What is called sea floor spreading?

• This phenomenon constitutes a fundamental aspect of plate tectonic theory. Its identification supplied a crucial mechanism for continental drift, a process that Alfred Wegener was unable to elucidate.
• In 1912, when Alfred Wegener postulated that the continents had previously existed as a unified landmass before fragmenting, the primary deficiency in his hypothesis was the lack of a significant process enabling continents to traverse oceanic basins.
• At that period, it was a notable consensus that oceans represented perpetual features, and during Wegener’s era, no essential explanation existed for how continents could have migrated through the rigid substrate of the ocean floor.
• However, in 1962, Harry Hess, a geologist and Rear Admiral in the U.S. Naval Reserve, introduced a notable solution.
• Instead of continents forcing their way through oceanic crust, Hess proposed that the oceanic crust itself was the primary agent separating the continents.
• He asserted that the location and topographical characteristics of the Mid-Atlantic Ridge were not coincidental.
• The Mid-Atlantic Ridge is an oceanic ridge situated along the floor of the Atlantic Ocean.
• • According to Hess, this ridge represented the site where new oceanic crust was being incorporated into the Earth’s lithosphere, thereby driving the continents apart. Hess designated this process as seafloor spreading.
• Hess argued that the Mid-Atlantic Ridge constituted a boundary where two lithospheric plates were diverging. As this divergence occurred, ascending magma from the upper mantle infiltrated the fissures generated within the Earth’s crust.
• The magma subsequently solidified into basalt and other igneous rocks; continued rifting further separated these rocks. Consequently, Hess conceptualized a magma-driven conveyor belt that persistently generated new oceanic crust, incrementally expanding the Atlantic basin and displacing the continents on either side.
• Thus, rather than continents plowing through oceanic crust, Hess maintained that the essential process involved the seafloor itself facilitating continental separation.
• Particularly, systematic mapping of the oceanic floor and paleomagnetic analyses of rocks from marine environments revealed the following significant findings:
  • It was determined that along the mid-oceanic ridges, volcanic activity is a notable phenomenon, resulting in substantial lava outpourings at these locations.
  • Rocks equidistant from the crest of mid-oceanic ridges exhibited remarkable uniformity in age, magnetic orientation, and chemical composition.
  • Rocks in proximity to the mid-oceanic ridges display normal magnetic polarity and are the youngest in age.
  • Oceanic crustal rocks are significantly younger in comparison to continental rocks.

Evidence for Seafloor Spreading

• The most important evidence derives from paleomagnetic rocks. The alignment or polarity of iron minerals in ancient rocks indicates a polarity that suggests the South Pole once existed between present-day Africa and Antarctica (paleomagnetism).
• Older rocks constitute the continents, whereas the younger rocks are located on the ocean floor. Continental rocks have been dated to 3.5 billion years, while the oldest oceanic crust is no more than 200 million years old.
• Progressing toward the ridges, increasingly younger rocks are encountered. This observation supports the effective expansion of the seafloor along oceanic ridges, which also serve as plate boundaries.
• The standard geothermal gradient on the seafloor is 9.4°C per 300 meters; however, near the ridges, this value increases, indicating an upwelling of magmatic material from the mantle.
• In oceanic trenches, where subduction occurs (convergent boundaries), the gravitational constant ‘g’ is less.
• This reflects a loss of material. For instance, gravity measurements in the vicinity of the Indonesian archipelago have demonstrated that substantial gravity anomalies are associated with the oceanic trench adjacent to Indonesia.
• The certainty that all plate boundary zones are regions of seismic and volcanic activity serves as crucial validation for the theory of seafloor spreading.