Subsurface basalts. Thick sequences of volcanic or volcanoclastic rocks (red) have been interpreted from geophysical studies on the U.S. continental margin, as shown on these geologic cross sections.  These are known as seaward-dipping reflections (SDR's), and are buried beneath several kilometers of Jurassic and younger strata in the Baltimore Canyon and Carolina troughs. The exact age of these volcanic rocks is not known, but, if they slightly older than the exposed basalts in the exposed rift basins, then they may have played a role in the mass extinctions at the Triassic-Jurassic boundary. Modified from Withjack et al. (1998).