Geologic History

Mountains like this one are fairly young in the grand scheme of things. The mountains in this monument formed from volcanoes. Those volcanoes rose from the ground as the earth churned uneasily underneath.

NPS photo

 

The study of the materials that make up the Earth is called geology. North America has fascinating geological formations that have been forming and changing for millions of years. Though this may sound like a long time, on the geological scale, it is a blink of an eye!

Imagine all of Earth’s geologic time placed evenly over a clock, with noon (12pm) marking the formation of Earth, and midnight (12am) marking the present. The North American continent only formed within the last 3-5 minutes (200-30 million years ago). Humans have only been around for the last fraction of a second before midnight (in reality, modern humans appeared around 350,000 years ago).

Learn more about the geologic history of Organ Pipe Cactus as you explore this page. If you want to explore more about the geologic history of the Sonoran Desert or learn more about geology across the United States, visit the geologic history of Organ Pipe Cactus and Geoscience Concepts page.

 

A general schematic of Basin and Range formation. The Farallon Plate moving to the right, being subducted under the North American Plate, moving to the left. Bubbling and thinning of the crust facilitated volcanic formations. To the right, signature crumpling of the surface occurs.

NPS Image

Basin and Range

Organ Pipe Cactus National Monument occupies a small part of the massive area called the Basin and Range Province. This region stretches from its eastern boundary in West Texas to its western boundary at the foothills of the Sierra Nevada range in California. It spans from deep western Mexico in the south to southern Oregon and Idaho in the north. The Basin and Range Province crosses almost every ecosystem imaginable, from wetland to desert, from forest to tundra, and from beaches to mountains, but it is unified by its unique geology.

A Time of Unrest

Between 80 and 30 million years ago, the continents that we know today were drifting into their modern places, but things looked very different. What would become Nevada and Arizona sat closer to the Pacific coast. Pieces of Earth’s crust called tectonic plates were busy dragging themselves into and out of the layer of Earth beneath them (the mantle). The Farallon Plate was an ancient piece of crust that lived under the Pacific Ocean. The east bound Farallon Plate collided with the edge of the west bound North American Plate and began slipping into the Earth through a process called subduction.

The Farallon Plate first made a deep dive under the North American Plate, then leveled out like a spatula under pizza. The plates kept moving east and west, creating pressure that squished and crumpled the western part of the North American Plate. As the North American Plate scraped over the Farallon Plate, crust swelled under it to create a massive area of highlands along the continent’s western edge. During this squishing and swelling, weak spots or “faults” formed where brittle rock split, exposing deeper, harder rock. As the Farallon Plate melted in the Earth, magma rose to the surface, creating volcanoes.

A Time of Ease

Eventually, the Farallon Plate was all but swallowed by the Earth (with a couple remnants) and the North American plate could relax. The crust began to spread and thin out, and the western highlands caved in, creating the repeating pattern of mountains and valleys that span western North America. Imagine playing an accordion, compressing it, then slowly pulling it apart. The ridges of the accordion are like the mountains and the spaces between are valleys.

 

After the Farallon Plate (dark green) was subducted, the hot mantle (light green) facilitated spreading of the North American Plate (orange), creating distinct valleys and mountains that are visible today, alongside remnants of volcanoes.

Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

From Then to Now

The valleys, of course, became the “basins,” and the mountains became the “ranges,” giving this area of North America its name. The climate and environment at the top of these mountains are often very different from those in the valleys. These high elevation environments, or “sky islands” stretch across the Basin and Range Province, isolated from each other by the basins between.

Ecosystems are further isolated from each other by rivers and streams. As the valleys formed in long lines, rivers that formed in them would rarely connect, and often had no outlet. Over time, rivers carried sediment, salt, and other loose objects and animals downstream, where the river would eventually end in salt flats or sand dunes. The Quitobaquito pupfish and Sonoyta mud turtle were stranded by this form of isolation and left to adapt to their local surroundings at Quitobaquito Springs.