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124 CHAPTER 8

• Pyroclastic density currents are fast-moving, Walker, G.P.L. & Croasdale, R. (1971) Two Plinian-
ground-hugging clouds of very hot clasts and gas type eruptions in the Azores. J. Geol. Soc. London
that flow like liquids. They contain a very wide 127, 17–55.
range of clast sizes. Their speeds can be at least Wilson, L. & Walker, G.P.L. (1987) Explosive volcanic
100ms −1 and their travel distance can be many eruptions – VI. Ejecta dispersal in Plinian eruptions:
tens of kilometers. Although they mainly flow controls of eruption conditions and atmospheric
along valley floors, these speeds imply that they properties. Geophys. J. Roy. Astron. Soc. 89, 657–
can climb over topographic obstacles in their 79.
paths a few hundred meters high if there is
no way to flow around. PYROCLASTIC DENSITY CURRENTS
• Smaller scale pyroclastic density currents can
Branney, M.J. & Kokelaar, P. (1992) A re-appraisal of
occur when only part of an eruption column
ignimbrite emplacement: progressive aggradation
collapses, or when a volcanic dome or lava flow
and changes from particulate to nonparticulate flow
becomes unstable and disintegrates into pyro-
during emplacement of high-grade ignimbrite. Bull.
clasts and released gas. These travel to smaller
Volcanol. 54, 504–20.
distances, although still at high speeds. A pyro-
Branney, M.J. & Kokelaar, P. (2002) Pyroclastic Density
clastic surge can form when the upper, more
Currents and the Sedimentation of Ignimbrites.
dilute, part of a pyroclastic density current be-
Memoir 27, Geological Society Publishing House,
comes detached from the lower, denser part
Bath, 143 pp.
when the flow meets an obstacle. Burgisser, A. & Bergantz, G.W. (2002) Reconciling
• As they travel, pyroclastic density currents pyroclastic flow and surge: the multiphase physics
deposit pyroclasts on the ground. The deposits of pyroclastic density currents. Earth Planet. Sci.
from large pyroclastic density currents dominated Lett. 202, 405–18.
by grain flow, called ignimbrites, tend to be mas- Bursik, M.I. & Woods, A.W. (1996) The dynamics and
sive, whereas surge deposits formed from more thermodynamics of large ash flows. Bull. Volcanol.
dilute currents exhibit internal stratification. 58, 175–93.
• The fronts and tops of pyroclastic density cur- Druitt, T.H. (1998) Pyroclastic density currents. In The
rents can incorporate air which is strongly Physics of Explosive Volcanic Eruptions (Eds J.S.
heated and rises above the body of the density Gilbert & R.S.J. Sparks), pp. 145–92. Special Publica-
current, carrying small clasts up with it to form a tion 145, Geological Society Publishing House, Bath.

co-ignimbrite or phoenix cloud which can itself Levine, A.H. & Kieffer, S.W. (1991) Hydraulics of
generate a fine-grained fall deposit. the August 7, 1980, pyroclastic flow at Mount
St. Helens, Washington. Geology 19, 1121–4.
Neri, A. & Macedonio, G. (1998) Numerical simulation
8.6 Further reading of collapsing volcanic columns with particles of two
sizes. J. Geophys. Res. 101, 8153–74.
Sparks, R.S.J., Wilson, L. & Hulme, G. (1978) Theore-
TEPHRA DISPERSAL IN STEADY ERUPTIONS
tical modelling of the generation, movement and
Carey, S. & Sparks, R.S.J. (1986) Quantitative models emplacement of pyroclastic flows by column col-
of the fallout and dispersal of tephra from volcanic lapse. J. Geophys. Res. 83, 1727–39.
eruption columns. Bull. Volcanol. 48, 109–25. Wilson, C.J.N. (1984) The role of fluidization in the
Sparks, R.S.J. (1986) The dimensions and dynamics emplacement of pyroclastic flows, 2: experimen-
of volcanic eruption columns. Bull. Volcanol. 48, tal results and their interpretation. J. Volcanol.
3–15. Geotherm. Res. 20, 55–84.
Sparks, R.S.J., Bursik, M.I., Ablay, G., Thomas, R.M.E. Wilson, L. & Head, J.W. (1981) Morphology and
& Carey, S.N. (1992) Sedimentation of tephra by rheology of pyroclastic flows and their deposits,
volcanic plumes. Part 2: controls on thickness and guidelines for future observations. In The 1980
and grain size variations of tephra fall deposits. Eruptions of Mount St. Helens, Washington.
Bull. Volcanol. 54, 685–95. U.S.Geol. Surv. Prof. Pap. 1250, 513–24.