Plate tectonics

• 1. Introduction

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  Plate tectonics describes how Earth's rigid outer shell has been slowly moving for somewhere between 3 and 4 billion years. The lithosphere, the planet's stiff outer layer of crust and upper mantle, is broken into seven or eight major plates and a scatter of smaller ones. They drift at speeds ranging from zero to about 10 centimeters a year, slow enough to seem still, relentless enough to remake the surface. Where they meet, the ground answers back with earthquakes, volcanoes, rising mountains, and the deep canyons of oceanic trenches. For most of human history, no one believed any of this was possible. Earth had a solid crust, a mantle, a liquid core, and seemingly no way for pieces of that crust to wander. So how did a planet so apparently fixed turn out to be in constant motion? What evidence finally forced the change of mind? And why, of all the worlds we know, is Earth the only one with active plate tectonics today?

• 2. A Conveyor Belt Of Crust

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  Subduction is the act of one plate sliding down beneath another and into the mantle, and it quietly removes surface area from the planet. Along these convergent margins, the edge of a plate is carried into the depths, shrinking Earth's total crust. That loss has to be balanced somewhere. New oceanic crust is born at divergent margins through seafloor spreading, and the two processes cancel out. The surface area of Earth stays constant, recycled endlessly in what amounts to a tectonic conveyor belt.

  Tectonic plates carry two kinds of crust on their backs. Oceanic crust forms at sea-floor spreading centers, while continental crust is built through arc volcanism and the accretion of terranes. Oceanic crust holds less silicon and more of the heavier elements, which makes it denser. Because of that density gap, oceanic crust tends to ride below sea level while continental crust floats buoyantly above it.

  Average oceanic lithosphere is typically about 100 kilometers thick, and its thickness depends on its age. At a mid-ocean ridge it can be as thin as 6 kilometers, thickening to more than 100 kilometers by the time it reaches a subduction zone. Continental lithosphere is heftier still, around 200 kilometers thick, though it varies widely between basins, mountain ranges, and the stable cratonic hearts of continents. Some fragments of oceanic crust never go down at all. Known as ophiolites, they failed to subduct and were instead shoved upward and preserved inside continental crust, a marker that an ocean once existed and vanished.

• 3. Three Ways To Meet

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  Divergent boundaries are where two plates slide apart, and at sea they build entirely new ocean basin through seafloor spreading. The Mid-Atlantic Ridge and the East Pacific Rise are made this way, the ridge rising at the spreading center while the basin widens. The same splitting can tear a continent: the East African Rift, the Baikal Rift, the West Antarctic Rift, and the Rio Grande Rift all mark places where land is pulling apart and may one day flood.

  Convergent boundaries bring plates crashing together. Where dense oceanic lithosphere meets a continent, it plunges beneath in ocean-to-continent subduction and raises mountain ranges like the Andes and the Cascade Range. Where ocean meets ocean, the older, colder, denser crust slips under and magma rises to build curving chains of volcanic islands, such as the Aleutian Islands, the Mariana Islands, and the Japanese island arc. As a subducted plate heats, it releases water from hydrous minerals into the mantle above, lowering its melting point and triggering volcanism. When two continents collide, neither one is dense enough to sink, so their edges fold and uplift instead, building the Himalayas and the Alps.

  Transform boundaries neither make crust nor destroy it. Here two plates grind past each other along transform faults, their relative motion either sinistral or dextral, and strong earthquakes can follow. The San Andreas Fault in California is the classic example, exhibiting dextral motion. Beyond these three clean categories lie messier zones, broad belts where the boundary is poorly defined and the movement shifts from one episode to the next.

• 4. What Moves The Plates

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  Slab pull is now thought to be the greatest force acting on the plates. When new crust forms at a mid-ocean ridge it is less dense than the asthenosphere beneath it, but as it cools and thickens with age it grows denser. Eventually the old, cold lithosphere sinks into the deep mantle at a subduction zone, and its sinking weight drags the rest of the plate along behind it. The weakness of the asthenosphere lets the plate slide toward the trench with little resistance.

  Mantle convection sits at the root of all this, the slow creeping motion of Earth's solid mantle driven by lateral density variations. For much of the early twentieth century, large-scale convection currents in the upper mantle were the leading explanation, a theory launched by Arthur Holmes and others in the 1930s. Seismic tomography later revealed that the mantle's density really does vary from place to place, whether through rock chemistry, mineral structure, or temperature. How exactly that convection couples to the plates above remains a matter of ongoing study in geodynamics.

  Gravitational sliding offers a quieter contribution, often misleadingly called ridge push. Newly formed lithosphere stands high at the spreading ridge and subsides as it cools, creating a slight downhill incline away from the axis. There is no real horizontal shove; tensional features actually dominate the ridges. Other proposals reach toward the sky. Tidal drag from the Moon and the Sun has been defended as a possible driver, and in 1973 George W. Moore of the USGS and R. C. Bostrom argued for a general westward drift of the lithosphere based on the steepness of subduction zones. A 2022 paper by Hofmeister and colleagues revived that lunar idea, even as the Coriolis and centrifugal forces are dismissed as negligible.

