Geography - Geomorphology - See Floor Spreading – Paleomagnetism October 29, 2017 Convectional Current Theory – Tectonics 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 mantle. Mapping of the Ocean Floor Detailed research during World Wars revealed that the ocean floor is not just a vast plain but it is full of relief with mountain ranges, deep trenches. The mid-oceanic ridges were found to be most active in terms of volcanic eruptions. The dating of the rocks from the oceanic crust revealed the fact that the latter is much younger than the continental areas (Rocks on ocean floor are much younger than those on the continents). Rocks on either side of the crest of oceanic ridges and having equidistant locations from the crest were found to have remarkable similarities both in terms of their constituents and their age. Distribution of Earthquakes and Volcanoes Volcanism and associated earthquakes at plate margins are a direct consequence of convection currents in the mantle. Dots in the central parts of the Atlantic Ocean and other oceans are almost parallel to the coastlines. In general, the foci of the earthquake in the areas of mid-oceanic ridges are at shallow depthswhereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones (deep focus earthquakes are more destructive). The map of volcanoes also shows a similar pattern. The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area. These observations (ocean floor and the distribution of earthquakes and volcanoes) led to the theory of See Floor Spreading. Convectional Current Theory is the soul of See Floor Spreading theory. Convectional Current Theory 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 convention currents in the mantle. Wherever rising limbs of these currents meet, oceanic ridges are formed on the sea floor and wherever the failing limbs meet, trenches are formed. Paleomagnetism It is the study of the record of the Earth’s magnetic field in rocks, sediment etc.. Why do we need to study this concept? Paleomagnetic rocks on either side of the submarine ridges provide the most important evidence to the concept of Sea Floor Spreading. Paleo == Rocks; Paleomagnetism == magnetism in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth’s magnetic field and the past location of tectonic plates. Paleomagnetists led the revival of the continental drift hypothesis and its transformation into plate tectonics. Paleomagnetic studies of rocks and ocean sediment have demonstrated that the orientation of the earth’s magnetic field has frequently alternated over geologic time. Periods of “normal” polarity (i.e., when the north-seeking end of the compass needle points toward the present north magnetic pole, as it does today) have alternated with periods of “reversed” polarity (when the north-seeking end of the compass needle points southward)[I have explained this in detail in the video]. As today’s magnetic field is close to the earth’s rotational axis, continental drift could be tested by ascertaining the magnetic characteristics of ancient rocks. Paleomagnetism: Strong evidence of See Floor Spreading and Plate Tectonics Some of the strongest evidence in support of the theory of see floor spreading and plate tectonics comes from studying the magnetic fields surrounding oceanic ridges. Rocks formed from this underwater volcanic activity were mainly basalt, which is low silica, iron-rich, volcanic rock 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 basically locks in a record of which way the magnetic field was positioned at the time that part of the ocean floor was created. Paleomagnetists [scientists who study past magnetic fields], took a look at the ocean floor going out away from oceanic ridges (either side of the oceanic ridges), they found magnetic stripes that were flipped so that one stripe would be normal polarity and the next reversed. How could this be? These oceanic ridges were actually boundaries with tectonic plates pulling apart. This movement of the plates allowed the magma to rise up and harden into new rock. As the new rock was formed near the ridge, older rock, which formed millions of years ago when the magnetic field was reversed, got pushed farther away, resulting in this magnetic striping. Rising magma assumes the polarity of Earth’s geomagnetic field before it solidifies into oceanic crust. At spreading centres, this crust is separated into parallel bands of rock by successive waves of emergent magma. When Earth’s geomagnetic field undergoes a reversal, the change in polarity is recorded in the magma, which contributes to the alternating pattern of magnetic striping on the seafloor. Concept of Sea Floor Spreading The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess. 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 convention currents in the mantle. Wherever rising limbs of these currents meet, oceanic ridges are formed on the sea floor and wherever the failing limbs meet, trenches are formed. 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 spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone. Evidences The mapping of the ocean floor and Paleomagnetic studies of rocks from oceanic regions revealed the following facts : Volcanic eruptions are common all along the midoceanic ridges and they bring huge amounts of lava to the surface in this area. The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest. The deep trenches have deep-seated earthquake occurrences while in the midoceanic ridge areas, the quake foci have shallow depths. It was on the basis of the continental drift theory, theory of sea floor spreading, that the theory of Plate Tectonics was formulated—first outlined by Morgan in 1968. So, next post will be a detailed explanation on Plate Tectonics.