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Easiest explanation of Sea Floor Spreading Theory !

Updated: Jul 24, 2021

It was given by Harry Hess.

Before reading the theory let us know some basics:

Lithosphere: The lithosphere is the solid, outer part of the Earth comprising of brittle upper portion of the mantle and the crust which forms the outermost layers of Earth’s structure. It is bounded by the atmosphere above and the asthenosphere (another part of the upper mantle) below.

Tectonic Plates: A tectonic plate (also called a lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. In general, it consists of the rocky part of the ocean floor and continents. The slab of rock is called a tectonic plate.

Sea Floor Spreading: As the name suggests it is spreading of the seafloor in divergent directions due to the upcoming of Molten Magma from within the earth. It is a geological process in which Tectonic plates (ie. Ocean Floor) splits apart from each other. It is a result of mantle convection.

Mantle convection is a process by which heat is transferred from the hot core of the earth to the planet's surface. Hot molten magma from the core rises up towards the upper layers of earth and cools down transferring heat to the upper layers. As it cools down it again comes down towards the core forming a loop of rising and subsiding. This causes a convection current.

When Pressurised Magma rises upwards, the earth's surface cracks and the ocean floor spreads making space for the magma to come up. In this process, the ocean plate is pushed aside and the water above cools down the upcoming magma to make igneous rocks which make the new seafloor. The seafloor spreading occurs at Divergent plate boundaries. The ocean plate pushed aside converges with the continental plate and being heavier it subsides down and converts into molten magma. This zone where the plate subsides is called the subduction zone. The Magma formed due to the melting of subsided ocean plate rises upwards and makes the mountains on the seafloor and other elevated uplands on the floor.

Seafloor spreading occurs along mid-ocean ridges—large mountain ranges rising from the ocean floor. The Mid-Atlantic Ridge separates the North American plate from the Eurasian plate, and the South American plate from the African plate and lies in the middle of the Atlantic ocean and hence the name.

Since the new crust keeps forming along the Mid-Atlantic Ridge the rocks closer to the ridge are newer and the ocean crust farther away from the ridge is older in age. Mid-ocean ridges and seafloor spreading can also influence sea levels. As oceanic crust moves away from the shallow mid-ocean ridges, it cools and sinks as it becomes denser. This increases the volume of the ocean basin and decreases the sea level.

Evidence of seafloor spreading :

  • 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.

  • The rocks of the oceanic crust near the oceanic ridges are much younger than the rocks of the continental crust.

  • The normal temperature gradient on the seafloor is 9.4° C/300 m, but near the ridges it becomes higher, indicating an upwelling of magma from the mantle.

  • 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.

What problems did Seafloor's spreading theory solve?

  1. It solved the problem of the younger age rocks found at the mid-oceanic ridges and older rocks being found as we go away from the middle part of the ridges.

  2. It also explained why the sediments at the central parts of the oceanic ridges are relatively thin.

  3. The sea-floor spreading also proved the drifting of continents as propounded by Alfred Wegener and helped in the development of the theory of plate tectonics.


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