All the silt and slimy living stuff has been removed from this basalt (volcanic rock) sample from Davidson Seamount. This photo shows how basalts are typically fine-grained, massive, very dark rocks. In fact, researchers find it difficult to tell much about oceanic basalts — even with a hand lens. That is why we use chemical analysis to determine which rocks are related to each other, and where they came from. Image copyright 2000 MBARI.