How was Devils Tower formed? What is the Geology of Devils Tower?
When you are driving across north eastern Wyoming and Devils Tower pops into view it is hard not to be struck by awe. Most people who see it are drawn closer and can’t help but want to climb it. Several American Indian tribes have legends about the formation of Devils Tower and you can read about them HERE and HERE. For many years geologist told us that Devils Tower was formed by the intrusion (the entry of molten rock into or between other rock formations) of igneous material, but there are several theories as to exactly how that process took place.
Scientists Darton and O’Hara from the early 1900’s decided that Devils Tower must be an eroded remnant of a laccolith (a large mass of igneous rock which is intruded through sedimentary rock beds but does not actually reach the surface, but produces a rounded bulge in the sedimentary layers on top). This made some sense, as the rock surrounding Devils Tower is a beautiful red sandstone commonly referred to as the “Red Beds, ” and that is where the Red Beds Trail gets its name.
Many suggested that Devils Tower is a volcanic plug neck of an extinct volcano, as it is easy to imagine the tower once being hot red magma. It looks just like the pictures of hot red magma shown in all of the text books that so many of us grew up with. There is though, no evidence of volcanic activity in the area. No volcanic ash, lava flows, or volcanic debris in the surrounding countryside.
Many people say that the rocks of Devils Tower are granite, but a more accurate name is phonolite porphyry. Say that fast 3 times! My geology friends describe phonolite porphyry as a light to dark-gray or greenish-gray igneous rock with conspicuous crystals of white feldspar. As the lava cooled, hexagonal columns formed. As the columns continued to cool, vertical cracks that all the rock climbers love, developed as the columns shrank horizontally in volume. As you stroll around the base of Devils Tower, there are a few of these great columns lying along the trail.
One thing geologist agree upon is that Devils Tower formed deep underground, was uplifted, and over eons of time, erosion exposed the rocks that we now see and climb on. The most recent theory suggest that instead of an intrusion the Tower is the eroded remains of a volcanic conduit which connected magma chambers to a lava lake.
All theories that I have read seem to agree that the soft rocks were worn away over the eons after the magma cooled, but the harder igneous stone (magma) stood fast against the erosion process, and now stands tall on the prairie, with its summit 5,112 feet (1,558 m) above sea level. The mighty Belle Fourche river flows some 1200 feet below the summit of Devils Tower. Every 10,000 years, so National Park Service rangers tell me, there is a major event where one of the columns falls and begins its very, very slow journey to the ocean. In the mean time, the twists and turns of the Belle Fourche river are mighty nice to look at while climbing the Durrance route or camp beside down in the willows. Those waters continue to carry sediment to the sea.
Article in Forbes offers the most recent theory on the formation of Devils Tower and can be found HERE.