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The surface of Mercury is geologically dead. Volcanic activity has never been observed on the surface of Mercury, nor has there ever been any evidence of it for geologically recent times. This was so much the case that it was once thought that Mercury was always a geologically dead world. That thinking has been changed by recently uncovered evidence.
The MESSENGER Probe
NASA's MESSENGER Probe began sending back photographs of Mercury's surface in January 2008. The results revolutionized the theories regarding Mercury's geology. For the first time, plains formed by large lava flows and the remnants of old volcanoes were directly observed. These details had been previously obscured by Mercury's heavily cratered, moon-like surface; and by both the low resolution of photography offered by previous probes, and the area in which they took their photographs. There had been some evidence of volcanism gathered in these previous probes, but it was murky and inconclusive. The probe also revealed evidence that Mercury has a molten core.
Mercury's Composition
Mercury is known to be the second densest planet in the solar system, right after the Earth. However, the planet Mercury is actually made out of more of the denser, metallic materials than the Earth is, being 70 percent metallic and 30 percent sillicate. The Earth surpasses Mercury in density only due to the effects of gravitational compression on the material deep inside the Earth. The most recent research, including that based on data gathered by MESSENGER, indicates that Mercury has a molten core that has a very high proportion of iron in it. It is this iron in the core that, despite its very slow rotation and small size, produces Mercury's weak magnetic field.
Why Did the Volcanism Stop?
Mercury still has a molten core, the essential precondition for volcanism. However, at some point all volcanic activity on the planet stopped. The reason Mercury ceased to be volcanically active, or when that happened, remains a mystery. There are not even adequate theories to explain the phenomenon of Mercury's volcanism shutting down.
Other Peculiar Effects
Mercury is subject to tidal forces, imposed upon it by the sun. On Earth, the moon's gravitational effects cause visible tides in the Oceans. Mercury is subject to tidal forces from the Sun's gravity, which are 17 percent stronger than those on Earth. The result's of these tidal forces are measurable distortions on the surface of Mercury, as the tidal force literally pulls part of the crust close to the Sun. What effect this might have had on Mercury's volcanism is unknown.