Page 105 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Explosive magmatic eruptions streaming may follow and produce vesicular juvenile
pyroclasts. Pyroclasts involved in repeated cycles of
These involve the exsolution and expansion of ejection followed by fallback into the vent ("milling") can
magmatic volatiles. Large volumes of pumice or scoria, have conspicuously rounded shapes. Pyroclasts are
and glass shards are generated. They are regarded as deposited by fallout from the eruption column and
relatively "dry" eruptions because steam contributed plume, and less commonly by small volume scoria and
from external water is only a minor component. The ash flows.
magma composition (especially volatile content) and
physical properties (mainly viscosity, temperature and Plinian eruptions are characterised by very powerful
density), and the vent geometry control the character release of gas and pyroclasts in high eruption columns
and behaviour of explosive magmatic eruptions (Walker and Croasdale, 1970; L. Wilson, 1972, 1976,
(McBirney and Murase, 1970; Settle, 1978; Wilson et 1980; Walker, 1973a, 1981a, 1981b; Sparks and Wilson,
al., 1978; Wilson et al., 1980; Head and Wilson, 1986; 1976; Sparks, 1986; Carey and Sparks, 1986; Carey and
Sparks, 1986; Wilson and Walker, 1987; Wilson et al. Sigurdsson, 1989; Woods and Bursik, 1991; Bursik et
1987; Woods,1988). As a result, explosive magmatic al., 1992; Sparks et al., 1992). They involve high
eruptions of basaltic, andesitic and silicic magmas tend viscosity (in most cases silicic), vesiculating magmas,
to exhibit characteristic styles, as described below capable of generating very large internal gas
(although there are always exceptions). pressures. When fragmentation takes place,
decompression of the gas generates a high-velocity jet
Strombolian eruptions are largely the result of of comminuted magma and hot gas that feeds a
intermittent bursting of large bubbles or groups of buoyant, convecting plume and uppermost, laterally
bubbles at the top of the magma column in an open spreading umbrella region. Observed plinian eruption
conduit (Walker and Croasdale, 1972; Walker, 1973a; columns have reached 30 km and theoretical calculations
Self et al., 1974; Blackburn et al., 1976; L. Wilson, suggest a maximum limit of about 55 km. Pyroclasts
1980; Houghton and Hackett, 1984). This eruption generated by plinian eruptions are relatively low-density
mechanism is restricted to magmas of low viscosity and pumice lapilli and ash, that are entrained in the high-
low gas content, hence the typical association of basaltic velocity jet and eventually deposited by fallout from
magma with Strombolian activity. Fragmentation is not the convecting plume and umbrella region. Intermittent
very efficient and only minor amounts of ash are or sustained collapse of plinian columns occurs if the
produced. If explosions are repeated at short vent widens significantly, or if the upward gas velocity
intervals, a convecting eruption cloud may form decreases, or if the gas content decreases. Widespread
above the vent and widely disperse entrained fine and voluminous pyroclastic flow deposits are formed as a
pyroclasts. However, most pyroclasts are deposited result of collapse of plinian-style eruption columns
close to the vent and build a cone of scoria lapilli, (Sparks et al., 1978).
blocks and bombs.
Phreatomagmatic eruptions
Hawaiian eruptions involve steady rather than
intermittent discharge, but are otherwise similar to Phreatomagmatic eruptions involve steam produced by
Strombolian eruptions and principally confined to low direct interaction of the magma (or lava) with external
viscosity basaltic magmas (Walker, 1973a; Wilson and water (Sheridan and Wohletz, 1981; Wohletz, 1983,
Head, 1981). Most pyroclasts are poorly to moderately 1986; Kokelaar, 1986). Explosions occur when
vesicular, relatively coarse, fluidal clots of magma that superheated water flashes to steam that rapidly expands
experience little cooling during eruption. They form and simultaneously fragments the magma. Quench
spatter deposits close to the vent, or else, in sustained fragmentation and magmatic-volatile driven
eruptions, may coalesce to form fountain-fed lava flows. fragmentation commonly operate at the same time to
varying extents. Special circumstances are required in
Vulcanian eruptions are characterised by discrete order for phreatomagmatic eruptions to occur: (1)
explosions repeated at intervals of a few minutes to There must be efficient transfer of magmatic heat to the
hours and usually involve basaltic andesite and andesitic external water. Efficiency depends on how the magma
magmas (Walker and Croasdale, 1972; Walker, 1973a; and water come in contact and the magma: water mass
Nairn and Self, 1978; Self et al. 1979; L. Wilson, 1980). ratio. (2) Steam generated must be able to expand.
Explosions result when gas confined beneath a High lithostatic or hydrostatic confining pressures will
temporary seal of degassed, congealed lava or a preclude explosive expansion of steam. The water
blockage of the conduit is suddenly released. The gas depth limits for steam explosivity are not well
pressure increases due to exsolution of volatiles from constrained, and estimates range from less than 500 m
rising fresh magma, and in some cases, steam from to about 2000 m (Kokelaar, 1986). Because these
heated ground water is important. The explosions eject conditions are not always met, phreatomagmatic
vertically directed slugs of pyroclasts and gas, and are eruptions do not always result when hot magma comes
sufficiently vigorous to generate eruption columns 5-10 in contact with water.
km high and convecting ash plumes. Initially, the
pyroclasts are dominated by fragments of the seal and Water-magma interaction may occur intermittently or
by dense accessory lithic pyroclasts, both of which may continuously during eruptions, involve magma of any
be finely comminuted. Periods of continuous gas composition and affect the products of otherwise "dry"

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