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Colorado Great Sand Dunes National Park and Preserve - Geology

小池清通 写真 Kiyomichi Koike Photography
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 History - Great Sand Dunes National Park and Preserve - Colorado

 Mosca, Colorado USA

Geologic Timeline (animation):
During the Paleozoic Era, over 500 million years ago, shallow seas and river systems deposited sedimentary materials throughout the region. The Sangre de Cristo mountain range east of the dunes is composed of Paleozoic rocks as well as metamorphic rocks from the much older Precambrian period.

Eons later, pressures from within the Earth pushed the landscape high above sea level, limiting sedimentary deposition and leaving a gap in the region's Mesozoic record.

Around 40 million years ago, an arid landscape dominated where rivers and intermittent lakes emerged and subsided upon the land. Evidence of this period are found in the silts, clays, sands, and gravels of the early-Tertiary Blanco Basin Formation.

Recurring lava flows poured onto the landscape about 34 million years ago from volcanic vents in the San Juan mountains, creating a subsurface layer that geologists call the Conejos Formation.

After the San Juan Mountain lava flows subsided, volcanic activity continued and for four million years ash flows blanketed the earlier lavas of the Conejos Formation. These ash-flow tuffs are visible on the west side of the Sand Luis Valley.

Rapid uplift of the Sangre de Cristo Mountains began about 19 million years ago. Rifting associated with the uplift widened the valley and caused its floor to drop significantly. (This process continues today.) Water-born and eolian sediments were deposited in the subsiding San Luis Valley, creating the Santa Fe Formation.

Around five million years ago, a new series of volcanic activity rocked the region, at times damming rivers that flowed out of the southern half of the San Luis basin. The Alamosa Formation formed during this time, made of lake, river, marsh sediments and wind-deposited layers of clay, sands and silts. Locally known as the "blue clay layer," the Alamosa Formation is a confining layer between aquifers found under the valley floor.

The Pleistocene began 1.8 million years ago as climate changed globally. Glaciers grew in mountain valleys, some pouring ice and rock far into the San Luis Valley. Many scientists consider the Pleistocene to be the period in which dune formation began in the San Luis Valley.

Cooler temperatures also caused the Earth's oceans to contract, creating, land bridges that permitted a worldwide diaspora of animals between North America and Asia.

Only about 12,000 years ago, a warming climate melted many glaciers worldwide and signaled the end of the Pleistocene. Large quantities of silt, gravel and sand were carried by rivers and streams into the San Luis Valley. Although the dunes existed long before the Pleistocene ended, glacial outwash materials provided new sources of sand for winds to rework into even greater sand dunes.

Today, the rivers and creeks continue to transport sediment into playa lake systems which are sources of sediment for dune-forming winds of the San Luis
Valley.

courtesy of National Park Service

Geologic Cross Section (animation): The Rio Grande Rift, one of the most notable geologic forces in the region, extends northward from Mexico into central Colorado. For almost 20 million years, this gaping crack in the Earth's crust has widened and the San Luis Valley floor has slowly dropped while "hinged" on the west to the San Juan Mountains. The sinking San Luis Valley has created a catch basin for river gravels, eolian sands, volcanic ash-flow tuffs and lake deposits and clays. About 15,000 feet of sedimentary fill is now nestled below the Great Sand Dunes, all of which has been deposited within the past 35 million years.

Wind Regime: Thirty-four miles away by car in Alamosa, prevailing winds are from the southwest throughout the year?as illustrated by arrowhead in the wind rose diagram. This wind pattern continues to the southwest corner of the dunefield.

Over the years, sand and small dunes migrate in this southwestern, unimodal wind pattern toward the main dunefield. Close to the dunefield, however, wind patterns change. It is not uncommon for winds to emerge from any of the three mountain passes?Music, Medano and Mosca?any time of the year. The dunefield's bimodal and complex wind regime contribute to two distinctive dune types: reversing and star dunes.

If you scrutinize the relief map above, take notice of the Sand Creek Star Dune complex in the northwest corner of the dunefield. Star dunes are oriented in many directions, which indicate the occurrence of a complex wind regime. And notice that a bimodal wind regime is apparent in the southern region of the dunefield by the existence of the north-south trending Medano Creek dune ridges.


Dune Types: reversing (animation), star, parabolic, barchan, transverse

Sand Recycling Grand Sand Recycling: The main dune field grows to enormous heights not only because of a complex wind regime, but also due to sand carried by Medano and Sand Creeks. These streams seasonally carve out and carry sand from the east and north side of the dunefield and re-deposit it where wind can transport it back to the dune field.

Saltation: Individual sand grains move in three ways: saltation, creeping, and suspension. Sand grains bounce or "saltate" slightly above the surface when blown by the wind. With each bounce, the grains usually reach heights of less than 5 cm above the surface, and move horizontally 5-10 cm. The average distance a grain bounces is the width between the crests of two ripples in the sand. Saltation accounts for about 95% of a sand grain's movement. A sand grain "creeps" when it collides with other grains, causing it to roll or make small jumps. Creep may account for about 4% of a grain's movement. Finally, a sand grain moves by "suspension" when it is blown high into the air. Suspension accounts for 1% or less of an individual sand grain's movement.

How Much Sand is There? More Sand than Meets the Eye!
How much sand is in the sand dunes? The simple answer is "a lot," approximately 4.8 billion cubic meters, but that is really just the tip of the iceberg. The dunefield is impressive with its tall dunes and scenic backdrop, but it is the smallest of three sand deposits in the Great Sand Dunes system, measuring just 30 mi2 (78 km2). In the valley surrounding the dunefield is the sand sheet, covering 180 mi2 (466 km2). Close examination shows how the sand sheet got its name; it is almost 100 percent sand and, despite some small hills and depressions, is primarily flat. Farther west in the valley, near the San Luis Lakes, lies the third sand deposit, the sabkha. It extends over 120 mi2 (311 km2) and is characterized by sand grains cemented together by minerals.


courtesy of National Park ServiceWhy are dunes only found on 30 of 330 mi2 (855 km2) of sand deposits? Dune formation requires several conditions. First, there must be sand! Wind strong enough to move the sand (>13 mph or 21 kmph) must also be present. Both sand and wind are plentiful, so the key factor is that sand must be loose and moveable. The sand in the sabkha is hardened by minerals, so the wind cannot shape it into dunes. On the sand sheet, vegetation anchors sand, preventing dune formation. If you look across the sand sheet, however, you can see tan-colored areas where the vegetation has lost its foothold and dunes are forming. Sand in the dunefield is completely exposed to the wind and, as a result, dune formation is very active.

The sand sheet and sabkha are subtle features, but you can discover them as areas undeveloped for human use. Coming from the west on Lane Six, the farmlands end where the sabkha starts. Travelers using Highway 150 will notice a lack of houses west of the highway until they near Highway 160; that is the sand sheet. Since the land in both areas is sand, it is not suitable for farming or home sites.

These varied sand deposits played an important part in the legislation passed in late 2000. The condition of the two larger deposits greatly influences whether new sand comes into the dunefield; the expansion is intended to help protect the natural processes of sand replacement occurring in those areas. by Andrew Valdez

[This website is created to show the beauty and wonder of a great american natural treasure. The contents are courtesy of the National Park Service. It is created in English and in Japanese. All the photo images shown here are taken by Kiyomichi Koike. The copy rights are reserved.]


A line of light lights up the dunes.

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