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Igneous Landforms -- Geology 101
This page describes igneous landforms. These are any forms volcanic in origin, or from other magma or lava flow phenomenon.
If you wish to return to the main geology page, please click here.
Igneous Land Forms
Includes Dikes, Necks, Calderas, Cinder Cones, Lava Domes, Composites, Lacolithic domes, Lava flows, stock.
Igneous landforms can usually be placed in one of two camps: Intrusive and Extrusive. Intrusive forms result from magma solidifying beneath the earth's crust, then later exposed by earthquake, erosion, glacial activity, etc. Extrusive landforms result when hot magma reaches the earth's surface in a liquid form, then solidifies.
Rock resulting from intrusive activity includes obsidian, fine grained basalt, etc. and other batholithic rock. Rock resulting from extrusive activity such as pumice, "lava rock" etc. frequently shows evidence of the outgassing that occurs as the magma cools.
Much of the igneous landforms described on this page can be best understood by referring to the illustration immediately below:
Some natural arch and bridge formations are the result of lava tubes, in which the majority of the tube has subsequently collapsed and a small section remains. If the length of the tube or tunnel is greater than the width, it is not technically an arch.
The Arch Millenium Website is an excellent resource for worldwide natural arch information from Guilain Debossens.
Large areas of intrusive igneous and/or metamorphosed rock exist on every continent. When these are exposed, usually by erosion or glacial activity, they are known as Precambrian shields.
Well known hunks of exposed batholithic formation, called "Monadnocks" or "Stocks" (see below) include Enchanted Rock in Fredericksburg, TX and Stone Mountain, GA (photo below). This link to Enchanted Rock leads to an extremely educational page by "Rob" at the University of Texas, and will provide a lot more information than you'll ever find here.
Most buttes are not igneous forms. Sometimes volcanic necks are locally referred to as a butte. More often, buttes are a combination of erosion forms and igneous forms. For example, some would call Devil's Tower a "butte," when it is actually a neck. Black Mesa and Cunningham Butte on the NM/CO border are the result of igneous forms, but are not necks; a caprock of basalt has protected the material beneath from being worn away...a butte remains.
Igneous forms seen in canyons are usually intrusives; the magma crystallized beneath the earth's crust but has since been exposed by whatever formed the canyon. Some igneous forms in canyons are extrusive, such as the lava flow in the western portion of Grand Canyon NP; the lava flowed after the canyon had been formed.
Igneous formed caves are also usually lava tubes. The "cave" or tube forms as the result of an additional matter being present...gas, water, etc.
Volcanic formations, sometimes called craters, sometimes buttes, sometimes cones...characterized by the presence of black volcanic sand. One of the best places to view this phenomenon is Craters of the Moon, an area in Idaho administered by the National Park Service. Another prime location is near Flagstaff Arizona, where dozens of black cinder cones can be explored.
Unfortunately this word is frequently used to describe two similar looking phenomena that couldn't possibly be more different in origin. One is the result of something blasting out of the earth -- while the other is the result of something blasting the earth. Both are called craters. Turn to northern Arizona, where Sunset Crater and Meteor Crater are within 50 miles of each other. Sunset "Crater" is actually an inactive volcano (see "cinder cones" above), while Meteor "Crater" is the site of a meteor impact.
When drainage channels create patterns across a relatively flat plain of old, metamorphosed batholiths. Eventually the channels narrow, and leave a checkerboard type pattern across the area. One of the most accessible and visible examples of dendritic drainage can be seen at Petrified Forest National Park in Arizona. (Specific location is the northernmost overlook at Blue Mesa.) See the definition of "stock" below; dendritic drainage surrounds many stocks (now monadnocks).
A volcanic vent or pipe formed by magma energized by gas, sometimes with explosive results. A well known example in the USA is Agathla Peak, near Monument Valley. A photo and explanation can be found here on Indiana Joe's visit to Monument Valley.
Dikes indicate volcanic activity. Appear frequently as "veins," but not necessarily. Intrusion in weaker or older cracked rock.
A significant upward flow of magma along a "neck" that spreads out and bulges upward in a subterranean mushroom shape. One of the best examples of an exposed laccolith is Sundance Mountain in Sundance, Wyoming. Nearby Lime Butte, Gypsum Butte, and Green Mountain are all laccoliths with varying degrees of exposure. See illustration below...dark area shows magma "neck" that flowed from batholith below to create laccolith.
Mesas due to igneous formations tend to be caprock (often basalt) that protects the "body" of the mesa from erosion.
Primary examples in the SW United States. These are features up to hundreds of feet high with steep sides and large, flat tops. Actually, mesas are what is left of what was once a flat plain; the valleys and canyons below have been eroded away. What's the difference between a mesa and a butte? Mesas have much larger "tops," and some will say that -- technically -- a mesa is a butte on which standing water can sometimes be found. This can be confusing, because after a rainstorm you can find water atop many buttes. It's likely that this definition came from a distinction made in the southwest USA in that you could find game on a mesa, but not a butte, which eventually translated to the idea that you could graze cattle on a mesa (and find water) but not on top of a butte. So, a good rule of thumb if you will, is that if you can fit a water tank and a small herd of cattle on it, it's a mesa.
See Stock, below. A Monadnock is an ancient, crystalline stone feature rising above a plain. Usually a mountain of a "solid" rock.
Necks are formations in which the magma solidified in the tube of the volcano. The volcanic activity ended, and eventually the cone material was stripped away by erosion (wind and or water). With the cinder cone gone, just the solidified plug remains. One of the best known volcanic neck formations is Shiprock (New Mexicon), near four corners. The topographical map below shows the solidified "neck" known as Shiprock (A) and, interestingly enough, a series of radial dikes (B) that are solidified magma that filled fissures in the long ago cone. These thin, ultra-hard dikes appear as long fins of rock that lead like spokes to the "hub," or neck known as Shiprock.
Highly metamorphosed batholithic rock that has been exposed over time.
Looking at the general igneous overview image at the top of this page, you'll notice that a stock is a portion of a batholith that bulges upward. Often this will be exposed as a large, "solid rock" mountain. The classic example of a stock formation -- complete with dead Confederates -- is Stone Mountain, near Atlanta Georgia. (Clicking that link takes you to an awesome Stone Mountain geology page created by Pamela Gore). Notice in the topographical map below that the surrounding Piedmont plain shows the lines of a typical dendritic wash, that is, the drainage channels have cut into the lower batholithic plain.
Yeah, well, you know what this is. BUT I bet you don't know about one of the coolest volcano websites going: The Cascades Volcano Observatory site, maintained by Lyn Topinka and the good people at the US Geological Survey. These folks are the spiritual heirs to the work of Mr. David Johnston, of Mt. St. Helens fame. Although it is focused on the volcanoes of the Pacific Northwest, this is one dandy website for anybody looking for info on the subject.
Mr. Johnston, for those of you who actually were born yesterday, was the scientist camped what he thought was a safe distance from Mt. St. Helens in May, 1980. He was tragically killed by a blast of 700 mph gasses, heat, debris, dust, rock, and heaven knows what else during the eruption on May 18. The ridge where his camp was located is now the home of the main visitors center at Mt. St. Helens National Monument, and is known as Johnston Ridge.
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.
Enchanted Rock an extremely educational page by "Rob" at the University of Texas about the geology of batholithic formations.
Stone Mountain geology page specific to this stock formation, by Pamela Gore.
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