Volcanic Tephra
Volcano Terms and Definition

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The most dazzling but destructive natural force on earth. Massive volcanic eruption can turn day into night, releasing the power of an atomic blast, spewing toxic avalanches of lava, gas, and ash. National Geographic Video transports you to some of the world's most notorious volcanoes

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Tephra erupted by Mount St. Helens on 18 May 1980 ranging in size from ash (left 2 piles) to lapilli (right 2 piles)

Volcanic ash falls to ground and creates darkness, Mount Pinatubo in the Philippines.

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Tephra includes large dense blocks and bombs, and small light rock debris such as scoria, pumice, reticulite, and ash.

As tephra falls to the ground with increasing distance from a volcano, the average size of the individual rock particles becomes smaller and thickness of the resulting deposit becomes thinner. Small tephra stays aloft in the eruption cloud for longer periods of time, which allows wind to blow tiny particles farther from an erupting volcano.

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

Tephra less than 2 millimeters in diameter.

Tephra between 2 and 64 millimeters in diameter.

Tephra greater than 64 millimeters in diameter.

Terms based on shape, composition, and mode of formation

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.

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.

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.

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.

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

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

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

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.

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.

Tephra deposit about 9 cm thick blankets former U.S. Clark Air Base, Philippines, about 25 km east of Mount Pinatubo. The pumice and ash fell to the ground on June 15 during the climactic eruption of Pinatubo. During the eruption, a massive typhoon made landfall and passed about 75 km northeast of the volcano. Typhoon Yuna blew tephra in all directions around the volcano, and a heavy rain saturated the tephra as it fell to the ground. Tephra-fall deposits 10-25 cm thick accumulated over a densely settled area of about 2,000 km 2 .

For a detailed summary of the tephra-fall deposits from the 1991 eruption of Mount Pinatubo, see Tephra Falls of the 1991 Eruptions of Mount Pinatubo , a report from Fire and Mud: eruptions and lahars of Mount Pinatubo, Philippines, that is available online.

This house was damaged by the heavy load of ash that accumulated on its roof during the eruption of Rabaul Caldera on September 19, 1994. Note the thickness of the ash deposits on top of the roof that didn't collapse (right side). Vulcan Volcano, one of two vents that erupted in the caldera, is visible to the left of the house.

Most damage to buildings from ashfall occurs when the load of ash exceeds the strength of either the roof-supporting structures or material used to cover the structure (sheet metal, plywood, etc.). Dry ash has a weight of 400-700 kg/m 3 (880-1,545 lb/yd 3 ), and rainwater can increase this by 50-100 percent if the ash becomes saturated. For a dry layer of ash about 10 cm (4 in) thick, the extra load on a building can range 40-70 kg/m 2 (120 to 200 lb/yd 2 ); a wet layer might reach 100-125 kg/m 2 (300-350 lb/yd 2 ).

During a heavy ashfall, which might be accompanied by rain and lightning, it is difficult and risky to remove ash from the top of a building or house. The risk to people trying to remove ash from roofs (for example, falling) may be greater than the risk from collapse of the building.

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40-minute video uses spectacular and unusual footage of erupting volcanoes from Hawaii and around the world to explain the features found in many of our volcanic national parks and monuments, and to show how they form. It presents up-to-date scientific ideas about lava flows: how they move, how they change, and how they create lava tubes

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Two hundred and thirty square miles leveled in moments Five hundred and forty million tons of ash and volcanic rock exploded twelve miles high One cubic mile of earth blasted from the crest of one of the world's most beautiful snowcapped domes Captured in rare and spectacular aerial photography and survivor's own words and pictures, witness the terrifying fury of the worst volcanic disaster in American history

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