Great Sand Dunes National Park

1. GEOMORPHOLOGY:
THE GREAT SAND DUNES NATIONAL
PARK
GEO 111-500 THOMAS RUSSELL

2. LANDFORMS
GEOMORPHOLOGY LANDFORM PROPERTIES
• “Geo” = “Earth” • Structure: Composition and
orientation
• “Morph” = “Change”
• Process: Events that have produced
• “Ology” = “Write about”
the landform as it appears currently
• Study of landform
• Slope: Angular relationship between
origin, properties, and changes over
the landform face and the earth’s
time. Including…
surface
• Structure, process, slope, drainag
• Drainage: “…[M]ovement of water
e
(from rainfall or snowmelt), either over
• Driven by internal or external the Earth’s surface or down into the
processes soil and bedrock.” 1
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3. GEOGRAPHIC LOCATION
35 Miles Northeast of Alamosa, Colorado in the San Luis Valley. Park is located at
37.73434, -105.640411 or 37 ̊ N 42’ 59.24” , 105 ̊ W 40’ 56.30”.
Figure 1
Winds blow sediment from the west side of the valley to the east side, where it accumulates as
the sand dunes at the base of the Sangre de Cristo Mountains.
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4. THE GREAT SAND DUNES
LANDFORM PROPERTIES
• Structure: Sand in the dunes is derived primarily from the San Juan Mountains
• Process: Large rocks in the mountains undergo erosion, and the resulting smaller sediment
is carried by the Rio Grande River to the San Luis Valley floor and then by the wind across to
the sand dunes
• Slope: Due to the fine-grained nature of the sand, the dunes form with low angles of repose
and broad parabolic summits
• Angle of Repose: Concerning the formation of sedimentary hills; Angular threshold at
which downward pull of gravity is offset by the static friction of the piled material.
Disturbance of the balance will cause the material to slide downward.
• Drainage: Runoff from the westerly Sangre de Cristo Mountains encapsulates the sand
dunes with Sand Creek on the north and Medano Creek on the south; both streams wash
away sediment as the dunes attempt to migrate across
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5. GEOLOGIC STRUCTURE
Figure 2
Figure 3
• Watershed: Main source of water for the sand • Precipitation in the watershed to the east runs
dunes—the Sangre de Cristo Mountains off in a series of streams around and under the
• Dunefield: Approximately 30 square miles in dunefield
area, this is where the tallest dunes are found • Water sinks under the dunes in a process
• Sand Sheet: Accounting for nearly 90% of the known as infiltration. It continues flowing
sand deposits in the National Park, the sand sheet outward under the sand sheet
contains small dunes and vegetation 2 • Water can be found mere inches below the
surface of even the highest the dunes and even
• Sabkha: Found where the water table meets the closer to the surface in the sand sheet
surface, creating marshlands. “The Sabkha
forms…when the water evaporates away in late • Mineral deposits in the Sabkha can either be
summer, [and] minerals similar to baking soda returned to the dunefield by winds or can be
cement sand grains together into a hard, white flushed further down the valley by fluvial forces.
crust” 3
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6. BUILDING THE GREAT SAND DUNES
Mechanical weathering in the form of frost wedging begins
displacement of large rocks
Rock can also be weakened through biological weathering
from lichens as the organisms extract minerals from the
rock
Mass wasting on mountainsides in the form of
mudflows, landslides, or rock falls can move large amounts
of material
Fluvial forces (e.g. rainfall, snowmelt, rivers) flush rocks
down mountains and through valleys, further weathering the
rocks
Once eroded down the Rio Grande River to the valley
floor, much of the finer sediment washes up on shore where
it can be swept away by the prevailing southwesterly winds
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7. MECHANICAL WEATHERING:
FROST WEDGING
• Occurring across the
mountains surrounding
the San Luis Valley, frost
wedging begins breaking
down large rocks
• Lichen can also form on
rocks and weaken their
Figure 4
structural integrity by
extracting minerals. This
is called biological
weathering
• Once rocks are
weathered down to a
manageable size, mass
wasting is the next
erosive process
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8. MASS WASTING & EROSION
• Mudflows occasionally • Erosion occurs by fluvial forces and gravity until
occur when a slope the fine sediment reaches the valley floor where it
becomes so quickly is then taken by the wind
oversaturated that the
ground is unable to • Fluvial forces include
absorb water. These can rainfall, snowmelt, streams, and rivers, all of which
move large amounts of also further break down rocks
sediment as well as large • Erosion eventually leads the sediment down to the
rocks San Luis Valley floor where it can wash up on the
shore, dry out, and be swept away in the wind.
