.


Volcanic ash, Brokeoff Volcano, California
These volcanic glass shards are found in a layer of ash known as the Rockland ash bed. These tiny glass shards are "pumiceous" because they consist of many gas-bubble holes (called "vesicles") that formed as gas dissolved in the magma expanded rapidly during eruption, similar to the vesicles contained in pumice. In the ash shards at left, some of the vesicles are oval and others are stretched into long, thin "capillary" tubes.

The ash was erupted about 600,000 years ago, probably by Brokeoff Volcano, northwest of Lassen Peak. The eruption produced local pyroclastic flows and widespread ash fall over western North America and the Pacific Ocean. Scientists have estimated that more than 120 km 3 of tephra was produced by this eruption, greater than that of the Mount Mazama eruption about 7000 years ago that formed Crater Lake, Oregon. The Rockland ash bed is found at many sites throughout northern California, northern Nevada, southern Oregon, and as far east as southeastern Idaho.


Volcanic ash, Yellowstone Caldera, Wyoming
These volcanic glass shards were erupted by Yellowstone Caldera about 2.1 million years ago during one its enormous caldera-forming eruptions. The glass shards are from the walls of gas bubbles (vesicles), and some are from the junctions of two or more vesicles. Note that the shards are slightly curved. The gas bubbles exploded apart during eruption from the pressure of expanding gas in the magma.

The sample in the SEM image is from the Falor Formation, marine sediments deposited on the floor of the Pacific Ocean. The sediments were subsequently uplifted above sea level and now form part of northwestern California. Some of the shards are pitted because they were partly dissolved by acidic groundwater that percolated through the sediments containing the ash layer.


Volcanic ash, Sonoma Volcanic field, California
These glass shards were erupted about 3.4 million years ago from the Sonoma volcanic field nearthe present Mount Saint Helena in central California. This image shows several types of glass shards, including pumiceous, bubble-wall, bubble-wall junction, and blocky. This eruption produced pyroclastic flows and extensive ash fall; the blocky shards were formed from "phreatomagmatic" activity--the explosive interaction of molten rock and groundwater.

The shards are from the Putah Tuff Member of the Tehama Formation found throughout northern and southern California, western Nevada, and in New Mexico. Tuff consists of volcanic ash, pumice, and other rocks erupted by a volcano that have been hardened together by either compaction, heat, or chemical cementation so that the initially loose material forms a hard rock.


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Source:
U.S. Department of the Interior