Terms Used to Describe Tephra

A variety of terms are used to describe the range of rock fragments erupted into the air by volcanoes. The terms classify the fragments according to size, shape, composition, or the way in which they form and travel.

Terms Based on Size

Ash

Tephra less than 2 millimeters in diameter.

Lapilli

Tephra between 2 and 64 millimeters in diameter.

Blocks

Tephra greater than 64 millimeters in diameter.

Terms based on shape, composition, and mode of formation

Lithics

Dense rock fragments of a pyroclastic deposit (formed by tephra fall, pyroclastic flow or surge) are referred to as lithics. Lithics may be subdivided into (1) cognate lithics, non-vesiculated magma fragments; (2) accessory lithics, rocks from along the magma conduit that have been explosively ejected during eruption; and (3) accidental lithics, rocks that have been eroded and picked up locally by pyroclastic flows and surges.

Accretionary lapilli

Spherical lapilli-sized particles that form as moist aggregates of ash in eruption clouds, usually by rain that falls through dry eruption clouds.

Ballistic fragment

An explosively ejected rock fragment that follows an arched ballistic trajectory.

Bomb

Bombs are thrown from vents in a partly molten condition and solidify during flight or shortly after they land. Bombs are named according to shape, such as ribbon bombs, spindle bombs, cow-dung bombs, and spheroidal bombs.

Breadcrust Bomb

An explosively ejected rock fragment with a fractured surface texture. The fractures develop as the interior of the rock expands and breaks the cooled brittle outer surface.

Cinder

A general term for a partly vesiculated lava fragment ejected during an explosive eruption; equivalent to scoria. Cinders typically consist of basaltic or andesitic composition.

Ejecta

A general term for rock particles ejected into the air by a volcano.

Glass shards

Tiny pieces of gas-bubble walls broken from magma during its explosive fragmentation are called glass shards. They exhibit a wide range of appearances, from slightly curved, thin glass plates broken from large, thin-walled spherical gas bubbles to hollow needles broken from stretching gas bubbles.

Lithics

Dense rock fragments of a pyroclastic deposit (formed by tephra fall, pyroclastic flow or surge) are referred to as lithics. Lithics may be subdivided into (1) cognate lithics, non-vesiculated magma fragments that have solidified from the magma; (2) accessory lithics, rocks from along the magma conduit and the margins of a magma reservoir that have been explosively ejected during eruption; and (3) accidental lithics, rocks that have been eroded and picked up locally by pyroclastic flows and surges.

Pele's hair

Thin strands of volcanic glass drawn out from molten lava. Because the strands look like human hair, they have long been called Pele's hair, named for Pele, the Hawaiian goddess of volcanoes.

Pele's tears

Small bits of molten lava in lava fountains can cool quickly and solidify into glass particles shaped like spheres or tear drops. The particles are called Pele's tears, named after Pele, the Hawaiian goddess of volcanoes.

Pumice

Pumice is a light, porous volcanic rock that forms during explosive eruptions. Pumice consists of many trapped gas bubbles.

Pyroclast

An individual volcanic particle ejected during an eruption is called a pyroclast. Examples of pyroclasts include individual fragments of ash, lapilli, and blocks.

Reticulite

Basaltic pumice in which nearly all gas-bubble walls have burst, leaving an open network of glass.

Scoria

A general term for a partly vesiculated lava fragment ejected during an explosive eruption; equivalent to cinder. Scoria typically consists of basaltic or andesitic composition.

Spatter

Very fluid fragments of molten lava ejected from a vent that flatten and congeal on the ground are called spatter. Spatter is almost always of basaltic composition.