Andrew on Nonmagmatic volcanism:
There are less well-known types of volcanism that don't involve magma: mud volcanism is one. Mud volcanoes come in two types. On land, hundreds of them occur in areas where hydrocarbons are abundant, like Trinidad or Azerbaijan (see this one in the Image Gallery). Under the sea, thousands of them occur near subduction trenches, where serpentinite mud is abundant (about serpentinization).
Another newly discovered form of volcanism involves asphalt. Asphalt flows were first documented on the floor of the Gulf of Mexico in 2003. No one knows how many of these tar volcanoes there are.
A few more references:
Supercritical water and Hydrothermal Chimneys
Lost City hydrothermal field
Mantle Serpentinization Rates
Carbon Degassing from the Lithosphere [and upper mantle]
Mud Volcano Tofu
Mantle Wedge Water Contents