• Landslides are the least • Ultimately, the wind carries the sediment to the
common but can occur if east edge of the valley and up to the base of the
a given area not Sangre de Cristo Mountains.
structurally sound is
subjected to sudden
stimuli
• Rockfalls occur when
Figure 6
rocks become dislodged
and fall downslope; the
resulting pile of rock is
called talus. Rockfalls
can be the result of frost
wedging, repeated
weathering from
rainfall, or sudden stimuli
Figure 5
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9. SEDIMENT RECYCLING
• Winds build up sand dunes from west to
east while water washes them down from
east to west
• With Sand Creek to the north and Medano
Creek to the south, the sand dunes are
slowly eroded back into the San Luis Valley
as they attempt to migrate across the
streams
• This is the process of sand recycling in the
Great Sand Dunes National Park
Figure 7
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10. BEYOND THE EXTERNAL PROCESSES
• Due to the unfathomable nature of geologic time, the present
geologic activity of the region seems to be controlled by
external processes. However, it will ultimately be internal
processes that control the sand dunes
• “Over the last 25 million years, a gigantic rift has ben
opening up at the southern end of the Rocky Mountains. It
stretches over 160,000 square miles and is known as the Rio
Grande Valley…as the rift opens, the mountains to each side
crumble into the valley.” 4
• Rifting is caused by a sinking and thinning of the earth’s
crust by “…lava from a source deep in the mantle [that]
periodically spreads across the surface. In the near geologic
future, several million years or so, a youthful ocean basin
may occupy this area.” 5
Figure 8
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11. DEMISE OF THE SAND DUNES:
RIO GRANDE RIFT
Figure 9
The San Luis Valley resides on the Rio Grande Rift and over the next several million years, as the
rift deepens and widens, the Sangre de Cristo and surrounding mountains will erode, and the
Great Sand Dunes will be lost to the wind
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12. IN THE DISTANT GEOLOGIC FUTURE….
Figure 10
Figure 11
• Millions of years in the • The sediments currently composing the Great
future, North America could Sand Dunes will eventually find their way into the
ocean, become compacted into the oceanic
split along the Rio Grande Rift crust, and be subducted back into the mantle
• The resulting in-land sea would
• As the oceanic crust melts in the mantle, the
be similar to the ancient sea sediments that once sat atop the Great Sand
that existed some 70 million Dunes will be liquefied and eligible to be returned
years ago to the surface in a volcanic eruption
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13. TOURISM: A CHANCE TO LEARN AND SEE
GEOGRAPHY IN ACTION
• Daily Entrance Fee: $3.00 per
adult, children are always free 6
• Annual Pass: $15.00 per adult, good
for whole family free of charge
• Visitor’s Center is full of educational
plaques, diagrams, videos, books, and
experiments
Figure 13
Figure 12
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14. BIBLIOGRAPHY
1. Tom McKnight. McKnight’s Physical Geography (New Jersey: Pearson Prentice
Hall, 2011), 367.
2. Stephen Trimble. Great Sand Dunes National Monument: The Shape of the Wind.
(Tucson: Western National Parks Association, 2000), 6.
3. National Park Service. “Great Sand Dunes System.”
http://www.nps.gov/grsa/naturescience/sand_system.htm. (Accessed July 18, 2012).
4. “How the Earth was Made: The Rocky Mountains.” http://www.history.com/shows/how-
the-earth-was-made/episodes#slide-9. (Accessed July 18, 2012).
5. “A Tapestry of Time and Terrain: The Rio Grande Rift.”
http://tapestry.usgs.gov/features/28riogrande.html. (Accessed July 18, 2012).
6. National Park Service. “Fees & Reservations.”
http://www.nps.gov/grsa/planyourvisit/feesandreservations.htm. (Accessed July
18, 2012).
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15. BIBLIOGRAPHY (CONT.)
Figure 1: Google Maps: San Luis Valley, Colorado. https://maps.google.com/maps?hl=en&q=great+sand+dunes&ie=UTF-8. Accessed July
18, 2012.
Figure 2: Great Sand Dunes System. http://www.nps.gov/grsa/naturescience/sand_system.htm. Accessed July 18, 2012.
Figure 3: Great Sand Dunes Cross-Section water flow.
http://www.handsontheland.org/grsa/resources/curriculum/high/images/hydro_cycle.htm . Accessed July 18, 2012.
Figure 4: Frost Wedging. http://itc.gsw.edu/faculty/bcarter/physgeol/weather/mechwth.htm. Accessed July 18, 2012.
Figure 5: Weathering and Erosion & Groundwater. http://www.semi.sd36.bc.ca/mleziva/unit4/U04L01.htm. Accessed July 18, 2012.
Figure 6: Weathering and Erosion & Groundwater. http://www.semi.sd36.bc.ca/mleziva/unit4/U04L01.htm. Accessed July 18, 2012.
Figure 7: Great Sand Dunes and Pike’s Peak Colorado. http://jugalbandi.info/2007/08/great-sand-dunes-and-pikes-peak-colorado/. Accessed
July 18, 2012.
Figure 8: Rio Grande Rift FAQ. http://cires.colorado.edu/science/groups/sheehan/projects/riogrande/faq /. Accessed July 18, 2012.
Figure 9: Rio Grande Aquifer System. http://pubs.usgs.gov/ha/ha730/ch_c/C-text4.html. Accessed July 18, 2012.
Figure 10: How the Earth was Made: Rocky Mountains. http://www.history.com/shows/how-the-earth-was-made/episodes#slide-9. Accessed
July 18, 2012.
Figure 11: Types of Collisions. http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-3101/EReport03/GroupD/Report1/web%20pages/
assignment_1.html. Accessed July 18, 2012.
Figure 12: Great Sand Dunes, N.P. http://alpineadventures.blogspot.com/2010/07/great-sand-dunes-np.html. Accessed July 18, 2012.
Figure 13: Medano Creek. http://usparks.about.com/od/parkphotographs/ig/greatsanddunes/grsa-medano_creek_dunes_her.htm. Accessed
July 18, 2012.
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