South America Geography

Maps show the east coast of South America and the west coast of Africa providing the evidence that the continents may have being joined together, then broken apart and moved to their present locations. In the earth, a fault is a line of fracture in the rocks where the two sides move by each other. The movement can be up, down or sideways, and it is caused by pressure and tension in the rock. When a sudden movement happens along one of these fault lines, an earthquake happens.

Most earthquakes occur along the boundaries of the large, rigid crustal plates that make up the outermost shell of the earth.  More topic at paper Person centred values.

These plates, which range in thickness from about 5km to 70km, are in motion relative to each other and to the earth’s interior. In some areas, such as along the Mid-Atlantic Ridge, the plates are moving apart and new oceanic crust is forming as molten rock rises from within the mantle.

These plates are part of a dynamic system, a gigantic recycling system; with new crust created at spreading centers and older crust thrust down and melted in the mantle at subduction zones. Plate boundaries, therefore, are where the most dynamic processes of plate tectonics take place, and where 95% of all the earthquakes in the world occur each year.

During the Mesozoic Era, about 200 million years ago, rifting took place along the east coast of North America as the Atlantic Ocean began to open, resulting in the continent being stretched or extended, and in the Reelfoot Rift being pulled apart in a new episode of rifting.

How the rifting of North America and Africa about 200 million years ago created the basins that were subsequently filled with sediments, some of which become traps for gas. During the Mesozoic Era about 200 million years ago, as the Atlantic Ocean was opening in the east, rifting was once again re-activated and intrusive igneous rocks were emplaced.

Again, the rifting failed and the continent remained intact, although with a significant zone of weakness. This rift is known as the Reelfoot Rift and coincides with the northernmost portion of the Mississippi embayment. Most of the seismicity is located from 5 to 25 km beneath the Earth’s surface. The current spatial configuration of the ocean basins is a by-product of plate tectonics. The creation of new oceanic crust at the mid-oceanic ridge moves the continents across the Earth’s surface and creates zones of subduction.

At the areas of subduction, oceanic crust is forced into the mantle after it collides with continental crust. Over the past 200 million years, the Atlantic basin has been the most active area of oceanic crust creation. The Atlantic Ocean formed about 200 million years ago as the Pangaean continent began rifting apart. 180 million years ago, North America separated from South America and Africa. North America then joined with Eurasia creating Laurasia. By 135 million years ago, South America began separating from Africa. North America and Eurasia split a few million years after

The rifting caused Gondwana and North America (Laurasia) to eventually pull apart creating the Rift basins over a broad band along the present coasts of North America, Africa and Europe. These rift basins continued spreading and became the Atlantic Ocean. Rifting occurred in stages: At first, there is bowing up of continent and lithosphere; the continent then pulls apart and rift basins form, that is blocks of the crust move down along faults; in those basins above sea level, sediments derived from the surrounding highlands are deposited in the basins by rivers; Basins below sea level fill with ocean water.

If the basin is shallow and evaporation in the basin is rapid enough compared to the flow of seawater into the basin, evaporate (gypsum and salts) deposits occur; The upwelling asthenosphere below the rift basin melts resulting in basaltic volcanic rocks at the surface and diabasic intrusive rocks as sills and dikes in the basins and surrounding basement rocks; As the basins subside sedimentation continues, leading to greater than 10,000 feet of sediments in some basins. Marine basins are characterized by; evaporates near the base, overlain by Jurassic sands and shales, finally covered by Cretaceous and younger sediments.

As the evaporites are buried deeper, they begin to form salt domes due to their low density relative to that of the overlying sediments. These basins were filled with sediments deposited subaerially, which are characterized by: conglomerates at the base which are overlain by river deposits consisting of sandstone and mudstones. The sandstones are channel deposits the mudstones are over bank deposits upon which soils were developed. About 200 million years ago, basaltic magma intruded forming dikes and sills and some of the magma reached the surface and formed basaltic lava flows.

The Palisades Sill in the Newark Basin and West Rock Ridge sill in the Hartford Basin are representatives of the intrusive phases. About this time, the basins developed large lakes, which regularly dried and expanded over periods of some 400,000 years. Marine sediments are classified as either neritic or pelagic based on where they are found neritic sediments are found near continental margins and islands and have a wide range of particle sizes. Most neritic sediments are eroded from rocks on land and transported to the coast by rivers.

Once they enter the ocean, they are spread across the continental shelf and down the slope by waves, currents, and turbidity currents. The largest particles are left near coastal beaches, while smaller particles are transported farther from shore. In many places, the ocean basin floor is a vast plain extending seaward from the base of the continental slope. It is flatter than any plain on land and is known as the abyssal plain. Sediments that fall from the surface and are deposited by turbidity form the abyssal plain

Currents to cover the irregular topography of the oceanic crust. An area of the abyssal plain that is isolated from other areas by continental margins, ridges, and rises is known as a basin, and some basins may be subdivided into sub basins by ridge and rise subsections. Low ridges allow some exchange of deeper water between adjacent basins, but if the ridge is high, both the deep water and the deep-dwelling marine organisms within the basin are effectively cut off from other basins. Abyssal hills and seamounts are scattered across the sea floor in all the oceans.

