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Gulf of California 2015, Leg 5 – Volcanoes and Seamounts

A fluted vase sponge is attached to a thick lava flow that had been cracked by a later lava intrusion from underneath.

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.

During the 2012 GOC expedition, researchers used the AUV to map the axis of the Alarcón Rise from the Pescadero to Tamayo transform faults at each end, and the ROV to explore and sample features identified in the maps. They discovered two active hydrothermal vent fields, enormous and relatively recent lava flows, abundant faulting, and an unusual, rugged volcanic dome. This dome is made of rhyolite, which is a lava type that had not been found on the seafloor previously but is common in volcanoes on land, particularly explosive arc volcanoes. Why it has formed on a spreading ridge where basalts are usually erupted, and whether similar eruptions might pose hazards by being so close to populated coastline, are among the questions the researchers will address during this dive series.

(left) Tectonic map of the Gulf of California showing the locations of Alarcón Rise, Pescadero Basin, and La Paz, Mexico. The northwestward movement of the Pacific Plate, on which the Baja Peninsula rides, relative to the North American Plate is responsible for the spreading of the East Pacific Rise mid-ocean ridge system, the Alarcón Rise, and the smaller spreading centers up in the gulf, and also for the strike-slip motion in the transform faults between spreading centers and eventually of the San Andreas Fault, which begins at the NW end of the gulf.
(right) Hydrothermal chimney in the Ja Sít vent field discovered on the Alarcón Rise in 2012. This image mosaic from ROV Doc Ricketts video shows the upper half of a 13-meter tall black smoker, and the colorful community of chemosynthetic bacteria, tube worms, crabs and fish supported by the >300º C, mineral-rich vent fluids.

ROV dives are planned on the rhyolite dome and other sites along the Alarcón Rise, at a nearby volcanic seamount with a large caldera, and in the Pescadero Basin to the east of La Paz, which formed by seafloor spreading but has never been explored. Lavas and hydrothermal fluids will be sampled for chemical analysis and sediments will be cored to analyze the deposits and to date the underlying flows.

This work will enable the researchers to learn how big the eruptions are and how often they occur, and to make a geologic map of the ridge, like is done routinely on land. These investigations will contribute to our general understanding of the mid-ocean spreading system in the Earth’s crust, and are especially important here because this area, close to shore yet not easily accessible and hidden in the deep sea, is geologically very active.

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.