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92 CHAPTER 6

the temperature of the material in the plume 6.9 Further reading
declines and a point will be reached where the
density of the plume material is equal to that of
GAS RELEASE AND FRAGMENTATION
the surrounding atmosphere. Above this point,
known as the level of neutral buoyancy, plume Gardner, J.E., Thomas, R.M.E., Jaupart, C. & Tait, S.
rise rapidly ceases and the plume spreads out. (1996) Fragmentation of magma during Plinian
This uppermost part of the plume is known as volcanic eruptions. Bull. Volcanol. 58, 144–62.
the umbrella region. Sparks, R.S.J. (1978) The dynamics of bubble forma-
• The height to which an eruption plume can rise tion and growth in magmas: a review and analysis.
within the atmosphere depends most strongly on J. Volcanol. Geotherm. Res. 3, 1–37.
the mass flux of the eruption. The larger the Thomas, N., Jaupart, C. & Vergniolle, S. (1994) On
mass flux, the more heat is supplied to the plume the vesicularity of pumice. J. Geophys. Res. 99,
in a given time and so the higher the plume can rise. 15633–44.
• Fragmentation generates a range of sizes of clasts
and these clasts are carried upwards in the gas
ACCELERATION IN THE DIKE SYSTEM
stream both within the dike system and then
above the vent in the eruption plume. Two com- Buresti, G. & Casarosa, C. (1989) One-dimensional
peting forces act on the clasts: the force of adiabatic flow of equilibrium gas-particle mixtures
gravity which tends to make the clasts fall and the in long vertical ducts with friction. J. Fluid Mech.
drag of the rising gas stream which tends to make 203, 251–72.
the clasts rise. A balance is reached in which the Wilson, L. & Head, J.W. (1981) Ascent and eruption of
clasts will fall at their terminal velocity through basaltic magma on the Earth and Moon. J. Geophys.
the gas stream. As long as the terminal velocity Res. 86, 2971–3001.
of the clast is smaller than the rise speed of the Wilson, L., Sparks, R.S.J. & Walker, G.P.L. (1980)
Explosive volcanic eruptions – IV. The control of
gas, the clast will be carried upwards relative to
magma properties and conduit geometry on erup-
the ground surface. Rise speed in an eruption
tion column behavior. Geophys. J. Roy. Astron. Soc.
plume decreases with height, and so a clast of
63, 117–48.
given size will be carried to a height at which the
rise speed becomes equal to the clast’s terminal
velocity and no higher. Large clasts have large PLUME RISE AND RISE HEIGHTS

terminal velocities and are, therefore, carried
only to relatively small heights within the plume. Carey, S.N. & Sparks, R.S.J. (1986) Quantitative
Smaller clasts have smaller terminal velocities models of the fallout and dispersal of tephra from
volcanic eruption columns. Bull. Volcanol. 48,
and will be carried higher above the vent before
109–25.
falling out from the eruption plume.
Morton, B.R., Taylor, G. & Turner, J.S. (1956)
• Most steady eruptions initially generate stable,
Turbulent gravitational convection from main-
convecting eruption plumes. In some cases,
tained and instantaneous sources. Proc. Roy. Soc.
however, an eruption plume may become un-
Ser. A 234, 1–23.
stable, i.e., it cannot achieve thermal buoyancy.
Settle, M. (1978) Volcanic eruption clouds and the
In such eruptions the plume will rise until it
thermal output of explosive eruptions. J. Volcanol.
no longer has any upward momentum and will
Geotherm. Res. 3, 309–24.
then collapse. The resulting pyroclastic foun-
Sparks, R.S.J., Bursik, M.I., Carey, S.N., Gilbert, J.S.,
tain can give rise to a pyroclastic density current Glaze, L.S., Sigurdsson, H. & Woods, A.W. (1997)
ultimately depositing an ignimbrite. Eruption Volcanic Plumes (Chapters 1–4). Wiley, Chichester,
plumes may become unstable due to an increase 574 pp.
in mass flux at the vent, a decrease in the gas Wilson, L. & Walker, G.P.L. (1987) Explosive volcanic
content of the magma, or a combination of eruptions – VI. Ejecta dispersal in Plinian eruptions:
both effects. the control of eruption conditions and atmospheric