S. Srivastava
Have you aver wondered: why the Atlantic coastlines of South America and Africa look as though they could fit together; why a chain of volcanoes lies along the west coast of Canada; and why most of the oil and gas offshore lie off the east, and not the west coasts of Canada? All of these phenomena can be explained by one theory known as Plate Tectonics.
Geological evidence indicates that ocean basins were formed by the drifting apart of large, rigid sections of the Earth’s crust called LITHOSPHERIC PLATES which contain the continents (Fig. 1). These plates are floating on semi-liquid material known as the ASTHENOSPHERE. The lithospheric plates are about 100 km thick and can be visualised as the rigid outer shell of the earth. There are about 12 to 14 such plates which interact with each other. Because they lie on hot molten material they are in continuous motion; like the movement of pieces of bread floating on thick boiling soup. The sizes of these plates change continuously. Where two plates move apart, lava (molten rock) comes up and hardens, making the plates larger (see plates a and b in Fig. 1(A)). Where plates collide, they overlap and therefore decrease in size (see plates b and c in Fig. 1(B)). As one of the plates, like plate b in the illustration, plunges deep under the other plate, it melts. The molten material pushes up through a crack to the surface forming volcanoes. Thus the belt of volcanoes along the west coast of Canada are created because the North American Plate is overriding the Pacific Plates.
The idea of continental drift has been in existence for a long time. The main difficult lay in finding the mechanisms of separation of these continents. In 1910 when Alfred Wegener proposed that these continents had drifted away from each other like rafts ploughing through sea, it met with great opposition, as scientists could not conceive of large continents ploughing through solid ocean floor.
The fact that the ocean floor is much younger than the continents was only shown in the 1950’s when it was discovered at a mountain belt, several thousand kilometres wide at the base and extending more than one-quarter the way around the globe, exists in ocean basins. In 1960 Harry Hess from Princeton University proposed that these ridges or mountains are formed by a rising current of material, which then spreads outward to form new ocean floors. In this way the continents did not move a self-contained entities but as rafts frozen into, and moving with, a sea floor as rigid as they were, contrary to Wegener’s idea. The process was termed "Seafloor Spreading" by Robert Dietz of U.S. Coast and Geodetic Survey. The reason you see the similarities in the outlines of the coasts of South America and Africa is because they were once joined together. The atlantic Ocean which lies between them is much younger (about 130 Million Years) and was formed due to separation of the South American Plate from the African Plate.
One may wonder why Geological Survey of Canada is interested in studying the evolution of these ocean basins? Canada’s coasts have wide continental shelves. Beneath these may be large amounts of minerals; oil, and gas, as in the Hibernia oilfield off Newfoundland. If we know when these offshore regions were formed in history of the Earth’s geology and climate, and where the continents were in those times, we can better predict where these natural resources can be found. So we need to determine the age of the ocean basins and direction along which the plates forming them have moved. When the volcanic rocks at the mid-ocean ridges come up to the surface and start to cool, its iron bearing minerals line up with the Earth’s magnetic field, as a compass points north.
When the lava solidified, these "mineral compasses" become frozen images of the Earth’s magnetic field at that time. It is now recognised that the Earth’s magnetic field has changed polarity many times in the past. Thus the volcanic rocks as the bottom of the ocean which come out like a conveyor belt on both sides of the mid-ocean ridges preserve these directions and form a continuous record of the history of the earth’s magnetic field, as shown in Fig. 2. When measured, the magnetic field over these rocks shows up as a series of positive and negative fluctuations in the magnetic field. Across mid-oceanic ridges they form positive and negative bands in magnetic fields which are symmetrical relative to the ridge axis. By determining the ages of the rocks using radiometric methods, the ages of reversals in the earth’s field have been determined. Hence magnetic anomalies in the ocean basins can be treated as lines of equal age. In this way, an age map of the ocean basin can be created as shown in Fig. 3.
Shifts in the mid-ocean ridge axis are prominent features on the ocean floor. Movement of these shifts as part of the plates are left as distinct traces on the ocean floor and are known as FRACTURE ZONES. They show the direction of plate movement. The fracture zones in Fig. 3 show an east-west direction of plate movement. Since we can tell the AGE and DIRECTION of movement in ocean basins, we can reconstruct the past position of plates to see what the ocean basin looked like. To reconstruct the ocean basin, we take matching bands of magnetic anomalies on opposite sides of mid-ocean ridge and "slide" them back towards each other along fracture zones until they meet at the ridge where they once emerged as lava. This eliminates the anomalies that formed since then, leaving a reconstruction of the ocean basin of a former age. Figure 4 is a reconstruction of the North Atlantic basin nine million years ago (9Ma), the time that anomaly 5 (central line) was formed.
Such operations if done on a computer are fast and accurate. We have reconstructed the North Atlantic basin for many geological periods on our computer and one of such reconstructions for the time of initial opening, when Africa and Europe had started to separate from North America, is shown in Fig. 5. Notice the good fit of Africa with the continental shelf of North America. The stippled regions mark the areas of overlap between plate boundaries along which they had initially separated and white regions mark gaps between them. These amount of overlaps are important as they tell us the amount of extension each plate was subjected to prior to their separation. Prior to their separation the plates undergo lots of stretching, like a toffee bar when pulled apart stretches before breaking into two pieces. As the plate stretches it thins and cracks forming deep troughs in which large amount of sediment accumulate later on, forming source material for oil and gas. These deep troughs full of sediments are called sedimentary basins. Reconstruction of the ocean basins thus is important not only for studying the correlation of geology between land masses which now lie across them but also for studying the formation of sedimentary basins which lie under their shelves and are the main source for oil and gas in offshore regions.