A mesa is a table of rock kept standing by a hard lid
Spanish speakers named these landforms after tables, and the comparison is exact: a flat top, sheer sides, and open ground all around. The secret is a hard cap of sandstone, limestone or old lava sitting on soft shale. Erosion eats the shale, the cap shields what lies directly beneath, and a table is left behind.
Geologists define a mesa as an isolated flat-topped hill or ridge ringed by steep cliffs and rising clearly above a plain. The geologist Kirk Bryan wrote in 1922 that such hills stand above the surrounding land the way a table stands on a floor. Unlike a plateau, which is broader and need not have horizontal rock layers, a mesa is built from flat-lying strata and stands alone. A butte is a smaller, more eroded version with only a little summit left, while a cuesta forms where layers tilt, giving one gentle slope and one cliff.
The shape comes from differential erosion. Layered rocks lifted by tectonic forces weather unevenly: shale crumbles readily, while sandstone, conglomerate, quartzite, basalt and especially lava flows and sills resist. Softer rock is carved into valleys that channel water away, and a resistant cap protects the column below it. The sides often form a staircase called cliff-and-bench topography, with hard layers making cliffs and soft ones gentler benches.
Mesas shrink by basal sapping. Runoff from the top, or groundwater seeping through permeable layers, erodes the shale at the base until it slumps and flows away. Unsupported, the cliff above collapses and retreats. When only a scrap of caprock remains, the mesa has become a butte; streams cutting back into a larger plateau can likewise slice off new mesas.
There is no agreed size cutoff. In Western Australia's Cockburn Range, mesas cover as much as 350 square kilometres, while in Germany's Elbe Sandstone Mountains hills as small as 0.1 square kilometres qualify. Venezuela's tepuis and Australia's jump-ups are local names for similar forms. Colorado's Grand Mesa is called the largest flat-topped mountain in the world, and Mars has mesas too, in fretted terrain, with relief ranging from about 2 kilometres down to 100 metres.
Source: Mesa
