Volcanic monoliths are a common sight in the landscape of the American West. Rising high above the often-desolate surrounding plains, these structures of igneous rock are testament to a violent volcanic past throughout this area of the world.
Here, just north of Kayenta, Ariz., this volcanic plug is known in the Dine' language as aghaat'q, meaning "place of wool and hair" due to all the animal fur once found on the sharp, volcanic stone. Today it is known as Agathla Peak or by the nickname "El Capitan" due to it location just south of Monument Valley. It is composed of volcanic breccias (rocks formed from broken minerals and other rocks that has been cemented together) thought to be some 25 million years old. This volcanic plug rises some 1,500 feet (457 meters) above the surrounding high desert plains.
Volcanic plugs are formed when molten magma solidifies in the pipe or neck of an active volcano. Over time, the agents of erosion wear the softer surrounding sediments of the volcanic cone away. Morro Rock, shown here, is a volcanic plug located in the Pacific Ocean at Morro Bay, Calif. The word "morro" comes from Spanish and refers to a rocky outcrop found in the shallow waters of a harbor.
Morro Rock is one of the Nine Sisters (also known as the Morros) found between Morro Bay and San Luis Obispo, Calif. The nine volcanic plugs range in height from Morro Rock's 576 feet (176 m) to Bishop Peak, which is 1,559 feet (475 m) tall. They were formed from volcanic activity that occurred over 20 million years ago.
Some volcanic monoliths look like volcanic plugs but are not. Such is the case of the legendary Weaver's Needle found in the Superstition Mountains east of Phoenix. Named after mountain man Pauline Weaver, who first explored this Sonoran Desert wilderness area in 1831, this thick layer of volcanic tuff rises 1,000 feet (300 m) above the desert floor.
Weaver's Needle is a part of the 160,000-acre Superstition Wilderness Area and is located at the northern end of Peralta Canyon. The fused volcanic ash (tuff) has been heavily eroded resulting in this distinctive monolith that can been seen from miles away. The volcanic activity that formed all of the Superstition Mountain Wilderness ranges in age from 3 million to 25 million years old. The shadow cast by Weaver's Needle plays a key role in the continual search for the legendary Lost Dutchman Gold Mine.
Some modern geologists think that the Superstition Mountains are the result of a large caldera collapse that occurred some 29 million years ago. Pre-Cambrian granites can be found in a few isolated areas formed from intrusive igneous rocks. The many eons of volcanic activity have resulted today in a wonderland of rock formations throughout the region with the volcanic monolith, Weaver's Needle, being the most prominent.
On the northeastern plains of Wyoming stands an enormous volcanic monolith known to the Lakota Indians as Mato Tipila, meaning "Bear Lodge." An 1875 U.S. Army interpreter translated the Lakota word to mean "bad gods tower" and soon the solders were calling it Devils Tower.
Rising some 867 feet (264 m) above the surrounding landscape, Devils Tower is considered the most remarkable feature of the American Great Plains. Devils Tower is a part of the Black Hills mountain chain and is located near the Belle Fourche River. The summit of Devils Tower has an elevation of 5,112 feet (1,558 m) above sea level.
Native Americans have long considered Devils Tower to be a sacred place. The 19th-century pioneers that travelled by covered wagon used Devils Tower as a guiding beacon as it can be seen above the surrounding plains from a distance of over 30 miles (about 50 km). Movie producer Steven Spielberg used the geological wonder as the backdrop for this 1977 hit movie "Close Encounters of the Third Kind."
The various Plains Indian tribes have several creation stories for Devils Tower. One story tells of seven young sisters playing in the woods who found themselves being chased by an enormous bear. They jumped onto of a flat rock and the Great Spirit caused the rock to grow and grow into an enormous size. The giant bear continued to jump up at the girls and his claws created the gouges in the rock walls that can be seen even today. The seven sisters became forever safe in the sky as the Pleiades constellation.
Sedimentary rock is the main classification of rocks that surround Devils Tower certainly normal for an area that once was covered by a shallow sea and that includes the red sandstone cliffs of the Belle Fourche River. Some 65 million years ago a continental uplift began the formation of the Rocky Mountains and the Black Hills. Magma flowing from deep within the earth accompanied this uplift.
Geologists long thought that Devils Tower was a classic volcanic plug of an ancient volcano from this long ago period of time. But a problem with the volcanic plug theory arises due to the lack of volcanic materials near or around Devils Tower. If an ancient volcano once stood here, the eroded remains of its cone would surely be found for miles around its volcanic plug.
A more favorable theory has arisen suggesting that Devils Tower is the remains of a lacolith. A lacolith is an intrusion of magma between two layers of sedimentary rock. The resulting pressure forces the sedimentary strata upward resulting in a dome-like form. Over millions of years, ancient rivers and the elements of erosion wore away the soft sedimentary rock, exposing the hardened magma lacolith. There is still a large amount of sedimentary materials near the base of Devils Tower that gives support to this theory. If this theory is correct, the top of Devils Tower first became visible some 1 million to 2 million years ago.
The igneous rock of Devils Tower is composed of phonolite porphyry, a greenish-gray rock with crystals of the mineral feldspar. As the magma cooled, eight-sided vertical columns also formed. As the cooling continued, these columns contracted in size and pulled away from each other, creating the unique vertical furrows found on the sides of Devils Tower from top to bottom.
Devils Tower itself is not immune to the forces of erosion. The large boulder field of igneous rocks around the base of the tower is evidence that Devils Tower is also slowly losing the battle with the eroding forces of nature.
The first successful climb of Devils Tower occurred on July 4, 1893, when local rancher William Rogers used a series of ladders held to the tower by pegs driven into the vertical furrows. Rogers may have been the first to scale the magnificent obelisk but he is surely not the last. Today only about 1 percent of the 400,000 annual visitors make the effort to use free climbing techniques with safety ropes and steel wedges to ascend to the top of Devils Tower.
Devils Tower, the crown jewel of the volcanic monoliths of the American West, was designated America's first National Monument by President Theodore Roosevelt on Sept. 24, 1906. When the official proclamation establishing Devils Tower National Monument was published, the apostrophe that was initially in "Devil's" was unintentionally dropped. This clerical error has never been officially corrected.