Clouds, Wind Pressure, and Fronts in The Great Sand Dunes

The Great Sand Dunes based on the name is composed of sand. Much of this sand is created from the weathering of the San Juan Mountain Range, which was explained in the last post. The process of erosion (movement of weathered particles)  is what carries the sand to the eastern boarder of the San Luis Valley where the sand dunes lie. The erosion process regularly occurs through wind currents, which are seen regularly in the valley. 

Picture Showing Wind Direction Within the San Luis Valley

Labeled in the above picture is the Alamosa Rose Wind. The Alamosa Rose is a Southwesterly wind which heads North up through the San Luis Valley until it finally reaches the base of the Sangre De Christo Mountains. The wind force (air pressure) is created through thermal variance in the valley. Since the San Luis Valley is relatively flat and low compared to the mountains surrounding it, the air in the valley is considerably warmer. Professor Allen's Third Law of Geography states wind always blows from high to low pressure, which can be seen in the San Luis Valley. Warm air (high pressure) wind blows the sand towards the mountains where the elevation is higher, resulting in cooler air masses. This process is commonly known as a cyclone because warm high pressure air from the valley flows to cool low pressure air in the mountains. As the winds rise up the Sangre De Christo Mountain Range, the heavier sand falls and is deposited in the base of the mountain. This occurs because the wind currents exit the valley through passes (Music, Medano, and Mosca) in the mountain range. This process is more noticeable during months where temperature varies drastically between the San Luis Valley and Sangre De Chriso Mountains. 

Picture Showing Condensation Level of Clouds

As the warm air from the San Luis Valley reaches and climbs the Sangre De Christo Mountains it cools and reaches dew point which is when clouds are formed. This process is known as orographic lifting (when an air mass is forced from low to high elevation). As the particles and moisture in the air cool they condense in the atmosphere. This process does not occur until the air reaches the Lifting Condensation Level, which is the dew point temperature.

Picture Showing the Rain Shadow Effect

The San Luis Valley is located on the windward side of the Sangre De Christo Mountains and the rainshadow side of the San Juan Mountains. This allows for the valley to receive between 8 and 10 inches of rain a year. This effect allows us to see clouds form close to the mountain peaks of the Sangre De Christo Mountains. Because the clouds contain so much moisture, which makes them heavy, they are unable to move until they release enough water for the wind to move them. 

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.

The Great Sand Dunes: Plate Tectonics and Volcanic Activity (Blog 1)

The Great Sand Dunes is a vast area containing several different landforms which have been created due to the many geological processes that have taken place. Two identifiable processes can easily been seen; plate tectonics and volcanic activity. I will discuss in detail how these processes are associated with the building of this national treasure.

The picture above shows the Sand Dunes themselves, as well as the San Juan mountain range in the background. The creation of these mountain is assumed to have take place around 30-35 million years ago. Based on information given by the NPS (National Park Service), the eruption of the La Garita caldera, the largest volcanic explosion in noted history was the source. After the initial explosion and several years of volcanic activity, such as lava flow took place, which created the flat-rounded San Juan mountains. The volcano eruption needed to create the mountain range is associated with that of a strato-volcano (Highly explosive). The strato volcano varies from cone and shield volcanoes due to its composition, number of vents, and the type of landforms it creates. There are many forms associated with strato volcanoes, which can be both beautiful and dangerous. Lava flow, pyroclastic flows (flying rocks), lahars (volcanic mudslides), and debris flow are just a few forms associated with the strato. With volcanic eruptions not being seen in thousands of years, the land forms created are evidence to show that activity is still present.

The San Luis Valley is a small portion of the Rio Grande Rift, which was created by diverging plate boundaries around 35-40 million years ago. The rift is very large in size and stretches from central Colorado to northern Texas. As the Rio Grande Rift separated it created Grabens (depressed blocks of land) and Horsts (raised blocks of land) in the San Luis Valley. As depicted in the image above, facing west towards the San Juan Mountains, you have the Monte Vista Graben; and to the east facing the Sangre de Cristo Mountains you have the Baca Graben. Between the two Grabens is the Alamosa Horst, which was formed by the extension of the earth's crust. Due to the location of the rift, geologists expect little to no action occurring the in the near future.


References:

http://www.nps.gov/grsa/naturescience/sanddunes.htm

http://www.geology.sdsu.edu/how_volcanoes_work/stratovolc_page.html

http://www.ux1.eiu.edu/~cfrbj/parks/GRSA/geo_cross_section_lg.jpg

http://newdeserttimes.com/wp-content/uploads/2010/01/bigstockphoto_great_sand_dunes_national_park_5539446.jpg

http://cires.colorado.edu/science/groups/sheehan/projects/riogrande/faq/

Intro

Hello, my name is Peter Runden, and I am an undergraduate student at the University of Colorado Business School. I am taking this class due to my interest in nature and the things that make the world go round. I hope to finish the course with a better understanding of geographical concepts and maybe some fun facts to help me win trivial pursuit.

When asked not only to choose a location over the next four months but to also blog about it, many locations came to mind. Instead of choosing a place I have already been, I decided to choose a place, that interests me but have yet to visit. This left me with the Great Sand Dunes National Park, which contains many different systems ranging from vast mountains, rift valleys, and of course sand dunes themselves. With all these systems working together many questions can be asked and hopefully answered. The above image shows just one of the many beautiful images created by the intertwined systems. Over the next semester I hope to provide information, which answers those questions, and finishing it all with a trip of my own to the location.