• 5. Fifty Years To Believe It

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  In 1912 the meteorologist Alfred Wegener described continental drift, an idea that would take fifty years to mature into modern plate tectonics. He expanded it in his 1915 book The Origin of Continents and Oceans, picturing the present continents as fragments of a single former landmass, later called Pangaea, drifting apart like icebergs of light sial on a sea of denser sima. The match between South America's east coast and Africa's west coast was old news, but Wegener marshaled the fossil evidence: the plants Glossopteris and Gangamopteris and the mammal-like reptile Lystrosaurus, scattered across South America, Africa, Antarctica, India, and Australia.

  Harold Jeffreys and Charles Schuchert ranked among the outspoken critics, and the central objection was fatal. There seemed to be no mechanism. No one could explain how continental rock might plow through the denser rock of the ocean floor, and Wegener's vindication did not come until after his death in 1930. The South African geologist Alex du Toit pressed on, gathering supporting evidence in his 1937 book Our Wandering Continents and stressing the links between the Gondwana fragments.

  Paleomagnetism cracked the door open. Rocks of different ages record different magnetic field directions, and the apparent path of the north pole could be read two ways: either the pole wandered, or the continents did. In a 1956 paper Keith Runcorn used this to argue for continental drift, work taken up by his students Ted Irving and Ken Creer. That same March, S. Warren Carey organized a symposium on continental drift in Tasmania, where some participants instead championed an expanding Earth, a theory ultimately set aside for lack of a convincing mechanism.

• 6. Reading The Ocean Floor

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  In 1947 a team led by Maurice Ewing aboard the Woods Hole research vessel Atlantis confirmed a rise running down the central Atlantic and found the seabed below the sediments was basalt, not the granite of the continents. The oceanic crust was also far thinner than continental crust. Mapping soon revealed that a whole system of mid-oceanic ridges circled the globe, and that new ocean floor was being created along it. Bruce Heezen described this Great Global Rift in a pivotal 1960 paper drawing on his work with Marie Tharp.

  Harry Hammond Hess, a Princeton geologist and Naval Reserve Rear Admiral, and Robert S. Dietz, who coined the term seafloor spreading, grasped what it meant. If crust was spreading away from the ridges in a conveyor-belt motion, it had to be consumed elsewhere, descending into the oceanic trenches along the rim of the Pacific basin. Hess concluded the Atlantic was expanding while the Pacific was shrinking, the ocean basins perpetually recycled. The idea neatly explained why Earth was not growing, why so little sediment had piled up on the floor, and why oceanic rocks are so much younger than continental ones.

  Magnetic instruments adapted from wartime submarine detection turned up the decisive clue. Basalt carries the magnetic mineral magnetite, and as new rock cools it locks in the direction of Earth's field at that moment. Ron G. Mason and co-workers published the resulting zebra pattern of alternating normal and reversed stripes in 1961 without explaining it. In 1963 Lawrence Morley, and independently Fred Vine and Drummond Matthews, tied those stripes to geomagnetic reversals. The bands are symmetrical around the ridge crest, youngest at the crest and older outward, turning the ocean floor into a natural tape recording of Earth's magnetic past.

• 7. The Theory Takes Shape

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  In 1965 Tuzo Wilson added the concept of transform faults, supplying the last class of fault the moving plates required. That same year a symposium at the Royal Society of London marked the official start of acceptance, and there Edward Bullard and co-workers used a computer to show how the continents on either side of the Atlantic best fit together, the famous Bullard's Fit. Wilson followed in 1966 with the paper that introduced the Wilson Cycle.

  W. Jason Morgan proposed in 1967, at the American Geophysical Union's meeting, that Earth's surface is made of 12 rigid plates moving relative to one another. Two months later Xavier Le Pichon published a full model built on six major plates and their relative motions, the step that sealed the scientific community's acceptance. McKenzie and Parker presented a similar model that same year, using translations and rotations on a sphere to define the motions. With the kinematics settled, the debate shifted to the forces, drawing on Holmes's convection and the slab-pull work of Elsasser, Solomon, Sleep, Uyeda, and Turcotte.

• 8. Worlds Without Plates

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  Water may be the reason Earth moves and its neighbors do not. Plate tectonics requires weak surfaces in the crust along which slices can slide, and water plays a central role in forming those shear zones. Earth's crust is soaked with it; Venus, where temperatures run too high for significant water, may never have developed that weakening, which is one explanation for its lack of active plate tectonics. Venus also has well-preserved impact craters suggesting it underwent an essentially complete volcanic resurfacing at least once in its distant past.

  Mars is smaller than both Earth and Venus and carries ice on its surface and in its crust. In the 1990s its Crustal Dichotomy was proposed to be tectonic, though scientists later traced it to mantle upwelling that built the Southern Highlands and Tharsis, or to a giant impact that carved the Northern Lowlands. The Mars Global Surveyor spacecraft detected magnetic striping in 1999, but the data failed a magnetic reversal test, leaving the case unproven. Beyond the rocky planets, Jupiter's moon Europa shows signs of ice crustal plates moving and interacting much like Earth's. The reach of the question keeps growing: according to a hypothesis from Robert Stern and Taras Gerya, plate tectonics may be a necessary condition for complex life, which is why it now figures into the search for life beyond Earth.