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Domes and Basins -- Geology 101
This page describes domes, cryptovolcanic domes, basins, salt domes, and local basins
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Dome structures are found where forces deep under the crust have thrust a portion of the earth upward. The cuestas or overlapping folds face inward. Basins are similar, except the overlaps face outward as the structure forms a depression.
The average slackpacker will hike across numerous domes and basins in her life, but will rarely be aware of it. The lower peninsula of Michigan, for example, is a giant basin formation. The city of Paris rests on a basin. More likely, the hiker will be aware of this phenomenon when encountering a smaller, local dome formation, such as Upheaval Dome in Canyonlands National Park.
The Michigan basin, as an example, clearly exhibits the large scale basin caused by gently depressed layers of different geologic materials. Most basins are on a large scale such as this; "localized" basins are rare.
Photo at right shows the Michigan basin. This type of formation does not appear on your average local USGS topo map, and will not be evident to the hiker. Understanding the overall formation will assist in understanding some of the smaller, local formations. As an example, the low sand hills you'll encounter hiking in parts of Michigan are evidence of glacially worn cuestas, and in some places the "outward" facing cuestas are still visible.
Most of the features in the western USA referred to as "basins" are more correctly called esplanades, but they will always be called basins anyway. They are not basins as described in this entry; they are actually broad, somewhat flat areas found below mesas. Narrower esplanades are known as "benches." The formation of these features is completely different from the basins caused by collapsing geology deep in the earth.
Many domes are too large to be understood in terms of a normal map. Southern Ohio/Northern Kentucky rests on a large scale dome. These are gently upward thrusts, created by movement deep within the earth (called subcrustal movement). Some domes are circular, some are quite irregularly shaped.
Not all domes are enormous. Some are small, localized structures. These can be caused by a variety of phenomena. They can be caused by upward thrust or "arching" over large batholithic or laccolithic intrusions, they can be caused by anticlines, or other deformations such as the salt dome illustrated below.
A well known example of a local dome structure is Upheaval Dome in the "Island in the Sky" section of Canyonlands National Park.
This illustration, made during John Wesley Powell's exploration of the Colorado River Plateau, is a "local basin structure" somewhere in southwest Wyoming. It shows the classic basin structure of a inward sloping depression, ringed with outward facing cuestas. Obvious local basins such as this are rare; their exact cause is seldom known. We can intelligently conjecture that some degradation and subsequent collapse is the cause.
The cross-section of a typical salt dome, left, shows how the plug of salt creates a dome at the surface. This is an approximation of what the salt dome below Weeks Island, Louisiana -- see map below -- looks like. Weeks Island is actually a salt dome that protrudes up slightly above the surrounding marsh plain. Note the mine shaft plunging into the salt plug, and the oil rigs taking advantage of the oil fields forced upward by the upthrusting plug.
Salt domes are believed to be formed when large plugs of salt, some five miles deep in the earth, are forced up, pushing up layers as they go up. As the illustration shows, salt domes are often the site of salt mines and oil wells.
Upheaval dome is an unusual feature amidst the canyons, mesas and esplanades of Canyonlands National Park, near Moab, Utah. It is a cryptovolcanic type of dome, which means that it was created by an upward volcanic thrust. The inward facing cuestas are extremely steep and quite obvious. Notice in the photo above, the center ring of cuesta looks almost like a volcano; many confuse "volcanic dome" with "volcanic cone." The dome, like any others, is the actual surface layers of earth forced upward by the unseen plug beneath. In person, this is a jagged, haphazardous and disorienting feature. Amidst the bizarre scenery of Canyonlands, it is striking, but certainly not the most striking feature in the park. After viewing the sweeping vistas of Island in the Sky, many tourists wonder why they bothered to stop at Upheaval Dome. Indeed, without a passing interest in geology, their time would be better spent visiting other landmarks in the park.
Roadside Geology -- If you travel at all, you absolutely have to start building your library of Roadside Geology Books. These are the fascinating geological wonders that professional geologists know about, but us amateurs drive right past without a clue. At the very least you ought to get the guide for your home state. If you are a rock collector, or just an armchair geologist, these books are more important than your GPS. The link goes to Amazon, so you can click safely. Your purchase earns a few cents toward operating this website, at no added cost to you.
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