VI. Products Of Volcanic Explosions

Airfall Ash Deposits

Tephra produced by a volcanic eruption may be distributed by fall through the atmosphere or by flow over the ground surface. Tephra may also be dispersed by ocean currents, where ash has fallen on seawater and coagulated. The most widespread pyroclastic product is airfall ash deposits. Tephra of basaltic eruptions are much less voluminous than those of intermediate to rhyolitic eruptions due to the less explosive style of basaltic volcanic activity.

Dispersal

An eruption column can carry ash-sized fragments to altitudes of 6-50 km above the vent. The dispersal of ash from the eruption column depends largely on the directions of winds at intermediate and high altitudes (4500-13000 m). At high levels, atmospheric flow is laminar, but at low levels, it is turbulent. Ash can be transported at 100-200 km/hr in the upper atmosphere. Once particles move into the upper atmosphere, however, velocity decreases due to:

(a)   gravity; and,

(b)   air resistance.

The rate of ash falling from the highest point of its trajectory increases until the acceleration of gravity is balanced by the decelerating effect of air resistance. Beyond that point, the velocity remains constant, and ash can remained suspended until the wind velocity drops below the particle's settling velocity, V_t, defined by Stokes Law:

V_t = {(8R_sg)/(3C_v)}^0.5

where R is the particle radius, _s is the particle density, C is the drag coefficient of the particle, and _v is the density of the transmitting medium.
In general, greater amounts of tephra fall out of the ash cloud near the vent, so that airfall deposits typically thin away from the vent. However, secondary thickness maxima may occur downwind. Airfall deposits typically have a circular or regular to irregular, fan-shaped distribution with respect to their source. The azimuth of the fan axis may change with distance from the source, and thickness may be skewed to one side, perpendicular to the fan axis. Moreover, the apex of the dispersal fan may no be on the volcano, such as at Mount St. Helens or White River, Yukon.

Structures Of Airfall Deposits

Because they form as atmospheric fallout, these deposits are characterized by what is termed mantle bedding, as they typically "mantle" or drape over the underlying topography except where it is rugged. Bedding planes are distinct where deposition is on weathered or erosional surfaces, or different rock types. They may be gradational if deposition is slow by small increments so that bioturbation, wind reworking, and other soil-forming processes dominate. The fabric in beds is commonly isotropic because elongate fragments are uncommon, with the exception of platy minerals and glass shards.
Airfall deposits are generally well-bedded and well-sorted, with bedding becoming more pronounced as sorting increases, and with size and sorting parameters varying geometrically with distance from the source within single layers. Inman parameters (s) are commonly 1.0 to 2.0 within both relatively coarse-grained as well as fine-grained tephra. Median particle diameters (Md) are commonly -1.0 to -3.0 (2 to 8 mm) or smaller (phi values) close to the source, but farther away may vary from 0.0 (1 mm) to 3.0 (1/8 mm) or more The sorting of airfall deposits depends on:

(a)   the distance from the vent;

(b)   variations in the strength and duration of eruptions;

(c)   length of quiescence between explosions;

(d)   changes in the direction of fragment ejection; and,

(e)   the direction and velocity of the wind.

Within products of a single eruption, bedding tends to show normal grading, but reverse grading may occur in waterlain pumiceous deposits, and cross-bedding may result due to shifts in wind strength and direction. Differences in the proportions and densities of lithic, crystal and vitric constituents produce both lateral and vertical variations in the size and nature of particles and nature of the deposits.
Vertical variations usually show increasingly basic compositions, often reflecting the tapping of compositionally zoned magma. Lateral variations reflect differences in the settling velocities of particles. Most ash deposits become more silica-rich with distance from the vent, as different minerals are winnowed from the ash cloud.

Morphology of Ash Particles

Ashes are best placed into two broad genetic categories: magmatic and phreatomagmatic. Ashes from magmatic eruptions are formed when expanding gases in the magma form a froth that loses its coherence as it approaches the ground surface. During phreatomagmatic eruptions, the magma is chilled and fractured on contact with ground or surface waters, resulting in violent steam eruptions. In low-viscosity magmas droplet shape is, in part controlled by surface tension, by acceleration of the droplets after they leave the vent, and by air friction. The ash particles consist of mostly sideromelane, translucent basaltic glass, or tachylyte, opaque Fe-Ti oxide charged glass. The sideromelane particles exhibit smooth, fluidal surfaces and a thin skin. In higher viscosity magmas, the morphology of ash particles is controlled primarily by vesicle density and shape, the vitric fraction generally consisting of very angular pumice fragments and thin vesicle walls broken from pumice fragments during or after eruption. The morphology of lithic fragments is dependent on the texture and fracture pattern of the rock type broken up during the eruption. The morphology of ash particles from phreatomagmatic eruptions is controlled by the thermal stresses within the chilled magma, which result in fragmentation of the glass to small blocky or pyramidal glass particles. Vesicle density and shape play a minor role in determining the morphology of phreatomagmatic ash particles.