Devils Tower

Current Opinion suggests that Devils Tower is not strictly speaking a volcano, since there is no unequivocal evidence for any eruption products that have been derived from this source. Nevertheless, Devils Tower, which rises nearly 400 meters from the surrounding terrain, is undisputedly one of the finest examples of an erosionally exposed cooled body of magma. The greenish-grey trachyte rock forming the column is a phonolite porphyry with about 60% SiO and forms columns hundreds of meters long, making them amongst the largest such columns in the world. Many other notable columnar features (e.g. Devils Stockpile, Giants Causeway) are made of columnar basalts which have a significantly lower silica content.

It is thought that about 65 million years ago, a body of magma rose through soft sedimentary rock layers leading to regional uplifting, without however actually penetrating the surface. Hence, Devils Tower is a monolithic igneous intrusion or laccolith, rather than a volcanic neck. As magma supply was discontinued, the body of magma cooled and formed the characteristic fracture patterns.

In 1906, Devils Tower became the first "National Monument" in the United States. Nevertheless, it is possible to obtain permits to climb the rock and this has become a hugely popular activity with several thousand climbers every year.

View from Northeast	View from South	View from East

View from East	View from WSW

Whilst the name Devils Tower seems suitable to describe this unusual landmark it actually derives from mistranslation of the original American Indian designations. These referred to it by the name "Bear Tower/Lodge" or similar designations, apparently deriving from local mythology explaining its origin. It was thought that Indian Girls were fleeing from a Grizzly Bear when the Great Spirit elevated the ground beneath them, forming the tower. As the bear tried to climb the tower, its claws scratched the rock (explaining the columnar structure). Even today, the place has a spiritual significance for the ancestors of the Indians, and a voluntary climbing ban is in place in June in recognition of this. In popular culture, the Tower gained fame by the Oscar-winning Spielberg movie "Close Encounters of the Third Kind" (1978), where it served as a focal point for the arrival of aliens on our planet.

View from Northeast	View from West (Car Park)	View from West

The formation of columnar structures in cooling magma/lava bodies is the result of cracking as a result of stresses caused by contraction during the cooling process. The dimensions and length of the columns are influenced by factors such as cooling rate and thermal gradients (Grossenbacher and McDuffie 1995. J. Volc. Geotherm. Res. 69, p.95-103). In particular, faster cooling is thought to result in narrower columns. In the case of Devils Tower, the surrounding sedimentary rock will have to some extent insulated the magma body, resulting in slow cooling and thus explaining the huge columns observed. The columns are largely hexagonal (6-sided) and pentagonal (5-sided) with larger examples having diammeters of up to 2.5m near the base, narrowing nearer the top to 1-1.5m. Narrower columns tend to be more commonly hexagonal. Columns with anything from 3-8 sides may be found. Due to the brittle nature of the columnar structure, the tower is surrounded by a Tallus Apron, largely consisting of columnar material broken off its flanks.

Close-up of Columns	Tower with Tallus Apron at base	Tower with Tallus Apron at base

The sedimentary rocks surrounding the tower are exposed in places. For example, near the Belle Fourche River, to the right of the road bridge just after entering the park, formations of red sandstone belonging to the Spearfish Formation are exposed. The red colour derives from iron oxides found in this layer. This layer was deposited in the Triassic period (about 225-195 years ago) and is mainly visible to the SE side of the tower. In places, this layer is still topped by more recent Jurassic gypsum deposits. At the base of the tower, these layers are overlain by the Sundance Formation, largely consisting of resilient yellowish fine-grained sandstone, limestone and grey-green shales.

The exact relationship between Devils Tower and the similar-aged nearby Missouri Buttes, further smaller laccoliths near to Devils Tower has been subject to debate. However, it today seems unlikely that the conduits underlying these formations were interconnected.

Red Sandstone of Spearfish Formation	Tower Rising above Sedimentary Deposits	2 of Missouri Buttes (left) and Devils Tower

The Devils Tower National Monument not only contains the Tower but also has a Prarie Dog Town located next to the road leading to the parking lots at the base of the Tower. Populations of the Black-Tailed Prarie Dog are under pressure due to poisoning on farm / ranch land, where it is often (wrongly) regarded as a pest, and also due to its susceptability to the Bubonic Plague and removal of young for the pet trade. Hence, the protected area around the Tower serves as an important haven for this species and for those species which rely on the Prarie Dog as prey.

Black-tailed Prarie Dog

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