Steep-sided dome, Pancake dome

Pancake domes are round features with steep margins and roughly flat tops. They often occur in groups, and many partly overlap one another.

Because the margins are steep, landslides are common (Bulmer and Guest 1996). They scallop the regular outline all around.

Many kinds of structures can occur on top of steep-sided domes: pits, radial, concentric or polygonal fractures, and smaller peaks. The top surface is convex or concave.

They are typically 10–35 km across, so they count as “intermediate” volcanoes, but one reaches 120 km. Estimates put their height at up to 4 km. Accurate heights, however, exist only for domes up to 1.6 km high.

There are 150–300 such structures on Venus.

What forms these flat, round lava features? Most researchers agree that they form when a highly viscous melt is pushed out onto the surface. The high viscosity may come from a high silica content, strong crystallisation of the lava and/or many gas bubbles. On Earth, such lavas build lava domes. Yet Earth’s silica-rich lava domes differ in many ways from the steep-sided domes of Venus. So these structures may be basaltic after all.

Conditions at the surface of Venus differ from those on Earth. The high temperature and the high pressure together may change the way lava builds landforms. On this basis, the most similar landform on Earth is the seamount. The setting adds to the likeness. The atmospheric pressure on Venus is 92 atmospheres, the pressure found in Earth’s oceans at a depth of 950 m (Lebonnois and Schubert 2017).

Some researchers think that a dome forms in a single eruption. Others think that the eruptions were renewed several times. In this view the domes were cone-shaped at first, and the lavas slowly widened them.

Steep-sided domes were first found on Magellan images by Head et al. (1991). They called them “pancake domes”, after the thick American pancake.

Where steep-sided domes occur may help to explain how they form. They are found almost everywhere on Venus. Their surroundings, however, are usually rich in volcano-tectonic and volcanic structures, and shield fields are often nearby. Atalanta Planitia, for example, has many steep-sided domes.

As a named feature type they take the descriptor term Farrum (plural Farra). The Latin word originally meant a kind of cake-shaped animal feed.