Dive and Discover : Expeditions to the Seafloor

Exploring for Mid-Ocean Ridge Eruptions
Join us for a 47-day cruise aboard the Research Vessel Melville during Dive & Discover’s Expedition 3—as scientists probe the dark depths of the eastern Pacific Ocean, looking for new volcanic eruptions on the crest of the mid-ocean ridge

The mid-ocean ridge is the Earth’s greatest mountain range. It is made of many active underwater volcanoes, where erupting lava cools when it hits cold seawater and solidifies to create new seafloor.

Seafloor lava flows have strange and wonderful shapes. Some look like they have been squeezed out of a tube of toothpaste to form long cylinders with deep grooves in them. Others form big, cracked pillows of rock that look like overstuffed sofas. Some lava flows look like ropy swirls of black taffy.

How did these lava flows form? When did they erupt? How do they build up to form the ocean crust of the Earth? How are hydrothermal vents, like the ones studied during Dive and Discover’s first two cruises, related to these undersea eruptions and lava flows? [See Expedition 1 or Expedition 2]

To answer these questions, scientists first have to find where underwater volcanoes are actively erupting. But in the vast, dark, deep ocean, how do you catch a submerged volcano in the act? Scientists have come up with an exciting new way that may work: they have set up an experimental array of hydrophones to monitor thousands of kilometers of the mid-ocean ridge and “listen” for and locate sound waves generated by underwater volcanic eruptions.

Over the past three years, data collected from the hydrophones have given us intriguing evidence that seafloor eruptions have taken place at four locations along the East Pacific Rise

, the mid-ocean ridge in the Pacific Ocean. This expedition will investigate whether eruptions have indeed occurred at those four sites and prove whether the hydrophones are reliable “ears” in the ocean.

A team of geologists and geophysicists from six institutions and government laboratories -- led by Dan Fornari, Mike Perfit and Maya Tolstoy -- will survey the target sites, between the Equator and about 10oN, aboard Scripps Institution of Oceanography’s RV Melville. Using the DSL-120 sonar and Argo II imaging systems, they will make detailed sonar and photographic maps of the seafloor. They will also use dredges, rock corers and CTD (conductivity, temperature, depth) instruments to sample lava flows and seawater, and sample hydrothermal vents that may have been created when the eruptions occurred.

Objectives
The primary objective of Expedition 3 is to study places on the seafloor where volcanic eruptions have occurred very recently. These underwater eruptions are one of the most fundamental processes that shape the Earth and life on it. They are the source of lava that creates new seafloor and of chemicals that provide energy for exotic life in the deep. To really understand how our planet works, we must understand where volcanic eruptions take place, how long they last, and how they affect the seafloor, seawater, and deep-sea organisms.

Scientists know that volcanic eruptions are happening all the time at different places on the mid-ocean ridge axis. But to study the new lava flows, we first have to find them. That is very difficult to do, because the eruptions occur in the dark depths of the vast ocean, where scientists have had no eyes or ears. Until now that is!

Our colleagues at the National Oceanic and Atmospheric Administration’s (NOAA) Hatfield Marine Center, in Newport, Oregon have developed an exciting new way to “listen” for sound waves generated by volcanic eruptions over thousands of kilometers of the mid-ocean ridge axis. It is called the Autonomous Hydrophone Array (or AHA), and consists of six hydrophones spaced many hundreds of kilometers apart. The hydrophones are placed in a special part of the upper ocean between 600 m and 1200 m depth, called the SOFAR (SOund Fixing And Ranging) channel. This channel acts almost like a pipeline for sounds in the ocean. Sound waves travel straight through the SOFAR channel without dispersing into ocean layers above and below. The sound waves can travel thousands of miles and still be detected. Since the mid-1940s when the SOFAR channel was discovered, scientists, engineers and the military from different countries have used this underwater pipeline to listen for submarines, whales, and to study how sound travels in the ocean. Now ocean scientists are using it to study volcanic processes on the mid-ocean ridge crest.

Hydrophones record sound waves that are generated by the cracking of the ocean crust as molten rock, or magma, travels to the seafloor within the ocean crust, or as the lava erupts on the seafloor. In 1996, six hydrophones were put out in the eastern Pacific Ocean to listen for volcanic eruptions between about 20°N latitude and 300°S latitude, a range of over 3000 miles (5400 kilometers). Every six months, NOAA scientists and technicians have been going out on ships to pick up the hydrophones, collect the data that they had recorded, put new batteries in the recorders, and put the hydrophones back in place so they can continue to monitor the ridge crest for eruptions.

Scientists in our research group have now analyzed the hydrophone data collected over the past three years and have located four sites on the East Pacific Rise axis between the Equator and 10oN latitude where we think seafloor lava eruptions have taken place. Will the AHA prove to be reliable “ears,” or are they giving us misleading signals? Answering this question is the primary goal of our expedition.

At each of the four target locations, we will use several different survey methods and types of sampling equipment to determine if recent volcanic eruptions have occurred and to see what effects those eruptions may have had on the seafloor, seawater, and seafloor life.