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Descent to the Mid-Atlantic Ridge

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Arcturus
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« on: July 31, 2008, 01:15:50 am »

Descent to the Mid-Atlantic Ridge



Reporting from the decks of the research ship Atlantis, from the middle of the Atlantic Ocean, join an international team of scientists as they spend almost four weeks exploring an unusual mountain called the Atlantis Massif, which is part of the extensive Mid-Atlantic Ridge. The Mid-Atlantic Ridge is one of the earth's largest undersea mountain ranges at a length of nearly 10,000 km.
 

The Earth's crust continuously forms along a network of oceanic spreading centers that circle the globe. As new seafloor forms, the earth's tectonic plates move apart in opposite directions at these spreading centers. As the tectonic plates move apart, rock is pulled up from depth at the spreading axis and melts as it depressurizes. The molten rock rises to the seafloor and cools to form the layer of crust that paves the ocean floor.

Fig. 5. Seafloor spreading at the Mid-Atlantic Ridge. Basemap adapted from image on Structural Geology and Tectonics website at Duke University, http://www.geo.duke.edu/Research/struct/MAR.html.

     Although this process occurs steadily over geologic time (tens or hundreds of thousands of years), on a day-to-day basis the formation of oceanic crust happens in fits and starts - small earthquakes occur as the plates break while pulling apart; some blocks of rocks are pushed up and some drop down; magma is squeezed through the fractures in the rocks and may extrude on the seafloor to form small volcanic cones; and time may pass with very little noticeable change in the landscape.
     The spreading centers themselves form broad central ridges that sink steadily with age and distance, starting at crestal heights of about 2,500 m water depth and sinking to typical open ocean depths of 4,000 meters or 12,000 feet. This unevenness of spreading and plate creation usually produces volcanic ridges and fault-bounded hills that rise up to several hundred feet above the surrounding seafloor, called "abyssal hills". These 'abyssal hills' can be thought of as the texture of the young seafloor, as wrinkles atop the broad ridge of oceanic spreading centers.
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Arcturus
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« Reply #1 on: July 31, 2008, 01:16:59 am »



Fig.6. A topographic section of Mid-Atlantic Ridge. The high flanks on both sides of the rift valley are shown in red. The highest areas are red and the lowest areas are blue. The rift valley is a linear low-lying area between the flanks of the ridge. The large features are covered with smaller-scale features that might be considered "texture". These are the abyssal hills. Image from Kenneth C. Macdonald and Paul J. Fox, University of California, Santa Barbara.
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Arcturus
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« Reply #2 on: July 31, 2008, 01:18:27 am »



The unusual thing about the area under study is a large mountain, called the Atlantis Massif, just west of the Mid-Atlantic spreading center at 30°N. The peak of the mountain is 1,700 m (5,000') higher than the usual spreading ridge crest. The width of the mountain is 4-6 times greater than that of most abyssal hills. It is clear that this mountain is a new addition to Earth's crust since it is part of very young and newly created seafloor. The mission is to find out why and how it formed. What forces are responsible for the great height to which rock as been uplifted at this site? What caused a change in the usual style of oceanic crustal formation? When might this area return to its normal state? These are the many questions the scientists seek to answer.

Figure 7. Topography of the area around the Atlantis Massif. Red indicates shallower areas than blue. Do you see that the massif is both smoother than the surrounding area and that it has east-west trending corrugations like the surface of a rippled potato chip?

     In 1996 Donna Blackman and Joe Cann, who joins this expedition from University of Leeds in the U.K., mapped the Atlantis Massif and recognized that its structure was unusual. Prior to this discovery, geologists had speculated that the way in which mountains form in the southwestern U.S., in an area called the Basin and Range province, could also take place near oceanic spreading centers. The scientists expected this because both settings are places where tectonic plates are being stretched apart - on the continent it just happens more slowly, the crust is thicker, and there's no seawater. Since their discovery in 1996, a number of similar undersea massifs have been charted in other places on the seafloor. Scientists think their formation is an occasional result of the overall plate spreading and creation process. Now the goal is to find out why.

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Arcturus
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« Reply #3 on: July 31, 2008, 01:19:32 am »

Theory 1
     As the two plates move apart at a spreading center, a fault cracks through the crust near the axis. Normally magma would fill the crack and the adjacent plates would inch away by just that amount. If for some reason the magma supply stops for a period of time the crust must stretch to match the plate motion. If the crack is not vertical, it almost never is, the lower part of the crust can be pulled sideways out from under the upper layer along a dipping fault and exposing deeper rocks at the seafloor. Once the load of the upper crust is removed from the lower crust, the balance of forces that act on the plate causes uplift of the high mountain.

Are the corrugations on the top of the mountain grooves or scrape marks that form as the top layer breaks away from the lower layer and slides along the fault? If so, we have a record of how long this has been going on and some information about how rock deform in these undersea conditions. This explanation of the corrugations is the most widely accepted hypothesis at present although unproven. Evidence from rocks and faults in the Basin and Range suggests that it could be correct. Barbara John from the University of Wyoming has worked extensively on settings on land and she will bring her knowledge of the mountain structures to the expedition.

