Tuesday, December 6, 2011

Some Examples of Weathering

Weathering can be defined as the gradual breakdown of rock materials.  It primarily results from the physical breakdown of rock material (mechanical weathering) or via the chemical breakdown of rock through chemical reactions (chemical weathering).

A nice example of mechanical weathering (especially pertinent for places like Minnesota) is through an action called ice wedging.  As liquid water flows into the cracks of rock materials and freezes during periods of low temperatures, the frozen water expands, widening the crack.  This action can reduce very large boulders to much smaller remants as shown in the pictures below.

These large granite boulders are found just outside Pipestone National Monument near Pipestone, Minnesota.  Granite is not native to the area and would have been deposited there after transport by glaciers.  The boulders are called the 'three maidens', at one point in time there would have been just three boulders of granite, but the repeated freezing and thawing of water have split the boulders into many pieces.  Largely because of how out of the ordinary granite is to the area, a Native American legend grew out of these large pieces of granite.  Native Americans believed that the granite boulders held the spirits of three maidens who required offerings before the quarrying nearby of catlinite (or pipestone) in what is now the National Monument.

Another form of mechanical weathering is abrasion, which is the grinding and wearing away of material through the action of wind or water.  The photographs below show great examples of abrasion at Iona's Beach, a Scientific and Natural Area maintained by the Minnesota Department of Resources along the Lake Superior shore.  On the north end of the beach a large rhyolite flow is found.  Waves break this rhyolite flow down and largely through wave action, these smaller pieces of rhyolite are rounded and smoothed before eventually being deposited on the beach.

Chemical weathering is the breakdown of rock material through a chemical reaction.  This occurs largely through weak acids that are found naturally in our rain or snow and through the oxidation of other materials.  The picture below (taken in Summit Cemetery, Waukesha County, Wisconsin) is a nice example of chemical weathering, over the last 150 years the rock has been exposed to a large portion of natural acids through precipitation.  A closer look at the headstone proves that the original carving into the stone has become much more difficult to read.

Another form of weathering is called differential weathering, which refers to how different rock materials weather (or breakdown chemically or mechanically) at different rates.  The two photographs below show a nice example of differential weathering, the pink feldspar crystals weather more slowly, and as such, seem to stand out from the rest of the granite.

Another very nice example of differential weathering is Devil's Tower National Monument in Wyoming.  The land area around Devil's Tower is comprised of sedimentary rocks, which weather (and then are eroded or transported away) at a much faster rate than the igneous rocks that comprise the monument.  The igneous rocks are much more resistant to weathering than the sedimentary rocks.  Devil's Tower formed as an intrusion of igneous material that, after the surrounding sedimentary rocks weathered and eroded away, was left standing over 1,200 feet above the immediate area.

In our classroom, students recently examined some examples of both mechanical and chemical weathering.  We used different rock types (limestone, rhyolite, basalt, sandstone, marble, gabbro) in our weathering lab.  Students placed these different rock types in weak solutions of carbonic acid to determine the effects of weathering.  The next class period, mechanical weathering through the process of abrasion was explored before comparing both activities.