These are typical pillow lavas, which form near the outer edges of lava flows, where the lava forms narrow fingers that crust over. When more lava pressure builds up from behind, the tip of the finger breaks out, forming another rounded lobe or "pillow" at the end. In this image you can see how several sets of pillow lavas have flowed over each other. Beneath the pillow lavas is a more blocky flow. Click image for larger view and image credit.
Geology of Davidson Seamount
David Clague, Senior Scientist
Monterey Bay Aquarium Research Institute
Davidson Seamount is one of a group of submarine volcanoes off the
continental margin offshore central California between Lat 37.5° to Lat
34.0°. Davidson, along with Guide, Pioneer, Gumdrop, and Rodriguez, are
morphologically unlike typical ocean island volcanoes.
All of the seamounts in the group are complex NE-SW trending structures,
consisting of parallel ridges separated by sediment-filled troughs. The
ridges are constructed parallel to the fossil spreading centers
abandoned when the San Andreas fault system formed. On each of these
seamounts, a series of knobs are aligned along the ridges; however,
craters like those commonly topping oceanic volcanoes are lacking. The
lack of collapse craters suggests that the lavas that built the
seamounts were never stored in chambers within the volcanoes, as is so
common at other oceanic volcanoes.
The manipulator arm of ROV Tiburon collects a piece of basaltic lava. In the background is a surface covered with blocky lava, which is fairly typical of Davidson Seamount (more typical than the pillow lavas shown in the other photo). Click image for larger view and image credit.
The surfaces of the seamounts are built mainly of blocky lava flows,
although some pillow lava has been seen on the their deeper flanks. Near
the summit, layered deposits of volcanic ash and pyroclastic materials
are found. These volcaniclastic rocks indicate mildly explosive eruptions of gas-rich lava near the summit of the volcano.
Ar-Ar laser fusion ages of mineral separates indicate that Davidson formed about 9 to 15 million years ago, so it formed 5 to 12 million years after the underlying ocean crust.
The volcanic rocks found on Davidson Seamount are predominantly differentiated alkalic basalt, hawaiite, mugearite, and trachyte. These evolved lavas are highly viscous (thick and pasty) and form short, thick flows and steep-sided knobby structures at the vents. The high viscosity of these lavas also likely inhibited the loss of gas bubbles, thereby making the eruptions more explosive. The lavas traveled to the surface carrying up rare xenoliths (foreign rocks) of mantle materials as well as a range of coarsely crystallized fragments of lava slowly solidified at depth.
A sample of basalt from Davidson seamount in the lab on the Western Flyer after all the silt and slimy living stuff has been cleaned off of it. This photo shows how basalts are typically fine-grained, massive, very dark colored rocks. In fact, it is hard to tell much about oceanic basalts even with a hand lens. That is why we have to use chemical analysis to determine which rocks are related to each other, and where they came from. Click image for larger view and image credit.
The relationship of Davidson Seamount to Monterey Bay and the coast. As you can see in this image, Davidson is located far offshore of the coast, beyond the continental borderland area (the continental shelf and the continental rise), with its dramatic submarine canyons. In fact, its base is probably partially buried in deep-sea muds. Click image for larger view and image credit.
