The technologies developed in the 1940s and 1950s also permitted more detailed mapping of the ocean floor and continental margins. A much better fit between the rifted continents is apparent when the shape of the continental slope is used instead of the continent's shoreline. Detailed mapping of distinctive rock units that extend out to sea along the South American and African coasts and North American and British coasts has shown that they would converge perfectly if the continents could be fitted together.
Sea floor spreading. In the 1960s, geologist Harry Hess proposed that the sea floor was moving outward from the midoceanic ridges. His theory of sea floor spreading maintained that new basaltic oceanic crust forms at a midoceanic ridge and is slowly pushed away on both sides toward the continents as more new crust is produced. (Measurements indicate that new crust moves away from a ridge at rates from 2 to 10 cm/year.)
A midoceanic ridge is called a spreading axis or spreading center. Subduction is the process by which the oceanic crust is pushed against, and finally underneath, continental or oceanic crust. Subduction zones are often marked by overlying chains of volcanic islands called island arcs. Geologists believe sea floor spreading results from convection in the mantle and lower crust that brings hotter, less dense, and more plastic material up toward the surface; the colder, more dense rock and sediment, such as subducted crustal material, sinks toward the mantle (Figure 1). These convective forces tear the ocean crust apart at the midoceanic ridge, forming a rift valley marked by high‐angle faults, basaltic lavas, and high heat flows. The Mid‐Atlantic Ridge is one of the best studied midoceanic ridges. It separates North America from Europe. Its 10,000‐foot‐tall mountain peaks lie about a mile below the surface of the ocean.
Sea Floor Spreading
As the hot mantle rock ascends toward a midoceanic ridge, it cools and starts to move laterally away from the ridge. This mantle movement drags the overlying oceanic crust along with it. The mantle material continues to cool, and eventually begins to sink. At this point, the oceanic crust begins to be subducted in deep oceanic trenches. Ocean trenches develop over the subduction zone where the subducted plate begins to bend and descend. Essentially great troughs, these are some of the deepest features on Earth, averaging about 10 kilometers in depth. The progressive cooling trend away from the ridge is illustrated by the low heat flows that have been measured at oceanic trenches. Subduction causes contact and friction with the overlying plate, which results in andesitic volcanoes and earthquakes along dipping Benioff zones.
The youngest oceanic crust is formed at the crest of a midoceanic ridge, and the crust becomes progressively older away from the ridge. The oldest oceanic crust is then subducted and reassimilated in the mantle. These events explain why oceanic crust is not as old as many continental rocks. Once cooled, the new crust is broken in half along the rift valley, each half moving away from the rift as new crust is formed. The oldest ocean crust is only about 150 to 200 million years old.
Magnetic anomalies. Magnetic surveys over the ocean floor in the 1960s revealed symmetrical patterns of magnetic “bands,” anomalies parallel to midoceanic rifts (Figure ). The same patterns in relation to midoceanic rifts are present in different oceans. The magnetic anomalies coincide with the episodes of magnetic reversals that have been documented from studies on land, indicating that the andesitic rocks that form new oceanic crust in the tensional setting of the rift valley record the earth's magnetic field as they cool. A rock has a normal (positive) polarity when its paleomagnetic field is the same as the earth's field today. The positive magnetism adds to the earth's magnetic field and creates a higher magnetic measurement at that location. Rocks are negatively polarized when the earth's field is reversed, which reduces the earth's net field strength. Since the ages of these anomalies are known from dating the paleomagnetic reversals on land, the rate of movement of the ocean floor can be calculated. The fact that new ocean crust moves away from the midoceanic ridge at speeds that range from 2 to 10 centimeters per year has also been documented using satellite measurements and radar. For example, if it is known that a segment of sea floor that formed 10.0 million years ago is now 50 kilometers (5.0 million cm) away from the crest of the ridge, it can be calculated that it traveled that distance at about 2 centimeters per year. By using the calculated ages for episodes of paleomagnetic reversal, scientists can construct sea floor age maps, which confirm that the youngest oceanic crust is presently being formed at midoceanic ridges and that the oldest is about 150 to 200 million years old, or late Jurassic in age. This older material is the farthest from the spreading centers and is the next crust to be subducted. Sea floor age maps have been proven correct by the age dates calculated from hundreds of rock samples gathered from the ocean floor.
Seismic studies. More proof for sea floor spreading comes from seismic studies indicating that earthquakes occur along the rift valley of a midoceanic ridge and the cross‐cutting fractures that offset it. Rift valley earthquakes occur only along transform faults, those portions of the fracture zone located between the offset sections of a ridge and rift valley. Because of the way in which the sea floor spreads (that is, away from both sides of a midoceanic ridge), transform faults are the only areas along the fracture zone in which sections of the oceanic crust pass one another in opposite directions. The concentration of earthquakes in the transform‐fault sections of the fracture zones further supports the concept of ocean crust moving away from a midoceanic ridge.
Modern plate tectonic theory. By the 1960s, the theories of continental drift and sea floor spreading were supported by reliable scientific data and combined to develop modern‐day plate tectonic theory. The theory maintains that the crust and uppermost mantle, or lithosphere, is segmented into a number of solid, rigid slabs called lithospheric plates. These slabs move slowly over the asthenosphere, the 200‐kilometer‐thick zone of more plastic mantle material that underlies the plates. New oceanic crust is created at the crests of the midoceanic ridges and pushed laterally away by new accumulations of crust. It begins to cool as it moves away from the high heat flows at the ridge. By the time it is subducted at the convergent boundary with another plate, it is cold and dense enough that it begins to sink back into the mantle. Subduction is also probably a function of a down‐turning mantle convection current below the converging plates.
Detailed geologic mapping on the continents has shown that crustal plates have been in motion, collided together to form new continental masses, and broken apart again many times in the past two billion years, and probably longer. Reconstructions of the paleoterrains in the geologic past reveal the same spatial relationships between terrain types that we see today along convergent, divergent, and transform boundaries. Suture zones that represent the lines along which separate crustal masses collided and joined together can be mapped in the field. Pangaea, the supercontinent that existed about 200 million years ago, consisted of other continental masses that had been welded together during previous plate tectonic collisions.
In summary, the compelling evidence for plate tectonic theory is
- the occurrence of most earthquakes, volcanic activity, and mountain‐building along plate boundaries
- the fit of continental edges
- the match of specific geologic features between continents, such as rock formations and fossils
- glacial striations on the southern continents
- the apparent variety of geographic pole locations
- the variety of magnetic pole locations
- the fact that midoceanic ridges parallel continental coastlines
- the symmetry of banded magnetic anomalies on the sea floor
- earthquake activity along transform faults
- “young” age dates of ocean floor rocks
- satellite measurements that indicate changes in distance between known points on different plates.