Volcanic flows
Lava flow, Festoon
Lava flows are volcanic materials that advance with lobate margins and pile up on one another. The term means both the active molten rock and the solid landform. A festoon is a lava flow with a series of folds on its surface.
Lava flows come in two basic types. Pāhoehoe has a smooth, shiny surface, often with a ropy texture. It tends to form thin layers and erupts at lower rates, and lava channels and lava tubes are common on it. ‘A‘ā lava is rough, blocky and brecciated. It forms thicker layers and comes from eruptions with higher rates. A pāhoehoe flow can turn into ‘a‘ā downslope.
On radar images of Venus the two types are easy to tell apart: pāhoehoe is radar-dark, ‘a‘ā radar-bright. Lava that broke up later can also be bright, however, and a lava lake can be dark.
In the heat of Venus, lava flows cool slowly. The dense atmosphere, on the other hand, carries heat away by convection more easily, which cools the lava faster. In the end, lava flows still cool more slowly than on Earth, so they can flow farther. Their average length is 140–300 km (Zimbelman 1998).
Several researchers have grouped lava flows by their shape. Crumpler et al. (1992) used these categories: amoeboid flows, fluctus-type flows and festoon flows (see below). Stofan et al. (2011) used these types: digitate flows; lava aprons (around a vent); lava fans; parallel flow fields (whose source cannot be identified); lava sheets (without internal structures); and filamentary flows (narrow, sometimes multiple, braided forms).
The surface of festoon flows (Crumpler 1992) is marked by pressure folds, or garlands, about 600 m apart and transverse to the flow direction (see the figure, with a fine lava delta). On the surface of the flow, viscosity slowly rises as the lava cools. Deep inside, the lava stays hot and fluid. The resulting difference in motion may form the folds (Theilig and Greeley 1987). Only three festoons on Venus are known from the literature. The plate shows lava flows with different radar properties around a shield field.
The term festoon comes from the Latin festum, a feast; a festoon is a festive garland of flowers (compare festival).
