Jökull - 01.12.1983, Síða 59

Lava Flows and Forms
GUNNAR BÖÐVARSSON
School of Oceanography, Oregon State University, Corvallis, OR 97331, U. S. A.
ABSTRACT
Assuming that many lavas behave as very thin flows of a
Newtonean viscous liquid, one can derive a non- linear
partial differential equation of the parabolic type that
govems the shape of the surface of such flows. Although the
boundary condition at thefront of the lavas still presents an
unresolved problem, the resulting equation can be applied to
estimating the viscosity offlowing lavas.
INTRODUCTION
The mechanism of lava flow is of both geoscien-
tific and direct practical interest. Basaltic layers
and other extrusions comprise a prominent part of
exposed igneous rock and the study of the mechan-
ism of formation of these structures is therefore im-
portant from the geological point of view. Lava
flows have also interfered with human affairs and
have occasionally caused damage to property. We
have only to recall the destruction due to the
eruption on Heimaey in Iceland in 1973. Walker
(1973) correctly emphasizes the real social need to
be able to predict the behavior of lava flows
advancing on populated areas.
Any attempts at gaining a better understanding
of the mechanism of lava flow will have to
commence with the collection of physical data and
with the development of relevant flow models. The
present paper has been written for the purpose of
presenting an elementary flow model which may be
of some interest in the present context.
SOME CHARACTERISTICS OF
LAVA FLOWS
A considerable number of geological and
physical data mainly on historical lava extrusions
are given by Walker (1973) where the factors that
affect the ultimate length of lava flows are discussed
at length. The author notes that while some low-
viscosity lava flows have reached lengths of 100 to
200 km their thickness varies only between 2 and 30
meters. Length/thickness ratios may thus amount
to as much as 10,000 and such extrusions behave as
truly thin-sheet flows. According to Hooper (1982),
some of the Columbia River basalt flows have
reached lengths in excess of 500 km with an average
thickness of only 30 m. Very briefly, some of the
main characteristics of such lava flow-sheets can be
listed as follows.
(1) Even relatively fluid lavas with viscosities in
the range 103 to 10® Pas advance at a slow rate
(Walker, 1973). Velocities range from meters to kilo-
meters per day. The mode of such flow is obviously
that of viscous creep where inertia forces can be
neglected.
(2) The rheological properties of lavas are
apparently complex and only poorly known.
According to Hulme (1974) there are indications
that magmas may behave as Bingham liquids, that
is, possess a yield strength. At stresses above the
Bingham limit, magmas probably behave as New-
tonean liquids, but the viscosity is highly tempera-
ture dependent. Moreover, the temperature and
chemistry of lavas may vary along the flow due to
chemical reactions and outgassing.
(3) As a consequence of the temperature
dependent viscosity, the heat balance of lavas is of
great importance for the rheology. In particular, it
is to be noted that lavas generally form a solid crust
and a solid bottom layer that reduce the eflective
height of the liquid section.
(4) Lava flows have a free surface and their
motion is therefore subject to a non-linear surface
condition.
(5) The mechanical processes in the fronts of lava
flows diífer from the situation in the liquid interior.
The solidified crust piles up at the front and forms a
type of barrier or wall that has to be partially pushed
ahead of the flow or under the advancing lava. This
condition remains to be quantified.
JÖKULL 33. ÁR 57