Weathering, Erosion, and Hydrology

Weathering and erosion are commonly defined as similar processes, but in fact vary greatly from one and another. Weathering is the breakdown or decaying of rocks, while erosion is the simple movement of that weathered material. This process is what turns large boulders and other various rocks (over a long period of time) into smaller particles such as sand. This is one process found abundantly in The Great SAND Dunes. The erosion process begins in the San Juan and Sangre De Christo mountains, as over time they begin to weather. Moved by either wind or water the weathered rocks, now pebbles and sand travel into the San Luis Valley or right to the sand dunes. There are many ways in which the mountain ranges weather. Weathering can be broken into two categories: physical or chemical. Physical weathering is defined as the breakdown of rocks through a mechanical process.

Diagram showing the process and effect of ice wedging.

A common type of physical weathering is thermal expansion. Thermal expansion happens when joints (cracks) fill with water. As the water freezes it begins to expand and crack the rock even more. As the weather warms and the ice melts back into water, it leaves a much larger area for more water to be filled with, and eventually frozen again. This process occurs several more times until the rock is eventually broken apart. Additional examples of physical weathering include root and crystallization wedging. These both use the same process as ice wedging but instead of ice, a root or crystal is formed in the crack.

Chemical weathering is the breakdown of rocks through chemical means such as acid rain among other processes.

After the weathering process takes place in the mountains it is then carried into the San Luis Valley through a system of rivers surrounding the park.

Map showing the physical features of Great Sand Dunes National Park

There are several rivers within the dunes which flow out of the Sangre De Christo Mountains into the sand dunes. The rivers carry small deposits of weathered rocks and minerals which are moved through the water to the base of the sand dunes. The transportation of the weathered rocks differs between the San Juan and Sangre De Christo Mountains. Since the San Juan Mountains are located far across from the sand dunes, the weathered rocks carried by the rivers are deposited into the San Luis Valley. With wind consistently being present because of the mountains and desert like atmosphere, the weathered material is moved across the San Luis Valley to the sand dunes.



Diagram of the Hydrologic Cycle

The Hydrologic cycle can clearly be seen in the sand dunes. As snow falls on the Sangre De Christo and San Juan Mountains, it begins to melt and creates a run off. As mentioned before, the run off, which feeds into the many rivers carries weathered rocks and minerals, which create the sand dunes. The sand dune field itself is very porous, which in turn allows it to absorb a considerable amount of water. As the San Luis Valley has characteristics of a desert, much of the water which reaches the area and is quickly evaporated or finds it's way to larger nearby rivers, to start the cycle over again.