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The Great Sand Dunes
in Alamosa, Colorado
BRENDAN FRERICH
The Agenda
• The Trip
• Geologic Timeline
• Formation and Erosion/Weathering
•
•
Tectonic Activity
Erosion and Weathering
• The End Result
•
•
Wetlands vs. Desert
Building of the Great Sand Dunes
The Trip
• On Sunday October 25, 2015 I went with my father to Great Sand Dunes
National Park, near the town of Alamosa in Southern Colorado.
• We explored the visitor’s center, exploring the areas geology and culture.
• The Great Sand Dunes are found in the San Luis Valley, at the foot of the Sangre
de Cristo mountain range. On the eastern side of the valley are the San Juan
mountains
The Clovis Native Americans lived
in this area for thousands of years,
calling it the land that moves back
and forth. They used Ponderosa
pine trees (right) for food and
medicine. (3)
Examples of the rock that was broken up to become the
sand in the dunes. While the majority is plain stone like
quartz and sandstone some of the sand is made from
amethyst and garnet.
The Trip (cont.)
• The Dunes, which can reach 750 ft. in height,
cover an area of 30 sq. miles (2)
• Visitor’s such as myself can hike in to the
mountains of sand
• Medano Creek plays and important role in
maintaining the landmark
Geologic Time:Precambrian 4,600-600 million years ago
Rio Grand Rift
Island arcs collide with the
northern landmass, creating
the base of gneiss (5)
Paleozoic 550-250 million years ago
The Ancestral Rockies form at the
collision of Gondwana and Laurentia (5)
Uplifts and unconformities occur. Paleozoic
Seas wash away many layers of sediment (5)
Mesozoic 250-70 million years ago
• The Mesozoic era is a massive
unconformity in the Rocky
Mountain area, as for most of
the Cretaceous the land was
covered by a sea. Many rock
layers were washed away,
leaving little fossil record of the
era. (5)
Cenozoic 70-0 million years ago
Rio Grande Rift Stops, Sangres
uplift continues (5)
Especially fast period of expansion along the Rift
The eruptions slow and release lighter materials, eg
higher amounts of ash, finishing the San Juan Range(1)
La Garita Caldera Erupts(1)
Rio Grande Rift forms, and begins to pull apart the land.
Small igneous intrusions in the Sangre’s are a result of
rifting
Plates collide, Sangre Uplift begins, as well as
the formation of the rest of the Rockies (5)
Recent Changes: 2 million years ago to present
• Starting 1.8 million years ago glaciers formed repeatedly,
carving out the Sangre de Cristos. Each time the glaciers
receded they left till called Bull Lake. (5)
• 30,000-12,000 years ago, the final expansion and recession of
the glaciers left glacial till, that is at this point only slightly
weathered. These younger deposits are called Pinedale.(5)
Formation, Erosion, and Weathering
• Tectonic Activity
•
•
La Garita Caldera and the San Juan Mountains
Sangre de Cristo Uplift
• Erosion and Weathering
Root/Frost Wedging
• Mountain Streams
• Saltation
•
La Garita Caldera Tectonic Forces
• Roughly 35-30 million years ago the La Garita Caldera erupted. It
was really a series of eruptions of mafic material that formed the
San Juan mountains.(1)
• 30-26 million years ago the eruptions consisted of more ash, and
less thick lava, completing the formation of the San Juan range.(1)
The yellow are represent the main caldera, and
the dark red areas represent smaller ones that
formed nearby. The red shading is the area
affected.
Rio Grande Rift Tectonic Activity
• Roughly 26 million years ago the Earth began to
pull apart where the San Luis now is. The Rio
Grande Rift acted like a rift valley, however it
was more localized. Con-Con convergence was
creating mountains at roughly the same time,
and it is not understood why rifting was
occurring. As the valley stretched, igneous
intrusions pushed into the Sangre Range. The
Rift did not cause the Sangre uplift, but helped
it along. It also formed a basin for water to flow
into. When these rivers and lakes dried up it
left dry, mobile sediment that was carried to
the Sangres to create the first Dunes.(5)
Sangre de Cristo Uplift Tectonic Activity
• Unlike the San Juan mountains, the Sangres are not volcanic, and
formed in a similar way to the rest of the Rockies. In the Cretaceous
and Tertiary time periods a Continental-Continental collision of
plates created the mountains, which were later carved out by
glaciers. (5)
Natural Wedging (Weathering)
Rock can break up easily when something breaks it from the inside. Wedging in
the Rocky Mountains comes in two main types, frost and root. Stone is broken
off into streams, and onto slopes, providing what eventually becomes sand for
the Dunes.
