DESARROLLO DE LOS PROCESOS DE SU PUESTO DE TRABAJO NOMBRE DEL PROCESO: Trámites IGSS

The forces that influence the movement of tectonic plates naturally affect the arrangement of the lithosphere; how- ever, they also have the ability to change other aspects of the Earth system. In the hydrosphere, plate movement can alter the shape of the ocean basins as well as the chem- istry of sea water. Tectonic activity can create, as well as

destroy, habitats. In addition, modifications to the arrangement of continents may alter the path of ocean currents and thus bring about climatic changes.

Earth’s Oceans

Sea Level

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 the 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 subduc- tion commences. Subduction preferentially consumes the oldest oceanic crust, so that the average age of oceanic crust becomes younger, and the material therefore becomes more buoyant. This increased buoyancy 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 of pure water, with the remaining 3.5 percent pre-

dominantly 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 con- tinents 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 ero- sion easily strip soluble elements such as sodium from their host minerals, and the relatively high concentration of sodium in ocean water is attributed to the effects of continental drainage.

Until the advent of plate tectonics, uncovering 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 ele- ment 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 tec- tonic processes occurring at oceanic ridges.

Life

The continuous rearrangement over time of the size and shape of ocean basins and continents, accompanied by changes in ocean circulation and climate, has 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 diminished when they were joined together.

Deep Sea Vents

Plate tectonics has influenced the evolution and propa- gation 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 make up about 1 percent of the world’s ecosystems. The exis- tence 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 utilized by bac- teria to convert inorganic carbon dioxide dissolved in seawater into organic compounds, a process known as chemosynthesis. Some scientists speculate that the cumulative effects of this process over time have had a significant effect on evolution. Others suggest that simi- lar processes may ultimately be responsible for the origin of life on Earth.

Evolution

When Laurentia began to rift apart and the Atlantic Ocean started to open during the middle Mesozoic, the differences between the faunas of opposite shores gradu- ally 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 connec- tion between Europe and North America persisted until about 60 million years ago.

After the breakup of Pangea, no land animal could become dominant because the continents were discon- nected. As a result, separate 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 mam- mals. However, about three 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 adapt- able 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 country of origin and found their last refuge to the south.

Extinction

Perhaps the most dramatic example of the potential impact of plate tectonics on life occurred toward the end of the Permian Period (about 300 to 250 million years ago). During this time, several extinction events caused the permanent disappearance of half of Earth’s known biological families. The marine realm was most affected, losing more than 90 percent of its species. This drop may be attributed in part to biogeographic changes associated with the formation of Pangea. Other factors, such as a

sharp decrease in the area of shallow-water habitats, or a change in ocean fertility due to a lack of upwelling of nutrient-rich deep currents, have also been invoked.

The extinction had a complex history. High latitudes were affected first as a result of the waning of the Permian ice age when the southern edge of Pangea moved off the South Pole. The equatorial and subtropical zones appear to have been affected somewhat later by a global cooling. On the other hand, the extinctions were not felt as strongly on the continent itself. Instead, the vast semiarid and arid lands that emerged on so large a continent, the shortening of its moist coasts, and the many mountain ranges formed from the collisions that led to the formation of the super- continent provided strong incentives for evolutionary adaptation to dry or high-altitude environments.

Earth’s Climate

Climate changes associated with the supercontinent of Pangea and with its eventual breakup and dispersal pro- vide an example of the effect of plate tectonics on paleoclimate. Pangea was completely surrounded by a world ocean (Panthalassa) extending from pole to pole and spanning 80 percent of the circumference of Earth at the paleoequator. The equatorial current system, driven by the trade winds, resided in warm latitudes much longer than today, and its waters were therefore warmer. The gyres that occupy most of the Southern and Northern hemispheres were also warmer, and consequently the tem- perature gradient from the paleoequator to the poles was less pronounced than it is at present.

Early in the Mesozoic, Gondwana split from its north- ern counterpart, Laurasia, to form the Tethys seaway, and the equatorial current became circumglobal. Equatorial surface waters were then able to circumnavigate the world and became even warmer. How this flow influenced

circulation at higher latitudes is unclear. From about 100 to 70 million years ago, isotopic records show that Arctic and Antarctic surface water temperatures were at or above 10 °C (50 °F), and the polar regions were warm enough to support forests.

As the dispersal of continents following the breakup of Pangea continued, however, the surface circulation of the oceans began to approach the more complex circula- tion patterns of today. About 100 million years ago, the northward drift of Australia and South America created a new circumglobal seaway around Antarctica, which remained centred on the South Pole. A vigorous circum- Antarctic current developed, isolating the southern continent from the warmer waters to the north. At the same time, the equatorial current system became blocked, first in the Indo-Pacific region and next in the Middle East and eastern Mediterranean and, about 6 million years ago, by the emergence of the Isthmus of Panama. As a result, the equatorial waters were heated less and the mid- latitude ocean gyres were not as effective in keeping the high-latitude waters warm. Because of this, an ice cap began to form on Antarctica some 20 million years ago and grew to roughly its present size about 5 million years later. This ice cap cooled the waters of the adjacent ocean to such a low temperature that the waters sank and initi- ated the north-directed abyssal flow that marks the present deep circulation.

Also, at about six million years ago, the collision between Africa and Europe temporarily closed the Strait of Gibraltar, isolating the Mediterranean Sea and restrict- ing its circulation. Evaporation, which produced thick salt deposits, virtually dried up this sea and lowered the salt content of the world’s oceans, allowing seawater to freeze at higher temperatures. As a result, polar ice sheets grew, and sea level fell. About 500,000 years later, the barrier

between the Mediterranean and the Atlantic Ocean was breached, and open circulation resumed.

The Quaternary Ice Age arrived in full when the first ice caps appeared in the Northern Hemisphere about two million years ago. It is highly unlikely that the changing configuration of continents and oceans can be held solely responsible for the onset of the Quaternary Ice Age, even if such factors as the drift of continents across the lati- tudes (with the associated changes in vegetation) and reflectivity for solar heat are included. There can be little doubt, however, that it was a contributing factor and that recognition of its role has profoundly altered concepts of paleoclimatology.