By Tom Schmidt

As one travels through northern Arizona, one of the first observations made might be the different types of rocks and physical landscapes. Indeed, the region is a land of geologic diversity, including the Grand Canyon and the red rock formations of Sedona. North of Prescott is the Granite Dells, an area of large boulders of eroded granite, while west of the city is Thumb Butte, a distinctive geological landmark. These awe-inspiring landscapes reflect Earth’s geological history that occurred not just in decades, but throughout millions of years.

According to Lou Abbott and Terri Cook in their book Geology Underfoot in Northern Arizona, the rocks around Prescott tell the history of the birth and evolution of the North American continent. This event is related to plate tectonics, a scientific theory that revolutionized geology during the 1960s. This principle states that Earth’s surface consists of several plates in constant motion, which collide, causing earthquakes and volcanoes as a result. Approximately 1,800 million years ago, one subduction zone, a region of the earth's crust where tectonic plates meet, began building a chain of volcanic islands in the Prescott area, which Abbott and Cook call the Prescott arc, while a separate chain that they label the Jerome arc formed several hundred miles away.

Clues of geological history can be found throughout the Prescott area. At the Stretch-Pebble Loop Natural Trail, located at the Highlands Center for Natural History, are phyllite deposits which form when rocks are subjected to high temperatures and physical pressure without melting in the process. Phyllite is very common around Prescott. Geologists have determined that phyllite’s original material is volcanic ash, which suggests proof of the Prescott volcanic arc’s existence. When a volcanic arc erupts due to gas pressure in its hot magma chamber, lava spews high in the air, where it forms volcanic ash. Phyllite rocks are created by differential, or unequal pressure, which is common in subduction zones, where rocks are squeezed the most in the direction that plates move.

The Stretch-Pebble Loop Natural Trail yields other signs of Prescott geological history. Hikers can view Prescott’s granite boulders, which, according to Abbott and Cook, are evidence of the Prescott arc’s collision with its nearby neighbor, the Jerome arc. The formations are the solidified remains of several magma chambers which fed the Prescott arc’s volcanoes at least 1,800 million years ago. Geologists have found other granite deposits underlying volcanoes of active subduction zones, including the Cascade Range of the Pacific Northwest. Along the trail is weathered, orange granite, which is another sign of a magma chamber that fed into volcanoes.

Of course, there is Thumb Butte, one of Prescott’s most visible landmarks seen in the background of numerous city photographs. At over 6,500 feet tall, it rises about 1,200 feet from downtown Prescott. Thumb Butte has often been defined as a volcanic plug, a geologic feature created after a larger volcano’s exterior erodes after millions of years and leaves behind its hardened center. Like the rock formations on the Stretch-Pebble Loop Natural Trail, Thumb Butte shows signs of geologic turbulence in Prescott’s past.

The nearby town of Jerome’s history is directly related to its geology. Copper mining dominated its history between 1876, when the first mining claims were staked, and 1953, when the United Verde mine closed. In 1914, several miners discovered a lode of 45 percent copper, a startling find that produced high-grade ore for a generation. At the height of copper production during the 1920s, the town’s population of 15,000 made it the second largest settlement in Arizona. The copper deposits beneath Jerome are some of the richest ever found, a result of a chain of geologic events taking place over nearly 2 billion years of earth’s history.

According to Abbott and Cook, Jerome, like Prescott, shows signs of its geological history. The reddish, orange rocks of Cleopatra Hill are among the oldest rocks in Arizona, at 1,750 million years old. Jerome sits atop the Verde Fault, the largest of several faults that slipped approximately 10 million years ago. As a result, the modern Verde Valley was created. This fault runs through downtown Jerome and is responsible for creating Cleopatra Hill’s steep slopes.

Dr. Phil Blacet began his geologic studies in the Prescott Region in 1959 and earned a doctorate from Stanford in 1968. For more than 30 years, he helped develop the Bagdad copper mine into one of the largest employers in Yavapai County. Please join Dr. Blacet for a free lecture on “Geology of the Prescott Region, Viewed Across Six Decades” at the Sharlot Hall Museum’s West Gallery on Saturday, June 16, 2018 at 2:00 p.m.

“Days Past” is a collaborative project of the Sharlot Hall Museum and the Prescott Corral of Westerners International (www.prescottcorral.org). This and other Days Past articles are also available at www.sharlothallmuseum.org/days-past. The public is encouraged to submit proposed articles and inquiries to dayspast@sharlothallmuseum.org. Please contact SHM Library & Archives reference desk at 928-445-3122 Ext. 2, or via email at archivesrequest@sharlothallmuseum.org for information or assistance with photo requests.