Are the corrugations just smaller fault blocks that jostle within the mountain range like uneven dominoes as the uplift occurs over time? If so, the way we interpret the massif structure is quite different. Jeff Karson has found evidence to suggest that this may be the case from part of a similar massif further to the south in the Atlantic.
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Arcturus
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« Reply #4 on: July 31, 2008, 01:20:58 am »

Theory 2
     As faults form in new crust seawater can flow into the cracks. If the cracks extend deep enough, the seawater can come into contact with mantle rocks that underlie the crust. The minerals (mainly olivine) in this kind of rock interact with seawater to form a new mineral (serpentine) that swells in size. This new, less dense material is lighter than the surrounding rock, which has not been altered by seawater, so it wants to rise towards the seafloor. This upward push may help create the high massif.

     Deborah Kelley has studied the chemical interactions between seawater and oceanic rocks and she will use new evidence collected on the expedition to determine if this theory can be proven correct (or incorrect!). Joe Cann has also studied rock samples from undersea mountains elsewhere in the Atlantic where it appeared that this rising, less dense serpentine might be pushing up the seafloor. Comparison between his previous findings and the new observations will help illustrate what is really going on.
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The expedition begins…
      At the expedition's height, the decks of R/V Atlantis will become a separate study in group dynamics as scientists, technicians and the ship's crew work in shifts around the clock to launch, operate and retrieving the various submersible vehicles; sort and study retrieved specimens; monitor scientific instruments and video screens; log data and transcribe their taped dictations during Alvin dives; strategize for the next days' activities; and somehow manage to find the time to eat and sleep. Join us on this journey of exploration to the frontiers of the seafloor and dive alongside scientists to restless undersea mountains of the Mid-Atlantic Ridge.

Check out the first transmissions from the ship on this website November 14'th!
 

 
http://earthguide.ucsd.edu/mar/intro2.html#anchor6
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Arcturus
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« Reply #5 on: July 31, 2008, 01:22:48 am »

November 10'th, 2000

Join Scripps researcher, Dr. Donna Blackman and the entire scientific team, as they guide the deep submersible Alvin to probe the depths of the North Atlantic - exploring undersea mountains of the Mid-Atlantic Ridge.

Drop-in twice a week to check out journals from the ship, posted Tuesday and Thursday between November 14 - December 14, 2000. An exception will be made for Thanksgiving day. Be part of our latest Earthguide Adventure!




Fig. 1. Dr. Donna Blackman. Join her and fellow chief scientists Dr. Jeff Karson and Dr. Deborah Kelley as they guide the expedition team.
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Arcturus
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« Reply #6 on: July 31, 2008, 01:24:12 am »



Reporting from the decks of the research ship Atlantis, from the middle of the Atlantic Ocean, we invite you to join an international team of scientists as they spend almost four weeks exploring an unusual mountain called the Atlantis Massif, which is part of the extensive Mid-Atlantic Ridge. The Mid-Atlantic Ridge is one of the earth's largest undersea mountain ranges at a length of nearly 10,000 km.

Fig. 2. Where we're going - 30 degrees north on the Mid-Atlantic Ridge.
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Arcturus
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« Reply #7 on: July 31, 2008, 01:25:43 am »




The team will use several high-tech tools to study the smooth and corrugated top of the mountain as well as its steep slopes where landslides and faults expose rocks that are usually hidden deep beneath the seafloor. The scientists plan to investigate the processes that might have brought these deep rocks to the surface in the form of an unusually large and elevated undersea mountain.
     For two weeks on site, the scientists will dive to the surface of the Atlantis Massif in the deep submersible Alvin to observe and chart its structure and collect rock samples. During several of these dives, the scientists will deploy an instrument called a gravity meter to measure very tiny changes in the pull of gravity that hint at the nature of buried faults.
     In addition, high performance sonar and video instruments will be towed from the research ship Atlantis. The sonar instrument images the surface of the undersea mountain using sound waves, similar to the way a camera uses light waves to make a photograph. The video instrument package will film rocks, faults and small volcanic features using very powerful lights to illuminate the blackness of the deep ocean.

Fig. 3. Watch the scientists deploy investigative tools at sea.
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Arcturus
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« Reply #8 on: July 31, 2008, 01:27:13 am »



The scientific expedition team
      Scientists from eight different universities will participate on this research cruise aboard the R/V Atlantis, one of several research ships in the U.S. academic fleet. Investigators from Scripps Institution of Oceanography, including chief scientist Donna Blackman, will bring expertise in geophysical mapping and gravity measurements. Colleagues from Duke University will be led by co-investigator Jeff Karson, a structural geologist. Deborah Kelley from the University of Washington, a geochemist, is the final member of the 3-person lead group for the project.

The expedition begins
      At the expedition's height, the decks of R/V Atlantis will become a separate study in group dynamics as scientists, technicians and the ship's crew work in shifts around the clock to launch, operate and retrieve the various submersible vehicles; sort and study retrieved specimens; monitor scientific instruments and video screens; log data and transcribe their taped dictations during Alvin dives; strategize for the next days' activities; and somehow manage to find the time to eat and sleep!

Fig. 4. Dive with the scientists aboard the deep submersible Alvin

     Join us on this journey of exploration to the frontiers of the seafloor and dive alongside scientists to restless undersea mountains of the Mid-Atlantic Ridge.

http://earthguide.ucsd.edu/mar/intro.html#anchor1
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