Powered by the Sun, under the influence of Gravity...
The exogenic system is comprised of external gradational processes that serve to shape the land by erosion and deposition.
The gradational processes work in opposition to tectonics by wearing down and/or filling in the landscape. These processes consist of:
The four agents of gradation are running water, wind, moving ice, and waves and currents. Running water is the most important, as it is responsible for most erosion. Water can move most sizes of sediments, depending on the strength of the force. Wind moves sand-sized and smaller pieces of rock through the air. Glaciers move all sizes of sediments, from extremely large boulders to the tiniest fragments. Gravity moves broken pieces of rock, large or small, down slope. These forces of erosion will be covered later.
While tectonic forces work to build huge mountains and other landscapes, the forces of weathering, erosion and mass wasting gradually wear those rocks and landscapes away, called degradation or denudation (think nude, to make bare). Deposition occurs when the material that is removed comes to rest, We refer to the filling in of the landscape with deposited material as aggradation. Tall mountains and deep valleys turn into hills and even plains through degradation and aggradation. The Appalachian Mountains along the east coast of North America are thought to have been once as tall as the Himalayas.
No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year? What changes would you see? What forces of weathering wear down that road, or rocks or mountains over time?
The gradational processes work in opposition to tectonics by wearing down and/or filling in the landscape. These processes consist of:
- Weathering: disintegration and decomposition of rock through physical and/or chemical processes.
- Erosion: the incorporation and transportation of material by mobile agents such as wind, running water, moving ice, waves and currents;
- Mass wasting: the transfer of rock material en-masse downslope under the influence of gravity;
- Deposition: occurs when the transferred material comes to rest.
The four agents of gradation are running water, wind, moving ice, and waves and currents. Running water is the most important, as it is responsible for most erosion. Water can move most sizes of sediments, depending on the strength of the force. Wind moves sand-sized and smaller pieces of rock through the air. Glaciers move all sizes of sediments, from extremely large boulders to the tiniest fragments. Gravity moves broken pieces of rock, large or small, down slope. These forces of erosion will be covered later.
While tectonic forces work to build huge mountains and other landscapes, the forces of weathering, erosion and mass wasting gradually wear those rocks and landscapes away, called degradation or denudation (think nude, to make bare). Deposition occurs when the material that is removed comes to rest, We refer to the filling in of the landscape with deposited material as aggradation. Tall mountains and deep valleys turn into hills and even plains through degradation and aggradation. The Appalachian Mountains along the east coast of North America are thought to have been once as tall as the Himalayas.
No human being can watch for millions of years as mountains are built, nor can anyone watch as those same mountains gradually are worn away. But imagine a new sidewalk or road. The new road is smooth and even. Over hundreds of years, it will completely disappear, but what happens over one year? What changes would you see? What forces of weathering wear down that road, or rocks or mountains over time?
Click below to access the individual units of this section:
Weathered Sandstone - New South Wales
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Meandering Glacial Outwash - Nunavut, Canada
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Sea Arch - Northland, New Zealand
Rippling Sand - Mojave National Preserve, CA, USA
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Matterhorn - Swiss Alps, Switzerland
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