August 2002 AGGMAN Geology

Continental Drift

Plate Tectonics — Building the Earth’s Surface

New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading: a. the spreading ridge about five million years ago; b. about two to three million years ago; and c. present day.

The plate, in geologic terms, is a large, rigid slab of solid rock. The word tectonics comes from the Greek root “to build.” Plate tectonics refers to how the Earth’s surface is built of plates.
Last month, this column described the events leading up to German meteorologist Alfred Wegener’s theory of continental drift (circa 1915) and how the scientific community berated his theory. Wegener’s theory smoldered for more than three decades. But eventually new ideas developed and the spark was rekindled. All the scientific studies described below happened during the 1950s and 1960s.
One of the most important discoveries was that of a great mountain range on the ocean floor. This vast submarine mountain range, called the mid-ocean ridge, winds its way around the globe like the seam on a baseball. The mid-ocean ridge is the most prominent topographic feature on the surface of our planet.
Scientists studying the floor of the ocean discovered that rocks have different magnetic properties depending where on the ocean floor the rocks occur. When scientists mapped the magnetic properties of the ocean floor rocks, the rocks formed a zebra-striped pattern along either side of the mid-ocean ridge. Scientists also discovered that the rocks get progressively older the farther away they occur from the mid-ocean ridge. They determined that this zebra-striped pattern occurs because, when lava erupts from the sea floor at the mid-ocean ridge, it solidifies and makes new crust on the floor of the ocean. The older crust is moved away from the ridge. When the Earth’s magnetic field reverses (changes from north to south) between the two eruptions, crust formed by different lava flows preserve those different magnetic properties.
Harry Hess, a Princeton University (USA) geologist and U.S. Naval Reserve rear admiral, and Robert Deitz, a scientist with the U.S. Coast and Geodetic Survey, working independently, took much of these data and published similar theories that became known as seafloor spreading. They proposed that magma, created by radioactive heat in the Earth, rises and erupts along the mid-ocean ridges to form new oceanic crust. That new crust continuously spreads away from the ridges, like two conveyor belts facing away from one another. Millions of years after the crust is extruded from the mid-oceanic ridge, the crust “reaches the end of the conveyor belt” and eventually descends into oceanic trenches—very deep, narrow depressions along the rim of the Pacific Ocean basin. The net effect is that the Atlantic Ocean is expanding with the creation of new crust at the ridges while the Pacific Ocean is shrinking through destruction of old crust at the trenches. All the while, the continents are riding along on top of the crust.
The hypothesis of seafloor spreading was confirmed with data from an interesting source. A worldwide array of seismometers had been installed to monitor compliance of the 1963 treaty banning above-ground testing of nuclear weapons. Studies of data from these instruments revealed that most earthquakes are aligned along the mid-ocean ridge and oceanic trenches. These earthquake studies helped confirm the seafloor spreading hypothesis by locating the zones where Hess had predicted oceanic crust is being generated (along the ridges) and the zones where oceanic lithosphere sinks back into the mantle (beneath the trenches).
In 1965, Canadian geophysicist Tuzo Wilson introduced the term plate for the broken pieces of the Earth’s lithosphere. In 1967, Jason Morgan, a geophysicist from Princeton University, described how the Earth’s surface consists of 12 rigid plates that move relative to each other. During 1968, Xavier Le Pichon, a French oceanographer, published a synthesis showing the location and type of plate boundaries and their direction of movement.
Plates are either primarily oceanic plates or primarily continental plates, and the plates are moving relative to one another, floating on a viscous layer (the asthenosphere) within the Earth. In some places, those plates “collide” with one another. The consequences of those collisions depend on the nature of the plates. Earthquakes occur in areas where two plates are passing side-by-side, such as in southern California. In areas where one oceanic plate plunges down under another oceanic plate, they form deep oceanic trenches like the Marianas Trench, which is deeper (about 36,000 ft. below sea level) than Mount Everest is tall (about 29,000 ft. above sea level).
Where a continental plate meets an oceanic plate, such as off the coast of Peru and Chile, the continental plate overrides the oceanic plate. The overriding plate is lifted up, creating the towering Andes Mountains, the backbone of South America. The uplifting of those mountains creates strong, destructive earthquakes.
Where two continental plates meet head-on, the crust tends to buckle and be pushed upward or sideways. The Himalayan mountain range, the highest continental mountains in the world, dramatically demonstrates this spectacular consequence of plate tectonics.
The theory of plate tectonics, like many theories, is the result of the efforts of numerous scientists, working independently, but building on the results of their colleagues and predecessors. How the theory will evolve in the future, and whether or not it will continue to be valid, remains to be seen. But for the moment, after centuries of observing nature, we have a concept that explains how mountains are created.
To learn more about Plate Tectonics, see “This Dynamic Earth”, a web page created by the U.S. Geological Survey at http://pubs.usgs.gov/publications/text/dynamic.html.

William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.

