Page 150 - Fundamentals of Physical Volcanology

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LAVA FLOWS 127

(although still high enough to cause the explosive
activity of course) so that the speeds of the clots
leaving the vent are not too large. “In sufficient
numbers” implies that the mass eruption rate from
the vent is high; this has two effects. First, if there
is a large mass flux there will be a large number of
clots in flight at any one time and so they form
a dense cloud, with clasts on the outside shielding
those near the middle from being able to “see” the
sky and radiate away their heat. Second, with a
large mass flux each clot that lands on the ground
will be buried by the later-arriving clots more
Fig. 9.3 A rhyolite flow forming the ∼80 m high Novarupta
quickly, again conserving its heat. When the accu-
dome marking the site of the vent for the June 1912
mulation rate of hot clasts is large enough they
eruption in Alaska that emplaced the Valley of Ten
Thousand Smokes ignimbrite. (Photograph taken by completely merge together, the slightly cooled
T. Miller, courtesy U.S. Geological Survey.) outer skins of the clots being reheated by the hot
interiors as heat is shared by conduction, and the
9.2 Origin of lava flows whole assemblage moves downhill away from the
vent as a rootless lava flow (Fig. 9.4). The tem-
The simplest way that a lava flow can form is when perature of such a flow may be only a few kelvins to
magma overflows from a volcanic vent onto the a very few tens of kelvins less than that of a flow
surrounding surface and moves downhill under formed directly by effusion from a vent.
gravity. We have seen that most magmas exsolve It is very common for basaltic magmas erupting
volatiles as they approach the surface, and thus as fire-fountains to form rootless lava flows as part
should not be surprised to find that most active lava of a spectrum of features. If clots lose almost no
flows contain significant numbers of volcanic gas heat, and coalescence is complete, a flow forms. If
bubbles which, after the flow has cooled and atmo- clots lose rather more heat, their skins may reach
spheric air has replaced the original gas in the inter- a high enough temperature, after heat sharing with
connected bubbles, are quite vesicular. We know the interiors, to weld together, but the interiors
from the evidence in Chapter 5 that the high juve- may then be too cool to deform significantly and so

nile volatile content of many magmas has the poten- no flow occurs; instead a welded spatter rampart
tial to make them erupt explosively. If the magmas forms (Fig. 9.5). Finally, if too much heat is lost in
erupt on the deep ocean floor on Earth, the pres- flight, the clots accumulate as a warm but un-
sure of the overlying water greatly reduces the welded scoria deposit (Fig. 9.6). Figure 9.7 shows
amount of gas that can exsolve, and so many of qualitatively how the combinations of magma volatile
these magmas behave as though they had a low content and mass flux through the vent control this
volatile content and do not erupt explosively. range of possible features.
However, even when volatile-rich magmas erupt In principle the same range of processes can occur
subaerially, there is still a way that lava flows can be in more evolved magmas. However, in dacitic
formed. All that is required is that at least some of magmas it is not common for the combinations
the clots of fragmented magma ejected through the of volatile content and eruption rate to favor this.
vent into a fire-fountain or into the base of an erup- Rhyolitic magmas, on the other hand, are much
tion column should fall back to the ground near the more likely to erupt at high mass fluxes and,
vent sufficiently quickly and in sufficient numbers. because of high volatile contents, very explosively.
“Sufficiently quickly” is important so that the clots The resulting great efficiency of magma fragmenta-
do not have time to cool much while they are in tion causes the average sizes of the pumice clasts
flight. Short travel times are encouraged by the that are formed to be small which, together with
magma having a relatively low volatile content the high mass flux, makes the jet of clasts and gas