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- To understand the concept of Seafloor Spreading, we must first understand some basic concepts that form the cornerstones for the concept of Seafloor Spreading. These cornerstones are Convectional Current Theory and Paleomagnetism.
- Convection Current Theory is the soul of Seafloor Spreading Theory.
- Arthur Holmes in 1930s discussed the possibility of convection currents in the mantle.
- These currents are generated due to radioactive elements causing thermal differences in the mantle.
Convection currents in the mantle
- According to this theory, the intense heat generated by radioactive substances in the mantle (100-2900 km below the earth surface) seeks a path to escape and gives rise to the formation of convection currents in the mantle.
- Wherever rising limbs of these currents meet, oceanic ridges are formed on the seafloor due to the divergence of the lithospheric plates (tectonic plates), and wherever the failing limbs meet, trenches are formed due to the convergence of the lithospheric plates (tectonic plates).
- The movement of the lithospheric plates is caused by the movement of the magma in the mantle.
- Paleomagnetism is the study of the record of earth’s magnetic field with the help of magnetic fields recorded in rocks, sediment, or archaeological materials.
- The polarity of the Earth’s magnetic field and magnetic field reversals are thus detectable by studying the rocks of different ages.
- Rocks formed from underwater volcanic activity are mainly basaltic (low silica, iron-rich) that makes up most of the ocean floor.
- Basalt contains magnetic minerals, and as the rock is solidifying, these minerals align themselves in the direction of the magnetic field.
- This locks in a record of which way the magnetic field was positioned at the time.
- Paleomagnetic studies of rocks have demonstrated that the orientation of the earth’s magnetic field has frequently alternated (geomagnetic reversal) over geologic time.
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Paleomagnetism: Strong evidence of Seafloor Spreading and Plate Tectonics
- Paleomagnetism led the revival of the continental drift hypothesis and its transformation into theories of Sea Floor Spreading and Plate Tectonics.
- The regions that hold the unique record of earth’s magnetic field lie along the mid-ocean ridges where the sea floor is spreading.
- On studying the paleomagnetic rocks on either side of the oceanic ridges, it is found that alternate magnetic rock stripes were flipped so that one stripe would be of normal polarity and the next, reversed.
- Hence, paleomagnetic rocks (paleo: denoting rocks) on either side of the mid-ocean or submarine ridges provide the most important evidence to the concept of Sea Floor Spreading.
- Magnetic field records also provide information on the past location of tectonic plates.
- These oceanic ridges are boundaries where tectonic plates are diverging (moving apart).
- The fissure or vent (in between the ridge) between the plates allowed the magma to rise and harden into a long narrow band of rock on either side of the vent.
- Rising magma assumes the polarity of Earth’s geomagnetic field at the time before it solidifies on the oceanic crust.
- As the conventional currents pull the oceanic plates apart, the solidified band of rock moves away from the vent (or ridge), and a new band of rock takes its place a few million years later when the magnetic field was reversed. This results in this magnetic striping where the adjacent rock bands have opposite polarities.
- This process repeats over and over giving rise to a series of narrow parallel rock bands on either side of the ridge and alternating pattern of magnetic striping on the seafloor.
The alternating pattern of magnetic striping on the seafloor
- Harry Hess proposed the idea of See Floor Spreading.
- When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere.
- Basaltic magma rises from the fractures and cools on the ocean floor to form new seafloor.
- The newly formed seafloor (oceanic crust) then gradually moves away from the ridge, and its place is taken by an even newer seafloor and the cycle repeats.
- With time, older rocks are spread farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.
Seafloor spreading helps explain continental drift in the theory of plate tectonics.
Nature of oceanic rocks around mid-ocean ridges
- Rocks on either side of the crest of oceanic ridges having equidistant locations from the crest were found to have similarities both in terms of their constituents, their age and magnetic orientation.
- Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest and the age of the rocks increases as one moves away from the crest (ridge).
- The rocks of the oceanic crust near the oceanic ridges are much younger than the rocks of the continental crust.
Distribution of Earthquakes and Volcanoes along the mid-ocean ridges
- The normal temperature gradient on the sea floor is 9.4° C/300 m, but near the ridges it becomes higher, indicating an upwelling of magmatic material from the mantle.
- Dots in the central parts of the Atlantic Ocean and other oceans are almost parallel to the coastlines. This indicates that the seafloor has widened with time.
- In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depths whereas, along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.