Page 154 - Fundamentals of Physical Volcanology

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LAVA FLOWS 131

the outside of a flow may be very cool, as long as it the top surface is raised. This occurs not just at the
is a meter or two thick the lava in the core of the margins but also, because the hot lava core trans-
flow can still be as hot as when that lava left the vent mits the pressure throughout the flow, all over the
many hours earlier. In this case we talk about the area covered by the flow unit – indeed, the thicken-
hot lava flowing within a lava tube. The lava may ing may be greater in the middle than at the edges.
drain out of a lava tube at the end of an eruption This process is called inflation, and can be a very
leaving a cave-like tunnel; “tide-marks” are com- important alternate to the formation of breakout
monly seen on the walls of such drained lava tubes flows in the growth of a lava flow field.
marking successive near-constant levels of the When an old flow unit ceases to move and forms
lava. a new flow unit as a result of a breakout somewhere
Hot, fluid lava transmits pressure changes with along the edge of the old flow, or when a new flow
perfect efficiency, and so if the front of a lava flow unit is initiated at the vent, we have the beginning
stops moving, the rising pressure as the vent con- of the formation of a compound lava flow field
tinues to supply lava is transmitted throughout the (Fig. 9.10). Each old flow unit continues to cool
core of the flow and can rupture the cooled skin at once magma has ceased to flow through it, and its
its weakest point and let a new flow begin to form stationary upper surface becomes rigidly frozen to
as a breakout. Breakouts from lava flows can occur its levées. However, as long as the core of the flow
for other reasons. Sometimes the eruption rate unit has experienced only minimal cooling it can
from the vent fluctuates, and if there is an increase still act as a pathway from the original vent to the
in flow rate the level of lava in the central channel currently active sites of emergence of lava onto the
of the flow rises and the lava overflows the existing surface, and it is now what is described as a lava
levées. However, the pressure due to the extra lava tube (Fig. 9.11). The fact that only a little heat is
depth may even be enough to push part of one of lost by conduction through the walls of a lava tube
the levées aside so that the original flow effectively means that a network of tubes in a compound flow
splits into two. Both the new and the old lava flow field can carry lava very much further from the orig-
may continue to be fed at the same time, although inal vent than it could possibly flow in a normal
of course the level of the lava in the original channel channelized flow.
downstream of the breakout will decrease due to
the diversion of part of the supply from the vent
into the new flow lobe. Sometimes the reason for

the overflow and breakout is that a section of the
inner part of one of the levées breaks away from the
main mass of stationary material and dams the exist-
ing channel. When this occurs we talk about an
accidentally breached flow.
When the front of a lava flow stops moving while
the vent is still supplying lava, it is not always the
case that a breakout occurs at one specific place
around the margin of the flow to create a new flow
unit. Instead, large numbers of small fractures may
form around the margins of the flow, each of which
allows the edge of the flow to expand sideways a lit-
tle but also, more importantly, raises the top sur-
face of the flow slightly. Lava from the hot core
Fig. 9.10 Compound pahoehoe lava flow forming on
oozes into each of these fractures and seals it again,
January 18, 2005, on the south flank coastal plain of Kilauea
even as new fractures are forming elsewhere. The
volcano, Hawai’I. (Photograph by Jon Castro, courtesy of
net effect is that the flow increases its area slightly, the photographer and U.S. Geological Survey, Hawaiian
but much more importantly it also gets thicker as Volcano Observatory.)