FROST WEDGING
ROOT WEDGING
• Water expands when it freezes. Water seeps
• The fine tips of tree roots work their way into
into cracks or pores in rock. When the liquid
a crack in rock. As the root grows it extends,
freezes it expands, pushing the rock apart, the
lengthening the fissure, and expands,
fissure growing larger each freeze thaw cycle,
widening it, until the rock gives way.
breaking stone apart.
Mountain Streams (Erosion and Weathering)
EROSION
WEATHERING
• As stone is carried
• Rocks of all sizes get dropped into
downstream it will
streams in the mountains of Colorado.
bounce against other
In the spring runoff the water can carry
stones, roll on the bed of
particles of all sizes downstream. Upper
the stream. This constant
Sand Creek and Medano Creek carry
motion and battering on
material to the Sand Dunes. In this
other rocks breaks the
stream that feeds the Dunes, you can
particles into ever smaller
see rounded boulders. The sharp edges
particles in a form of
have worn away, and been carried
mechanical weathering.
downstream.
Eventually it can carve out
V shaped valleys, and in
the case of Zapata Falls in
the San Luis Valley, small
caves (right).
Saltation (Erosion and Weathering)
EROSION
WEATHERING
• Saltation is the movement of
particles over uneven ground by
turbulent wind. Wind carries
particles deposited in the San Luis
valley across the valley, and releases
them in the dunes.(2)
• While the wind blows particles hit
each other and drag on the ground,
such as in a stream, and are ground
finer and finer until they become
the powdery sand found in the
Dunes.(2)
The End Result
• Ecosystems
•
•
Animals specific to the Dunes
Wetlands
• Building of the Great Sand Dunes
Great Sand Dunes Ecosystem
•
(4)Many
organisms are unique
to the Sand Dunes, such as the:
Great Sand Dunes Tiger Beetle
• Nouctid Moth
• Werner and Triplehorn Flower
Beetles
•
Wetlands
• Surrounding the Dunes to the
South and West are diverse
wetlands. They are home to
many species, in contrast to
the relatively bare sand. Water
from the Sand and Medano
creeks soak into the ground,
and travel past the dunefield,
creating a oasis in the desert.
As a result of the water being
wicked away, dry sediments
left by the creeks are free to
join the dunes.(2)
Building of the Great Sand Dunes
• Southwesterly winds blow
sediments from the San Juan
mountains and San Luis Valley
toward the Sangre range.
Weaker Easterly winds coming
from over the Sangre de
Cristo’s conflict and forces the
sand to drop out of the air. The
airflow whips the sand
deposits into true dunes. (2)
Weaker Easterly Wind
Conflict forms
Dunes
San Juan Mountains
Strong Southwest winds
Sangre De
Cristo
Mountains
Sand Recycling Building of the Dunes (cont.)
• The waterways circling the
Dunes carry away sediment
from the sand sheet. At the
ends of Sand and Medano
creek the sand is deposited,
and the wind blows the
sediment directly back at the
Sand Dunes. (2)
Sand carried to the
end of
Sand/Medano Creek
Sand blown back to
Dunes
Sand picked up by
Sand/Medano Creek
Sources
• (1) Steven, Thomas A., Peter W. Lipman, and Harald H. Mehnert. "Volcanic History of the San Juan
Mountains, Colorado, as Indicated by Potassium–Argon Dating." Volcanic History of the San Juan
Mountains, Colorado, as Indicated by Potassium–Argon Dating. The Geologic Society of America, 10 Apr.
1970. Web. 30 Nov. 2015. http://gsabulletin.gsapubs.org/content/81/8/2329
• (2) United States. National Park Service. "Geology." National Parks Service. U.S. Department of the
Interior, 29 Nov. 2015. Web. 30 Nov. 2015. http://www.nps.gov/grsa/learn/nature/sanddunes.htm
• (3) United States. National Park Service. "History & Culture." National Parks Service. U.S. Department of
the Interior, 28 Nov. 2015. Web. 30 Nov. 2015. http://www.nps.gov/grsa/learn/historyculture/index.htm
• (4) "Insects, Spiders, Centipedes, Millipedes." National Parks Service. United States, n.d. Web. 30 Nov.
2015. http://www.nps.gov/grsa/learn/nature/insects.htm
• (5) Lindsey, David A. "The Geologic Story of Colorado’s Sangre De Cristo Range." The Geologic Story of
Colorado’s Sangre De Cristo Range (n.d.): n. pag. United States Geologic Survey. Web. 30 Nov. 2015.
http://pubs.usgs.gov/circ/1349/pdf/C1349.pdf