Meseta Central

(also Mexican Highland), a highland in the southern part of North America, occupying most of Mexico. It consists of a vast plateau and the mountain ranges rimming it on the east, south, and west. In the north, near the US border, the highland gradually gives way to the Great Plains and the Colorado Plateau, divided by ranges and basins. It covers an area of about 1.2 million sq km, and prevailing elevations range from 1,000 m to 2,000 m.

The eastern rim of the highland is formed by the Sierra Madre Oriental (4,054 m), which descends abruptly to the lowlands along the Gulf of Mexico. The western rim, the Sierra Madre Occidental (3,150 m), is broken by faults running parallel to the Pacific coast, to which it descends by stages. A chain of extinct and active volcanos, known as the Transverse Volcanic Axis, marks the southern edge of the highland. The principal volcanoes are Orizaba (5,700 m, the highest peak in the highland and all of Mexico), Colima, Popocatépetl, and Ixtacihuatl. The Jorullo volcano first erupted in the 18th century, and the currently active volcano Paricutin, in February 1943. The interior of the highland comprises two regions, the northern and central mesas. The northern mesa consists of relatively flat areas, called bolsons, with elevations of 900–1,200 m, divided by low short ranges. The central mesa to the south comprises volcanic plateaus (2,000–2,400 m), separated by mountain uplifts and depressions. The highland is composed chiefly of limestone, sandstone, and marl. Lava sheets cover a large part of the highland, and alluvialtalus deposits are found on the plateau. The Meseta Central has deposits of silver, lead, antimony, zinc, mercury, iron and manganese ores, gold, copper, and other minerals.

The climate is tropical, but it varies considerably from one region to another depending on the relief and on proximity to the Atlantic and Pacific oceans and to the vast continent to the north. Influenced by the summer trade wind from the Gulf of Mexico, the south has a mild and humid climate. In the north the climate is dry and continental. Average January temperatures range from 9°C to 14°C. In the north cold northern winds may lower the temperature to — 20°C. Average July temperatures range from 15°C to 20°C. Annual precipitation ranges from 200–400 mm in the north and interior regions to 2,000–3,000 mm on the outer mountain slopes. In the south there are three climatic zones, depending on elevation: tierra caliente, tierra templada, and tierra fría. The tierra caliente, occurring at elevations up to 1,400 m, has continually hot weather, with sunny skies and alternately dry and wet phases in winter and very humid and rainy summers. The tierra templada, rising to elevations of 3,000 m, has moderately warm weather throughout the year. In winter and spring the skies are clear, and in the autumn cloudy and rainy weather prevails. The tierra fría is the cold zone of the upper slopes of high mountains. The snow line is at 4,500 m, and the highest peaks are snow-capped.

There is no surface runoff in the dry northern part of the Meseta Central. Streams are temporary and generally flow into the bolsons, resulting in the accumulation of a thick layer of sediments in the bolsons and in the formation of a number of temporary interior-drainage salt lakes. An exception is Rio Conchos, a tributary of the Rio Grande (Mexican name, Rio Bravo del Norte), flowing along the northern border. In the south the rivers are deeper, with steep descents. In regions of active volcanism and frequent earthquakes there are many lakes dammed up by lava and tectonic lakes (Chapala, Cuitzeo).

In the arid north soils are primitive, frequently consisting of slightly altered gypsum and carbonaceous weathering crusts. In the south these soils gradually give way to mountain sierozems and to the gray-cinnamonic, cinnamonic, and red soils of the tall-grass tropical savannas. Mountain cinnamonic soils predominate over large areas in the southern part of the highland and on the mountain slopes.

As one of the regions where American flora developed, the Meseta Central has a rich vegetation, with 8,000 endemic species. In the north sparse vegetation predominates, represented by some 500 species of cacti, ranging from tiny round plants to giant species up to 4 m high, agaves (140 species), yuccas, dasylirions, and thorny shrubs. To the south this vegetation gives way to savannas with sparse stands of acacias, species of Amaranthaceae, and indigos. Wet tropical forests grow on windward slopes in the south. Broadleaf varieties, such as evergreen oaks and plane trees, predominate to elevations of 1,000 m; here also grow trees of the Myrtaceae family, laurels, and species of Annonaceae, with magnolias and ligneous ferns in the underbrush. Between 1,000 m and 2,000 m the tropical forests are supplanted by mixed forests, and at higher elevations, by coniferous forests of pines and, at the upper limits, firs. A zone of subalpine and alpine meadows lies between 4,000 m and 4,500. Most of the animals belong to the Nearctic zoogeographic region. Animals found throughout most of the Meseta Central include white-tailed and other species of deer, prong-horn antelopes, and various rodents (squirrels, gophers). Among predators are red lynxes, pumas, wolves, foxes, skunks, otters, and raccoons. Various birds of the order Passeriformes are common, and there are many reptiles. In the south the jaguar, ocelot, kinkajou, peccary, armadillo, and anteater are encountered.

Although most of the highland is sparsely populated, about half the population of Mexico lives in the fertile valleys and basins in the south.