Continental drift is the hypothesis, developed by Alfred Wegener, that the continents had once been joined together to form a single supercontinent (Pangaea) and then drifted apart. Harry Hess proposed that as continents drift apart, new ocean floor forms between them by seafloor spreading. Hess and others suggested that continents move toward each other when the old ocean floor between bends down and sinks back down into the Earth’s interior, a process now called subduction. In the model encompassing continental drift, seafloor spreading, and subduction, the Earth’s lithosphere consists of about 20 distinct plates that slowly move relative to each other. This model is known as plate tectonics.
Wegner provided multiple lines of evidence for continental drift. The coastlines of the continents suggested they once fit together. Sediments and striations indicative of late Paleozoic glaciation occurred in widely separated areas that, with the exception of Antarctica, do not currently lie in cold polar regions, but fit together within a southern polar ice cap on a map of Pangaea. Late Paleozoic sedimentary rock layers include material that define climate belts at appropriate latitudes (e.g., equatorial and subtropical) on a map of Pangaea. Fossils representing land-dwelling species that lived during the late Paleozoic and early Mesozoic exist on continents now separated by oceans; this distribution of fossils requires that the continents were once adjacent to each other. The same types of Precambrian rock assemblages occurred on the eastern coast of South America and the western coast of Africa, which would have been adjacent to each other on Pangaea and therefore could have composed continuous blocks or belts; also, features of mountains in North America closely resemble mountains on other continents, regions which would have lain adjacent to each other on Pangaea. Thus, Wegner’s model of a supercontinent that later broke apart explained the distribution of ancient glaciers, coal, sand dunes, rock assemblages, and fossils.
Bathymetric maps reveal several important features. The floor beneath all major oceans includes abyssal plains, which are broad, relatively flat regions of the ocean that lie at a depth of about 4 to 5 km below sea level, and mid-ocean ridges, submarine mountains whose peaks lie only about 2 to 2.5 km below sea level. Along much of the perimeter of the Pacific ocean, and in a few additional localities, the seafloor reaches depths greater than 5 km, referred to as trenches. Trenches border volcanic arcs, curving chains of volcanoes. Fracture zones, narrow bands of vertical cracks and broken-up rock, cut the ocean floor of mid-ocean ridges. Numerous volcanic islands poke up from the ocean floor and sonar has detected seamounts, isolated submarine mountains.
Many important characteristics of the seafloor demonstrate that oceanic crust differs from continental crust. The layer of sediment composed of clay and shells of dead plankton becomes progressively thicker away from the mid-ocean ridge axis; also, the sediment layer just above the basalt gets progressively older with increasing distance from the ridge axis. Oceanic crust consists primarily of basalt; it does not contain the great variety of rock types found on continents. More heat rises beneath mid-ocean ridges than elsewhere. Earthquakes occur in distinct seismic belts, such as along trenches, mid-ocean ridge axes, and fracture zones; these features are places where motion of one part of the Earth, relative to the adjacent part, is taking place. The ridge axis of some mid-ocean ridges is marked by a narrow trough. From these characteristics, Harry Hess suggested that magma rises upward at mid-ocean ridges, and that this material solidifies to form the basalt of oceanic crust. The new seafloor then moves away from the ridge axis, leading to the widening of the oceanic basin (seafloor spreading). The old ocean floor is consumed at deep-ocean trenches, where the seafloor sinks back into the mantle (subduction). Earthquakes at trenches are evidence of this movement. Seafloor spreading provided the explanation for how continental drift occurs. Continents passively move apart as the seafloor between them spreads at mid-ocean ridges, and they passively move together as the seafloor between them sinks back into the mantle, or subducts, at trenches. (Geologists now realize that the whole lithosphere moves, not just the crust.)
An understanding of paleomagnetism provided proof of both continental drift and seafloor spreading. The magnetic axis is not parallel to the axis of rotation. The magnetic field is generated by flow in the outer core. The magnetic pole and the geographic pole do not coincide. The difference is magnetic declination. The position of the magnetic poles has changed over time. Each continent has its own unique polar-wander path, therefore the continents must move with respect to each other. The polarity of Earth’s magnetic field reverses every now and then, and quickly. During some intervals of time, the Earth has normal polarity, with the north magnetic pole near the north geographic pole. During other intervals of time, the Earth has reversed polarity, with the north magnetic pole near the south geographic pole. These polarity reversals cause magnetic anomalies over the seafloor, proving seafloor spreading takes place. The polarity of basalt in the oceanic crust depends on the polarity of the Earth’s magnetic field at the time the crust forms.
The modern theory of plate tectonics evolved from these discoveries. Geoscientists distinguish 12 major plates and several microplates. Plate tectonics theory can be summarized as follows:
- The Earth’s lithosphere is divided into plates that move relative to each other. As a plate moves, its internal area remains mostly, but not perfectly, rigid and intact.
- Rock along plate boundaries undergoes intense deformation (cracking, sliding, bending, stretching, and squashing) as the plate grinds or scrapes against its neighbors or pulls away from its neighbors.
