Plate Boundaries

The plate boundaries are the borders of tectonic plates, which are the huge moving slabs of the lithosphere. The lithosphere in turn consists of the crust and the upper mantle, being divided into plates that drift about over a sea of molten rock. There are three types: 1) convergent boundary, which is the point where the edges of two tectonic plates meet as they collide; 2) divergent boundary, which is when their borders separate from one another; 3) transform boundary, where the edges of two plates slide by, sideways without collision.

Convergent Boundary

It is the geological phenomenon when one plate collides against another. As it crashes against the other plate, it slips violently under its edge. The convergence of two slabs of the lithosphere releases a huge amount of energy, causing strong seismic and volcanic activities, with the formation and/or elevation of mountains. When an oceanic crust plate collides against a continental crust block, it sinks and slides under the continental plate. This is so, because the oceanic crust plate is much denser and heavier that the continental one. A good example of this is the convergence of the Nazca Plate, which form the floor of the South Pacific Ocean, and the South American Plate, with the former pushing and slipping under the latter. This collision gave birth to the Andes Mountain Range and it has caused the strongest earthquake in the world, affecting the Chilean coastal cities.

Divergent Boundary

It is the space marked when two tectonic plates drift away from one another. When this happens, new crust crops up in the form of molten magma from the depth of the upper mantle to fill in the gap. Thus, most divergent boundaries are located along mid-ocean ridges in the ocean floor where this separation takes place. In this case there is no seismic activity. The plates separation and the rising of new crust form a long ridge of rock; an example of this is the ridge in the Atlantic Ocean, where the South American Plate drifts away from the African Plate. When divergent boundaries take place on land, they form deep valleys, which are called rift valleys; an example of this is the Great Rift Valley in Eastern Africa, which marks the separation of the African Plate from the Arabian Plate.

Transform Boundary

It is formed when two plates slides past each other as they move in opposite direction, but they do not collide. In this type of boundary, there is no formation of new crust and the long line that mark the sliding is called fault. The San Andreas Fault in the State of California is a good example of this phenomenon and it is where the Pacific Plate slips sideways northwards by the North American Plate. Earthquake is also quite common along transform boundary.

Dow below, the Andes Mountain Range between Argentina and Chile. It was shaped by the convergent boundary between the Nazca Plate and the South American Plate.

 

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Orinoco River Basin

The Orinoco River Basin is one the three major river basins in South America, along with the Amazon, and Parana-Rio de la Plata basin. With 2,700 km in length, the Orinoco River and its tributaries constitute a major fluvial system that drains almost the whole of the Venezuela territory and part of northern Brazil. About 75% of its waters are pluvial waters from the Amazon rain forest.

The Orinoco originates in the Guiana Highlands, near Mount Delgado Chalbaud, right on the political border between Brazil and Venezuela. Then it flows in a southeast-northwest direction, meandering as it runs. When it receives the fresh waters from its first important tributary of the left hand, the Guaviare River, it turns to the right, making a large arch. Thus, at this point, the Orinoco changes its direction and now flows northwards.

In Central Venezuela, another important river of its left hand side flows into it; the Meta River. When the Apure River joins it, the Orinoco begins to run eastwards, towards the Atlantic Ocean, into which it flows. It must be said that part of the waters from both the Meta and Apure River originates from melting snow on the Andes Mountain Range. Flowing down the mountain sides, these streams and creeks flow into one another to become the basin’s tributary rivers of the left hand side that flow across the grassy plains of Venezuela, called ‘Llanos’.

The most important rivers of the Orinoco’s right hand side are the Ventuari, Caura, and Caroni, which are entirely pluvial (rain water streams from the Amazon jungle).

Below, a map of Venezuela, showing the Orinoco River Basin, with its networks of tributaries.

 

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Rio de la Plata Basin

The Rio de la Plata basin, also known as the River Plate Basin in British text books, is the largest fluvial system in Argentina. It is also the second biggest basin in South America, right after the Amazon Basin. It is an exorheic river system which drains a huge portion of Argentina's territory. This territory includes the northeastern, a part of northwestern, and central Argentina, as well as Uruguay, Paraguay, and the southern portion of Brazil.

The Rio de la Plata basin is drained by a complex network of rivers, of which the Parana and Uruguay River are the most important ones. These two rivers confluence into one another to form the Rio de la Plata, which is the widest river in the world; as a matter of fact, it is really an estuary, with 190-km wide at the point where it flows into the Atlantic Ocean.

The Parana River originates in southern Brazil as Paranaiba River, which becomes the High Parana after it receives the waters of an eastern tributary, the Tiete River (still in Brazil). As it flows southwards, the High Parana makes the international border between Argentina and Paraguay. At this point, another eastern tributary flows into it; the Iguazu River, which makes the Iguazu Falls just as it flows into the northeast of Argentina. Then the High Parana turns westward, flowing in this direction for about 400 km.

Right at the bend where it turns south, the High Parana receives its main tributary, which is the Paraguay River that flows from north to south across the nation of Paraguay. At this meeting point, it becomes the Middle Parana, which runs from north to south. Having received small tributaries, the Middle Parana gets about 50% of the rain water of the Northwest of Argentina through the Salado River, which flows into it at the level of the city of Santa Fe, in the Province of Santa Fe.

As it takes an eastward course, turning around the big bend of the province of Entre Rios, it becomes the Low Parana, also called the Parana River Delta, where it gets fluvial water from the Uruguay River. When these two important water courses flow into one another, they form the Rio de la Plata (River Plate).

“Fluvial”, from Latin Fluvium, which means ‘river’. “Exorheic” means ‘draining towards the sea’.

Below, a map of South America showing the rivers that make up the Rio de la Plata Basin

 

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