Carbon-based lavas...

are flowing from Tanzania's Oldoinyo Lengai volcano, arguably the world's most unusual. When this type of lava crystallizes it forms a rare and peculiar volcanic rock termed carbonatite. Key bit:

"The lava expelled from the volcano is highly unusual in that it contains almost no silica and greater than 50 percent carbonate minerals. Typically lavas contain high levels of silica, which increases their melting point to above 900°C (1652°F). The lavas of Oldoinyo Lengai volcano erupt as a liquid at approximately 540°C (1004°F). This low silica content gives rise to the extremely fluid lavas, which resembles motor oil when they flow."

I've got to put this place on my "to do" list, after all, life is just one big geology field trip.

UPDATE: Here's an additional link with images about this bizarre volcano.