Geography

external processes that set into motion air, water & ice all powered by solarenergy- the fluid realm of the earths enviornment.

Most sedimentary rocks are derived from existing rocks or from organic materials such as bone or shell. The exogenic process of weathering and erosion provide the raw materials needed to form sedimentary rocks. Bits and pieces of former rocks- pricpally quartz, feldspar, and clay minerals-are eroded and then mechanically transported ( by ice, wind, and gravity) to other sites, where they are deposited. In addition, some minerals go into solution and form sedimentary deposits by precipitating from those solutions; this is an mportant process in the oceanic enviornment.

Divergent boundaries are characteristics of seafloor spreading centers, where upwelling material from the mantle forms new seafloor, and crustal plates spread apart. These are zones of tension. An example is the Nazca plate and the pacific plate. Although most divergent boundaries occur at mid-ocean ridges, there are a few within continents themselves. An example would be the Great Rift Valley of East Africa. where continental crust is pulling apart.

Convergent boundaries are characteristics of collision zones, where areas of continental and/or oceanic crust collide. These are zones of compression and crustal loss. Examples include the subduction zone off the west coast of South and Central America and the area along the Japan and Aleutian trenches. Along the western edge of South America, the Nazca plate collides with and is subducted beneath the South American plate, creating the Andes Mountain chain and related volcanoes. The collision of India and Asia is another example of a convergent boundary.

Transform boundaries occur where plates slide laterally past one another at right angles to a sea-floor spreading center, neither diverging or converging, and usually with no volcanic eruptions. These are the right-angle fractures streching across the mid-ocean ridge system worldwide.

Individual sites of upwelling material which arrive at the surface in tall plumes from the mantle, sometimes producing thermal effects in the groundwater and the crust. Iceland is an example of an active hot spot sitting astride a mid-ocean ridge.

An example of an isolated hot spot is the one that has formed the Hawaiian-Emperor Islands chain. The Pacific plate has moved across this hot, upward erupting plume for almost 80 million years, with the resultig string of volcanic islands moving northwestward away from the hotspot. Thus, the age of each island or seamount in the chain increases northwestward from the island of Hawaii.

A caldera is a large, basin-shaped deression. A caldera forms when summit material on a volcanic mountian collapses inward after an eruption or other loss or magma. A caldera may fill with rainwater, as it did in beautiful Crater Lake in southern Oregon.

The world's oceanic trenches are subduction zones and are the deepest features on the Earth's surface. The deepest is the Mariana Trench near Guam, which descends to -36,198 feet below sea level.