VI. Products Of Volcanic Explosions

Terminology and Classification

Fragments which are thrown out by volcanic explosions are referred to collectively as ejecta, and accumulations of these fragments are known as pyroclastic rocks or tephra depending on whether they are consolidated or not. Different sorts of volcanic explosions produce somewhat different types of ejecta. Instantaneous or long-continued, weak or violent, all volcanic explosions are the result of escape of gas from the magma, but they may be subdivided according to the origin of the gaseous component as:

(a)   if magmatic or juvenile gases, then magmatic explosions;

(b)   if steam generated where water comes in contact with hot rock or magma, then phreatic explosions; or,

(c)   if both gas sources, then phreatomagmatic explosions.

Ejecta produced by these types of volcanic explosions have been classified by several different criteria:

(a) origin;

(b) fragment size;

(c) condition at time of ejection and at time of striking the ground; and,

(d) degree of consolidation of the deposit.

Origin

Considering origin first, it must be recognized that ejecta may be derived from the molten magma or from rock that was already solid (non-magmatic ejecta). Non-magmatic ejecta may represent:

(a) already solidified magma of the same eruption;

(b) rocks of the same volcano but formed during earlier eruptions; or,

(c) rocks derived from the underlying crust and unrelated to volcanic activity (termed accidental ejecta).

Magmatic ejecta and Type I non-magmatic ejecta, which are derived from molten magma of the same eruption, are termed essential ejecta, and are typically partly or entirely glass (vitric). Type II non-magmatic ejecta or fragments of older rocks formed during previous eruptions are termed accessory ejecta, and most are partly or wholly crystalline (lithic). Some accessory ejecta consist of coarse-grained clots of several minerals that represent cognate material that was torn from the conduit walls or from parts of magma crystallized at depth.

Fragment Size

The most important classification of pyroclastic rocks is based on fragment size, although fragments greater than 5 cm in average diameter are subdivided further on the basis of shape, which reflects their physical condition at the time of ejection:

(1)   Bombs, typically composed of low-viscosity basaltic magma, are ejecta larger than 64 mm in average diameter that are thrown out of the vent in a molten state. Highly to moderately fluid, magma may be ejected both as long, irregular strips or as discrete blebs. Because they are fluid, their shape is typically modified during flight through the air and such fragments are typically termed fusiform:

(a)   Strips that break up into short segments form cylindrical or ribbon bombs, which are more less circular or flat in cross-section, and typically show twisted, longitudinal fluting.

(b)   Large blebs pulled up into spheres by the surface tension of the magma form spherical bombs.

(c)   Fragments that spin during flight form spindle- or almond-shaped bombs, characterized by longitudinal fluting, and one side smoother and broader than the other. The smooth or "stoss" side represents the front side as the bomb fell through the air, whereas the "lee" side is produced by frictional resistance of the magma dragging the still-plastic skin of the bomb toward this side. this resistance often forms a thin projecting rim along the edge of the stoss side:

(d)   Bombs of very fluid magma, that is projected only to moderate heights and strikes the ground while still liquid, flatten or even splash to form pancake or cow-dung bombs.

At the other end of the spectrum, very viscous bombs are not rounded during flight, and although their outside is nearly solid, the inside is still plastic enough to expand as gases escape and produce a skin that is broken by deep cracks, forming what is called bread-crust bombs. Most bombs are simply an irregular and generally extremely vesicular lumps, which are described as cinder or scoria. Fusiform bombs may only form at the very end of an eruption, because they represent denser material than scoria or cinder, and they form at a stage when the amount of gas in the magma started to decrease.
Most bombs in cross-section are at least somewhat vesicular, and they are often characterized by concentric layers of greater and lesser vesicularity. Exceedingly vesicular cinder are called pumice. Pumice of rhyolitic magma are characterized by vesicles that are stretched out into long very thin tubes, giving the fragment a silky appearance. In contrast, far less abundant basaltic pumice typically consists only of thin glass threads that mark the intersections of vesicles. These form the lightest rock (0.3 gm/cm³) known to exist, what is called thread-lace scoria or reticulate. Bombs that have formed around a core of older, solid accessory or accidental fragments, are termed cored bombs. Showers of still-fluid blebs striking ground around the vent may flatten and mod themselves to the underlying surface, forming an accumulation of flattened and welded fragments called spatter or agglutinate. Masses of tephra containing a large proportion of bombs is called agglomerate.
Most bombs are less than 25 cm in diameter, but some may be exceptionally large, e.g. 6 m irregular, elongate bombs at Paricutin; up to 1 m fusiform bombs at Mauna Loa; and, up to 1.3 m cow-dung bombs at Stromboli (1965).

(2)   Blocks: These are angular ejecta larger than 64 mm in average diameter that are thrown out of the vent in a solid state. Blocks typically are formed by the disruption of the crust of a lava pool or a dome. They may be entirely cold or still warm and incandescent when deposited. Accumulations of blocks are called breccia, and it is often desirable to specify whether the deposits are pyroclastic breccia or phreatic breccia depending on the type of volcanic explosions responsible.
  
(3)   Lapilli: These are ejecta between 2 and 64 mm in average diameter, may be essential, accessory or accidental in origin, and may be ejected in either a liquid or solid state. Lapilli are the most abundant type of fragment in cinder deposits. Special forms characterize drops of basaltic lava that are ejected in a very fluid condition and solidified in the air:

   (a) droplet-shaped fragments form Pele's Tears, and

   (b) fragments drawn out into threads form Pele's Hair.

An unusual type of lapilli-sized fragment is known as accretionary lapilli, which grow by accretion or addition of concentric layers of fine moist material to a nucleus, like the growth of hailstones.

(4)   Ash: This is tephra less than 2 mm in average diameter, may also be essential, accessory or accidental in origin, and ejected in either a liquid or solid state. Depending on the material that composes the ash, it can be described as:
  
(a) lithic, composed dominantly of solid rock;

(b) vitric, composed dominantly of glass; or,

(c) crystal, composed dominantly of crystals.

   The most common type of ash is vitric. Unconsolidated deposits of ash-sized material are referred to as ash layers, ash beds or ash blankets. Consolidated ash deposits are described as tuffs, or more specifically, depending on the predominant constituent as:

   (a) lithic tuff;

   (b) vitric tuff; or

   (c) crystal tuff.

   Some ash deposits contain moderately to very abundant lapilli, blocks or bombs, and are termed lapillistone, lapilli tuff, tuff breccia or tuff agglomerate.