Mesa

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Ground-level view of mesas in the Canyonlands National Park, Utah, known as the "Islands in the Sky" IslandInTheSky.JPG
Ground-level view of mesas in the Canyonlands National Park, Utah, known as the "Islands in the Sky"
Aerial view of mesas in Monument Valley, on the Colorado Plateau Over Monument Valley, Navajo Nation.jpg
Aerial view of mesas in Monument Valley, on the Colorado Plateau

A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock, e.g. shales overlain by sandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock can consist of either sedimentary rocks such as sandstone and limestone; dissected lava flows; or a deeply eroded duricrust. Unlike plateau , whose usage does not imply horizontal layers of bedrock, e.g. Tibetan Plateau, the term mesa applies exclusively to the landforms built of flat-lying strata. Instead, flat-topped plateaus are specifically known as tablelands . [1] [2] [3]

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As noted by Bryan [4] in 1922, mesas …stand distinctly above the surrounding country, as a table stands above the floor upon which it rests. It is from this appearance the term mesa was adopted from a Spanish word meaning table. [2]

A mesa is similar to, but has a more extensive summit area than, a butte. However, there is no agreed size limit that separates mesas from either buttes or plateaus. For example, the flat-topped mountains, which are known as mesas, in the Cockburn Range of North-Western Australia have areas as much as 350 kilometres (220 mi). In contrast, flat topped hills, which are as small as 0.1 kilometres (0.062 mi) in area, in the Elbsandsteingebirge, Germany, are described as mesas. [1] [2] [3]

Cockburn Range, Kimberley, Western Australia, Australia Cockburn Range.jpg
Cockburn Range, Kimberley, Western Australia, Australia

Less strictly, a very broad, flat-topped, usually isolated hill or mountain of moderate height bounded on at least one side by a steep cliff or slope and representing an erosion remnant also have been called mesas. [3]

In the English language geomorphic and geologic literature, other terms for mesa have also been used. [1] For example, in the Roraima region of Venezuela, the traditional name, tepui , from the local Pomón language, and the term table mountains have been used to describe local flat-topped mountains. [5] [6] Similar landforms in Southern Australia are known as tablehills, table-top hills or tent hills. [7] The German term Tafelberg has also been used in the English scientific literature in the past. [8]

Formation

Har Qatum, a mesa located on the southern edge of Makhtesh Ramon, Israel HarQatum.jpg
Har Qatum, a mesa located on the southern edge of Makhtesh Ramon, Israel

Mesas form by weathering and erosion of horizontally layered rocks that have been uplifted by tectonic activity. Variations in the ability of different types of rock to resist weathering and erosion cause the weaker types of rocks to be eroded away, leaving the more resistant types of rocks topographically higher than their surroundings. [9] This process is called differential erosion. The most resistant rock types include sandstone, conglomerate, quartzite, basalt, chert, limestone, lava flows and sills. [9] Lava flows and sills, in particular, are very resistant to weathering and erosion, and often form the flat top, or caprock, of a mesa. The less resistant rock layers are mainly made up of shale, a softer rock that weathers and erodes more easily. [9]

The differences in strength of various rock layers is what gives mesas their distinctive shape. Less resistant rocks are eroded away on the surface into valleys, where they collect water drainage from the surrounding area, while the more resistant layers are left standing out. [9] A large area of very resistant rock, such as a sill may shield the layers below it from erosion while the softer rock surrounding it is eroded into valleys, thus forming a caprock.

Differences in rock type also reflect on the sides of a mesa, as instead of smooth slopes, the sides are broken into a staircase pattern called "cliff-and-bench topography". [9] The more resistant layers form the cliffs, or stairsteps, while the less resistant layers form gentle slopes, or benches, between the cliffs. Cliffs retreat and are eventually cut off from the main cliff, or plateau, by basal sapping. When the cliff edge does not retreat uniformly, but instead is indented by headward eroding streams, a section can be cut off from the main cliff, forming a mesa. [9]

Basal sapping occurs as water flowing around the rock layers of the mesa erodes the underlying soft shale layers, either as surface runoff from the mesa top or from groundwater moving through permeable overlying layers, which leads to slumping and flowage of the shale. [10] As the underlying shale erodes away, it can no longer support the overlying cliff layers, which collapse and retreat. When the caprock has caved away to the point where only a little remains, it is known as a butte.

