Leg 1 Overview
Galapagos Rift: The Hotspot Paradox
Scientists were stunned by the discovery of hydrothermal venting and strange biology on the Galápagos Rift in 1977. Since then, explorations on midocean ridges and volcanic arcs have found similar hot springs throughout the oceans. Ridges and volcanoes are where new ocean crust is built by molten lava, erupting onto the seafloor from deep in the Earth. Scientists have learned that vents are most common wherever the supply of lava—and thus heat—is most abundant.
The Galápagos Rift is also a rare example of a ridge that overlies a mantle “hotspot.” Hotspots are locations where greatly increased lava output lasts millions of years. More than 40 hotspots are known, including those that are building Hawaii, Iceland, and the Galápagos Islands, but only a few lie beneath a ridge (Figure 1). Our first hydrothermal survey of the Galápagos Rift, in 2005-2006, tested the hypothesis that the increased supply of lava there would create an abundance of active vent sites. Paradoxically, we found that vents were not abundant but unusually scarce. Because the 2005-2006 survey covered only the middle third of the ridge, we will explore the eastern third during the GALREX expedition to further test this hypothesis (Figure 2).
Imaging the Seafloor
Testing our hypothesis requires an appropriate search strategy and a basic set of tools. Our first requirement is a detailed map of the seafloor. The Okeanos Explorer’s high-resolution multibeam is particularly well suited for exploration of the Galápagos Rift. The ship’s Kongsberg EM302 system is designed to operate in water depths up to 7000 meters, deep enough to cover most of the world’s seafloor. The water depths over the spreading center are 1750 to 2400 meters. In this depth range, the EM302 system obtains a single depth for every 15 x 30 meter section of the seafloor. Although this may seem like a large area, it is far smaller than previous multibeam systems could achieve.
Our maps thus provide a more realistic image of the seafloor than has ever been seen before. Can we see individual chimneys or other individual hydrothermal vents? No, not likely even with the EM302. However, the EM302 does create high-resolution maps of geological features—such as faults and small volcanoes—where we are most likely to find vents.
Perhaps the most important feature is the shallow axial valley that marks the actual spreading axis of the rift (Figure 6). Lava eruptions always occur near a ridge axis, and so do vent fields. On Leg 1, the Okeanos Explorer made the first high-resolution map of the western third of the Rift, and collected much-improved bathymetry of the eastern third (Figure 2). With these new maps we will be able to plan for future work to complete our hydrothermal exploration of this remarkable ridge.
Tracking Down the Vents
Seafloor hot springs are a submarine version of Yellowstone National Park. Cold seawater in deep cracks is heated by hot lava, becomes buoyant, and rises to the seafloor. Instead of forming a warm pond, as at Yellowstone, the hot fluid rises like campfire smoke into the cold ocean. Our tools are specialized sensors on a standard Conductivity-Temperature-Depth (CTD) package. The sensors detect hydrothermal particles (“smoke”), chemicals, and heat. We “tow-yo” the CTD package along the ridge axis to construct a 2D map of the hydrothermal tracers (Figure 5).
The tow-yos conducted by the Okeanos Explorer during Leg 1 were some of the longest continuous tows ever conducted for hydrothermal vent detection. From the afternoon of June 23rd to the evening of June 29th, 24-hour tow-yo operations occurred over a distance of 408 kilometers (220 nautical miles). Figure 5 shows a small section of one of our tows. Near 88.56°E, the CTD began to sense an increased concentration of hydrothermal “smoke” in the water. Within about 10 km, the thickness of the plumes had more than doubled. This newly discovered area of hydrothermal plumes is a primary target of ROV operations during Leg 2.
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