A natural bridge or arch is where host rock has been eroded away leaving a void underneath. In fact it is essentially a really short cave!
Weathering
Weathering is the natural breakdown of rock into smaller pieces either by physical or chemical means. There are two types of weathering: mechanical and chemical
Mechanical Weathering
Mechanical weathering is when rock is physically broken into smaller rocks. This can be due to several causes:
-Sandblasting by wind-carried particles
-Abrasion due to the sheer weight of ice and the particles contained within scraping on a rock face
-Ice wedging due to water seeping into cracks, freezing, expanding, and prying portions of rock off of the main rock
In Jasper National Park, the most striking type of erosion was caused by the Wisconsin Glaciation which ended around 11,000 years ago. This is the main "ice age" known to most people and saw most of the North American continent north of the 49th parallel covered in a glacier over a kilometer thick. The weight of this glacier was so immense that the continent actually deflected downward and just like a a river, a frozen river flows downhill too, albeit slowly. Combined with the weight of the ice and particles of sand, rock, and boulders embedded in the ice sheet, this force tore the surface of the earth apart resulting in smooth U-shaped valleys upon the glacier's retreat.
Chemical Weathering
Chemical weathering is when rock is broken down and the chemical composition of the rock is changed, not merely broken into smaller pieces. Some rocks are more susceptible to chemical weathering than others. Limestone for example is broken down very easily due to weak acids dissolved in water.
The three types of chemical weathering are as follows:
-Oxidation: Oxygen combines with elements in the rock to form a new substance (i.e. iron oxide [rust]).
-Hydrolysis: Water combines with elements to form a new substance (i.e. Othoclase + Water -> Kaolinite + Silicic Acid + Potassium).
-Carbonation: Carbon Dioxide combines with elements in the rock to form new substances (i.e. Calcite + Carbonic Acid -> Calcium ion + Bicarbonate) [CaCO3 + H2CO3 → Ca2+ + 2HCO3].
These new substances are usually weaker than they used to be and physical weathering breaks them down easier.
The Queen Elizabeth Range
The range in which this arch is embedded in is in the Front Ranges, a sub-range of the Rocky Mountains. These mountains are formed from what was the bottom of an ancient sea, where the topmost layer of the mountain is limestone. Due to tectonic drift, the continental plate was "scrunched" up due to the plate being compressed up to 30 km in length, and when that happens, there is only one direction to go. Up.
Many Front Range Mountains show a three layered sandwich of Rundle Group limestone summits overlying the soft shales of the Banff Formation, which in turn lay atop the limestones of the Palliser Formation. (http://www.mountainnature.com/ecology/DefineTheRockies.htm).
What is Limestone?
Limestone is formed from the gradual deposition of skeletons of ancient sea creatures over time which have been compressed into rock. Looking up you will see many different layers in the mountain and these cracks between the layers make the mountain susceptible to frost wedging. In fact you will see that the arch is a section which has been "popped" out. Since limestone has a high calcium content, it is vulnerable to acids and will be broken down by easily. When limestone is compressed under immense heat and pressure, it is converted into Marble, a very hard, commonly used building material. All fossil records are destroyed in the process.
To Log This Earthcache:
E-mail me the answers to the following questions through the geocaching.com messenger. Please send your answers within two weeks of logging or I reserve the right to delete your log.
1.) Estimate the length of the bridge of the arch
2.) Is the arch made of the same material as the mountain? Do you notice a difference between the rocks?
3.) What type of weathering formed this arch? Mechanical, chemical, both?
References
Cameron, W. (2005). Defining the Canadian Rockies. www.mountainnature.com. Retrieved June 16, 2015, from http://www.mountainnature.com/ecology/DefineTheRockies.htm.
Mamo, M., Kettler, T., & McCallister, D. (2009). Soil Genesis and Development, Lesson 2 - Processes of Weathering. Retrieved June 16, 2015, from http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1124303183&topicorder=4&maxto=7
Musieux, R., & Nelson, M. (1998). A Traveller's Guide to Geological Wonders in Alberta. Edmonton, AB: Provincial Museum of Alberta.
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