Sea-floor spreading is the process in which the ocean floor is extended when two plates move apart. Plates are huge slabs into which the crust as well as the upper part of the mantle (the intermediate zones between the Earth’s crust and core) is divided. Sea-floor spreading is a process that occurs at mid-ocean ridges (general term for an underwater mountain system that has various mountain ranges, typically with a valley in between known as a rift), where new oceanic crust (the thin outer most layer of the earth, forming both the continents and the ocean floors) is formed through volcanic activity and then gradually moves away from the ridge. Sea-floor spreading helps explain continental drift. Continental drift is the movement of the Earth’s continents relative to each other.
Earlier theories of continental drift were that continents “plowed” through the sea. The idea that the sea-floor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess form Princeton University in the 1960s. The theory is well-accepted now, and the phenomenon is known to be caused by convection (movement of molecules within fluids like liquids, gases) currents in the plastic, very weak upper mantle.
Sea-floor spreading occurs where oceanic plates are diverging from one another. Plates are huge slabs into which the crust as well as the upper part of the mantle (the intermediate zones between the Earth’s crust and core) is divided. As the plates move apart, the rocks break and form a crack between the plates. Earthquakes occur along the plate boundary. Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano. Magma is a very hot, molten rock which is formed within the earth by partial melting of the mantle. As a result, the age of the rocks increases as one move away from the rift-zone (place where the earth’s crust is pulled apart). The mid-oceanic ridge is the primary site for sea-floor spreading. Earthquakes and volcanoes are where sea-floor spreading occurs.
The process begins when the crust is heated from beneath by up welling in the mantle, sometimes known as mantle plum. This creates a three-armed rift called a triple junction, usually with each rift at a 120 degree angle to the rest. The crust along the length of the rift is heated by the underlying mantle, becoming more plastic and less dense. The crust rises, creating a broad dome and widening the rift. The heated region pushes the crust outwards form the rift ever so slightly, at a rate of 5-10 cm a year, about the same speed fingernails grow, causing the sea-floor spreading. As the sea-floor spreading continues, magma rushes up to fill the gap, creating a sea volcanoes and mountains. Rifts are the site of geological activity, including earthquakes. Many of the world’s hydro-thermal vents are found on or near the rifts that cause sea floor spreading. When a new rift starts to spread on land, water will rush in, creating a sea. This can be seen in the Red sea, which separates Eurasia -Saudi Arabia -from Africa- Egypt, Sudan, and Ethiopia. Over million years, this rift would continue to spread, creating a new ocean.
Over million years, the process drives continental drift, bringing together continents, then dividing them, in a process called the super continent cycle. This process runs its course every 250 to 500 million years. The last super continent was Pangaea, about 200 million years ago. Within next 200 million years, the world’s continents are predicted to evolve again, in a new super continent called Pangaea Ultima.
In a typical rift system, two of the rifts will continue spreading while one; a “failed rift” will stop spreading. If on land, the failed rift will become a rift valley. Rifts that keep spreading are guaranteed to create an ocean, even if they begin on land. The Atlantic Ocean was created when a rift opened between the continents of North America, Africa, and Europe. This rift still exists today, in the form of the “Mid-Atlantic Rift”, one of the major mountain ranges on earth.
On the whole, sea-floor spreading is basically volcanic, but it is a slow and regular process, without the explosive outbursts of the volcanoes on land.