Page 151 - Fundamentals of Physical Volcanology

9780632054435_4_009.qxd 12/10/2007 12:31PM Page 128

128 CHAPTER 9

Fig. 9.5 View of one side of a welded spatter rampart on
Kilauea volcano, Hawai’I. (Photograph by Lionel Wilson.)

Fig. 9.4 A rootless lava flow being formed from a lava
fountain at the Pu’u ‘O’o vent on Kilauea volcano, Hawai’I,
in 1984. (Photograph by J.D. Griggs, courtesy U.S.

Geological Survey, Hawaiian Volcano Observatory.)

emerging from the vent very opaque, thus conser-
ving heat. The same mass of material broken up into
a large number of small particles, rather than a
small number of large particles, forms a much more
opaque cloud: compare walking through mist or
cloud, consisting of tiny water droplets, to walk-
ing through a normal rain shower. Thus in some
explosive rhyolitic eruptions complete welding
and some flowage of pumice takes place, forming
Fig. 9.6 An unwelded deposit of scoria clasts. The scale is
a rheomorphic deposit. This can happen in the in centimeters. (Photograph courtesy of Rebecca Horne.)
proximal parts of plinian fall deposits, but is much
more likely to happen in ignimbrites (Fig. 9.8). The 9.3 Types of lava flow
great viscosity of rhyolitic magmas ensures that
such rheomorphic flows do not normally travel for Empirically, the morphological shapes of lava flows
more than a few meters to tens of meters. can be classified into a few general categories.