August 2002 Carved in Stone Plate Tectonics — Building the Earth’s Surface				Continental Drift Plate Tectonics — Building the Earth’s Surface By Bill Langer New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading: a. the spreading ridge about five million years ago; b. about two to three million years ago; and c. present day. The plate, in geologic terms, is a large, rigid slab of solid rock. The word tectonics comes from the Greek root “to build.” Plate tectonics refers to how the Earth’s surface is built of plates. Last month, this column described the events leading up to German meteorologist Alfred Wegener’s theory of continental drift (circa 1915) and how the scientific community berated his theory. Wegener’s theory smoldered for more than three decades. But eventually new ideas developed and the spark was rekindled. All the scientific studies described below happened during the 1950s and 1960s. One of the most important discoveries was that of a great mountain range on the ocean floor. This vast submarine mountain range, called the mid-ocean ridge, winds its way around the globe like the seam on a baseball. The mid-ocean ridge is the most prominent topographic feature on the surface of our planet. Scientists studying the floor of the ocean discovered that rocks have different magnetic properties depending where on the ocean floor the rocks occur. When scientists mapped the magnetic properties of the ocean floor rocks, the rocks formed a zebra-striped pattern along either side of the mid-ocean ridge. Scientists also discovered that the rocks get progressively older the farther away they occur from the mid-ocean ridge. They determined that this zebra-striped pattern occurs because, when lava erupts from the sea floor at the mid-ocean ridge, it solidifies and makes new crust on the floor of the ocean. The older crust is moved away from the ridge. When the Earth’s magnetic field reverses (changes from north to south) between the two eruptions, crust formed by different lava flows preserve those different magnetic properties. Harry Hess, a Princeton University (USA) geologist and U.S. Naval Reserve rear admiral, and Robert Deitz, a scientist with the U.S. Coast and Geodetic Survey, working independently, took much of these data and published similar theories that became known as seafloor spreading. They proposed that magma, created by radioactive heat in the Earth, rises and erupts along the mid-ocean ridges to form new oceanic crust. That new crust continuously spreads away from the ridges, like two conveyor belts facing away from one another. Millions of years after the crust is extruded from the mid-oceanic ridge, the crust “reaches the end of the conveyor belt” and eventually descends into oceanic trenches—very deep, narrow depressions along the rim of the Pacific Ocean basin. The net effect is that the Atlantic Ocean is expanding with the creation of new crust at the ridges while the Pacific Ocean is shrinking through destruction of old crust at the trenches. All the while, the continents are riding along on top of the crust. The hypothesis of seafloor spreading was confirmed with data from an interesting source. A worldwide array of seismometers had been installed to monitor compliance of the 1963 treaty banning above-ground testing of nuclear weapons. Studies of data from these instruments revealed that most earthquakes are aligned along the mid-ocean ridge and oceanic trenches. These earthquake studies helped confirm the seafloor spreading hypothesis by locating the zones where Hess had predicted oceanic crust is being generated (along the ridges) and the zones where oceanic lithosphere sinks back into the mantle (beneath the trenches). In 1965, Canadian geophysicist Tuzo Wilson introduced the term plate for the broken pieces of the Earth’s lithosphere. In 1967, Jason Morgan, a geophysicist from Princeton University, described how the Earth’s surface consists of 12 rigid plates that move relative to each other. During 1968, Xavier Le Pichon, a French oceanographer, published a synthesis showing the location and type of plate boundaries and their direction of movement. Plates are either primarily oceanic plates or primarily continental plates, and the plates are moving relative to one another, floating on a viscous layer (the asthenosphere) within the Earth. In some places, those plates “collide” with one another. The consequences of those collisions depend on the nature of the plates. Earthquakes occur in areas where two plates are passing side-by-side, such as in southern California. In areas where one oceanic plate plunges down under another oceanic plate, they form deep oceanic trenches like the Marianas Trench, which is deeper (about 36,000 ft. below sea level) than Mount Everest is tall (about 29,000 ft. above sea level). Where a continental plate meets an oceanic plate, such as off the coast of Peru and Chile, the continental plate overrides the oceanic plate. The overriding plate is lifted up, creating the towering Andes Mountains, the backbone of South America. The uplifting of those mountains creates strong, destructive earthquakes. Where two continental plates meet head-on, the crust tends to buckle and be pushed upward or sideways. The Himalayan mountain range, the highest continental mountains in the world, dramatically demonstrates this spectacular consequence of plate tectonics. The theory of plate tectonics, like many theories, is the result of the efforts of numerous scientists, working independently, but building on the results of their colleagues and predecessors. How the theory will evolve in the future, and whether or not it will continue to be valid, remains to be seen. But for the moment, after centuries of observing nature, we have a concept that explains how mountains are created. To learn more about Plate Tectonics, see “This Dynamic Earth”, a web page created by the U.S. Geological Survey at http://pubs.usgs.gov/publications/text/dynamic.html. William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.