That which has been blasted violently out of a volcano, and becomes airborne. Tephra does not include gasses, only solid materials. Occasionally the term tephra will be used to exclude ash, but this is uncommon. Underwater volcanoes can be said to have tephra too.

Tephra is often categorized by size:

  • Blocks: Tephra that measures more than 64mm on its longest side. These are rocks blasted up by the explosion, not flaming balls of magma. They are part of the Earth's crust that has been blown off by the force of the explosion. Blocks are also often formed when water spills into a volcanic vent, and the resulting explosion of steam blasts chunks of rock into the air.
  • Volcanic bombs: globs of magma greater than 64mm. These are still somewhat fluid ('partially or entirely plastic') when they leave the vent, and come in many shapes, from spherical to long ribbons. (These are sometimes called lava bombs.)
  • Lapilli: (singular is lapillus) 2 to 64mm in diameter, these include both small rocks from the crust and small globs of molten magma. Most cinders will also fall into this category.
  • Ash: particles less than 2mm. Some lapilli are actually formed from ash that has clumped together, and tuff is rock formed from volcanic ash, so we have to be careful to avoid confusion; when we speak of tephra, ash is composed of independent, unclumped particles. Once it's on the ground, ash refers to that which was once only small particles, whatever form it is in now. Volcanic ash is not the same as the ashes you make when you burn wood in a fire; it is made up of small particles of rock and volcanic glass.
In addition to these there are other classifications based on type of rock (scoria, pumice, reticulite, etc.) and forms (volcanic bombs may be categorized into a number of forms, including breadcrust bombs, spindle bombs, and cow-dung bombs, among others). If the tephra is still molten when it hits the ground, it is referred to as spatter. Sometimes formations such as Pele's hair and limu o Pele may be considered tephra, although they form off of lava flows after the main eruption.

Once the tephra settles to the ground it may remain as individual bits of rubble, or may be compressed into various types of rock, such as tuff and breccia. Despite the general dullness of its definition, tephra is a word that geologists and volcanologists use quite a lot.

Both of these sites have lots of great photos; you should check them out.

Was this guide not nitpicky enough for you? Well, you're in luck! Here's the Encyclopedia of Geology's classification of pyroclastic materials. (The phi measurement refers to the Krumbein phi scale, which has not yet been noded. Note that it is a logarithmic scale, hence the oddities of the measurements.)

  • 3 phi (0.12 mm) and under -- very fine ash.
  • 3 to 1 phi (0.12-0.5 mm) -- fine ash.
  • 1 to 0 phi (0.5-1 mm) -- medium ash.
  • 0 to 1 phi (1-2 mm) -- coarse ash.
  • -1 to -3 phi (2-8 mm) -- fine lapilli.
  • -3 to -5 phi (8-32 mm) -- medium lapilli.
  • -5 to -6 (32-64 mm)-- coarse lapilli.
  • -6 phi and larger (64+ mm) -- blocks and bombs.
Encyclopedia of Geology Elsevier Academic Press, edited by Richard c. Selley, L. Robin M. Cocks {sic}, and Ian R. Pilmer. 2005.