The floors of sea are not one contiguous basin. It just like the world that you see, with valleys, plateaus and mountains, except that it is all covered with water. Moreover, there are areas in the sea floor that are more ‘active’ than the rest. These areas are prone to faulting, fracturing, volcanic activity and spreading. Sea floor spreading is a basic premise that helps postulate the theory of plate-tectonics.
The first stage in plate separation is the initiation of a new pattern of convection within the Earth’s mantle, which brings hot mantle material to high levels inside the Earth (fig 1). The elevated temperature and buoyant effect of the rising plume arches up the oceanic crust, causing it to extend. As the plates continue to diverge, further fracturing of the thinned oceanic crust occurs.
The new oceanic crust cools and moves away on either side of the spreading axis. As it cools, it becomes denser and subsides, gradually generating the low lying ocean floor, which becomes imprinted with magnetic anomalies as polarity reversals take place.
The seafloor lava acquire a veneer of marine sedimentary rocks, produced by marine organisms. The margins of the ocean are marked by normally faulted continental edges, partly due to subsidence of the oceanic crust. Continental shelves develop along the continental edges and receive sediments worn by erosion of the adjacent continental crust.
Magnetic anomalies in ocean floor lava reveal polarity reversals. Because the cooled crust is brittle, the convection pattern is accommodated by transform faults which offset the ridge axis
Fig 1: Diagrammatic representation of sea floor spreading
Discovery of sea-floor spreading
Development of highly sophisticated seismic recorders and precision depth recorders in the 1950s led to the discovery in the early 1960s that the magnetism of mid-ocean ridges resulted in the process of seafloor spreading.
Basalt, the once-molten rock that makes up most new oceanic crust, is a fairly magnetic substance, and scientists began using magnetometers to measure the magnetism of the ocean floor in the 1950s.
What they discovered was that the magnetism of the ocean floor around mid-ocean ridges was divided into matching ‘stripes’ on either side of the ridge. The specific magnetism of basalt rock is determined by the Earth’s magnetic field when the magma is cooling.
Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust.
In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquakes and volcanoes.
Fig 2: Plate margins and zones of spreading indicated in red.
The oldest rocks are represented in blue.