Mesa (geology)

A mesa is an isolated highland with a flat top and steeply sloping cliffs or gentle slopes. The cap is made of harder rocks than the sides, and provides protection for the softer slopes or cliffs, preventing them from eroding quickly. The word mesa is Spanish for “table,” referencing the resemblance to the furniture, and mesas are also called “table mountains.” Mesas are common in Arizona, Colorado, New Mexico, and Utah in the United States and in Australia, South Africa, and Spain. The largest mesa in the world is Grand Mesa, which is in Colorado. Mesas can be thousands of feet taller than the surrounding area and are usually significantly wider than they are tall. Buttes are similar but smaller than mesas and usually about as wide as they are high; often buttes are the remains of mesas that have been significantly reduced by erosion.

Because of their distinctive form, mesas often hold spiritual significance for Indigenous peoples. They may be held to be sites where deities have been or continue to be active.

Background

Humans have observed geological features for thousands of years. The field of geology emerged in the eighteenth century. Scottish naturalist James Hutton is regarded as the founder of modern geology. Hutton, who ran two family farms and was keenly interested in how wind and weather affected the land, carefully observed the natural world and developed the theory that Earth is constantly being formed. Hutton reasoned that the history of the world could be discerned in this ongoing formation of landscapes by learning to understand how the forces that build and destroy the land work in modern times. Many scholars of his time believed Earth was only about six thousand years old and that fossils were records of animals that died in the great flood described in the Bible.

Many naturalists understood that sediment was deposited in water and eventually formed stone layers, but Hutton estimated that process took much longer than others believed. He thought the oldest rocks were made of the sediment of long-gone continents and were in the process of breaking down, a cycle he believed occurred repeatedly. He found evidence of this constant change and the forces of tectonic movement in vertical layers of gray shale and overlying horizontal layers of red sandstone in cliffs at Siccar Point on the east coast of Scotland. The only scientific explanation for this arrangement was that the gray shale formed from sediments that were deposited and compressed, then uplifted, tilted, and eroded, and finally covered by an ocean where deposits collected and formed the sandstone. He presented his theory in a paper before the Royal Society of Edinburgh in 1788.

Stratigraphy is the branch of geology that studies rock layers, how they formed, and how they change over time. This area of study emerged in the nineteenth century with geologist Charles Lyell’s Law of Superposition. The law, which is used by both geologists and archaeologists, states that soils that are deeply buried were laid down earlier and are older than the soils on top of them.

Overview

Mesas are usually found in areas where rock layers are horizontal and the climate is dry. They are usually wider than they are tall. The life cycle of a mesa begins when an area becomes separated from a plateau. Over its lifetime it gradually erodes and crumbles, transforming into a cluster of buttes. Finally the buttes themselves disintegrate and all that remains is the debris, including scattered boulders.

Most mesas form when horizontal rock stratification is pushed upward by tectonic forces. Over millions of years, weaker layers of rock erode away, leaving behind varieties that are less affected by weathering. The harder type of rock that is exposed becomes the cap of the mesa. Common types of rock that cap mesas are basalt, chert, conglomerate, and sandstone. It weathers more slowly than the underlying layers. The steepness of the sides of the mesa is determined by the types of rock layers. Layers that erode more slowly tend to form cliffs, while rocks less resistant to erosion form slopes. Basal sapping also affects the form of the mesa. This occurs when weathering or erosion at the base of the formation undercuts the slope, causing it to retreat further. Streams also cause erosion that shapes mesas; they tend to flow in fractures or where rocks are the weakest. The base of a mesa is usually concealed under rock materials that have crumbled and fallen away from the cliffs.

Grand Mesa in Colorado illustrates the development and qualities of many mesas, although it did not emerge as a result of tectonic forces pushing horizontal layers of rock upward. This mesa, which is an extinct volcano, is about 40 miles (64 kilometers) long and covers about 500 square miles (1,295 square kilometers). Although Grand Mesa stands taller than the surrounding landscape, millions of years ago it was the lowest part of the landscape, the bottom of a valley. The mesa, which looms about 6,000 feet (1,800 meters) above the surrounding river valleys, has a cap of basalt. This rock cap formed from lava flowing into valleys about ten million years ago. Once the lava flows hardened, the soil of the valley walls eroded, but the harder lava weathered far more slowly. Battlement Mesa, also in Colorado, shares many of the same geological characteristics.

The layer directly beneath the basalt cap of Grand Mesa is Eocene shale (formed between about 56 and 34 million years ago), while the lowest layers are Mancos shale from the Late Cretaceous Period (about 100.5 to 66 million years ago). Other layers are sandstone. In many areas around the base of Grand Mesa are slump blocks called Toreva blocks. These blocks slid off the face of the cliff and tilted toward the mesa, creating escarpments or fault scarps up to 500 feet (152 meters) tall. The slump blocks trap water, forming lakes. The Colorado and Gunnison Rivers are the primary forces that carved the mesa, chipping away at the shale and sandstone.

Another well-known mesa is found in Australia. Mount Conner was formed by deposits in a shallow sea during the Neoproterozoic Era (from 1 billion to 538.8 million years ago). Its rocks are mostly hard conglomerate and quartzite forming the cap. Below this is mostly sandstone, with some shale. Aboriginal Australians believe ice beings that cause cold weather live atop the mesa, which they call Artilla or Attila.

Bibliography

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DiPietro, Joseph A. Geology and Landscape Evolution: General Principles Applied to the United States, 2nd ed., pp. 389 – 428. Elsevier, 2018.

“Geology of Mesa Verde National Park.” US Geological Survey, www.usgs.gov/geology-and-ecology-of-national-parks/geology-mesa-verde-national-park. Accessed 24 Mar. 2023.

Hirst, K. Kris. “Stratigraphy: Earth’s Geological, Archaeological Layers.” Thought Co., 25 Feb. 2019, www.thoughtco.com/stratigraphy-geological-archaeological-layers-172831. Accessed 27 Mar. 2023.

Migon, Piotr, Milena Rozycka, Kacper Jancewicz, and Filip Duszynski. “Evolution of Sandstone Mesas—Following Landform Decay Until Death.” Progress in Physical Geography: Earth and Environment, vol. 42, no. 5, 2018, DOI: 10.1177/0309133318795837. Accessed 24 Mar. 2023.

Palmisano, Laura. “On a Drive Around Grand Mesa a Geologist Explains Why It’s Prone to Slide.” KVNF Community Radio, 5 June 2014, www.kvnf.org/news/2014-06-05/on-a-drive-around-grand-mesa-a-geologist-explains-why-its-prone-to-slide. Accessed 27 Mar. 2023.

Williams, Matt. “What Is a Butte?” Universe Today, 31 Jan. 2017, www.universetoday.com/73855/what-is-a-butte/. Accessed 27 Mar. 2023.