A variety of shield volcanoes can be found in selected locations on the lunar surface, such as on Mons Rümker. These are thought to be formed by relatively viscous, possibly silica-rich lava, erupting from localized vents. The resulting lunar domes are wide, rounded, circular features with a gentle slope rising in elevation a few hundred meters to the midpoint. They are typically 8-12 km in diameter, but can be up to 20 km across. Some of the domes contain a small pit at their peak.