Ride down the highway with a geologist, and you will never look at roadcuts in the same way. These dynamited cliffs along roadsides expose rocks that had previously been inaccessible. Geologists studying these road cuts can decipher the processes that shaped these rocks and the surrounding landscape. Marine geologist Kathy Gillis studies the seafloor equivalent of road cuts. Faults along the seafloor at the mid-ocean ridges have ripped open the ocean crust, creating rifts in the seafloor that are as deep as six kilometers. For Kathy, the exposed ocean crust provides a window into the processes that take place under the seafloor.
Kathy in Oman - writing notes as she is mapping the geology. Oman has an ophiolite, a sequence of rocks that is thought to be similar to the oceanic crust.
The ocean crust is born at the mid-ocean ridges. There the earth’s tectonic plates spread apart and volcanic eruptions bring magma to the surface. Like a giant conveyor belt, the plates then carry the newly formed ocean crust away from the ridges until it is subducted beneath another plate. During this 100 million-year journey, the crust undergoes many changes. One of the chief culprits is seawater that circulates beneath the seafloor. This water triggers chemical reactions and deposits and removes chemicals and minerals from the crust.
The greatest changes to the crust take place in the beginning of the journey at the mid-ocean ridges. Magma heats up the water that has seeped down into the crust to as high as 750°C. The extreme heat triggers many chemical reactions. Metals and other elements leach from the crust. The scalding seawater then rises back up through the crust and spews forth from the seafloor at hydrothermal vents, carrying with it the metals leached from the crust. These metals form ore deposits on the surface of seafloor.
Kathy is particularly interested in the paths that the water takes beneath the seafloor and how it interacts with the crust. She is finding the answers in the outcrops and rifts on the seafloor.
Unfortunately, these outcrops and rifts are much harder to get to than road cuts. Kathy has dived in research submarines such as Alvin to observe and photograph these bluffs and to collect samples. She has examined cores from the Ocean Drilling Program. She also studies formations of ocean crust that are now on land. Although these structures, called ophiolites, differ somewhat from typical ocean crust, it is possible to visit them again and again.
Back in the lab, Kathy dissects the rock samples. Based on the minerals that have formed and the presence or absence of certain metals, she can determine the amount of seawater that was within the crust and the temperature of that water. She also examines tiny capsules of fluid trapped in the minerals to see how these fluids changed as they reacted with the crust.
Photomicrograph of a thin section of a rock from the lower part of the oceanic crust.
On a recent research cruise, Kathy dove on the walls of the Hess Deep; a rift that is five kilometers deep. There she observed half-a-million years of history. There were solidified magma chambers, sheeted dikes, which are the feeders that bring magma to the seafloor, ancient conduits where hot fluids flowed, and crust that was between half-a-million and a million years old. Some of the preliminary results were surprising. It had long been assumed that most of the reactions between water and crust took place deep in the ocean crust close to the magma chambers. However, Kathy and her colleagues have found evidence that sheets of lava (dikes) moving up towards the seafloor may have heated the water enough to trigger many of these reactions close to the surface. In fact, Kathy is working on a hypothesis that water may even enter the hot magma chamber. If true, her next challenge would be to figure out how this affects the formation of ore deposits on the seafloor.
- Associate Professor, Earth and Ocean Sciences
- University of Victoria
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