Tectonics of South America (Nazca Plate Subduction Zone)

The South American arc extends over 7,000 km, from the Chilean margin triple junction offshore of southern Chile to its intersection with the Panama fracture zone, offshore of the southern coast of Panama in Central America. It marks the plate boundary between the subducting Nazca plate and the South America plate, where the oceanic crust and lithosphere of the Nazca plate begin their descent into the mantle beneath South America. The convergence associated with this subduction process is responsible for the uplift of the Andes Mountains, and for the active volcanic chain present along much of this deformation front. Relative to a fixed South America plate, the Nazca plate moves slightly north of eastwards at a rate varying from approximately 80 mm/yr in the south to approximately 65 mm/yr in the north. Although the rate of subduction varies little along the entire arc, there are complex changes in the geologic processes along the subduction zone that dramatically influence volcanic activity, crustal deformation, earthquake generation and occurrence all along the western edge of South America.

 Subduction of the Nazca plate is geometrically complex and impacts the geology and seismicity of the western edge of South America. The intermediate-depth regions of the subducting Nazca plate can be segmented into five sections based on their angle of subduction beneath the South America plate. Three segments are characterized by steeply dipping subduction; the other two by near-horizontal subduction. The Nazca plate beneath northern Ecuador, southern Peru to northern Chile, and southern Chile descend into the mantle at angles of 25° to 30°. In contrast, the slab beneath southern Ecuador to central Peru, and under central Chile, is subducting at a shallow angle of approximately 10° or less. In these regions of “flat-slab” subduction, the Nazca plate moves horizontally for several hundred kilometers before continuing its descent into the mantle, and is shadowed by an extended zone of crustal seismicity in the overlying South America plate. Although the South America plate exhibits a chain of active volcanism resulting from the subduction and partial melting of the Nazca oceanic lithosphere along most of the arc, these regions of inferred shallow subduction correlate with an absence of volcanic activity.

Comments

Post a Comment

Popular posts from this blog

Rock Cycle

Image
The rock cycle is a model of the natural changes that occur in rocks and rock materials on the earth. The diagram to the right distinguishes between those processes that take primarily at Earth's (external processes) and those that take place within the Earth (internal processes).  Internal processes, such as compression and heating require the conditions of extreme temperature and pressure that occur deep within the Earth. The rock cycle illustrates several important principals of geology. First, nearly all rocks are made from the remains of other rocks (coal and other organic sedimentary rocks are important exceptions).  Second, rocks are classified on the basis of their origin.  Finally, there are a variety of ways that rocks can change in response to changing conditions at the surface or within Earth. Homework  due Wednesday, April 6, 2011 Review Book:  read pages 55-56; do questions 54-60 on pages 56-57, and questions 1-10 on pages 59-60.
Read more

Igneous Rock Classification

Image
The most common igneous rocks are classified using just two characteristics: Crystal size and color. For example, basalt is relatively dark in color (gray to black) and usually contains crystals to small to see without the aid of an optical lens.  On the other hand, granites are lighter in color (pink to gray) and have crystals that can be seen by the naked eye.  Igneous rocks are the only kind of rock that forms from hot magma, and all rock that crystallize from melted rock are igneous.  
Read more

Weather Map Project

Image
In the past two weeks, we have been studying the properties of the atmosphere and the different weather variables. This week we will begin constructing weather maps.  We will also do a weather map project. The details are as follows: Project Due Date :  Friday, February 18, 2011 Task: Your assignment is to construct a weather map, for a seven day period, across the United States. Criteria : You may select the weather for a seven day period of your choosing. The project must be illustrated on poster paper and may include a typed report. The project must include the following items: o Title of Assignment o Two weather maps that track the changes in weather for the seven day period (from either newspaper or internet). o Map must include a general description of the weather patterns, for example, a description of air masses likely to create fronts (cP, continental polar, mT, marine tropical, etc.). o Indicate on map where unstable weather is occurring across the nati...
Read more