Abyssal hills are less than 1000 m high, and seamounts are steep-sided volcanoes rising abruptly and sometimes piercing the surface to become islands. Most abyssal hills are probably volcanic, but some may have been formed by other movements of the sea floor. Submerged, flat-topped seamounts, known as guyots, are found most often in the Pacific Ocean. Pelagic sediments are fine-grained and collect slowly on the deep-sea floor. The thickness of elagic sediments is related to the length of time they have been accumulating or the age of the sea floor they cover.

Consequently, their thickness tends to increase with increasing distance from mid-ocean ridges. The patterns formed by the sediments on the sea floor reflect both distance from their source and processes that control the rates at which they are produced, transported, and deposited. Seventy-five percent of marine sediments are terrigenous. The majority of terrigenous sediments are initially deposited on the continental margins but are moved seaward by the waves, currents, in addition, turbidity flows that move across the continental shelves and down the continental slopes.

The terrigenous sediments of coastal regions are primarily lithogenous, supplied by rivers and wave erosion along the coasts. Finer particles are held in suspension and are carried farther away from their source. Volcanic ash is present in seafloor sediments and can be found in layers of significant thickness associated with past volcanic events. Loose sediments on the sea floor are transformed into sedimentary rock through lithification that is as one layer of sediment covers another, the weight of the sediments puts pressure on the lower sediment layers, and the sediment particles are squeezed more and more tightly together.

The particles begin to stick to each other, and the pore water between the sediment particles, with its dissolved solids, moves through the sediments. As it does so, minerals precipitate on the surfaces of the particles and, in time, act to cement the sediment particles together into a mass of sedimentary rock. Sedimentary rocks are found beneath the sediments of the deep-sea floor, along the passive margins of continents, and on land where they have been thrust upward along active margins or formed in ancient inland seas.

If sediments are subjected to greater changes in temperature, pressure, metamorphic rock are formed. Oil and gas deposits are usually associated with marine sedimentary rocks and are believed to be produced by the slow conversion of marine plant and animal organic matter to hydrocarbons. Conditions must be just right for marine organic material to eventually be converted to oil and gas. It must first accumulate in relatively shallow, quiet water with low oxygen content. Anaerobic bacteria can then utilize the organic matter to produce methane and other light hydrocarbons.

As these simple hydrocarbons are buried beneath deeper layers of sediment, they are subjected to higher pressure and temperature. Over a period of millions of years, they can be converted to oil or gas. Oil forms if the depth of burial is approximately about 2 km. If the organic material is buried deeper for a longer period at higher temperature, gas is produced. Because oil and gas are very light, they migrate upward over time, moving slowly out of the source rock and into porous rocks above. This upward migration continues until the fluids reach an impermeable layer of rock.

The oil and gas then stop their ascent and fill the pore spaces of the reservoir rock below this impermeable layer. Petroleum-rich marine sediments are more likely to accumulate during periods of geologic time when sea level is unusually high and the oceans flood extensive low-lying continental regions to create large shallow basins. Much oil and gas are found in marine rocks that formed from sediments deposited during a relatively short period of time during the Jurassic and Cretaceous, between about 85 million and 180 million years ago, when sea level was high.

Major offshore oil fields are found in the Gulf of Mexico, the Persian Gulf, and the North Sea, and off the northern coast of Australia, the southern coast of California, and the coasts of the Arctic Ocean. Now, many U. S. companies are finding it more profitable to drill for oil and gas in foreign waters and are moving their rigs to the waters of the North Sea, West Africa, and Brazil. Conclusion Marine sediments and the skeletal materials in them provide important information about processes that have shaped the planet and its ocean basins over the past 200 million years.

The study of the oceans through an analysis of sediments is called paleoceanography. Sediment classifications are based on their size, location, origin, and chemistry. Sediment particles are broadly categorized in order of decreasing size as gravel, sand, and mud. Within each of these categories, particles can be further subdivided by size. The sinking rate and distance traveled in the water column are related to sediment size, shape, and currents. Small particles sink more slowly than large particles. Sediments that accumulate on continental margins and the slopes of islands are called neritic sediments.

Sediments of the deep-sea floor are pelagic sediments. pelagic sediments accumulate very slowly and neritic sediments accumulate more rapidly. Loose sediments are transformed into sedimentary rock in which the layering of the sediments may be preserved. Oil and gas are the most valuable of all seabed resources. Large deposits of gas hydrates are being studied to determine their potential as economically important sources of methane gas. These deposits are ice like accumulations of natural gas and water that form at low temperature and high pressure on the sea floor.

On occasion, the pressure of the buoyant methane hydrate and free methane gas can break through the domal structure to erupt as a mud volcano. Produced by the dissociation of large quantities of methane hydrate, mud volcanoes form structures that resemble typical volcanic cones. However, unlike ordinary volcanoes, they erupt no hot magma or ash. Some of these ordinary volcanoes are active; many more are extinct. Instead, mud volcanoes are the result of water and gas forcing its way through overlying mud.

They form when sediments containing large amounts of fluid are compressed and the fluid squeezed out and upwards through faults and cracks in overlying sediments.

Work cited Richardson, N. J. , Underhill, J. R. 2002. “Controls on the structural architecture and sedimentary character of syn-rift sequences, North Falkland Basin, South Atlantic. ” Marine & Petroleum Geology, 417-443. Alt, D. , J. M. Sears, D. W. Hyndman. 1988. The origins of large basal plateaus, hotspot tracks, and spreading ridges. Journal of Geology 96:647-662. Marshall, J. E. A, 1994. “The Falkland Islands: a key element in Gondwana palaeography. ” Tectonics, 13; 499-514.

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