The vast majority of Venus' surface (
%) is composed of
gently rolling plains that are thought to be primarily due to
effusive eruptions of basaltic lavas. Over much of their surfaces,
morphologic features indicative of lava flows (e.g., lobate flow
margins) are faint or indistinguishable in radar images of the
surface. Much of this surface could be due to eruptions similar to
those that created the canali. However, there are many other
kinds of volcanic constructs found on the surfaces of the plains,
each of which may play some role in the formation of this major
geologic province. Besides the various types of channels, there
are several types of small volcanic constructs found on the
surface of the plains, including small shields, cones, and
steep-sided domes, and large complexes of individual lava flows
called ``flow fields'' [ Head et al., 1992;
Guest et al., 1992].
Small shields and cones (usually defined as being <20 km in diameter) are the most ubiquitous of venusian constructs, with a population nearing and possibly exceeding 100,000 [ Head et al., 1992]. They occur as isolated constructs or in large groups, called shield fields, and appear to represent relatively small-volume effusive eruptions. Shield fields are commonly associated with large shield volcanoes and coronae, possibly because near-surface magma is relatively abundant in the vicinity of such features. Although small constructs themselves can represent no more than a small fraction of Venus' crustal volume, they may be analogous to the plains-forming volcanism as epitomized by the Snake River Plain on Earth [ Guest et al., 1992], and thus may be representative of a volumetrically significant style of volcanism for Venus.
Magellan data allowed the first identification of steep-sided
domes, informally known ``pancake domes,'' on Venus. These
features are most commonly 10--30 km in diameter and 
km in
height (thus, 
km
in volume) and, as their name
suggests, tend to be near-circular in plan view and approximately
cylindrical in cross-section, with flat tops and steep sides
[ Pavri et al., 1992]. Somewhat similar features
are seen on Earth, usually due to eruptions of highly silicic,
highly viscous magmas. The terrestrial features, however, tend to
be considerably smaller (
km in diameter, 
km
in volume) and more irregular in shape [ Pavri et
al., 1992]. Large-volume (
km
) eruptions of silicic
magmas on Earth are most commonly associated with either large
flows or with large, explosive ignimbrite-forming events. On
Venus, the high atmospheric pressure may inhibit the exsolution
and expansion of volatiles in the magma to the extent that such
eruptions nearly always occur effusively. Although interesting
because they may represent differentiated magmas, there no more
than about 200 such constructs identified on the surface and they
are unlikely to represent a large fraction of crustal composition.
In addition to these discrete constructs, Head et al. [1992] recognize approximately 50 regions, typically a few hundred to one thousand kilometers across, in which the dominant or sole characteristic of the surface is one or (usually) more lava flows. These lava flow fields commonly cover a few hundred thousand square kilometers, contain lava channels, and are not directly associated with large shield volcanoes. Perhaps contrary to expectation, neither are they strongly concentrated in the vicinity of volcanic rises such as Atla and Beta Regiones. Instead, most are found at the edges of a variety of highlands or at the edges of regional topographic depressions.
Similar to the canali and sinuous rilles discussed above, flow
fields may represent high effusion rates. Extrapolating results
from studies of terrestrial lava flows whose lengths are limited
by flow cooling, Roberts et al. [1992] estimate
effusion rates for flows at one such flow fields (Mylitta Fluctus
) to be in the range of 
to 
m
sec
. Such high rates may be attained in flood basalt
eruptions, but have not been found in other effusive terrestrial
volcanic environments (e.g., volcanic hotspots). Effusion rates
may be significantly lower if the flows are not cooling-limited
but are instead tube-fed or otherwise effectively insulated from
atmospheric cooling over a significant fraction of their length.