Volcanic glass shards are fragments of the molten part of magma that cooled and solidified during eruption without mineral crystallization. Glass shards are typically remnants of tiny gas bubbles that developed and grew in size during the final ascent of magma toward the surface; such shards may consist of many gas bubbles or only a portion of a single gas bubble. During eruption, the expanding gas broke the bubbles and surrounding glass into shards of various sizes and shapes. Shards formed by phreatomagmatic eruptions (see eruption style) often have a particularly angular shape resulting from the violent explosive interaction between magma and water. Glass is relatively hard (5 on Moh's scale), and the more angular the glass shards the more abrasive the ash.
Rising magma will incorporate pieces of different types of rocks (often called lithics) through which it moves, including rocks located deep beneath a volcano and within the volcano itself. The rapid ascent of magma during explosive eruption will often rip fragments from the walls of the magma conduit, which are ejected and fragmented further by the explosive expansion of volcanic gases. These non-magmatic rock fragments are found in varying abundances within ash deposits and often have a shape and texture distinctly different than glass shards.
Minerals within volcanic ash are primarily derived from the magma. These minerals crystalized within the magma while it was below the earth's surface. The type of minerals within an ash deposit depends upon the chemistry of the magma from which it was erupted. Most minerals in ash have not been shown thus far to cause any long term adverse health effects in humans, but they will affect the composition of soil into which they become incorporated possibly affecting livestock and agriculture. The hardness of individual minerals varies, with the harder minerals being more abrasive.
| Magma composition | Minerals typically present |
|---|---|
| Rhyolite | Quartz, feldspar, +/-mica, +/-orthopyroxene,+/-amphibole |
| Dacite | Quartz, feldspar, +/-mica, +/-orthopyrocene, +/-clinopyroxene,+/-amphibole |
| Andesite | Feldspar, clinopyroxene, +/-quartz, +/-orthopyroxene,+/-amphibole |
| Basalt | Feldspar, clinopyroxene, +/-olivine, +/-orthopyroxene,+/-amphibole |
Fine minerals may also grow on the walls of expanding gas bubbles in the magma prior to fragmentation into ash-sized particles. Crystobalite is a type of silica crystal that forms in this way. Crystobalite is known to cause silicosis in humans, which is typically contracted by people working for prolonged periods in industries that expose workers to fine rock dust. See the health section for more information on health effects of ash, including who is most at risk from ash inhalation and how to minimize exposure to volcanic ash.
Ash chemistry is directly related to the chemistry of the source magma. Learn more about magma characteristics.
Volcanic glass is relatively high in the element silicon compared to mineral crystals, but relatively low in non-silicic elements (especially Mg and Fe). Both glass and most minerals almost always contain Si, Al, K, Na, Ca, Mg and/or Fe.
See Gases & aerosols for a discussion of soluble component contribution to ashfall via ash coatings derived from aerosols.