Geology of the Ocean and How The Seafloor Was Created

    Mid-oceanic ridges are the seams of the world. They are boundary lines between the earth's plates, the large
    chunks of lithosphere in which pieces of seafloor and continents are embedded.

    They are also the places where the plates are accreted as magma flows up to create new oceanic crust. How this
    process works, however, is not well understood. Does it require, for example, that a magma chamber be
    constantly present beneath the ridge? The answer to this kind of question will be important in understanding the
    dynamic evolution of the plates.

    In order to learn more about how oceanic crust is made, scientists used seismic reflection profiling to probe and
    map with sound waves the internal structure of oceanic crust over 110 million years old in the western North
    Atlantic. Their seismic lines, which had greater resolution and a longer extent than other data to date, crossed three
    fracture zones, or transform faults, that run from the American continent through the Mid-Atlantic Ridge and on to
    Africa and Europe.







    The researchers discovered that the structure of the oceanic crust changed gradually and systematically along this
    line. At the fracture zones themselves, they observed that the crust is thin, less than a third of its normal thickness,
    and that the seismic maps revealed little internal complexity. Moving away from a fracture zone toward the north
    or south, however, the crust becomes thicker and a new structure appears in the profile that the researchers have
    dubbed "horizon R." Crustal material is successively added beneath horizon R (which is always horizontal) as the
    midsection between fracture zones is approached.

    The horizon R is the reflection boundary between two types of rocks that form at a magma chamber: gabbros,
    coarse-grained igneous rocks that crystallize at the upper chamber walls, and cumulates, which settle out in layers
    at the bottom of the chamber. Between fracture zones, he argues, the presence of horizon R with its underlying
    layered cumulates implies that when this region was at the ridge, a large and persistent magma chamber once
    existed to create the thickest oceanic crust.

    Near fracture zones, the thin crust, lack of cumulates and absence of horizon R indicate little or no magma source.
    The magma supply might have become diminished and intermittent, he says, because it was chilled by the crust on
    the other side of the fracture zone, which researchers have found almost always to be of a different age and hence
    temperature.

    The seafloor has been likened to a giant tape recorder, because as it is churned out conveyor-belt-style at mid-
    oceanic ridges, it becomes imprinted with the earth's changing magnetic field. The resultant "magnetic stripes" that
    line the ocean floor enable scientists to reconstruct the past positions of the continents as they, and the plates upon
    which they sit, move around the globe.

    But these magnetic lines are not the only oceanic record of relative plate motions. Studies indicate that the motions
    are reflected on a much finer scale in the structure of fracture zones that cut across ocean basins, perpendicular to
    mid-oceanic ridges. While scientists have focused on one fracture zone, they suspect the structures of all fracture
    traces are very similar. What's more, the researchers think they see evidence in fracture zone structure for global
    changes in plate motions every few million years.

    Seismology Was Used To Study The Ocean Floor
    Using seismic reflection and bathymetry (seafloor depth) data, scientists examined the detailed structure of two
    700-kilometer-long segments situated on each side of the Mid-Atlantic Ridge. They located spots where the
    fracture zone had changed direction or where it had been blocked -- perhaps by the upwelling of molten rocks at
    times when changes in plate motion caused the crust near the ridge axis to be stretched out. It was found that the
    kinks and bends on the western segment correlated remarkably well with similar structural changes in the eastern
    limb.

    Researchers also compared a 450-km-long Kane segment straddling the ridge with similar segments in the Pacific
    and Indian oceans. There are plate motion changes recorded in all of these oceans at roughly the same times -- at
    about 4.5 million years, 2.5 million years and 1 million years, and the Kane fracture zone is a global geological
    response to pressures within the earth.























    The earliest portion of the Pacific plate dates back 170 million years to Earth's Jurassic period. This piece of
    seafloor, identified during a drilling expedition in the Pigafetta basin southeast of Japan, provided much information
    about the history of the world's oceans.

    During the Jurassic, a huge ocean stretched uninterrupted across most of the planet while the continents sat
    huddled to one side. Almost all of the seafloor from that majestic superocean has since disappeared into the Earth's
    interior through the process of subduction, and until now scientists have lacked any seafloor rocks from middle of
    that ocean.














