- Plate boundaries
- Convergent margins
- Plate boundaries
- Alfred Wegener and the concept of continental drift
- Renewed interest in continental drift
- Toward a unifying theory
Magnetic anomalies, transform faults, hotspots, and apparent polar wandering paths permit rigorous geometric reconstructions of past plate positions, shapes, and movements. Although some important controversies remain, these paleogeographic reconstructions show the changing geography of Earth’s past and can be determined with excellent precision for the past 150 million years. Before that time, however, the absence of the ocean-floor record makes the process significantly more challenging. A variety of geologic data are used to help determine the proper fit of continents through time. Some of the methods used to test these reconstructions are based on matching patterns from one continental block to another and are similar to the approach of Wegener. However, modern geoscientists have more precise data that help constrain these reconstructions. Of the many advances, perhaps the most significant are the improved analytical techniques for radiometric dating, allowing the age of geologic events to be determined with much greater precision. One of the most common methods used measures the radioactive decay of uranium to lead in the mineral zircon by comparing the ratio of one to the other in the sample of zircon. Zircon is a common accessory mineral in igneous, metamorphic, and sedimentary rocks. Modern techniques typically yield age determinations with an estimated error of 2 million years or less, even for rocks of Archean age.
Since the 1990s the database has improved so that reasonably constrained reconstructions can now be made as far back as 1 billion years. For example, the abundance of continental-collisional events about 1.1 billion years ago is one of the principal lines of evidence suggesting the presence of a supercontinent that is given the name of Rodinia. (See also Tonian Period.) By about 750 million years ago a number of continental-rift sequences had developed, suggesting that Rodinia had begun to break up. Between about 650 million and 550 million years ago, however, a number of mountain belts formed by continental collision, which resulted in the amalgamation of Gondwana, the supercontinent originally identified by Du Toit in 1937. The continental fragment that rifted away from Laurentia did not return to collide with North America as predicted by a simple Wilson cycle. Instead, it rotated counterclockwise away from Laurentia until it collided with eastern Africa.
Interactions of tectonics with other systems
As plate tectonics changes the shape of ocean basins, it fundamentally affects long-term variations in global sea level. For example, the geologic record in which thick sequences of continental shelf sediments were deposited demonstrates that the breakup of Pangea resulted in the flooding of continental margins, indicating a rise in sea level. There are several contributing factors. First, the presence of new ocean ridges displaces seawater upward and outward across the continental margins. Second, the dispersing continental fragments subside as they cool. Third, the volcanism associated with breakup introduces greenhouse gases in the atmosphere, which results in global warming, causing continental glaciers to melt.
As an ocean widens, its crust becomes older and denser. It therefore subsides, eventually forming ocean trenches. As a result, ocean basins can hold more water, and sea level drops. This changes once again when subduction commences. Subduction preferentially consumes the oldest oceanic crust, so that the average age of oceanic crust becomes younger. Younger oceanic crust is therefore more buoyant and has a higher elevation, a circumstance that causes sea level to rise once more.
Composition of ocean water
Water’s strong properties as a solvent mean that it is rarely pure. Ocean water contains about 96.5 percent by weight pure water, with the remaining 3.5 percent predominantly consisting of ions such as chloride (1.9 percent), sodium, (1.1 percent), sulfate (0.3 percent), and magnesium (0.1 percent). The drainage of water from continents via the hydrologic cycle plays an important role in transporting chemicals from the land to the sea. The effect of this drainage is profoundly influenced by the presence of mountain belts. For example, the erosional power of the Ganges River, which drains from the Himalayas, carries 1.45 billion metric tons of sediment to the sea annually. This load is nine times that of the Mississippi River. The processes of weathering and erosion strip soluble elements such as sodium from their host minerals, and the relatively high concentration of sodium in ocean water is attributed to the weathering and erosion that accompanies continental drainage.
Until the advent of plate tectonics, discovering the source of chlorine was problematic because chlorine is present in only very minor amounts in the continental crust. Scientists hypothesized that the source of this element may lie in underwater volcanic activity. In the late 1970s three scientists investigating the oceanic ridge off the coast of Peru from a submersible craft documented the occurrence of superheated jets of water, up to 350 °C (660 °F), continuously erupting from chimneys that stood 13 metres (43 feet) above the ocean floor. These hot springs were found to be rich in chlorine and metals, confirming that the source of chlorine in the oceans lay in the tectonic processes occurring at oceanic ridges.
Plate tectonics has influenced the evolution and propagation of life in a variety of ways. The study of oceanic ridges revealed the presence of bizarre life adjacent to the chimneys of superheated water that together makes up about 1 percent of the world’s ecosystems. The existence of these life-forms in the deep ocean cannot be based on photosynthesis. Instead, they are nourished by minerals and heat. The energy released when hydrogen sulfide in the vent reacts with seawater is used by bacteria to convert inorganic carbon dioxide dissolved in seawater into organic compounds, a process known as chemosynthesis. Some scientists speculate that the cumulative influence of this process over time has had a significant effect on evolution. Others suggest that similar processes may ultimately be responsible for the origin of life on Earth.
The continuous rearrangement of the size and shape of ocean basins and continents over geologic time, accompanied by changes in ocean circulation and climate, had a major impact on the development of life on Earth. One of the first studies of the potential effects of plate tectonics on life was published in 1970 by American geologists James W. Valentine and Eldridge M. Moores, who proposed that the diversity of life increased as continents fragmented and dispersed and diversity diminished when the continents were joined together.
When Pangea began to rift apart and the Atlantic Ocean started to open during the middle Mesozoic, the differences between the faunas of opposite shores gradually increased in an almost linear fashion—the greater the distance, the smaller the number of families in common. The difference increased more rapidly in the South Atlantic than in the North Atlantic, where a land connection between Europe and North America persisted until about 60 million years ago.
After the breakup of Pangea, no land animal or group of animals could become dominant, because the continents were disconnected. As a result, the separated landmasses evolved highly specialized fauna. South America, for example, was rich in marsupial mammals, which had few predators. North America, on the other hand, was rich in placental mammals.
However, about 3 million years ago, volcanic activity associated with subduction of the eastern Pacific Ocean formed a land bridge across the isthmus of Panama, reconnecting the separate landmasses. The emergence of the isthmus made it possible for land animals to cross, forcing previously separated fauna to compete. Numerous placental mammals and herbivores migrated from north to south. They adapted well to the new environment and were more successful than the local fauna in competing for food. The invasion of highly adaptable carnivores from the north contributed to the extinction of at least four orders of South American land mammals. A few species, notably the armadillo and the opossum, managed to migrate in the opposite direction. Ironically, many of the invading northerners, such as the llama and tapir, subsequently became extinct in their region of origin and found their last refuge to the south.