Radially patterned intermediate volcano, Anemone

Craters already have petal-shaped ejecta blankets. So it was hard to find words for these volcanic features, which are also built of radially arranged parts – but we managed.

These features barely rise above their surroundings. A central round or elongate vent is surrounded by lava flows that reach outwards radially, like petals. The “petals” are of uniform size (Head et al. 1992). Their ends are radar-bright, which suggests rougher, blocky lava there. The centre is radar-dark, possibly because of ash deposits.

They are 20–90 km across and 200–580 m high. The plate shows a group in Atla Regio with particularly rich forms. The structures themselves are not common: only about 25 have been counted.

One theory explains them by Hawaiian-style lava fountaining, which releases gases (White 2010). This idea rests on where they lie. They are often found on higher ground, usually at least 800 m above the mean level of Venus, where the air pressure is relatively low. Another key to their origin may be that they typically form near fault lines. There, their usually elongate vent feeds fissure eruptions.

Radially patterned volcanoes are “intermediate” volcanoes. On Venus, volcanoes 20–100 km across count as intermediate.

Their original informal name comes from the Magellan mission, which discovered them: “anemone volcano”. The name refers to the sea anemone. The radiating lava flows resemble its spreading tentacles.

None of the radially patterned volcanoes has a geographical name.