Sinuous valley.

Widespread lowlands.

The speed and direction of an object.

The measure or property of a material to be resistant to flow; the internal friction of a material. Materials with a high viscosity are more resistant to flow, while those with a low viscosity are more fluid.

The process whereby topographic features become subdued over time due to the flow of the surrounding geologic material.

As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

Produced or discharged from a volcano.

The eruption of molten material or gas-driven solid fragments onto the surface of a planetary body. On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism. (Image: A volcano blasts material off the surface of Jupiter's moon Io.)

A mountain formed from the eruption of igneous matter through a source vent.