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9 Lava flows

9.1 Introduction control the features of both individual lava flows,
called flow units after they have come to rest, and
A lava flow is probably the first kind of feature of collections of flow units, called lava flow fields.
that comes to mind when most people think about
volcanic activity. The kinds of lava flows most
commonly illustrated in documentary films are the
basaltic flows that form on the shield volcanoes of
Hawai’I (Fig. 9.1). There are, however, other kinds
of lava flow, especially those formed by magmas of
more evolved composition (Figs 9.2 & 9.3), and
flows can differ greatly in terms of length (from sev-
eral meters to literally a few hundred kilometers),
volume, geometry, advance rate (from less than mil-
limeters per second to several meters per second),
and potential threat (see Chapter 11). This chapter
first describes the general morphological features
of lava flows and then considers the factors that

Fig. 9.1 A pahoehoe toe lobe advancing over older lava.
Note the rope-within-rope structure of the central part of
the ∼2 m wide lobe and the smoother surface of the small Fig. 9.2 A dacite dome formed during the eruption of
break-outs. (Photograph by Pete Mouginis-Mark, University Unzen volcano, Japan, that began in May 1991. (Photograph
of Hawai’I.) by K. Scott in 1994, courtesy U.S. Geological Survey.)