    Scientists found that the seafloor had all been subducted, except for this part of the ocean. This was discovered by
    using a technique known as seismic reflection profiling which bounces acoustic waves off rock formations under
    the seafloor. By doing this scientists were able to penetrate volcanic layers and drill several hundred meters into the
    underlying Jurassic material.


















    The Jurassic rocks offered clues to the climate and biology of that era. The scientists say the sedimentary rocks
    contain abundant fossils from silica-shelled plankton but none from carbonate-shelled plankton. Since silica-shelled
    organisms withstand nutrient-poor conditions better than their carbonate counterparts, it told scientists that the
    Jurassic ocean had weak current systems that delivered only meager nutrient supplies from deeper waters.

    The Pacific plate today covers about one-quarter of the Earth's surface, but during the late Jurassic it was hardly
    bigger than the United States. The rest of the ocean floor consisted of other, unknown plates that have since
    disappeared as the Pacific plate grew.



















    Volcanic Eruptions Aided the Development of Ocean Life
    Despite their reputation as deadly scourges, giant volcanic eruptions may have profoundly benefited life on Earth
    by triggering two of the greatest biological revolutions in the planet's history.

    Some scientists believe massive outpourings of lava, particularly those in the deep ocean, spurred bursts of
    evolution early in the Paleozoic era, about 550 million years ago, and in the middle of the Mesozoic era, around 200
    million years ago, and that the biological changes took place while extensive undersea eruptions were creating new
    ocean basins and ripping apart oversized continents.

    Below scientists are witnessing and videotaping for the first time the eruption of an undersea volcano in the Pacific
    Ocean in the Northern Mariana Islands near Guam.

    The early Paleozoic revolution produced the first predators, burrowing animals, and creatures able to harvest
    minerals to form skeletons. Starting in late Precambrian time, this biological blast extended into the middle
    Ordovician period. The Mesozoic revolution ran from the late Triassic period through the late Cretaceous. It
    brought a proliferation of marine creatures, including important new types of plankton that could grow mineralized
    shells. On land, social insects and flowering plants began to conquer the continents.

    The coincidence between geologic and biological events suggests that the submarine eruptions loosened the
    restrictions on organisms living at the time by increasing the availability of energy and nutrients. Volcanic eruptions
    enhanced the supply of raw materials in the environment and provided organisms with easier access to such
    commodities.

    Undersea eruptions aided life on Earth by releasing massive amounts of carbon dioxide, which produced periods of
    greenhouse warming. In the warmer climates, biologically important chemical reactions could proceed much more
    readily, enabling organisms to boost their metabolisms. Warmer weather and increased amounts of carbon dioxide
    in the atmosphere also enhanced erosion on the continents, thereby freeing important nutrients from rocks.

    The volcanic activity altered the landscape as well. Eruptions raised sea levels by producing hot new ocean crust
    that sat higher than the old ocean floor. As the swollen oceans flooded continental coastlines, the area of shallow
    seas grew markedly. These regions, with their abundant nutrients and sunlit waters, would have provided fertile
    new habitats for ocean organisms.

    The question of what sparks biological revolutions has long captivated paleontologists and evolutionary theorists.
    Some researchers have favored so-called intrinsic explanations, which link evolutionary bursts to genetic or
    biochemical innovations such as the origin of see-bearing plants. Other scientists tie life revolutions to external
    causes, such as chemical changes in the ocean, climate changes, or mass extinctions.

    In theory, external factors explain only part of the story. Ecological forces, such as competition and predation
    among organisms, also play an important role in evolution. The timing and rates of evolution are dictated by
    extrinsic factors, but the directions of evolution are largely determined by what other organisms are doing.

    References
    Hancock, G. 2003. Underworld: The Mysterious Origins of Civilization.

    Wegener, A. 2011.Origin of Continents and Oceans.

    Stow, D. A. V. 2012. Vanished Ocean: How Tethys Reshaped the World.
Earth's Crust
Plates Collide
Underwater Volcano
This video shows  an Inferno vent at
Axial Volcanowhich the NEMO
Project studied on the Juan de Fuca
and Gorda Ridges deep on  the
ocean floor.