Mexico's geography: two mountain walls and a plateau between them

What the two sierras enclose

Two ranges frame the country and everything else sits in relation to them.

The Sierra Madre Occidental begins about 50 kilometres from the United States border and runs about 1,250 kilometres south to the Río Santiago. The Sierra Madre Oriental starts at the Big Bend region of the Texas border and continues 1,350 kilometres to Cofre de Perote.

Between them lies the Mexican Altiplano, the high plateau stretching from the northern border down to the volcanic belt. Its northern section is crossed by narrow isolated ridges and dotted with depressions, the largest being the Bolsón de Mapimí. Its southern section is higher than its northern one, which matters: the plateau is not a level surface but a ramp rising towards the middle of the country.

Narrow lowlands run along the Pacific and Gulf of California flanks of the western range, and the same arrangement holds on the eastern side. Mexico's coasts are strips at the foot of mountains, which is the single most important thing to know about how the country is settled. A ship arriving anywhere on either shore is arriving at the bottom of a wall.

A fourth range, the Peninsular Ranges, crosses the northern half of the country as a southern extension of the California coastal ranges, forming the spine of Baja California.

What the volcanic belt does

The Cordillera Neovolcánica runs 900 kilometres long and 130 wide, from the Pacific to the Gulf of Mexico, beginning at the Río Grande de Santiago, running south to Colima, then turning east along the nineteenth parallel to central Veracruz.

It is the country's most consequential single feature. It carries considerable seismic activity and Mexico's highest volcanic peaks, including three exceeding 5,000 metres: Pico de Orizaba, also called Citlaltépetl, and Popocatépetl and Iztaccíhuatl near Mexico City.

It is also regarded as the geological dividing line between North America and Central America. A country that spans that line is a country whose northern and southern halves belong to different physiographic provinces, and the difference shows in the climate, the vegetation, the agriculture and the history.

Mexico City sits on this belt, at altitude, between active volcanoes, on a seismic zone. The reasons the Aztec capital was founded there and the reasons the modern city is difficult to run are the same reasons.

How the south differs

South of the volcanic belt the country breaks into several distinct ranges rather than continuing the two-sierra pattern.

The Sierra Madre del Sur extends 1,200 kilometres along the southern coast from the southwestern end of the volcanic belt to the isthmus of Tehuantepec, averaging 2,000 metres in elevation and 100 kilometres wide, widening to 150 in Oaxaca. A narrow coastal plain runs between it and the Pacific.

The Sierra Madre de Oaxaca begins at Pico de Orizaba and runs southeast for 300 kilometres to the same isthmus, with peaks averaging 2,500 metres and some exceeding 3,000.

South of the isthmus, the Sierra Madre de Chiapas runs 280 kilometres along the Pacific coast to the Guatemalan border, averaging 1,500 metres with one peak, Volcán de Tacaná, exceeding 4,000. The Meseta Central de Chiapas extends 250 kilometres through the centre of the state.

Every one of those southern ranges runs close to the Pacific, which leaves the southern coastal plain narrow and the interior of the south broken into basins between ridges. It is the most topographically complicated part of the country and, not coincidentally, the part with the greatest concentration of distinct indigenous languages.

The isthmus of Tehuantepec is the pinch point: nearly flat, narrow, and the place where every one of these ranges stops or starts. It is the natural break between the Mexican core and the southeast.

What the coasts are for

Mexico's 9,330 kilometres of coastline divide unevenly: 7,338 kilometres face the Pacific and the Gulf of California, and 2,805 face the Gulf of Mexico and the Caribbean.

More than twice as much coast faces west as east, and yet the country's historical and economic orientation has been substantially eastward, because that is where the Gulf ports connect to Europe and to the United States. Geography offered the Pacific and history took the Atlantic.

The coasts are also thin. Because the ranges run close to both shores for much of the country's length, the coastal plains are narrow strips rather than broad lowlands, and the population, the cities and the farmland sit above them on the plateau.

That is unusual. Most maritime countries put their people at sea level. Mexico put them at two thousand metres and connected them to the sea by roads that climb.

What the far ends are like

The southeastern peninsula belongs to none of the patterns above and deserves its own paragraph.

The Yucatán Peninsula is low, flat and made of limestone, which puts it below the 1,000 metre line that governs climate in the southern half of the country and gives it a high year-round median temperature. It has no mountains, and it drains underground rather than on the surface, because limestone dissolves.

