TEPHRA AND PYROCLASTIC ROCKS

The rapid eruption of expanding gases results in the obliteration and fragmentation of magma and rock. The greater the explosivity, the greater the amount of fragmentation. Individual eruptive fragments are called pyroclasts ("fire fragments"). Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.

CLASSIFICATION OF PYROCLASTS - Tephra is classified on the basis of pyroclast size:

	ASH -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Courtesy of USGS.
	LAPILLI -- Pea- to walnut-size pyroclasts (2 to 64 mm). They often look like cinders. In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli (left). Courtesy of USGS.
	BLOCKS AND BOMBS -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks (not shown) are ejected as solid fragments with angular shapes. Courtesy of J.P. Lockwood, USGS.

Within this size classification, specific types of tephra can be further defined by physical attributes. For example, lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli called Pele's tears. During strong winds, these molten fragments can be drawn out into fine filaments called Pele's hair. Nonexplosive Hawaiian-type eruptions often produce lapilli- to bomb-size fragments, called spatter which remain airborne for only a short amount of time so that are still liquid when they hit the ground surface. Some lapilli- to bomb-size pyroclasts maintain a frothy appearance due to escaping volcanic gases and the rapid accumulation of bubbles (vesicles). These highly vesicular rock types include mafic and felsic varieties. Vigorous gas escape in felsic lavas produces pumice, whereas similar gas escape in mafic lavas produces reticulite. The high vesicularity of pumice lowers the density of this rock type so that it can literally float on water. Reticulite has a still lower density, with vesicles occupying up to 98% of the total volume. Unlike pumice, however, most of the bubble walls in reticulite are broken so that it sinks in the presence of water. Reticulite, however, is not as common as scoria, a denser mafic rock containing a smaller abundance of relatively large vesicles. Click the links below to view images of these pyroclast types.

Pele's tears	Pele's hair	Spatter

Pumice	Reticulite	Scoria

NON-GENETIC CLASSIFICATION OF PYROCLASTIC ROCKS

In some cases, unconsolidated pyroclasts can be welded, compacted, or cemented into a coherent pyroclastic rock. The nongenetic classification of pyroclastic rocks is partly based on the relative abundance of the incorporated pyroclast types discussed above. For example:

• Ash tuff - rock dominated by ash; sometimes simply referred to as tuff.
• Lapilli tuff - rock dominated by lapilli.
• Tuff breccia - rock containing 25% to 75% blocks and/or bombs.
• Pyroclastic breccia - rock containing at least 75% blocks and bombs.
• Agglomerate - rock containing at least 75% bombs.
• Agglutinate - rock composed of fused, largely unrecognizable, basalt spatter fragments.

GENETIC CLASSIFICATION OF TEPHRA DEPOSITS

Pyroclastic airfall deposits (tephra) contain pyroclasts that are coarser near the vent (bomb and lapilli size) and become increasingly finer grained away from the vent (lapilli to ash size). The mafic to felsic composition of these deposits will vary with eruption type. Two end-member genetic types of airfall deposits are recognized:

• Scoria-fall deposits -- These are derived from Strombolian eruptions of scoria cones. The deposit is composed of basaltic to andesitic vesiculated pyroclasts (scoria) lying near the eruptive vent.
• Pumice-fall deposits -- These are derived from Plinian eruptions of stratovolcanoes. The deposit is composed of highly vesiculated dacitic to rhyolitic pyroclasts (pumice) which can be destributed for hundreds of square kilometers away from the vent.

Strombolian scoria-fall deposit

Plinian pumice-fall deposit  Dark pyroclasts of basaltic scoria are typical of Strombolian airfall accumulations, whereas light colored pyroclasts of felsic pumice and ash are typical of Plinian airfall accumulations. Courtesy of Peter Francis.

In addition to these two end-member types, some airfall deposits can be very highly fragmented to generate ash-fall deposits. Such fine-grained deposits are common in highly explosive Vulcanian eruptions and hydrovolcanic eruptions.