Soil Formation on Crosier Mountain

The aim of this EarthCache is to look at a large boulder just off of the Crosier Mountain trail, and to examine the processes that are taking place in the formation of soil on this boulder. You will then need to find another place along the trail where soil is being formed and describe the process to me. This EarthCache is located just below the summit of Crosier Mountain, and will take several hours of strenuous hiking to reach. Please take enough water for your hike.

Soil is formed from the breakdown of inorganic rock combined with organic materials. Rock is broken down through weathering, which is the decomposition of rocks and minerals through contact with the atmosphere. Weathering is generally classified into two different types: mechanical weathering and chemical weathering.

Mechanical weathering occurs when there is no chemical change in the rock as it breaks down. Forces involved in mechanical weathering include temperature, water, ice, and pressure. The most common type is the freeze-thaw action of water. As water freezes, it expands outward, cracking the rock and increasing the surface area for more weathering to take place. Temperature change is another type of mechanical weathering. Rocks will expand as they are heated and contract when they are cooled. This heating and cooling causes cracks, which can then be exploited by other means of weathering. The force of pressure is most evident in the granite exfoliation domes which can be seen near Estes Park. These granite domes were once covered by metamorphic rocks several miles thick. After the overlying rock eroded away, the granite was exposed and the pressure was released. The outer parts of the rock expand, causing parallel fractures on the rock surface. This is called sheeting, or exfoliation.

Chemical weathering involves a change in the chemical makeup of the rock. These reactions change the bonds that hold the rock together. This type of weathering is much more active in areas where there is abundant water, which helps to facilitate the chemical reactions. Oxidation, dissolution, and hydrolysis are common types of chemical weathering. Oxidation occurs when oxygen combines with metallic ions to form weaker oxides. The most common example of this is when oxygen and iron combine to form iron oxide (rust). Dissolution occurs when rain falls and combines with gases such as carbon dioxide, sulphur dioxide, and nitric oxides to create weak acids. These acids dissolve cracks in the rock. Hydrolysis occurs when water combines with silicate minerals to form other minerals which are softer than the original rock types. This is what occurs when feldspar breaks down into clay.

A close look at this boulder will show that biological weathering, a combination of mechanical and chemical weathering, is working to break things down further. Lichens cling to the rock and break down the surface by secreting chemicals and covering portions of the surface with their root systems. Wind-blown soils have been caught by the lichens in small depressions on the rock. Over time, grasses and forbs begin to grow in the depressions and help to catch more soil, as well as send down roots of their own. Insects, spiders, and other small animals add organic materials through their feces and dead bodies, helping to enrich the soils which are forming. Eventually, trees begin to grow with roots that help to break the rocks down further.

It takes a long time to change inorganic rock into a form on which plants can grow. At one time, the boulder in front of you was bare with no hint of organic material. Now it is a thriving, living community due to weathering, and the creation of soil.

In order to log this EarthCache, complete the following:

1.) Find the boulder at the coordinates and examine the weathering processes that are taking place.

2.) Find another boulder or solid rock face along the Crosier Mountain Trail on which weathering is acting to create soil. Using the information given above, send me an email describing the mechanical, chemical, or biological weathering that is taking place to create this soil.

3.) In your online log, include the coordinates of the second location.

Please consider posting photos of yourself, or the local geology, when you log this EarthCache. Photos can be an additional rewarding part of your journey, but posting them is not a requirement for logging this EarthCache, and is strictly optional.

The above information was compiled from the following sources:

2004. Harris, A.G., Tuttle, E., and Tuttle, S.D. Geology of National Parks, 6th Ed.

2004. Hopkins, R.L., and L.B. Hopkins. Hiking Colorado’s Geology.

PhysicalGeography.Net. Weathering. Online at http://www.physicalgeography.net/fundamentals/10r.html

Geography4Kids. Weathering. Online at http://www.geography4kids.com/files/land_weathering.html

Wikipedia. Weathering. Online at http://en.wikipedia.org/wiki/Weathering

Thanks to the Roosevelt National Forest for allowing placement of this EarthCache!