That single geological difference produces a landscape with no rivers to speak of and a water supply held in the rock itself. Settlement there has always been organised around access to groundwater rather than around a river valley, which is the opposite of the arrangement everywhere else in Mexico.

It is also the part of the country furthest from the volcanic belt, the plateau and the two sierras, and its separation is more than physiographic: the isthmus of Tehuantepec sits between it and the Mexican core, and the peninsula faces the Caribbean rather than either of the country's main coasts.

North of the Tropic of Cancer and behind the two sierras, the interior is dry, and the depressions of the northern Altiplano are the driest ground in the country.

The Bolsón de Mapimí is the largest of them. Narrow isolated ridges cross the plateau there, and the basins between them have no outlet, which is what makes them depressions rather than valleys: water that arrives does not leave except by evaporation.

The consequence is an arid interior with a small population, large land holdings and an economy of extraction and grazing rather than of cultivation. The northern border sits at the top of it, and the region's orientation has historically been northward across that border as much as southward towards the capital, for the straightforward reason that the ground between it and the capital rises and the ground to the north does not.

Those two regions, the limestone peninsula in the southeast and the closed basins of the northwest, are the country's extremes: one with water it cannot see and one with almost none at all.

Where the plates meet

Almost all of Mexico sits on the North American Plate, with small parts of the Baja California Peninsula in the northwest lying on the Pacific and Cocos plates.

That boundary is the source of the volcanism and the earthquakes. The Cocos plate descending beneath the North American plate is what raised the Neovolcanic belt and what continues to make the belt seismically active, and Baja California's separation from the mainland along the Gulf of California is the visible result of the Pacific plate moving northwest.

So the country's most densely populated corridor sits on its most active geology, and this is not a coincidence: volcanic soils are fertile, upland basins hold water, and people settle where the land supports them regardless of what it costs occasionally.

Why the plateau is a ramp

The Altiplano is described as a plateau and behaves as a slope, and the difference matters for everything built on it.

Its southern section is higher than its northern one, so the surface rises from the United States border towards the volcanic belt. A traveller moving south across the interior is climbing continuously, and the cities of the Mexican core sit near the top of that climb rather than in the middle of a level tableland.

That gradient does three things. It makes the south of the plateau cooler and wetter than the north, which is why the agricultural and demographic weight sits there. It puts the capital at the junction of the rising plateau and the volcanic belt, which is the highest and most seismically active combination available. And it means the country's internal transport has been a matter of climbing in every direction: from either coast up to the plateau, and from the northern plateau up to its southern end.

The Bolsón de Mapimí and the other closed basins of the north sit at the bottom of that ramp, which is part of why they are dry.

How the Tropic of Cancer splits the climate

The Tropic of Cancer effectively divides the country into temperate and tropical zones.

Land north of the twenty-fourth parallel experiences cooler winter temperatures. South of it, temperatures hold steadier through the year, and altitude replaces season as the thing that changes the climate.

Below that line, areas up to 1,000 metres, which is to say the southern parts of both coastal plains and the Yucatán Peninsula, hold a high year-round median temperature. Above 1,000 metres in the same latitudes the climate is temperate despite being tropical by position.

That produces the arrangement that defines Mexican settlement: altitude substitutes for latitude. A city at 2,000 metres inside the tropics has a mild climate all year, which is why the Altiplano and the volcanic belt carry the population rather than the coasts below them.

The north is a different problem. Beyond the tropic and behind the two sierras, the interior is dry, and the depressions of the northern Altiplano are the driest part of the country.

What the geography arranged

Three consequences follow, and all three run through the rest of this catalogue's articles on Mexico.

The population lives high and inland. Two ranges with narrow coastal strips, and a plateau at an altitude that makes a tropical latitude comfortable, put the cities where they are. Every port is at the bottom of a climb.

The north and the south are different countries physically. The Tropic of Cancer, the volcanic belt and the isthmus of Tehuantepec are three separate dividing lines running roughly parallel through the middle of the territory, and they separate a dry temperate north from a wet tropical south with a highland core between them.

The hazards are structural. A country whose densest settlement sits on a volcanic belt above a subducting plate has earthquakes and eruptions as permanent conditions, and both coasts are exposed to tropical storms. Nothing about that changes with policy.

The country's high point is Citlaltépetl at 5,636 metres, on the volcanic belt. Its low point is Laguna Salada at 10 metres below sea level, in the northwest near the Baja peninsula. Between them lies almost every terrestrial environment that exists.