New Guide to Geologic History of Tucson Mountains

Lori Stiles
Dec. 13, 2002

If your holiday plans include a hike or horseback ride through the Tucson Mountains, a drive through Saguaro National Park West or a visit to the Arizona-Sonora Desert Museum, tuck a copy of "Desert Heat – Volcanic Fire" in your pack.

This brief, attractive, easy-to-understand new book, published by the Arizona Geological Society at $16, describes the spectacular geologic events that shaped the familiar and popular Tucson Mountains, where fantastical prehistoric creatures once roamed across a much different southern Arizona landscape than the one we now see.

The Tucson Mountains are not impressively high. They rise to only 4,687 feet. They are hot and rugged -- "a very good mirror of the character of Tucson" -- which explains much of their allure, according to University of Arizona planetary scientist David A. Kring, author of "Desert Heat – Volcanic Fire."

Kring said he began working on the book 14 years ago 'just for the fun of it. I am a geologist and wanted to learn about the geology around here. And I wanted to share the geologist's perspective with students and non-scientists. I hope this book will bring the Tucson Mountains volcanic caldera to life for all its visitors."

The Tucson Mountains are largely the remains of an immense, more than 15-mile-long volcanic caldera, or collapsed volcano. Towering stratovolcanoes erupted here with far greater explosive force than did modern Mount St. Helens, spewing several hundreds of cubic kilometers of hot volcanic debris across the landscape about 70 million years ago.

Tucson Mountain-area rock layers record long ago landscapes that include a large, shallow, algal mat-covered, oyster-rich freshwater lake, located west and south of what today is Wasson Peak. Dinosaurs roamed around the lake, which lasted perhaps a million years before the land rose and the Tucson Mountain area was plunged into a cycle of repeated catastrophic volcanic eruptions by several different types of volcanoes through geologic time.

Across the reshaped later landscapes, certainly, "camels walked across a mudflat, mastodons fed on grasses while possibly under attack by saber tooth tigers, and giant ground sloths moved in and out of rocky clefts while herds of miniature horses galloped across grasslands," Kring noted.

The volcanic caldera cycle that created the Tucson Mountains rock formations also gave rise to the Silver Bell Mountains, Sierrita Mountains, Santa Rita Mountains, Chiricahua Mountains, Superstition Mountains, Galiuro Mountains, Dos Cabezas Mountains and other southern Arizona ranges, he added. "By examining the easily accessible Tucson Mountains, one can learn how the entire region was shaped over the last 100 million years."

The book is aimed at 7th graders on up to more technically minded adults interested in how plate tectonics and volcanism shaped the mountains in our own back yard. It answers simple questions (What is an igneous rock? How long does it take a volcano to grow?), while also providing more detailed information for the mineral enthusiast and budding geologist.

"Desert Heat – Volcanic Fire: The Geologic History of the Tucson Mountains and Southern Arizona," 104 p., softbound, is available from the Arizona Geological Survey, 416 W. Congress, Suite 100, Tucson 85701 ($17.22 including tax), or mail order a copy through the Arizona Geological Survey, tel: 520-770-3500, FAX: 520-770-3505 ($23.22 including tax, shipping, handling). It should be in the bookstores soon, but maybe not by Christmas.