Monday, 1 February 2016

The Discovery of Sea-Floor Spreading

The Discovery of Sea-Floor Spreading

New Images of Sea-Floor Bathymetry 

Bathymetry of mid-ocean ridges and abyssal plains.
Military needs during World War II gave a boost to sea-floor exploration, for as submarine fleets grew, navies required detailed information about bathymetry, or depth variations. The invention of echo sounding (sonar) permitted such information to be gathered quickly. Echo sounding works on the same principle that a bat uses to navigate and find insects. A sound pulse emitted from a ship travels down through the water, bounces off the sea floor, and returns up as an echo through the water to a receiver on the ship. Since sound waves travel at a known velocity, the time between the sound emission and the echo detection indicates the distance between the ship and the sea floor. (Recall that  velocity distance/time, so distance velocity s time.) As the ship travels, observers can obtain a continuous record of the depth of the sea floor. The resulting cross section showing depth plotted against location is called a bathymetric profile (figure above a, b). By cruising back and forth across the ocean many times, investigators obtained a series of bathymetric profiles and from these constructed maps of the sea floor. (Geologists can now produce such maps much more rapidly using satellite data.) Bathymetric maps reveal several important features.

Other bathymetric features of the ocean floor.
  • Mid-ocean ridges: The floor beneath all major oceans includes abyssal plains, which are broad, relatively flat regions of the ocean that lie at a depth of about 4 to 5 km below sea level; and mid-ocean ridges, submarine mountain ranges whose peaks lie only about 2 to 2.5 km below sea level (figure above a). Geologists call the crest of the mid-ocean ridge the ridge axis. All mid-ocean ridges are roughly symmetrical bathymetry on one side of the axis is nearly a mirror image of bathymetry on the other side. 
  • Deep-ocean trenches: Along much of the perimeter of the Pacific Ocean, and in a few other localities as well, the ocean floor reaches depths of 8 to 12 km deep enough to swallow Mt. Everest. These deep areas occur in elongate troughs that are now referred to as trenches (figure above b). Trenches border volcanic arcs, curving chains of active volcanoes. 
  • Seamount chains: Numerous volcanic islands poke up from the ocean floor: for example, the Hawaiian Islands lie in the middle of the Pacific. In addition to islands that rise above sea level, sonar has detected many seamounts (isolated submarine mountains), which were once volcanoes but no longer erupt. Volcanic islands and seamounts typically occur in chains, but in contrast to the volcanic arcs that border deep ocean trenches, only one island at the end of a seamount and island chain remains capable of erupting volcanically today. 
  • Fracture zones: Surveys reveal that the ocean floor is diced up by narrow bands of vertical cracks and broken-up rock. These fracture zones lie roughly at right angles to mid-ocean ridges. The ridge axis typically steps sideways when it intersects with a fracture zone.

New Observations on the Nature of Oceanic Crust 

By the mid-20th century, geologists had discovered many important characteristics of the sea-floor crust. These discoveries led them to realize that oceanic crust differs from continental crust, and that bathymetric features of the ocean floor provide clues to the origin of the crust. Specifically: 
  • A layer of sediment composed of clay and the tiny shells of dead plankton covers much of the ocean floor. This layer becomes progressively thicker away from the mid-ocean ridge axis. But even at its thickest, the sediment layer is too thin to have been accumulating for the entirety of Earth history. 
  • By dredging up samples, geologists learned that oceanic crust is fundamentally different in composition from continental crust. Beneath its sediment cover, oceanic crust bedrock consists primarily of basalt it does not display the great variety of rock types found on continents. 
  • Heat flow, the rate at which heat rises from the Earth’s interior up through the crust, is not the same everywhere in the oceans. Rather, more heat rises beneath mid-ocean ridges than elsewhere. This observation led researchers to speculate that hot magma might be rising into the crust just below the mid-ocean ridge axis. 
  • When maps showing the distribution of earthquakes in oceanic regions became available in the years after World War II, it became clear that earthquakes do not occur randomly, but rather define distinct belts (figure below). Some belts follow trenches, some follow mid-ocean ridge axes, and others lie along portions of fracture zones. Since earthquakes define locations where rocks break and move, geologists realized that these bathymetric features are places where motion is taking place.
A 1953 map showing the distribution of earthquake locations in the ocean basins. Note that earthquakes occur in belts.

Harry Hess and His “Essay in Geopoetry” 

Harry Hess's basic concept of sea-floor spreading. Hess implied, incorrectly, that only the crust moved. We will see that this sketch is an oversimplified.
In the late 1950s, Harry Hess, after studying the observations described above, realized that because the sediment layer on the ocean floor was thin overall, the ocean floor might be much younger than the continents. Also, because the sediment thickened progressively away from mid-ocean ridges, the ridges themselves likely were younger than the deeper parts of the ocean floor. If this was so, then somehow new ocean floor must be forming at the ridges, and thus an ocean basin could be getting wider with time. But how? The association of earthquakes with mid-ocean ridges suggested to him that the sea floor was cracking and splitting apart at the ridge. The discovery of high heat flow along mid-ocean ridge axes provided the final piece of the puzzle, for it suggested the presence of very hot molten rock beneath the ridges. In 1960, Hess suggested that indeed molten rock (basaltic magma) rose upward beneath mid-ocean ridges and that this material solidified to form oceanic crust basalt (figure above). The new sea floor then moved away from the ridge, a process we now call sea-floor spreading. Hess realized that old ocean floor must be consumed somewhere, or the Earth would have to be expanding, so he suggested that deep ocean trenches might be places where the sea floor sank back into the mantle. Hess suggested that earthquakes at trenches were  evidence of this movement, but he didn’t understand how the movement took place. Other geologists, such as Robert Dietz, were coming to similar conclusions at about the same time.
Hess and his contemporaries realized that the sea-floor-spreading hypothesis instantly provided the long sought explanation of how continental “drift” occurs. Continents passively move apart as the sea floor between them spreads at mid-ocean ridges, and they passively move together as the sea floor between them sinks back into the mantle at trenches. (As we will see later, geologists now realize that it is the lithosphere that moves, not just the crust.) Thus, sea-floor spreading proved to be an important step on the route to plate tectonics the idea seemed so good that Hess referred to his description of it as “an essay in geopoetry.” But first, the idea needed to be tested, and other key discoveries would have to take place before the whole theory of plate tectonics could come together.