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gaseous emission from incandescent magmatic particles.
When fluidization and drag-reduction cease, which may not
be for many kilometers, the still-hot PDFs stops, cools, suf-
fers progressive autocompaction , and contracts to form
columnar cooling joints (Section 4.14). However, as with
turbidity currents (Section 4.12), it is a great mistake to assume
that any PDFs just represents local flow deceleration - deposi-
tion from moving flows may be cumulative at any point due
to nonuniformity of overall flow. The characteristic deposits
of pumice-rich pyroclastic flows are called ignimbrites .
We may ask why Plinian-type eruptions and pyroclastic
flows involving intermediate composition melts do not
occur so commonly in other basic and acidic magmas.
Volatile species, concentration, and diffusion rate are the
keys here: in basic melts the volatile phase is predominantly
CO 2 rather than H 2 O and the diffusion rate is orders of
magnitude faster, so smaller and more numerous fast-
growing bubbles can advect to safety at the surface through
the low-viscosity (1-10 Pa s) melt much faster than in
intermediate melts. At the free surface, the coalesced
bubbles simply escape, contributing to fire-fountaining in
Hawaiian/Strombolian eruptions, while at depth they are
thought to ascend and accumulate as frothy layers at the
top of magma chambers. Calculations indicate that ascent-
related decompression may enhance local pressure by more
than 10 MPa, triggering roof fragmentation and eruption.
In acid melts of rhyolitic composition with high volatile
content the diffusion coefficient is very low and bubbles
have great difficulty in nucleating and growing in the rap-
idly chilling acidic glass ( obsidian ). However, significant
volatile loss is thought to occur in Vulcanian (unblocking)
vents subject to explosive rhyolitic eruptions. Here, intense
shear in conduit boundary layers is believed to induce
increased permeability during vitrification/fragmentation
reactions.
Finally, we must stress that our discussion of melting
and volcanism has concentrated on the physical processes
involved; we may have given the impression that volcanoes
are rather “one-track,” either acid, intermediate, or basic;
effusive or explosive; gas-rich or gas-poor. The immense
basaltic shield volcanoes, like those of Hawaii and Iceland,
are indeed mostly monogenetic, in this case made up of
basalt lava flows. This is far from the general picture.
Geological sections through ancient and active volcanoes
reveal that their eruptive products are far from uniform.
Magma-mixing and fractionation see to it that time trends
exist in magma composition and a volcanic edifice or cen-
ter may reveal an internal architecture made up of every
combination of eruptive product; ash-fall, ash-flow, and
lava flow. These strictly volcanic deposits are all mixed up
with the sedimentary products of lahar flows that record
hyperconcentrated stream flow, reworking of volcanic ash
that often dominate the outer flanks and peripheries of
volcanic vents (Fig 5.34). This architectural complexity is
best seen in that most abundant of active volcanoes, the
immense stratovolcanoes that dominate the volcanic arcs of
the world. These are made up of intermediate composition
of lava flow/pyroclastic flow/lahar/ash fall deposits built
up over time, often millions of years. Their summits often
show caldera morphology (Figs 5.18 and 5.21a), record-
ing periods of complete or partial edifice collapse. Within
the caldera walls multivent subsidiary cones may occur.
5.2
Plate tectonics
We saw in our introductory chapters that the outer 100 km
or so of the solid Earth comprises a mechanical layer called
the lithosphere . Evidence from seismology (Section 4.17)
makes it clear that this more-or-less rigid layer comprises
the crust and the upper part of the mantle. The minerals
quartz and feldspar dominate the crust while olivine
dominates the mantle. Although the lithosphere is rigid and
behaves rheologically in a brittle-elastic fashion (Section
4.14), under high loads and over longer time spans it may
also deform as a plastic material. What is even more remark-
able is that the lithosphere is discontinuous, in the sense that
it comprises a number of constituent fragments, called
plates , which are in constant relative motion with respect to
each other. There are seven major and several minor plates
making up the outer solid Earth (Fig. 5.35). Plate cycling
makes Earth a highly distinctive planetary body; hence our
previous references to the mode Cybertectonica .
5.2.1
Definitions and other facts
Plates are able to move as rigid bodies over the surface of
the planet in response to driving forces (see below) because
they lie over another weaker mechanical layer termed the
asthenosphere , whose topmost part has a tiny proportion
(less than 1 percent) of partial melt. The base of the oceanic
lithosphere occurs at a temperature of c .1,000
C, just above
the onset of the partial melting that characterizes the top
asthenospheric low velocity zone (LVZ; see Section 4.17.4).
It is this “accident” of the upper mantle geotherm
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