Lobate plains, Smooth plains

Lava that floods large areas forms two basic types of plain. Lobate plains are built of overlapping lava flows that are easy to tell apart. On smooth plains, the boundaries of the individual floods cannot be distinguished.

Lobate plains often consist of alternating radar-bright and radar-dark flows. Each flow is hundreds of kilometres long and a few tens of kilometres wide. They are younger than the wrinkle ridge plains, because along the boundary of the two plain types they flow around the wrinkle ridges. So they record the youngest volcanic activity on Venus (Ivanov and Head 2011).

They form from several large-volume eruptions from different centres. Such plains cover 8% of the surface of Venus, an area about 400 times the size of Hungary. They are usually linked to a volcanic or fracture belt or to a rise, and they are absent from tessera areas. As a stratigraphic unit, the Bell Formation consists of lobate plains.

Smooth plains are usually featureless, uniformly radar-dark areas. They show no traces of tectonic events (that is, fractures). They have no special landforms at all, such as lava channels, apart from small lava domes in places. All this suggests that they are the sites of the most recent lava floods on Venus, since no later event has affected them. They appear on the surface as patches 10–100 km across.

Smooth plains occur in three main settings. The first is in young volcanic areas and along their margins, where lobate plains also occur (see the plate). The second is inside tessera areas, where they are probably lava fills in depressions. The third is around impact craters. These smooth plains are probably not volcanic, however. They are the radar-dark traces of fine-grained ejecta that fell back to the surface.

As a stratigraphic unit, the Gunda Formation is made of smooth plains (Ivanov and Head 2011).

On the plate, they are wherever nothing in particular can be seen.

Because they are so featureless, there is nothing more to say about them.