How do plates form

Oceanic crust is most commonly formed when basaltic magma rises to the surface at mid-ocean ridges, such as the Mid-Atlantic spreading ridge, and is thinner — typically about 7 kilometers thick — and denser than continental crust. Today, continental crust is formed mainly along subduction zones, where partial melting of descending slabs forms granitic and andesitic magmas at volcanoes on the overriding plate.

This process produces thicker — up to 70 kilometers thick — and more buoyant crust that is not as easily subducted. But it is not known how continental crust formed in the past. Subduction zones form where two plates converge and one begins sliding under the other. As old lithosphere is recycled back into the mantle at subduction zones and new lithosphere is formed at spreading centers, the balance of lithosphere on Earth remains relatively constant.

Today, the planet has eight major plates defined as those with areas over 20 million square kilometers and dozens of minor plates between 1 million and 20 million square kilometers and microplates less than 1 million square kilometers. While some plates are composed solely of oceanic or continental crust, most major plates contain portions of both. While continental crust that is billions of years old still exists on Earth's surface, most oceanic crust is less than million years old Ma.

Older oceanic crust, which is more dense than continental crust, has long since been recycled in the process of subduction. Credit: U. Geological Survey. Mantle convection is driven by temperature differences between the hot interior and the gradually cooling outer layers of the planet. Cooler, denser material sinking down into the mantle is thought to be the primary driver of circulation, while hotter, less dense material rising to the surface in the form of mantle plumes and upwellings provides a secondary driver.

The forces generated by these vertical movements result in horizontal shifts of the tectonic plates at the surface at rates of about a few centimeters per year.

One of the big questions about the onset of plate tectonics is how subduction got started. Geologists think that the lithosphere of the pretectonics Earth existed as a single plate that covered the whole planet. Massive forces would have been needed to break this single lithosphere into multiple plates and to initiate plates descending into the mantle.

Minerals in lithospheric slabs restructure as slabs descend into the mantle, releasing water and increasing the slabs' densities. The dense, downgoing slabs pull on the parts of the plates still at the surface, driving plate tectonics. Some subducting slabs stall at the transition zone, while others descend toward the core-mantle boundary. Credit: both: K. Cantner, AGI. The forces involved are incredible.

Modern plate tectonic movement is driven primarily by the descent of the subducting limb of a plate, called a slab, pulling the rest of the plate down behind it. The momentum of the massive sinking slabs overcomes the friction generated by the upper mantle adjacent to the slabs as they descend. New origin seen for Earth's tectonic plates. Nature Download citation. Published : 06 April Anyone you share the following link with will be able to read this content:. Sorry, a shareable link is not currently available for this article.

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Download PDF. Subjects Geology Geophysics Mathematics and computing Physics. Continual diving of crust into mantle is sufficient to explain formation of plate boundaries.

This interaction of tectonic plates is responsible for many different geological formations such as the Himalaya mountain range in Asia, the East African Rift, and the San Andreas Fault in California, United States. The idea that continents moved over time had been proposed before the 20 th century. However, a German scientist named Alfred Wegener changed the scientific debate.

Wegener published two articles about a concept called continental drift in He suggested that million years ago, a supercontinent he called Pangaea began to break into pieces, its parts moving away from one another. The continents we see today are fragments of that supercontinent. To support his theory, Wegener pointed to matching rock formations and similar fossils in Brazil and West Africa.

In addition, South America and Africa looked like they could fit together like puzzle pieces. Despite being dismissed at first, the theory gained steam in the s and s as new data began to support the idea of continental drift. Maps of the ocean floor showed a massive undersea mountain range that almost circled the entire Earth.

An American geologist named Harry Hess proposed that these ridges were the result of molten rock rising from the asthenosphere. As it came to the surface, the rock cooled, making new crust and spreading the seafloor away from the ridge in a conveyer-belt motion. Millions of years later, the crust would disappear into ocean trenches at places called subduction zones and cycle back into Earth.

There was one nagging question with the plate tectonics theory: Most volcanoes are found above subduction zones, but some form far away from these plate boundaries. As the plates rub against each other, huge stresses can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California.

Virgin Islands. Home Ocean Exploration Facts What features form at plate tectonic boundaries?