Page 138 - Fundamentals of Physical Volcanology

9780632054435_4_008.qxd 12/10/2007 12:29PM Page 115

PYROCLASTIC FALLS AND PYROCLASTIC DENSITY CURRENTS 115

that is a perfect cone. If you take a series of closely to fall deposits from the convecting eruption
spaced horizontal slices through the cone, then the clouds above Hawaiian eruptions, but little work
average of the areas of the top and bottom of a slice, has been done on this yet.
multiplied by the slice thickness, is a good approx- We now turn our attention to the second family
imation to the volume of the slice. In the same of pyroclastic deposits, those emplaced not by fall-
way, the average of the areas of any two isopachs, out from a high eruption column but rather by flow
multiplied by the difference between the deposit of a mixture of clasts and gas as density currents
thicknesses that they represent, is a good approx- close to the ground.
imation to the volume of the deposit between
the isopachs. Adding up the contributions from
8.4 Pyroclastic density currents and
all pairs of isopachs gives the total deposit volume.
their deposits
Of course, it is necessary to estimate the loca-
tion of the zero-thickness isopach; and the more
closely spaced the isopachs the more accurate The deposits from pyroclastic density currents
the volume. include some of the smallest volcanic layers pre-
The resulting volume is the volume of the deposit served in the geological record and also the depos-
as it lies on the ground. It is normal to convert this its from what are undoubtedly the largest-volume
to the equivalent volume that it would occupy if all and most destructive volcanic phenomena on Earth.
of the pore space within the pyroclasts and all of However, they have certain key properties in com-
the vacant space between the rather loosely packed mon which is why they are discussed together.
clasts were removed – the so-called dense rock
equivalent (DRE) volume. Multiplying the DRE
8.4.1 Nature of the deposits
volume by the density of the solid version of the
erupted magma gives the total mass of the deposit. The three main terms used to describe these depos-
These are important adjustments to make, because its are ignimbrite, pyroclastic surge deposit
they allow the duration of the eruption to be esti- and block-and-ash flow deposit, and the magmas
mated. This is done by dividing the mass of the giving rise to them are almost always of evolved
deposit by the mass eruption rate derived from composition. The deposits are dominated by juve-
fitting the field data to the curves in Fig. 8.6. The nile material but commonly also contain lithic
estimate of the duration may not be very accurate fragments torn from the walls of the dike and vent

because, as mentioned earlier, the mass flux may system feeding the eruption. The juvenile clasts
have changed during the eruption, and so the fit to include intact lapilli and blocks of vesicular pumice,
the theoretical curves may not give a good estimate glassy shards which represent fragments of pumice
of the average eruption rate. clasts which have been broken up, and broken
pieces of crystals which grew in the magma prior to
eruption and which were released when pumices
8.3.5 Fall deposits: summary
shattered. Ignimbrites are generally ash-rich and
When used on prehistoric eruptions, the kinds very poorly sorted, forming extensive sheets or
of analyses described above, from which we can fans (Fig. 8.8) that cover large areas, up to tens of
estimate magma volatile content, mass eruption thousands of square kilometers. They bury or
rate, eruption cloud height, and wind speed and drape pre-existing topography and thicken in local
direction during the eruption, are best applied to depressions. Pyroclastic surge deposits are some-
the deposits from large Plinian eruptions. This is what better-sorted than ignimbrites, although still
because the large dispersal area of such eruptions poorly sorted, and show distinct internal stratifica-
maximizes the chances of significant amounts tion (Fig. 8.9). They may occur within or immedi-
of the deposit being preserved in the geological ately adjacent to ignimbrite deposits. Block-and-ash
record and exposed for us to examine. There is flow deposits differ from the other two types in
clearly the potential to apply the same methods generally containing clasts that are less vesicular.