838 Encyclopedia of the Solar System
quickly establish a pattern in which lava moves downhill in
a central channel between a pair of stationary banks called
lev ́ees. Also, lavas do not flow downhill indefinitely after
the magma supply from the vent ceases: They commonly
stop moving quite soon afterward, often while the front of
the flow is on ground with an appreciable slope and almost
all the lava is still at least partly liquid. Also, liquid lava
present in a channel at the end of an eruption does not
drain completely out of the channel: A significant thickness
of lava is left in the channel floor. These observations led to
the suggestion that no lavas are Newtonian, and attempts
were made to model flows as the simplest non-Newtonian
fluids, Bingham plastics.
The basis of these models is the idea that the finite thick-
ness of the lev ́ees or flow front can be used to determine
the yield strength of the lava and that the flow speed in the
central channel can be used to give its apparent, and hence
Bingham, viscosity. Multiplying the central channel width
by its depth and the mean lava flow speed gives the vol-
ume flux (the volume per second) being erupted from the
vent. Laboratory experiments were used to develop these
ideas, and they have been applied by numerous workers to
field observation of moving flows on Earth and to images
of ancient flows on other planets. For flows on Earth, it is
possible to deduce all the parameters just listed; for ancient
flow deposits, one can obtain the yield strength unambigu-
ously, but only the product of the viscosity and volume flux
can be determined.
There is a possible alternative way to estimate the vol-
ume flux if it can be assumed that the flow unit being ex-
amined has come to rest because of cooling. An empirical
relationship has been established for cooling-limited flows
on Earth between the effusion rate from the vent and the
length of a flow unit, its thickness, and the width of its ac-
tive channel. If a flow is treated as cooling-limited when
in fact it was not (the alternative being that it was volume-
limited, meaning that it came to rest because the magma
supply from the vent ceased at the end of the eruption),
the effusion rate will inevitably be an underestimate by an
unknown amount. Cooling-limited flows can sometimes be
recognized because they have breakouts from their sides
where lava was forced to form a new flow unit when the
original flow front came to rest.
Lava rheologies and effusion rates have been estimated
in this way for lava flows on Mars, the Moon, and Venus. It
should be born in mind, when assessing these estimates, that
a major failing of simple models like the Bingham model is
that they assign the same rheological properties to all the
material in a flow, whereas it is very likely that lava that has
resided in a stationary lev ́ee near the vent for a long period
will have suffered vastly more cooling than the fresh lava
emerging from the vent and will have very different prop-
erties. More elaborate models have been evolved since the
earliest work, including some that apply to broadly spread-
ing lava lobes that do not have a well-defined lev ́ee-channel
structure, but no model yet accounts for all the factors con-
trolling lava flow emplacement. With this caution, the val-
ues found suggest that essentially all the lavas studied so
far on the other planets have properties similar to those of
basaltic to intermediate (andesitic) lavas on Earth. Many
of these lavas have lengths up to several hundred kilome-
ters, to be compared with basaltic flow lengths up to a few
tens of kilometers on Earth in geologically recent times, and
this implies that they were erupted at much higher volume
fluxes than is now common on Earth. There is a possibility,
however, that some of these flow lengths have been over-
estimated. If a flow comes to rest so that its surface cools,
but the eruption that fed it continues and forms other flow
units alongside it, a breakout may eventually occur at the
front of the original flow. A new flow unit is fed through
the interior of the old flow, and the cooled top of the old
flow, which has now become a lava tube, acts as an excel-
lent insulator. As a result, the breakout flow can form a new
unit almost as long as the original flow, and a large, com-
plex compound flow field may eventually form in this way.
Unless spacecraft images of the area have sufficiently high
resolution for the compound nature of the flows to be clear,
the total length of the group of flows will be interpreted
as the length of a single flow, and the effusion rate will be
greatly overestimated.
There are, however, certain volcanic features on the
Moon and Mars that may be more unambiguous indica-
tors of high effusion rates: the sinuous rilles. The geomet-
ric properties of these meandering channels—widths and
depths that decrease away from the source, lengths of tens
to a few hundred kilometers—are consistent with the chan-
nels being the result of the eruption of a very fluid lava
at a very high volume flux for a long time. The turbulent
motion of the initial flow, meandering downhill away from
the vent, led to efficient heating of the ground on which it
flowed, and it can be shown theoretically that both mechan-
ical and thermal erosion of the ground surface are expected
to have occurred on a timescale from weeks to months.
The flow, which may have been∼10 m deep and moving
at∼10 m/s, slowly subsided into the much deeper channel
that it was excavating. Beyond a certain distance, the lava
would have cooled to the point where it could no longer
erode the ground, and it would have continued as an ordi-
nary surface lava flow. The volume eruption rates deduced
from the longer sinuous rille channel lengths are very sim-
ilar to those found for the longest conventional lava flow
units; modeling studies show that the turbulence leading
to efficient thermal erosion was probably encouraged by
a combination of unusually steep slope and unusually low
lava viscosity. A few sinuous channels associated with lava
plains are visible on Venus, but the lengths of some of the
Venus channels are several to ten times as great as those
seen on the Moon and Mars. It is not yet clear if the ther-
mal erosion process is capable of explaining these channels
by the eruption of low-viscosity basalts, or whether some