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DISTRIBUTION OF OCEANS AND CONTINENTS 29 mountain ranges as well as deep trenches, Abyssal Plains mostly located closer to the continent margins. These are extensive plains that lie between the The mid-oceanic ridges were found to be most activeintermsofvolcaniceruptions.Thedating continental margins and mid-oceanic ridges. of the rocks from the oceanic crust revealed The abyssal plains are the areas where the continental sediments that move beyond the the fact that they are much younger than the margins get deposited. continental areas. Rocks on either side of the crest of oceanic ridges and having equi-distant locations from the crest were found to have Mid-Oceanic Ridges remarkable similarities both in terms of their This forms an interconnected chain of constituents and their age. mountain system within the ocean. It is the longest mountain-chain on the surface of the Ocean Floor Configuration earth though submerged under the oceanic Inthissectionweshallnoteafewthingsrelated waters. It is characterised by a central rift to the ocean floor configuration that help us insystem at the crest, a fractionated plateau and the understanding of the distribution of flank zone all along its length. The rift system continents and oceans. You will be studying at the crest is the zone of intense volcanic the details of ocean floor relief in Chapter activity. In the previous chapter, you have been 12. The ocean floor may be segmented into introduced to this type of volcanoes as mid- three major divisions based on the depth oceanic volcanoes. as well as the forms of relief. These divisions are continental margins, deep-sea basins and Distribution of Earthquakes and Volcanoes mid-ocean ridges. Study the maps showing the distribution of seismic activity and volcanoes given in Figure 4.2. You will notice a line of dots in the central parts of the Atlantic Ocean almost parallel to thecoastlines.ItfurtherextendsintotheIndian Ocean. It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana. You will notice that this line of dots coincides with the mid- oceanic ridges. The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean. In general, the foci of the earthquake in the areas of mid-oceanic ridges Figure 4.1 : Ocean Floor are at shallow depths whereas along the Alpine-Himalayan belt as well as the rim of the Continental Margins Pacific, the earthquakes are deep-seated ones. The map of volcanoes also shows a similar These form the transition between continental pattern. The rim of the Pacific is also called rim shores and deep-sea basins. They include continentalshelf,continentalslope,continental offireduetotheexistenceofactivevolcanoesin thisarea. rise and deep-oceanic trenches. Of these, the deep-oceanic trenches are the areas which are C ONCEPT OF S EA F LOOR S PREADING of considerable interest in so far as the distribution of oceans and continents is As mentioned above, the post-drift studies concerned. providedconsiderableinformationthatwasnot Reprint 2026-27