Suggested Student Grouping: Students work as individuals. Framework Integration: Themes: Patterns of change: over time, new sea-floor is created by the upwelling of magma at mid-ocean spreading centers; old ocean floor is destroyed by subduction at deep sea trenches. Science skills and processes: Inferring from a model. Integrating with other disciplines: Physical science: dipole magnets and magnetic fields; convection. Oceanography: topography of the ocean floor. Life Science: animals found at hot-water vents on the ocean floor. Related Activities: Submarine Mountains. The outer km or so is a rigid layer called the lithosphere , which is made up of the crust and uppermost mantle. The lithosphere is broken into a number of large and small plates that move over the asthenosphere , a plastic layer in the upper mantle.
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Earthquakes and volcanoes are concentrated at the boundaries between lithospheric plates. It is thought that plate movement is caused by convection currents in the mantle Fig. Lithosphere plates are moving at rates of a few cm per year. If a plate of oceanic lithosphere collides with thicker and less dense continental lithosphere, the denser oceanic plate will dive beneath the continent in a subduction zone Fig.
Return to top. The Mid-Atlantic Ridge is the longest mountain chain on Earth.
Letter. 40Ar/39Ar dating of oceanic plagiogranite: Constraints on the initiation of seafloor spreading in the South China Sea.
A test of the Vine—Matthews Hypothesis, which required determining the age of the seafloor, became a test of seafloor spreading. Dating the ocean floor using magnetic anomalies detected by magnetometers towed behind ships and core samples extracted during the Deep-Sea Drilling Project confirmed the hypothesis. With a theory for predicting the depths of oceans, it was also possible to understand the history of sea-level changes.
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Paving the SeafloorBrick by Brick
Not a MyNAP member yet? Register for a free account to start saving and receiving special member only perks. The following sections review the scientific accomplishments and new fields of inquiry that scientific ocean drilling results have fostered in our knowledge of the solid Earth. Some of the remaining challenges about the solid Earth to which scientific ocean drilling can contribute are also discussed.
Harry Hammond Hess: Spreading the seafloor theory was strengthened further when dating studies showed that the seafloor becomes older.
In the s, scientists found evidence that new material is indeed erupting along mid-ocean ridges. The scientists dived to the ocean floor in Alvin , a small submarine built to withstand the crushing pressures four kilometers down in the ocean. Such rocks form only when molten material hardens quickly after erupting under water. These rocks showed that molten material has erupted again and again along the mid-ocean ridge.
When scientists studied patterns in the rocks of the ocean floor, they found more support for sea-floor spreading. You read earlier that Earth behaves like a giant magnet, with a north pole and a south pole. If the magnetic poles suddenly reversed themselves today, you would find that your compass needle points south. Magnetic Stripes.
In fact, scientists are mapped more of the surface of the Moon, Mars, and Venus than the surface of our ocean. You may have heard this fact where, and while true, there is a logical explanation as to why. Seafloor mapping, in its earliest, most primitive form, consisted of lowering weighted lines and measuring how where the sunk.
Since sea-floor spreading is a continuous process on a geological timescale, the age data (derived by radiometric dating of rocks dredged from the sea floor).
Anyone can learn for free on OpenLearn, but signing-up will give you access to your personal learning profile and record of achievements that you earn while you study. Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available. At the time that sea-floor spreading was proposed, it was also known from palaeomagnetic studies of volcanic rocks erupted on land that the Earth’s magnetic polarity has reversed numerous times in the geological past.
During such magnetic reversals , the positions of the north and south magnetic poles exchange places. In the late s, a series of oceanographic expeditions was commissioned to map the magnetic character of the ocean floor, with the expectation that the ocean floors would display largely uniform magnetic properties. Surprisingly, results showed that the basaltic sea floor has a striped magnetic pattern, and that the stripes run essentially parallel to the mid-ocean ridges Figure 6.
Moreover, the stripes on one side of a mid-ocean ridge are symmetrically matched to others of similar width and polarity on the opposite side.
