Junction Butte in Canyon-lands National Park, Utah. Part of the Colorado Plateau, the landscape of sedimentary sandstone in this area was eroded into countless canyons, mesas, and buttes by the Colorado River and its tributaries
Spanish explorers in the mid-sixteenth century ranged over the American Southwest. They had come north from Mexico, looking for gold and gems and the legendary Seven Cities of Cibola (pronounced SEE-bow-lah), allegedly filled with such treasures. In their quest, they found neither gold nor riches. They did, however, become the first Europeans to view the geological wonders of the area, and they were amazed at what they saw.
Among the canyons, plateaus, and rock towers and arches, the explorers saw landforms that appeared plateaulike, only smaller and isolated. They called these geologic features mesas (pronounced MAY-suz), which means table in Spanish, because the explorers thought the landforms resembled tables with their smooth, flat tops and sides that drop away steeply. Populating the spare, arid (dry) landscape of the area along with mesas were still smaller landforms that had a similar appearance. At the beginning of the nineteenth century, the word butte (pronounced BYOOT) was coined from the French word meaning mound or hillock to describe these solitary landforms.
The shape of the land
A mesa is an isolated, flat-topped hill or mountain with steep sides that is smaller in area than a plateau. A butte is also a flat-topped hill with steep sides, though smaller in area than a mesa. Definitions of the surface areas of mesas and buttes vary. One source states that a mesa has a surface area of less than 4 square miles (10 square kilometers), while a butte has a surface area less than 11,250 square feet (1,000 square meters). Another source states that the surface area of a mesa is larger than 1 square mile (2.59 square kilometers); the surface area of a butte is smaller than that dimension. Some simply define a mesa as a landform that is wider than it is high and a butte as one that is higher than it is wide.
A mesa’s and butte’s characteristic shape—flat top and clifflike sides—is due to the layers of rock forming them. These landforms are most often composed of sedimentary rock, formed by the accumulation and compression of sediment (which may consist of rock fragments, remains of microscopic organisms, and minerals). This type of rock covers more than 75 percent of Earth’s land surface. Most sedimentary rocks occur in layers, called strata, that are mostly horizontal or flat when first formed. Forces within Earth that rupture the surface to form volcanoes, mountains, plateaus, and many other topographical features (physical features on the planet’s surface), may later cause these layers to tip, fold, warp, or fracture.
The top layer of a mesa and a butte is a hardened layer of rock that is resistant to erosion, which is the gradual wearing away of Earth surfaces through the action of wind and water. Sometimes this top layer, called the cap rock, is not sedimentary rock but is cooled and hardened lava that had spread out across the landscape in repeated flows from fissures or cracks in the ground. Beneath this flat protective cap of rock are horizontal layers of softer sedimentary rock. To varying degrees, these layers are not as resistant to erosion.
These landforms are found in arid and semiarid regions. Arid regions are defined as those that receive less than 10 inches (25 centimeters) of rain per year; semiarid regions receive 10 to 20 inches (25 to 50 centimeters) of rain per year. Precipitation in these regions often comes in the form of sudden, heavy rainfalls. Because water evaporates quickly in these normally dry environments, plants and other ground cover are scarce. Left exposed to the action of running water, the bare sides of the softer rock layers of mesas and buttes are eroded away over time. The base of these landforms is often gently sloped, contrasting with the almost-vertical sides leading down from the top. Rock material that has been eroded from the sides is carried downward, forming this sloping base.