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Gulf of California 2015, Leg 5 – Volcanoes and Seamounts – Log 3

Sunburst radial jointing in the interior of a large pillow lava exposed in a fault scarp. This extreme radial jointing is not commonly seen in basaltic lavas.

Gulf of California 2015, Leg 5 – Volcanoes and Seamounts – Log 3

Pillow lavas

Chrissy Maschmeyer writes: The purpose of our expedition today was to determine if volcanic rocks containing silica occurred along the spreading ridge south of the previously identified rhyolitic lava dome. Oceanic ridges typically erupt basalt, which is denser and contains less silica than continental crust, and is considered to be more directly derived from the Earth’s mantle.

The Alarcón Rise is a unique spreading ridge because it contains a wide range of lava compositions, including basalt, dacite, andesite, and rhyolite. The range of lava compositions, in which increasing silica is just a part, is due to fractionation or contamination of the source magmas over time.

Sunburst radial jointing in the interior of a large pillow lava exposed in a fault scarp. This extreme radial jointing is not commonly seen in basaltic lavas.

Earlier today we observed extreme sunburst radial jointing in the interior of a pillow lava that had been exposed by a fault. This pattern suggests the lava is likely of a higher-silica composition. With this encouraging observation, we were certain we would find high-silica lavas away from the center of the spreading ridge and south of the rhyolitic dome. I was especially excited because my master’s research is the creation of a classifier that can accurately predict lava composition based on lava morphology in maps of mid-ocean ridges.

While in South Carolina, I was able to determine a site that I thought contained high-silica rocks based on visual analysis of MBARI’s high-resolution AUV maps and initial results from my classifier. Our chief scientist, Dave Clague, had independently identified the same area as a site of interest. Today, we explored that region with ROV Doc Ricketts and found the contact between basaltic pillow lavas and lavas that contain higher levels of silica.

Contact between basaltic and higher-silica pillow lavas. The lavas to the right have a smooth surface and are relatively small. The lavas to the left are deeply grooved and large.

In the control room, the ROV pilots and science team referred to the grooved pillow lavas as having a “bread crust” texture. Similar samples collected in 2012 in the same area were composed of dacite, andesite, and even rhyolite. When we return to land, geochemical analyses will confirm the composition of these rocks, but we are confident that this bread-crust texture seems to be a morphological signature we can use to distinguish the higher-silica rocks at spreading ridges.

A large sea anemone (possibly of the genus Bolocera) is attached to a ridged pillow lava. The red laser dots are 29 centimeters apart for scale.
Chrissy Maschmeyer in the lab, examining a lava sample from the Alarcón Rise.

—Chrissy Maschmeyer

About Gulf of California 2015, Leg 5 – Volcanoes and Seamounts

Leg 5 of this expedition returns to the mouth of the Gulf of California, to study the seafloor-spreading ridges of the Alarcón Rise and Pescadero Basin and volcanic seamounts nearby. Researchers will map, explore, and sample lavas, sediments, and hydrothermal deposits and vent fluids. Their objective is to understand the tectonic and volcanic processes of the ridges and the transition from spreading ridge to bounding transform faults.