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.

We don’t yet know what minerals make up these mounds. They extended over a vast area along the fault.

We don’t yet know what minerals make up these mounds. They extended over a vast area along the fault.

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.

This sample will be analyzed to understand what minerals are present, which will help us understand the geochemical processes that made the rock. For scale, the tray under the rock is about 40 centimeters (16 inches) long.

This sample will be analyzed to understand what minerals are present, which will help us understand the geochemical processes that made the rock. For scale, the tray under the rock is about 40 centimeters (16 inches) long.

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.

Lamellibrachia worms live among the mounds.

Lamellibrachia worms live among the mounds.


We found white, fuzzy bacterial mats growing on the mounds.

We found white, fuzzy bacterial mats growing on the mounds.

—Susan von Thun