- As plates move, so do continents that form part of the plates.
- Because of plate tectonics, the map of the Earth’s surface constantly changes.
Earthquake (seismic) belts define the position of plate boundaries because the fracturing and slipping that occurs along plate boundaries generates earthquakes. Active margins are plate boundaries; passive margins are not. A boundary at which two plates move apart from each other is a divergent boundary. A boundary at which two plates move toward each other so that one plate sinks beneath the other is a convergent boundary. A boundary at which two plates slide sideways past each other is a transform boundary.
Open space does not develop between diverging plates. Rather, as the plates move apart, new oceanic lithosphere forms continually along the divergent boundary. This takes place at mid-ocean ridges. As seafloor spreading takes place, hot asthenosphere rises beneath the ridge and begins to melt, and molten rock, or magma, forms. The rock from this magma becomes new oceanic crust. New crust moves away from the ridge axis and more magma rises from below, so still more crust forms. The youngest seafloor occurs at the ridge axis, and seafloor becomes progressively older away from the ridge. The oldest seafloor on our planet underlies the western Pacific Ocean; this crust formed at about 200 million years ago. The tension (stretching force) applied to newly formed solid crust as spreading takes place breaks the crust, resulting ini the formation of faults. Movement (slip) on the faults causes divergent-boundary earthquakes. As the newly formed oceanic crust moves away from the ridge axis, the crust and uppermost mantle directly beneath it gradually cool and becomes, by definition, part of the lithosphere. As oceanic lithosphere continues to move away from the ridge axis, it continues to cool, so the lithospheric mantle grows progressively thicker. The rate at which lithospheric thickening occurs decreases progressively with increasing distance from the ridge axis. As lithosphere thickens and gets cooler and denser, it sinks down into the asthenosphere. So the ocean becomes deeper over older ocean floor than over younger ocean floor. That’s why abyssal plains are deeper than mid-ocean ridges.
Convergent boundaries are also known as subduction zones. The amount of oceanic plate consumption worldwide, averaged over time, equals the amount of seafloor spreading worldwide, so the surface area of the Earth remains constant through time. Subduction occurs because oceanic lithosphere is denser than the underlying asthenosphere. The downgoing plate must be composed of oceanic lithosphere. The overriding plate can consist of either oceanic or continental lithosphere. Continental lithosphere cannot be subducted because it is too buoyant. Some continental crust has survived more than 3.8 billion years. Earthquakes are generated when downgoing plates grind along the base of overriding plates. Volcanic arcs can form on the edge of a continent or as a chain of islands in the sea.
Mid-ocean ridges consist of short segments that appear to be offset laterally from each other by narrow belts of broken and irregular seafloor called fracture zones. At a transform boundary, one plate slides sideways, relative to its neighbor, on a vertical fault. The slip direction on the fault is horizontal, so no new plate forms and no old plate is consumed. Most transform boundaries link segments of mid-ocean ridges, but some cut across continental crust.
A triple junction is the point where three plate boundaries intersect. At volcanic arcs, all of the volcanoes are active. However, not all volcanoes are plate-boundary volcanoes. There are about 100 volcanoes that exist as isolated points called hot spots. Active hot-spot volcanoes occur at the end of a chain of extinct hot-spot volcanoes. The active volcano represents the present-day location of the heat source, whereas the chain of extinct volcanic islands represents locations on the plate that were once over the heat source but progressively moved off. The position of the heat source is fixed, relative to the moving plate above. The suggested heat source is a mantle plume, a column of very hot rock rising up through the mantle to the base of the lithosphere.
Most new divergent boundaries form when a continent splits and separates into two continents, a process called rifting. A continental rift is a linear belt in which continental lithosphere pulls apart; the lithosphere stretches horizontally and thins vertically. Blocks of rock slip down the fault surfaces, leading to the formation of a rift valley. Eruption of molten rock produces volcanoes along the rift. If rifting continues long enough, the continent breaks in two, a new mid-ocean ridge forms, and seafloor spreading begins.
Collision is the process during which two buoyant pieces of lithosphere, such as a continent or island arc, converge and squeeze together. When a collision concludes, the convergent boundary that once existed between the two colliding pieces cease to exist. Collisions yielded mountain ranges such as the Himalayas and the Alps. The boundary between what was once two separate continents is called a suture.
Plate tectonics is driven by convection along with ridge-push force and slab-pull force. Convection occurs on a very broad scale as in some places deeper, hotter asthenosphere is rising (upwelling) and in other places shallower, colder asthenosphere is sinking (downwelling). Ridge-force push develops because the lithosphere of mid-ocean ridges lies at a higher elevation than that of the adjacent abyssal plains; gravity causes the elevated lithosphere at the ridge axis to push on the lithosphere that lies farther from the axis, making it move away. Slab-pull force develops because the lithosphere is denser than the underlying asthenosphere and sinks; once an oceanic plate starts to sink, it gradually pulls the rest of the plate along behind it. Plates move at rates of about 1 to 15 cm per year.
—January 2021
—March 2023
Lance Jones 