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LAVA FLOWS 129

(A)
Fast Lava ponds

Pyroclast accumulation rate Moderate Brittle cinders (B) Partly welded spatter
Lava flows
Rootless flows

Completely welded spatter

Plastic
Slow cinders Fluid “plops”

Cold Warm Hot
Mean pyroclast temperature

Fig. 9.7 Diagram showing how the mass flux erupted from the vent and the exsolved volatile content of the magma control
the type of deposit produced near the vent in an explosive eruption. The pyroclast accumulation rate is directly proportional
to the erupted mass flux. A high volatile content causes pyroclasts to be erupted at high speeds and so to travel further before
landing, giving them a greater opportunity to cool. Hot deposits form preferentially from low volatile-content, low-speed
eruptions. (Adapted from fig. 5 published in Journal of Volcanology and Geothermal Research, Vol. 37, Head, J.W. and
Wilson, L., Basaltic pyroclastic eruptions: influence of gas-release patterns and volume fluxes on fountain structure, and the
formation of cinder cones, spatter cones, rootless flows, lava ponds and lava flows, 261–271, copyright Elsevier (1989).)

the ground over which the lava is flowing, but it is
convenient to describe the general features first.
Many lava flows, especially those of basaltic com-
position but also some more intermediate flows,
consist of lava moving in a central zone bounded by
a stationary accumulation of rock on either side
which consists of lava that has been carried down
the channel to the flow front and then pushed
aside to make way for the liquid following it. Flows
with these properties are called channelized lava
flows, and the stationary banks bordering the cen-
tral channel are called levées (French for “raised”
features) because the depth of lava in the channel
is often (but not always) less than the height of
Fig. 9.8 A rheomorphic ignimbrite deposit in the Trans-
the levée (see Fig. 9.1). In other cases, the boundary
Pecos volcanic province of southwest Texas, USA. Note the
extreme flattening and stretching of dense, highly alkaline between channel and levée is much less distinct,
juvenile (pumice) clasts caused by the lateral flow of the and all of the lava over almost the entire width of
hot, welded deposit. (Image courtesy of Stephen Self.) the flow is moving. In this case we have a sheet
flow (Fig. 9.9). There is a strong tendency for sheet
These morphologies are intimately connected with flows to be wider, for a given thickness, than chan-
the rheological properties of the flow materials, as nelized flows, and there is some indication that they
will be seen shortly, and also depend on the mass or are commoner on the ocean floor than on land. This
volume eruption rate of the lava and the slope of may be at least partly related to the fact that sheet