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Seabed spreading is caused by upwelling in the mantle, creating a triple junction and heating the crust, which rises and widens the seafloor. This process drives continental drift and creates oceans, mountains, and geological activity. The world’s continents are expected to agglomerate into a new supercontinent, Pangea Ultima, within the next 200 million years.
Seabed spreading is a constant geological phenomenon and a major cause of continental drift. The process begins when the crust is heated from below by upwelling in the mantle, sometimes called the mantle plume. This creates a three-armed rift called a triple junction, usually with each rift at a 120° 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 large dome and enlarging the crack.
The heated region pushes the crust out of the rift very gently, at a rate of 5 to 10 cm per year, about as fast as fingernails grow, causing the seafloor to widen. Over millions of years, this process drives continental drift, bringing continents together, then splitting them, in a process called the supercontinent cycle. This process runs its course every 250-500 million years. The last time there was a supercontinent, Pangea, was about 200 million years ago. Within the next 200 million years, the world’s continents are expected to agglomerate again, into a new supercontinent called Pangea Ultima.
In a typical rift system, two of the rifts will continue to spread while one, a “failed rift”, will stop spreading. If down, the failed rift will become a rift. Rifts that keep spreading are guaranteed to make an ocean, even if they start 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 Earth’s major underwater mountain ranges.
As seafloor spreading continues, magma rises upwards to fill the void, creating volcanoes and mountains. Rifts are the site of geological activity, including earthquakes. Many of the world’s hydrothermal vents are located on or near the fissures that cause the seabed to spread.
When a new rift begins to spread over 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 millions of years, this rift will continue to spread, creating a new ocean.
Seafloor spreading pushes the oceanic tectonic plate boundaries under the continental plates, which are paler. This crust is subducted into the mantle, where it melts and becomes magma. In this way, the oceanic crust is constantly replenished. The oldest seafloor crust is only about 200 million years old, in contrast to continental crust, which can be billions of years old.
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