These are hellish planets, often termed 'Venusian' after the prototype world in the Sol System, their trademark thick atmospheres formed by unrelenting volcanic activity and the buildup of greenhouse gases over several hundred million years. As such, the planetary carbon cycle has broken down and completely ceased due to the resulting loss of water from the surface and the atmosphere. Initially, these worlds likely began with large amounts of water, but its loss causes the tectonic cycle to lock up and the atmosphere is no longer processed into the planet's crust, while out-gassing volcanoes continue to add to the atmosphere.
Eventually an atmosphere hundreds of time as thick as a standard Gaian world's is formed, while the surface is riddled with volcanoes. These volcanoes also thicken the surface crust, eventually to the point where much of the activity is literally penned beneath the surface. Every few hundred million years, however, this activity reaches a point where the entire surface of the planet turns over in a planet-wide catastrophe, and the thickening process begins again once the internal heat has been released.