SEM image provided by A.M. Sarna-Wojcicki
Tic marks 100 microns apart

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 km³ 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.

SEM image provided by A.M. Sarna-Wojcicki
Tic marks 100 microns apart

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.

SEM image provided by A.M. Sarna-Wojcicki
Tic marks 100 microns apart

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.