Mysterious Devils Tower, called Mato Tipila in Lakota, is an igneous landform. It may be a volcanic plug that formed in the throat of an ancient volcano, but more likely it is a laccolith, an igneous intrusion that formed when molten rock pushed between two layers of sediment and formed a bubble that slowly turned to stone.
Mato Tipila is composed of phonolite porphyry, which shrank as it cooled to form tall rock columns. These resemble similar basalt columns found around the world.
Laccoliths, volcanic plugs, and basalt columns are some landforms created by igneous rock. Others include sills, dikes, lava flows, and volcanoes.
A laccolith is an igneous intrusion, formed when molten magma intrudes into older rock. It forces it way between horizontal layers of sedimentary rock, pushing up the layers above it to form a dome. The base of a laccolith is usually flat. Laccoliths form beneath the ground, but appear aboveground when the relatively soft sedimentary rock surrounding them has been eroded away. Though the Devils Tower is not dome-shaped, it is possible that its sides eroded away to shape the plug we see today. Crown Butte in Montana is another example of an exposed laccolith.
A volcanic plug forms when magma solidifies in a vent of a volcano. Once the volcano erodes away, the plug is isolated as a high point in the landscape. Morro Rock in coastal California is an example of one of these mysterious formations. Standing at the ocean-side entrance to Morro Bay, the Rock is the most northwestern of the Nine Sisters, a series of volcanic plugs that march from Morro Bay southeast to beyond San Luis Obisbo. Morro Rock is now a peregrine falcon sanctuary.
Columnar jointing formed many famous landforms. Giants Causeway in Ireland, Devils Postpile in California, and a site in the Marte Vallis on Mars are examples. Basalt columns, often hexagonal, occur when molten basalt cools. It shrinks as it solidifies, as almost all substances do, and therefore takes up less space. To accommodate this contraction, cracks form in the rock. Similar many-sided cracks can often be seen on the surface of mud that dries up in the sun. The cracks in basalt begin at the surface, where it is coolest. These cooling cracks gradually lengthen as cooling proceeds through the basalt and it draws together, to form lengthening columns. Rock that cools more slowly is able to form larger columns, and rock that cools quickly forms more slender piers.
Sills, Pipes, Veins, and Dikes
Dikes, pipes, veins, and sills are all intrusions into sedimentary layers. Sills are horizontal (at least originally) layers of rock that form when magma flows between layers of older rock. They do not lift the rock above them to form domes the way that laccoliths do, and they do not cut across layers of rock.
Pipes, veins, and dikes cut across layers of older rock. Pipes are columns of magma that feed laccoliths or volcanoes. Veins are intrusions of magma that travel upward to fill small irregular cracks in older rock.
Igneous dikes are sheet-shaped intrusions into older rock. They usually form in vertical or near-vertical positions. They appear as stripes in older rock, once it is exposed, or as walls on the surface when surrounding softer rock has been worn away. They may curve around volcanoes, as ring dikes, or they may ray out from them, in radial dike swarms. Dikes stripe some of the walls in the Black Canyon of the Gunnison in Colorado, and ray out from Shiprock in the Navaho desert.
Lava flows are extrusive igneous rocks: they are extruded onto the surface of the earth. (Lava is the term for magma that reaches the surface of the earth.) They cool more quickly than the intrusive formations that are insulated and under pressure, and therefore their crystals tend to be smaller than rock that has cooled in intrusive formations. Big crystals don't have time to grow. Sometimes, as in the case of volcanic glass, crystals are non-existent. Lava may flow from fissures without creating a volcano, and spread out to create a plateau, like the Siberian Traps, formed over a period of 200,000 to 1 million years.
Volcanoes are generally cone-shaped landforms. They may be broad-based shield volcanoes, built of fluid lava, or stratovolcanoes built of alternating layers of loose materials and lava. Famous volcanoes include Etna, Vesuvius, and Mauna Loa.
Smaller spatter cones are built up from accumulated slag and cinders. Many are found at Craters of the moon National monument in Idaho. Cinder cones form from volcanic ash and tuff thrown from a small vent. They form in groups, including a cluster northeast of Mt. Lassen in California.
Landforms made of igneous rock sometimes dominate the landscape. Then they add a sense of mystery and power, and remind us of the earth's great age.