Amesa is an isolated, flat-toppedelevation,ridge orhill, which is bounded from all sides by steepescarpments and stands distinctly above a surroundingplain. Mesas characteristically consist of flat-lying softsedimentary rocks capped by a more resistant layer or layers of harderrock, e.g.shales overlain bysandstones. The resistant layer acts as a caprock that forms the flat summit of a mesa. Thecaprock can consist of either sedimentary rocks such as sandstone andlimestone; dissectedlava flows; or a deeply erodedduricrust. Unlikeplateau, whose usage does not imply horizontal layers ofbedrock, e.g.Tibetan Plateau, the termmesa applies exclusively to the landforms built of flat-lyingstrata. Instead, flat-topped plateaus are specifically known astablelands.[1][2][3]
As noted by geologist Kirk Bryan in 1922, mesas "...stand distinctly above the surrounding country, as a table stands above the floor upon which it rests".[4] It is from this appearance that the term mesa was adopted from theSpanish wordmesa, meaning "table".[2]
A mesa is similar to, but has a more extensivesummit area than abutte. 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 ofNorth Western Australia have areas as large as 350 km2 (140 sq mi). In contrast, flat topped hills with areas as small as 0.1 km2 (0.039 sq mi) in theElbe Sandstone Mountains,Germany, are described as mesas.[1][2][3]
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 termtable mountains have been used to describe local flat-topped mountains.[5][6] Similar landforms in Australia are known astablehills,table-top hills,tent hills, orjump ups (jump-ups).[7][8][9] The German termTafelberg has also been used in the English scientific literature in the past.[10]
Mesas form byweathering anderosion of horizontally layered rocks that have been uplifted bytectonic 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.[11] This process is called differential erosion. The most resistant rock types includesandstone,conglomerate,quartzite,basalt,chert,limestone,lava flows andsills.[11] Lava flows and sills, in particular, are very resistant to weathering and erosion, and often form the flat top, orcaprock, of a mesa. The less resistant rock layers are mainly made up ofshale, a softer rock thatweathers anderodes more easily.[11]
The differences in strength of various rock layers are what give 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.[11] 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".[11] 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, orplateau, by basal sapping. When the cliff edge does not retreat uniformly but instead is indented byheadward eroding streams, a section can be cut off from the main cliff, forming a mesa.[11]
Basal sapping occurs as water flowing around the rock layers of the mesa erodes the underlying soft shale layers, either assurface runoff from the mesa top or fromgroundwater moving through permeable overlying layers, which leads toslumping and flowage of the shale.[12] 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 little remains, it is known as abutte.
A transitional zone onMars, known asfretted terrain, lies between highly cratered highlands and less cratered lowlands. The younger lowland exhibits steep walled mesas andknobs. 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 (1.2 mi) to 100 m (330 ft) depending on the distance they are from the escarpment.[28]
^abcDuszyński, F., Migoń, P. and Strzelecki, M.C., 2019.Escarpment retreat in sedimentary tablelands and cuesta landscapes–Landforms, mechanisms and patterns.Earth-Science Reviews,' no. 102890. doi.org/10.1016/j.earscirev.2019.102890
^abcMigoń, P., 2004a.Mesa. In: Goudie, A.S. (Ed.),Encyclopedia of Geomorphology. Routledge, London, pp. 668.ISBN9780415272988
^abcNeuendorf, Klaus K.E. Mehl, James P., Jr. Jackson, Julia A.. (2011).Glossary of Geology (5th Edition). American Geosciences Institute.ISBN9781680151787
^Bryan, K. (1922). "Erosion and Sedimentation in the Papago Country, Arizona".US Geological Survey Bulletin (730):19–90.
^Briceño, H.O. and Schubert, C., 1990.Geomorphology of the Gran Sabana, Guayana Shield, southeastern Venezuela.Geomorphology, 3(2), pp.125-141.
^Doerr, S.H., 1999.Karst-like landforms and hydrology in quartzites of the Venezuelan Guyana shield: Pseudokarst or" real" karst?.Zeitschrift fur Geomorphologie, 43(1), pp.1-17.
^Jack, R.L., 1915.The Geology and prospects of the Region to the South of the Musgrave Ranges, and the Geology of the Western Portion of the Great Australian Artesian Basin.Geol. Survey South Australia Bulletin 5, pp. 72.
^abReading, H. G. (1954) The stratigraphy and structure of the syncline of stainmore, Durham theses,Durham University. Available at Durham E-Theses Online:http://etheses.dur.ac.uk/9349/
^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.
