Printed from “The Discovery of Hydrothermal Vents - 25th Anniversary
CD-ROM” ©2002 Woods Hole Oceanographic Institution
MAJOR DISCOVERIES: New Understanding of Earth
Solving the mystery of an unchanging sea
People often say the ocean is eternal and unchanging—and, chemically speaking, that’s true. The chemical composition of seawater—that is, the kinds and relative amounts of chemical elements in it—hasn’t changed very much over millions of years. Scientists had figured this out, but they couldn’t figure out how this was possible. Every year, the world’s rivers dump millions of tons of elements dissolved from rocks and sediments into the oceans. How could the ocean’s chemistry stay the same?
The discovery of hydrothermal vents provided a way—which had never been thought of before—to remove some of these excess elements from the ocean. Hydrothermal vents also added some elements to the ocean.
Samples from the first vents discovered in 1977 showed that fluids coming out of the vents were very different from seawater. MIT’s John Edmond explained what happens: Seawater percolating into seafloor cracks is heated up by underlying magma or hot rock. This launches chemical reactions between the hot seawater and volcanic rocks in the ocean crust. The seawater gives up certain elements and takes in other elements from the rocks. After these exchanges, seawater is no longer seawater. It has chemically changed into hydrothermal fluid.
Edmond and colleagues showed, for example, that the vents remove elements such as magnesium and sulfur from seawater (which are put there by rivers). These elements get incorporated into seafloor crust. At the same time, the vents add to seawater some elements leached out of seafloor rocks.
Scientists estimate that the entire volume of the world’s oceans percolates through mid-ocean ridge hydrothermal vent systems every 10 million years or so. Hydrothermal circulation at mid-ocean ridges draws in seawater, rearranges the seawater’s chemical composition, and spews out chemically different fluids. The vents act as great chemical reactors that help regulate Earth's ocean chemistry.
Building
Earth’s great ore deposits
Besides being great chemical reactors for our planet, hydrothermal systems on mid-ocean ridges are also great furnaces that produce many mineral deposits on Earth.
The basic process is the same one that produces seafloor black smoker chimneys. Beneath the seafloor, chemical reactions between hot hydrothermal fluids leach metals (such as iron, zinc, copper, lead, and cobalt) out of ocean crust rocks. These metals are concentrated in hydrothermal fluids and carried up to the seafloor. There, they encounter near-freezing seawater that is full of oxygen. This sparks new chemical reactions. The metals come out of solution, or precipitate, forming hydrothermal deposits that are enriched with metals.
Hydrothermal venting produced multi-colored, mineral-rich sediments in the
Red Sea that first intrigued scientists in the mid-1960s, and the curious
mineral-speckled Galápagos mounds discovered by the 1972 Southtow
expedition.
Since then, scientists have found that seafloor mineral deposits come in many varieties. In 1984 and 1985, German scientists aboard the research vessel Sonne unexpectedly discovered a hydrothermal deposit near the Galápagos quite unlike anything seen before. The site had irregular white heaps and towers that measured 1 to 2 meters (3 to 6 feet) in height and 0.5 to 1 meter (1.5 to 3 feet) in diameter. They were made also entirely of white silica and resembled gigantic cauliflowers. The site was called “Cauliflower Garden.”
In 1980 and 1981, scientists found massive mineral deposits at a site near the Galápagos where hydrothermal venting has been occurring for a long time. Layers of rock that had once been under the seafloor were exposed, and scientists could see branching pathways of big and small cracks and faults—the “plumbing system” through which hydrothermal fluids flow to the surface.
In the mid-1980s, scientists discovered a massive mineral deposit on the Mid-Atlantic Ridge that they named the Trans-Atlantic Geotraverse (TAG) site. The iron- and copper-rich TAG site measures 150 meters (500 feet) in diameter and rises 50 meters (165 feet) high. It has an estimated 4 million tons of metal-rich rocks in it. The Ocean Drilling Project drilled 17 holes into the TAG mound to sample rocks deep inside the mound. They found that the mound was like a big layer cake-with different layers filled with different types of minerals.
The great copper mines of Cyprus were probably formed the same way that TAG was. We have discovered that some of the best mineral deposits on land are made at mid-ocean ridges by hydrothermal venting.