They are believed to be analogous to Earth's fold mountains such as the Rockies or the Himalayas, formed by the collision and buckling of tectonic plates, or to subduction zones like the Andes, where upwelling lava (or cryolava) from a melting descending plate rises to the surface. One possible mechanism for their formation is tidal forces from Saturn. Because Titan's icy mantle is less viscous than Earth's magma mantle, and because its icy bedrock is softer than Earth's granite bedrock, mountains are unlikely to reach heights as great as those on Earth. In 2016, the Cassini team announced what they believe to be the tallest mountain on Titan. Located in the Mithrim Montes range, it is 3,337 m tall. If volcanism on Titan really exists, the hypothesis is that it is driven by energy released from the decay of radioactive elements within the mantle, as it is on Earth. Magma on Earth is made of liquid rock, which is less dense than the solid rocky crust through which it erupts. Because ice is less dense than water, Titan's watery magma would be denser than its solid icy crust. This means that cryovolcanism on Titan would require a large amount of additional energy to operate, possibly via tidal flexing from nearby Saturn. The low-pressure ice, overlaying a liquid layer of ammonium sulfate, ascends buoyantly, and the unstable system can produce dramatic plume events. Titan is resurfaced through the process by grain-sized ice and ammonium sulfate ash, which helps produce a wind-shaped landscape and sand dune features. Titan may have been much more geologically active in the past; models of Titan's internal evolution suggest that Titan's crust was only 10 kilometers thick until about 500 million years ago, allowing vigorous cryovolcanism with low viscosity water magmas to erase all surface features formed before that time. Titan's modern geology would have formed only after the crust thickened to 50 kilometers and thus impeded constant cryovolcanic resurfacing, with any cryovolcanism occurring since that time producing much more viscous water magma with larger fractions of ammonia and methanol; this would also suggest that Titan's methane is no longer being actively added to its atmosphere and could be depleted entirely within a few tens of millions of years. Many of the more prominent mountains and hills have been given official names by the International Astronomical Union. According to JPL, "By convention, mountains on Titan are named