On Mars

A mesa in Noctis Labyrinthus on Mars, viewed by HiRISE. PIA21585 - A Mesa in Noctis Labyrinthus.jpg
A mesa in Noctis Labyrinthus on Mars, viewed by HiRISE.

A transitional zone on Mars, known as the fretted terrain, lies between highly cratered highlands and less cratered lowlands. The younger lowland exhibits steep walled mesas and knobs. The mesa and knobs are separated by flat lying lowlands. They are thought to form from ice-facilitated mass wasting processes from ground or atmospheric sources. The mesas and knobs decrease in size with increasing distance from the highland escarpment. The relief of the mesas range from nearly 2 km to 100 m depending on the distance they are from the escarpment. [11]

See also

Related Research Articles

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An escarpment is a steep slope or long cliff that forms as a result of faulting or erosion and separates two relatively level areas having different elevations. Usually scarp and scarp face are used interchangeably with escarpment.

Inselberg Isolated rock hill or small mountain that rises abruptly from a relatively flat surrounding plain

An inselberg or monadnock is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, an Afrikaans word from the Dutch word kopje. If the inselberg is dome-shaped and formed from granite or gneiss, it can also be called a bornhardt, though not all bornhardts are inselbergs.

Drakensberg Mountain range in South Africa

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Palo Duro Canyon canyon system in the US state of Texas

Palo Duro Canyon is a canyon system of the Caprock Escarpment located in the Texas Panhandle near the cities of Amarillo and Canyon. As the second-largest canyon in the United States, it is roughly 120 mi (190 km) long and has an average width of 6 mi (9.7 km), but reaches a width of 20 mi (32 km) at places. Its depth is around 820 ft (250 m), but in some locations, it increases to 1,000 ft (300 m). Palo Duro Canyon has been named "The Grand Canyon of Texas" both for its size and for its dramatic geological features, including the multicolored layers of rock and steep mesa walls similar to those in the Grand Canyon.

Stack (geology) A geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion

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Butte Isolated hill with steep, often vertical sides and a small, relatively flat top

In geomorphology, a butte is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word "butte" comes from a French word meaning "small hill"; its use is prevalent in the Western United States, including the southwest where "mesa" is used for the larger landform. Because of their distinctive shapes, buttes are frequently landmarks in plains and mountainous areas. In differentiating mesas and buttes, geographers use the rule of thumb that a mesa has a top that is wider than its height, while a butte has a top that is narrower than its height.

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Cuesta A hill or ridge with a gentle slope on one side and a steep slope on the other

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Dip slope topographic surface which slopes in the same direction as the dip of the underlying strata

A dip slope is a topographic (geomorphic) surface which slopes in the same direction, and often by the same amount, as the true dip or apparent dip of the underlying strata. A dip slope consists of the upper surface of a resistant layer of rock, often called caprock, that is commonly only slightly lowered and reduced in steepness by erosion. Dip slopes form the backslopes of cuestas, homoclinal ridges, hogbacks, and flatirons. The frontslopes of such ridges consist of either an escarpment, a steep slope, or perhaps even a line of cliffs. Generally, cuestas and homoclinal ridges are asymmetrical in that their dip slopes are less steep than their escarpments. In the case of hogbacks and flatirons, the dip of the rocks is so steep that their dip slope approaches the escarpment in their steepness.

Tuya A flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet

A tuya is a type of distinctive, flat-topped, steep-sided volcano formed when lava erupts through a thick glacier or ice sheet. They are somewhat rare worldwide, being confined to regions which were covered by glaciers and had active volcanism during the same period.

Cape Fold Belt A late Paleozoic fold and thrust belt in southwestern South Africa

The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa. It was originally continuous with the Ventana Mountains near Bahía Blanca in Argentina, the Pensacola Mountains, the Ellsworth Mountains and the Hunter-Bowen orogeny in eastern Australia. The rocks involved are generally sandstones and shales, with the shales persisting in the valley floors while the erosion resistant sandstones form the parallel ranges, the Cape Fold Mountains, which reach a maximum height of 2325 m at Seweweekspoortpiek.

