Mysterious rock mounds
May 12, 2015
Susan von Thun writes: Today we explored the San Clemente Fault Bank (see map) and were intrigued by what we found. Along the fault, we found a vast expanse of rock mounds made of some kind of precipitated mineral. The mounds formed in areas where chemicals seeping through the seafloor met with seawater, resulting in large rocks with branches extending upward away from the seafloor.
Pieces of the rock mounds were easily broken off by the ROV manipulator arm and put into the drawer. With these rock samples, we can run geochemical analyses to see what minerals make up this rock. The rock is porous, light, and brittle.
Among these mounds, we also saw living tubeworms. The Lamellibrachia worms obtain nutrition from sulfur-eating bacteria in their guts. The worms’ long roots can burrow up to two meters into sediment or bedrock in search of methane and hydrogen sulfide for their bacterial food providers. We also saw bacterial mats growing on top of some of these mounds. The presence of these organisms gives us more clues about what chemicals are seeping out of the seafloor in this area.
—Susan von Thun