Continents move, carried on huge slabs, or plates, of dense rock about km 62 miles thick over a low-friction, partially melted zone the asthenosphere below. In the oceans , new seafloor, created at the globe-circling oceanic ridges , moves away, cools, and sinks back into the mantle in what are known as subduction zones i. Where this occurs at the edge of a continent, as along the west coast of North and South America, large mountain chains develop with abundant volcanoes and their subvolcanic equivalents.
Radiometric dating later showed that oceanic rocks were very young compared to those on Diagram of sea-floor spreading as put forward by Arthur Holmes.
Radioisotopic dating of volcanic minerals is a powerful method for establishing absolute time constraints in sedimentary basins, which improves our understanding of the chronostratigraphy and evolution of basin processes. The relative plate motions of Greenland, North America, and Eurasia changed several times during the Palaeogene. However, the timing of a key part of this sequence, namely the initiation of compression between Greenland and Svalbard, is currently poorly constrained.
The formation of the Central Basin in Spitsbergen is inherently linked to changes in regional plate motions, so an improved chronostratigraphy of the sedimentary sequence is warranted. The timing of basin formation is broadly coeval with depositional changes at the Danian-Selandian boundary around the other margins of Greenland, including the North Sea, implying a common tectonic driving force.
Furthermore, these stratigraphic tie points place age constraints on regional plate reorganization events, such as the onset of seafloor spreading in the Labrador Sea. The onset of compression between Greenland and Svalbard in the Palaeocene led to the eventual formation of the West Spitsbergen fold-and-thrust belt in the Eocene, with a rapidly subsiding foreland basin forming adjacent to the mountain range 1 — 3 Fig. The basin infill is named the Van Mijenfjorden Group; a 2.
Subsidence in the Central Basin began before the formation of the fold-and-thrust belt and with no clear hiatus in sediment deposition. Therefore, a detailed understanding of the basin stratigraphy can be used to refine the chronology of plate reconfigurations in the run up to the Eurekan deformation and the opening of the northeast Atlantic. A map of the study area, the eastern Central Basin in Svalbard.
The lithostratigraphic inlay shows the main formations of the Van Mijenfjorden Group 5.
The global tectonic rock cycle
Abundant evidence supports the major contentions of the seafloor-spreading theory. First, samples of the deep ocean floor show that basaltic oceanic crust and overlying sediment become progressively younger as the mid-ocean ridge is approached, and the sediment cover is thinner near the ridge. Second, the rock making up the ocean floor is considerably younger than the continents, with no samples found over million years old, as contrasted with maximum ages of over 3 billion years for the continental rocks.
This confirms that older ocean crust has been reabsorbed in ocean trench systems. By the mids studies of the earth’s magnetic field showed a history of periodic reversals in polarity see paleomagnetism. A timescale for normal and reversed polarity was established, showing magnetic flip-flops in the past 76 million years.
U-Pb DATING AND TRACE ELEMENT COMPOSITION OF ZIRCON FROM The new age inferred for seafloor spreading and ocean crust.
Seafloor spreading , theory that oceanic crust forms along submarine mountain zones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. This idea played a pivotal role in the development of the theory of plate tectonics , which revolutionized geologic thought during the last quarter of the 20th century. Shortly after the conclusion of World War II , sonar -equipped vessels crisscrossed the oceans collecting ocean-depth profiles of the seafloor beneath them.
The survey data was used to create three-dimensional relief maps of the ocean floor, and, by , American oceanic cartographer Marie Tharp had created the first of several maps that revealed the presence of an underwater mountain range more than 16, km 10, miles long in the Atlantic—the Mid-Atlantic Ridge. The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in As the magma cools, it is pushed away from the flanks of the ridges. This spreading creates a successively younger ocean floor, and the flow of material is thought to bring about the migration, or drifting apart, of the continents.
The continents bordering the Atlantic Ocean , for example, are believed to be moving away from the Mid-Atlantic Ridge at a rate of 1—2 cm 0. See also continental drift. A veritable legion of evidence supports the seafloor spreading hypothesis.