Why are We Weathered? EarthCache

French to follow (just working on it right now)

What is Weathering?

Disintegration of rock near the surface of the earth is commonly called weathering. Some of the major causes of weathering are plant and animal life, atmosphere and water. Weathering breaks down and loosens the surface minerals of rock so they can be transported away by agents of erosion such as water, wind and ice. There are two types of weathering: mechanical and chemical.

Mechanical Weathering

Mechanical weathering is the disintegration of rock into smaller and smaller fragments. There are many types of mechanical weathering. Thermal expansion, frost disintegration, pressure release and biological weathering are just a few. Frost action is an 'effective' form of mechanical weathering. When water trickles down into fractures and pores of rock, the water then freezes into ice and the volume increases by about 10%. This volume increase causes outward pressure to the rock and causes the rocks to crack. When the ice thaws, the water can flow further into the rock. When the temperature drops below freezing point and the water freezes again, the ice enlarges the cracks further. Repeated freeze-thaw action weakens the rocks which, over time, break up along the cracks into pieces. Pressure release occurs when the overlying rocks are removed. The built up pressure on the rocks below is slowly released and causes cracking.

Living organisms, such as lichens, trees and even humans, can also cause biological weathering. When lichens grow on rocks, it creates a more humid environment and also the roots of the plants will create cracks into which water and chemicals can seep into the rock and cause frost disintegration. Animal and human activity on or near rocks can remove the soil covering to the bedrock surface thus further increasing water exposure to oxidation processes.

 

An example of pressure release to granite.	An example of biological weathering.

Chemical weathering transforms the original rock into a substance with a different composition and different physical characteristics by way of a chemical reaction. The new substance is typically much softer and more susceptible to agents of erosion than the original material. The rate of chemical weathering is greatly accelerated by the presence of warm temperatures and moisture. Also, some minerals are more vulnerable to chemical weathering than others. For example, limestone is far more reactive than granite.

Differential weathering occurs when some parts of a rock weather at different rates than others. For example, Hopewell Rocks in the Bay of Fundy are outstanding examples of differential weathering, the water from the ocean has worn away the rocks from the bottom and left towers of rocks.

Another form of weathering is the dissolving of the rocks, or solution weathering. One of the most well-known solution weathering processes is carbonation. Atmospheric carbon dioxide leads to solution weathering. This process occurs on rocks which contain calcium carbonate, such as limestone and chalk. This takes place when rain combines with carbon dioxide or an organic acid to form a weak carbonic acid which reacts with calcium carbonate (the limestone) and forms calcium bicarbonate. This process speeds up with a decrease in temperature and therefore is a large feature of glacial weathering.

The reactions as follows:

Carbonation on the surface of well-jointed limestone produces a dissected limestone pavement which is most effective along the joints, widening and deepening them.

Plants and animals can also have a biological weathering effect on rocks. As plants grow on rocks they can release acids which will break down the rocks. Also, when plants or animals decompose they release carbon dioxide into the soil, which causes carbonation.

This area, along with much of the exposed bedrock in northern North America, is a result of the Wisconsin Glacial Episode. Over the period of three ice ages, the oldest dating back more the 70,00 years ago, there was much disruption to the landscape. During the last big melt, about 10,000 years ago, the bedrock began to be exposed. This exposed bedrock has had lots of weathering, and this location one can see some great examples of weathering.

To log this cache, you'll have to publish a picture of you and your GPSr, with the weathering in the background , at the posted coordinates, and send me by email the answers to the following questions:

1. - Determine the area of the large weathered rocks at the posted coordinates?
2. - Determine the area of a nearby 'hollow' rock at the posted coordinates?
3. - What is the prominent element which can be seen at the surface in the area? and what is the process which releases it?

Thanks Cron for the html template and model12 and danieloliveira for the Earthcache idea.

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