Great Escarpment, Southern Africa Major topographical feature in southern Africa

The Great Escarpment is a major topographical feature in Africa that consists of steep slopes from the high central Southern African plateau downward in the direction of the oceans that surround Southern Africa on three sides. While it lies predominantly within the borders of South Africa, in the east it extends northwards to form the border between Mozambique and Zimbabwe, continuing on beyond the Zambezi River valley to form the Muchinga Escarpment in eastern Zambia. In the west, it continues northwards into Namibia and Angola.

Hogback (geology) A long, narrow ridge or a series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks

In geology and geomorphology, a hogback or hog's back is a long, narrow ridge or a series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks. Typically, the term is restricted to a ridge created by the differential erosion of outcropping, steeply dipping, homoclinal, and typically sedimentary strata. One side of a hogback consists of the surface of a steeply dipping rock stratum called a dip slope. The other side is an erosion face that cuts through the dipping strata that comprises the hogback. The name "hogback" comes from the Hog's Back of the North Downs in Surrey, England, which refers to the landform's resemblance in outline to the back of a hog. The term is also sometimes applied to drumlins and, in Maine, to both eskers and ridges known as "horsebacks".

Table Mountain Sandstone A group of rock formations within the Cape Supergroup sequence of rocks

The Table Mountain Sandstone (TMS) is a group of rock formations within the Cape Supergroup sequence of rocks. Although the term "Table Mountain Sandstone" is still widely used in common parlance, the term TMS is no longer formally recognized; the correct name is the "Peninsula Formation Sandstone", which is part of the Table Mountain Group. The designation "Table Mountain Sandstone" will, however, in deference to the title, continue to be used in the rest of this article. The name is derived from the famous landmark in Cape Town, Table Mountain.

A homoclinal ridge or strike ridge is a hill or ridge with a moderate, generally between 10° to 30°, sloping backslope. Its backslope is a dip slope, that conforms with the dip of a resistant stratum or strata, called caprock. On the other side of the other slope, which is its frontslope, of a homoclinal ridge is a steeper or even cliff-like frontslope (escarpment) that is formed by the outcrop of the caprock. The escarpment cuts through the dipping strata that comprises the homoclinal ridge.

Flatiron (geomorphology) A steeply sloping triangular landform created by the differential erosion of a steeply dipping, erosion-resistant layer of rock overlying softer strata.

Traditionally in geomorphology, a flatiron is a steeply sloping triangular landform created by the differential erosion of a steeply dipping, erosion-resistant layer of rock overlying softer strata. Flatirons have wide bases that form the base of a steep, triangular facet that narrows upward into a point at its summit. The dissection of a hogback by regularly spaced streams often resulted in the formation of a series of flatirons along the strike of the rock layer that formed the hogback. As noted in some, but not all definitions, a number of flatirons are perched upon the slope of a larger mountain with the rock layer forming the flatiron inclined in the same direction as, but often at a steeper angle than the associated mountain slope. The name flatiron refers their resemblance to an upended, household flatiron.

Scarp retreat

Scarp retreat is a geological process through which the location of an escarpment changes over time. Typically the cliff is undermined, rocks fall and form a talus slope, the talus is chemically or mechanically weathered and then removed through water or wind erosion, and the process of undermining resumes. Scarps may retreat for tens of kilometers in this way over relatively short geological time spans, even in arid locations.

Shinarump Conglomerate

The early Late Triassic conglomerate called the Shinarump Conglomerate, formally the Shinarump Member of the Chinle Formation, is a highly resistant coarse-grained sandstone and pebble conglomerate, sometimes forming a caprock because of its hardness, cementation, and erosion resistance. The Shinarump is found throughout the Colorado Plateau with significant exposures as the canyon rimrock in the vicinity of Canyon De Chelly National Monument, at the north-northeast of the Defiance Plateau/Defiance Uplift. At Canyon De Chelly the Shinarump Conglomerate was laid down upon De Chelly Sandstone-(280 Ma, an erosion unconformity of 50 my), in a region at the west foothill region of the mostly north-south trending Chuska Mountains of northeast Arizona – northwest New Mexico.

References

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  11. Baker, David M. Morphological Analyses of Mesas and Knobs in the Northwest Fretted Terrain of Mars; Constraints on the Presence and Distribution of Ice-Facilitated Mass-Wasting. Ed. Alexander K. Stewart and James W. Head. Vol. 40. Issue 2. pp. 72. United States: Geological Society of America (GSA) : Boulder, CO, United States, 2008.