Pyroclastic deposits
Much of our understanding of explosive eruptions comes from
detailed studies of the deposits they produce. Explosive activity produces fragments of solidified magma
and debris from the volcano (both of these types of fragments are referred to
as pyroclasts, from the Greek words for
fire and broken). These pyroclasts
are subsequently deposited around the volcano by a range of processes,
including fallout from volcanic plumes that may reach tens of kilometers into
the atmosphere, by simple ballistic ejection from a vent, and by mixtures of
gases and pyroclasts that flow along the ground (often referred to as
pyroclastic flows). The deposits
of a given type of activity might be limited to within tens or hundreds of
meters from a volcanic vent and have volumes of hundreds to thousands of cubic
meters, as commonly produced by Strombolian activity, or they might extend
hundreds of kilometers and comprise hundreds to thousands of cubic kilometers
of volume, as produced by large caldera-forming eruptions.
Pyroclastic
deposits are both sedimentary (produced by deposition of particles) and igneous
(derived directly from magmatic processes). Like other sedimentary deposits, much about their
emplacement can be learned by detailed studies of facies characteristics and
variations, fabrics and textures, and grain size parameters. The properties of a given pyroclastic
deposit depend both on the eruptive processes at the vent(s) and their
variation with time, and on the complex range of transport and depositional
processes that can occur in these energetic events. Similarly, as with igneous rocks, the material properties of
the source magmas, such as viscosity and crystal and bubble content, also play
a key role the final product.
Field-based studies of pyroclastic deposits must be combined with experimental
and theoretical studies in order to quantify their parent processes (see also Volcano
fluid dynamics & modeling).
My interests in pyroclastic deposits range from improving
our understanding of the controls on eruptive styles at small basaltic
volcanoes such as scoria cones (see also Basaltic
volcanism) to the mechanisms and fluid
dynamics that result in large-volume pyroclastic flow deposits that cover wide
areas such as in the western United States. In the latter case, much remains poorly known about how
these flows are able to transport their pyroclastic load over such large
distances. A research topic that I
am pursuing over the next few years is aimed at improving our understanding of
the initial conditions of pyroclastic flows in areas proximal to their sources
(see also Volcano
fluid dynamics & modeling) and what
clues we can find in proximal deposits that might help us understand transport
and emplacement in more distant areas.