In the informal slang sometimes used by earth scientists, individuals can describe their field of study as either hard-rock or soft-rock geology. Hard-rock geology encompasses areas of inquiry related to crystalline rocks. These rocks are more resistant to erosion and more difficult to break apart. Both igneous (solidified from melted or molten rock) and metamorphic (formed through solid-state recrystallization at very high temperatures and pressures) are crystalline. Soft-rock geology examines sedimentary rocks that are made of grains that were transported by wind and water, deposited on the earth’s surface, then buried and solidified. The La Sal Mountains with sedimentary rocks in the foreground.
The canyon country surrounding Moab, and in fact that of nearly the entire Colorado Plateau (e.g., the Four Corners region of Utah, Colorado, New Mexico, and Colorado), is largely a land of soft rocks. In southeastern Utah, a particular type of sedimentary rock is so predominant that the nearly national park could be renamed Sandstonelands; a name that aptly describe the park’s lithology (rock type) as “Canyonlands” describes its landscape.
Although sedimentary rocks are predominant here in southeastern Utah, hard rocks are important parts of the geologic tableau of this region. Despite the fact that all the bedrock in Arches and Canyonlands National Parks as well as in the immediate vicinity of Moab is all sedimentary, igneous rocks and cobbles are very common in Moab itself, especially along Pack and Mill Creeks. Crystalline rocks are found as cobles all along the Colorado River. These rocks’ very hardness and their resistance to weathering and erosion means that they make durable cobbles that are transported by the river and in other drainages, whereas the softer sedimentary rocks eroded from the canyons themselves are not very tough and more readily disaggregate to sand and other small particles.
As it turns out, the hard rocks found on the Colorado Plateau are actually diverse among themselves. For example, they are either geologically ancient or quite young*.
Hard rocks in Colorado National Monument. The pink colored band is made of igneous rock (granite) and the darker colored rock is metamorphic. Similar basement rocks are found closer to Moab in Westwater Canyon.
Metamorphic and igneous rocks make up the basement of the continent. These rocks are very old, being Precambrian in age and mostly forming around 1,700 million years ago when this part of North America was being constructed through plate tectonic collisions. These basement rocks are exposed at the bottom of Grand Canyon, but also in Westwater Canyon upstream of Moab, and in Colorado National Monument near Grand Junction.
Young igneous rocks are found on and around the Colorado Plateau, including both intrusive (plutonic) rocks that crystallized underground and volcanic (extrusive) rocks that were erupted onto the Earth’s surface. The closest occurrences of igneous rocks are in the La Sal Mountains just east of Moab, which are igneous intrusions. The La Sals are mountainous both because magma (molten rock) was injected close to the earth’s surface so that it domed up the sedimentary rock layers that it intruded and also because the crystalline igneous rock is much more resistant to erosion than the surrounding sedimentary rocks so that it forms a topographic high.
Although the closest volcanic rocks are more than 100 miles from Moab, this rock type is not uncommon along the outer margins of the Colorado Plateau. Zion and Capitol Reef National Parks mostly contain sedimentary rocks like parks near Moab, but they also have young volcanic rocks. Sunset Crater Volcano National Monument near Flagstaff, Arizona, contains the youngest volcanic rocks in the Southwest, with the eruption occurring in 1085 CE (common era). Most of the volcanoes along the Colorado Plateau erupted basalt, a dark-colored rock that can form extensive lava flows and that is also the same type of volcanic rock found in Hawaiʻi.
Diversity in rock type adds to diversity in the landscape. Sedimentary, igneous, and metamorphic rocks all play a role in the formation of one of the most scenic places on Earth: the canyon country of the Moab region and the greater Colorado Plateau.
*Young is a relative term. In this case, young means less than about 30 million years old; however, some volcanic rocks on the Colorado Plateau are substantially younger and are mostly a few million years old. The most recent eruptions occurred only about a thousand years ago.
Part of a lava flow from Sunset Crater Volcano in front of the Sunset Crater cinder cone, a small volcano made up of fragments of volcanic rock that were blasted into the air and then fell around the vent..
Streams flowing down from the La Sal Mountains carried cobbles and boulders of igneous rocks to Moab. This deposit of boulders is near the bowling alley (Gravel Pit Lanes). Almost every boulder is made of a igneous rock called diorite from the La Sals because these rocks are so much more durable than sedimentary ones. The close up shows the salt and pepper texture of the diorite with light-colored feldspar and dark hornblende crystals..
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.