Leg 7: April 24, 2012

Day 4: Big expectations

It has been beautiful, glassy calm, and sunny for the trip so far. Today a little wind riffle picked up and there is a slight swell. It is probably still dry, dry, dry on shore in the desert, but it is hot and humid out here on the water. The air conditioning on the ship is barely keeping up—it is 30 degrees Celsius (85 degrees Fahrenheit) in the lab from where I write. However, these are perfect conditions for diving, and we continue to have successful remotely operated vehicle (ROV) operations more than 2,300 meters (7,500 feet) below our little ship.

There is some haze obscuring the southern tip of Baja, almost 100 kilometers (62 miles) off to the west, throughout the day until the sun sets behind the peninsula and the mountains pop into view. Smoke from a fire on shore adds some color to the sky.

There is some haze obscuring the southern tip of Baja, almost 100 kilometers (62 miles) off to the west, throughout the day until the sun sets behind the peninsula and the mountains pop into view. Smoke from a fire on shore adds some color to the sky.

The active hydrothermal vents and young, glassy lava flows we found during our April 22 dive are on the shallowest and broadest area of the Alarcón Ridge neovolcanic zone. The dive sites yesterday and today are to the south and north of that, respectively, where the axis deepens and narrows. Scientists have determined elsewhere on the ridge system that if the magma chamber is under only part of a ridge segment, heat and volcanic activity will be focused above it and a bulge will develop. On the deeper parts of the ridge there will be less heat and reduced volcanic activity. We can see in the maps from our autonomous underwater vehicle (AUV) that Alarcón’s broad inflated area is paved with extensive, unfractured flows, and to the north and south of the inflated zone the flows tend to be smaller and some of them are highly fractured. The fracturing is due to the ridge continuing to spread since the individual flows erupted, and the extent of fracturing suggests the relative amount of time that has passed (see yesterday’s log).

So our expectation was that the flows we explored yesterday and today should be, on average, older than those we explored April 22, and we found plenty of confirming evidence. In addition to large fractures in the flows, the evidence included hydrothermal chimneys that are no longer actively venting, deep accumulations of pelagic sediment, and steep mounds of pasty pillow lavas rather than vigorous, fluid sheet flows.

—Jenny Paduan

Map of the Alarcon Rise spreading ridge, showing its full extent from the southern to the northern transform faults (through the center of the map from the lower left to upper right). The starting points of our ROV dives on the ridge are the labeled dots. ROV dive D392 was on the inflated bulge of the broadest, most volcanically active part of the ridge and the start depth for the dive was 2,297 meters (7,536 feet). The start depth for D393 was 2,379 meters (7,805 feet), and D394 was 2,373 meters (7,785 feet). Bathymetry data after Castillo et al. (2002).

Map of the Alarcon Rise spreading ridge, showing its full extent from the southern to the northern transform faults (through the center of the map from the lower left to upper right). The starting points of our ROV dives on the ridge are the labeled dots. ROV dive D392 was on the inflated bulge of the broadest, most volcanically active part of the ridge and the start depth for the dive was 2,297 meters (7,536 feet). The start depth for D393 was 2,379 meters (7,805 feet), and D394 was 2,373 meters (7,785 feet). Bathymetry data after Castillo et al. (2002).

We have started the day with big expectations; there’s a possibility to find some hydrothermal vents and chimneys, maybe not as impressive as the ones we found on Sunday, but finding these features on the seafloor is always a reason to be excited. Our submersible seems like a spy, revealing the secrets of the deepest part of the oceans. Our first image of the seafloor is a cracked pillow lava field with fine sediments on top. Like someone getting used to a new environment, the submersible looks around and then starts its amazing journey along the ocean ridge. So come with us and be part of this incredible world.

Cracked pillow lava flow.

Cracked pillow lava flow.

As we fly above the ocean ridge we encounter some corals and crustaceans. It is time to take a sample from the lava lying on the floor so the submersible slowly lands on top of the lava. Like a person waking up in the morning and stretching his arms, the submersible stretches his mechanical arm for the first time in the day and very gently gets a fragment of lava and deposits the sample in a box container. Isn’t this exciting? In a few moments we are going to have in our hands a piece of new seafloor that not long ago was magma coming from the earth’s interior!

Lava sample taken by submersible.

Lava sample taken by submersible.


White coral (Chrysogorgia sp.) and a small crustacean (galatheid crab) behind the coral on the right side of the image.

White coral (Chrysogorgia sp.) and a small crustacean (galatheid crab) behind the coral on the right side of the image.

The submersible continues its journey and we see some strange-looking worms—more than 30 centimeters (12 inches) long—living in the sediment deposited on the top of the lava flows. Suddenly, we find some hydrothermal chimneys, but they appear to be inactive. Surrounding these regions we can see a reddish color that reveals the presence of minerals that precipitated when the chimneys were active. As the day passes, the submersible continues its journey above the seafloor, and we see white corals, crabs, sea cucumbers, seastars, fissures in the seafloor, lava flows, fault scarps, and so many other features and living organisms that it is not possible to mention all of them right now. Clearly the seafloor near the mid-ocean ridges is one of the most dynamic places on our planet’s surface, where only special and unique organisms can survive, and where hydrothermal vent fields are like an oasis in the desert, full of life!

—Rigoberto Guardado

Inactive hydrothermal chimney.

Inactive hydrothermal chimney.


Reddish minerals precipitated when the hydrothermal chimney was active.

Reddish minerals precipitated when the hydrothermal chimney was active.

MBARI's mapping AUV, the

Equipment

Gulf of California 2012 Expedition