Volcanic rises are thought to form when a portion of the deep mantle is heated to several hundred degrees hotter than its surroundings. This portion of the mantle is now less dense than its surroundings and begins to rise. Such a rising bubble of material is often called a mantle plume.
As it nears the surface, the plume may take on a mushroom-like shape, as shown in the above sketch. It also begins to generate magma (deep inside the planet the pressure was too great to allow the material to melt) some of which escapes to the surface to form volcanos and lava flows. In addition, the buoyant plume pushes the surface upward, forming a rise, and possibly contributing to the formation of rift valleys. This form of plume volcanism is similar in many ways to terrestrial plume volcanism.
There are three particular types of volcanic rises:
- These are rises that contain two or more deep rift valleys. These valleys cut through most of the rise itself, but also extend several hundred kilometers beyond the rise itself. Atla Regio and Beta Regio are rift-dominated rises.
- Volcano-Dominated Rises
- These rises lack deep rift valleys. They do contain one or more large shield volcanoes. Western Eistla Regio and Imdr Regio are volcano-dominated rises.
- Corona-Dominated Rises
- These rises lack large rift valleys, like volcano-dominated rises. But instead of being dominated by shield volcanoes, their surfaces are dominated by coronae. Corona-dominated rises include Eastern Eistla Regio and Themis Regio.