Plate Tectonics(Adapted from an article by Becky Oskin, LiveScience 2017)

From the deepest ocean trench to the tallest mountain, plate tectonics explains the features and movement of Earth's surface in the present and the past.

Plate tectonics is the theory that Earth's crust is divided into several plates that glide over the mantle (the rocky middle layer above the core). The plates act like a hard, rigid shell. This strong outer shell is called the crust, which is about 20 miles thick. Below the crust is the mantle, which is hot enough that the rock is melted into a liquid. And due to convection currents, that molten rock moves around. But how it moves around is an evolving idea.

History:Plate tectonics is the modern version of “continental drift,” a theory first proposed by scientist Alfred Wegener in 1912. Wegener noticed that the various continents on Earth seemed to fit together, almost like a jigsaw puzzle. The corner of South America fit nicely into the eastern side of Africa, and India also fit snugly between Africa and Antarctica. In 1915, Wegener published a book that proposed his theory of “continental drift,” arguing that all the continents were once joined together as a single landmass and had since drifted apart.

But Wegener could never explain how continents could move around the planet. Fast forward about 50 years, scientists began to collect rock samples, fossils, and other evidence. And all of it seemed to support Wegener’s idea. In the modern day, the theory is almost universally accepted. We now call it plate tectonics. “It’s the unifying theory of geology,” said Nicholas van der Elst, a seismologist at Columbia University, “Before plate tectonics, people had to come up with explanations of the geologic features in their region that were unique to that particular region. Plate tectonics unified all these descriptions and said that you should be able to describe all geologic features as though driven by the relative motion of these tectonic plates.”

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How Many Plates Are There?There are nine major plates, according to World Atlas. These plates are named after the landforms found on them. The nine major plates are North American, Pacific, Eurasian, African, Indo-Australian, Australian, Indian, South American and Antarctic.

The largest plate is the Pacific Plate at 39,768,522 square miles. Most of it is located under the ocean. It is moving northwest at a speed of around 2.75 inches per year. There are also many smaller plates throughout the world.

How Plate Tectonics Works:The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth's core rises, and the colder rock sinks. Sort of like boiling water on a hot stove. These convection currents drive plates tectonics through a combination of pushing and pulling. At mid-ocean ridges, the plates spread apart and at subduction zones, the plates stack on top of each other. Scientists continue to study and debate the mechanisms that move the plates.

Mid-ocean ridges are gaps between tectonic plates, similar to seams on a baseball. Hot magma wells up at the ridges, forming new ocean crust and shoving the plates apart. At subduction zones, two tectonic plates meet and one slides beneath the other back into the mantle (the layer underneath the crust). The cold, sinking plate pulls the crust behind it downward. Many spectacular volcanoes are found along subduction zones, such as the “Ring of Fire” that surrounds the Pacific Ocean.

Plate Boundaries:

Subduction zones, or convergent margins, are one of the three types of plate boundaries. The others are divergent and transform margins. At a divergent margin, two plates are spreading apart, as at seafloor-spreading ridges or continental rift zones such as the East Africa Rift. Transform margins mark slip-sliding plates, such as California's San Andreas Fault, where the North America and Pacific plates grind past each other with a mostly horizontal motion.

Reconstructing the Past:While the Earth itself is 4.54 billion years old, because oceanic crust is constantly recycled at subduction zones, the oldest seafloor is only about 200 million years old. The oldest ocean rocks are found in the northwestern Pacific Ocean and the eastern Mediterranean Sea. Specific fragments of continental crust are much older, with large chunks at least 3.8 billion years found in Greenland.

With clues left behind in the form of rocks and fossils, geoscientists can reconstruct the past history of Earth's continents. Most researchers think modern plate tectonics began about 3 billion years ago, based on ancient magmas and minerals preserved in rocks from that period. “We don't really know when plate tectonics as it looks today got started, but we do know that we have continental crust that was likely scraped off a down-going slab [a tectonic plate in a subduction zone] that is 3.8 billion years old,” Van der Elst said. “We could guess that means plate tectonics was operating, but it might have looked very different from today.”

As the continents jostle around the Earth, they occasionally come together to form giant supercontinents, a single landmass. One of the earliest big supercontinents, called Rodinia, assembled about 1 billion years ago. Its breakup is linked to a global glaciation called Snowball Earth. A more recent supercontinent called Pangaea formed about 300 million years ago. Africa, South America, North America and Europe nestled closely together, leaving a characteristic pattern of fossils and rocks for geologists to decipher once Pangaea broke apart. The puzzle pieces left behind by Pangaea, from fossils to the matching shorelines along the Atlantic Ocean, provided the first hints that the Earth's continents actually move.

Plates bumping into each other can also cause mountain ranges. For example, India and Asia came together about 55 million years ago, which created the Himalaya Mountains, according to National Geographic. And on the scarier side, plate boundaries can also create earthquakes (as plates get stuck and the slip past each other), volcanoes (if any magma escapes to the surface), and tsunamis (if an earthquake happens underwater, look out!) So study hard, everyone. One of the keys to predicting these disasters is understanding plate tectonics!

Did You Know?One of the most fascinating effects of plate tectonics is the distribution of fossils. One example is an extinct reptile called Mesosaurus. It was a sea-dwelling creature that inhabited shallow coastal waters and flourished around 275 million years ago. This fossil is only found in two locations, in southern Africa and near the southern tip of South America. Today, these areas are separated by almost 5,000 miles, the entire Atlantic Ocean!