seafloor spreading hypothesis

seafloor spreading hypothesis

      theory that oceanic (oceanic ridge) crust forms along submarine mountain zones, known collectively as the midocean ridge system, and spreads out laterally away from them. This idea played a pivotal role in the development of plate tectonics, a theory that revolutionized geologic thought during the last quarter of the 20th century.

      The seafloor spreading hypothesis was proposed by the American geophysicist Harry H. Hess in 1960. On the basis of new discoveries about the deep-ocean floor, Hess postulated that molten material from the Earth's mantle continuously wells up along the crests of the midocean ridges that wind for 60,000 km (37,000 miles) through all the world's oceans. 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 (0.4–0.8 inch) per year, thus increasing the breadth of the ocean basin by twice that amount. Wherever continents are bordered by deep-sea trench systems, as in the Pacific Ocean, the ocean floor is plunged downward, underthrusting the continents and ultimately reentering and dissolving in the Earth's mantle from which it originated.

      A veritable legion of evidence supports the seafloor spreading hypothesis. Studies conducted with thermal probes, for example, indicate that the heat flow through bottom sediments is generally comparable to that through the continents except over the midocean ridges, where at some sites the heat flow measures three to four times the normal value. The anomalously high values are considered to reflect the intrusion of molten material near the crests of the ridges. Research has also revealed that the ridge crests are characterized by anomalously low seismic-wave velocities, which can be attributed to thermal expansion and microfracturing associated with the upwelling magma.

      Investigations of oceanic magnetic anomalies have further corroborated the seafloor spreading hypothesis. Such studies have shown that the strength of the geomagnetic field is alternately anomalously high and low with increasing distance away from the axis of the midocean ridge system. The anomalous features are nearly symmetrically arranged on both sides of the axis and parallel the axis, creating bands of parallel anomalies.

      Measurements of the thickness of marine sediments (marine sediment) and absolute age determinations of such bottom material have provided additional evidence for seafloor spreading. The oldest sediments so far recovered by a variety of methods, including coring, dredging, and deep-sea drilling, date only to the Jurassic Period—that is, they do not exceed 208 million years in age. Such findings are incompatible with the doctrine of the permanency of the ocean basins that had prevailed among earth scientists for so many years.

* * *


Universalium. 2010.

Игры ⚽ Нужно сделать НИР?

Look at other dictionaries:

  • Seafloor spreading — Age of oceanic crust; youngest (red) is along spreading centers. Seafloor spreading is a process that occurs at mid ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor… …   Wikipedia

  • Vine–Matthews–Morley hypothesis — The Vine–Matthews–Morley hypothesis, also known as the Morley–Vine–Matthews hypothesis was the first key scientific test of the seafloor spreading theory of continental drift and Plate tectonics. Geophysicist Frederick John Vine and the Canadian… …   Wikipedia

  • Morley-Vine-Matthews hypothesis — The Morley Vine Matthews hypothesis , also known as the Vine Matthews Morley hypothesis was the first key scientific test of the seafloor spreading theory of continental drift. Geophysicists Frederick John Vine and Lawrence W. Morley… …   Wikipedia

  • plate tectonics — plate tectonic, adj. Geol. a theory of global tectonics in which the lithosphere is divided into a number of crustal plates, each of which moves on the plastic asthenosphere more or less independently to collide with, slide under, or move past… …   Universalium

  • Plate tectonics — The tectonic plates of the world were mapped in the second half of the 20th century …   Wikipedia

  • Atlantic Ocean — an ocean bounded by North America and South America in the Western Hemisphere and by Europe and Africa in the Eastern Hemisphere. ab. 31,530,000 sq. mi. (81,663,000 sq. km); with connecting seas ab. 41,000,000 sq. mi. (106,100,000 sq. km);… …   Universalium

  • North America — North American. the northern continent of the Western Hemisphere, extending from Central America to the Arctic Ocean. Highest point, Mt. McKinley, 20,300 ft. (6187 m); lowest, Death Valley, 276 ft. (84 m) below sea level. 400,000,000 including… …   Universalium

  • continental drift — Geol. the lateral movement of continents resulting from the motion of crustal plates. Cf. plate tectonics. [1925 30] * * * Large scale movements of continents over the course of geologic time. The first complete theory of continental drift was… …   Universalium

  • Oceanic trench — Oceanic crust is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at trenches. The oceanic trenches are hemispheric scale long but narrow topographic depressions of the sea floor. They are also the… …   Wikipedia

  • Ordovician Period — Interval of geologic time, 490–443 million years ago, the second oldest period of the Paleozoic Era. It follows the Cambrian and precedes the Silurian. During the Ordovician, many of the landmasses were aligned in the tropics. Life was dominated… …   Universalium

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”