The volcanic eruptions, always numerous even in historical times (the Colima, the Jorullo, the Popocatepetl, the Orizaba and others are still active), denote that this part of the Measico is still a field of very active telluric movements, something also proven by the frequency and extension of seismic phenomena, particularly intense and disastrous in the belt along the Pacific from Manzanillo to Tehuantepec.
Between the two Sierre Madri extends the plateau which, higher towards the south in the Mesa del Centro or Anahuac (city of Mexico m. 2277, Puebla m. 2155, Toluca m. 2680), gradually widening and lowering towards the north (Queretaro m. 1880, Saltillo m. 1598, Chihuahua m. 1400, Ciudad Juárez m. 1150). The name of plateau, however, should not be misleading, since it is not already a high flat region, but rather a series of shelves and basins at different altitudes, often separated by trunks of chains running from east to west, that is in an almost direction. normal to the general course of the marginal folds. The Mesa del Norte, or northern section, with a more arid climate, largely constitutes a series of closed basins with no outlet to the sea: the elevated plateaux are distinguishedof Zacatecas (2440), of Durango (2000) and of Chihuahua (1400) which lean on the western Cordillera and the belt of the basins (bolsones) in which the internal basins end, the bottom of which sometimes drops below 1000 m. The Mesa del Centro, or southern section, is characterized above all by the presence of large volcanic systems and by the extension of the lava fields: the closed basins are higher and narrower and have almost all been conquered by the marginal rivers (Río Santiago and Río Pánuco) which, due to the increased rainfall, were able to widen their basin more rapidly in the interior of the country.
According to searchforpublicschools, the geological history of Mexico, according to the current state of knowledge, appears as follows: along the Pacific coast there are islets of archaic and metamorphic rocks belonging to an ancient continent that occupied the westernmost part of present-day Mexico, and which, corrugated by effect of very ancient movements and subjected to a very long subaerial erosion, as would be shown by the deposits dating back to the Permic and Triassic periods of Zacatecas, Sonora, Puebla, Oaxaca and Chiapas, was partly submerged in the period from the Jurassic to the Tertiary, during which sedimentary strata formed which generally rest directly on the archaic strata; of them the largest part belongs to the Cretaceous period.
In the Tertiary sector, the action of the tangential forces pushed these masses of deposits against the archaic lands, causing them to wrinkle; the folds with trend NO. – SELF. they constitute the main feature of the town and the volcanic alignments are also related to them. The wrinkling of the Tertiary principle was followed by a lifting motion accompanied by fractures, which was repeated successively throughout the Tertiary, interspersed with periods of erosion renewing with each new uplift. The subsequent periods of uplift were accompanied by large volcanic manifestations that began in the Eocene and then continued until the present period, with repeated paroxysms that first brought to light the large masses of andesites that occupy the western half of the country.
In more recent periods the volcanic activity was reduced in the southern part of the plateau between 22 ° and 28 ° of lat. N. by raising the major cones, mostly basaltic, on the Cretaceous strata. In the north-eastern part of the country there are no superficial volcanic formations, but it seems that the intrusive masses are of considerable importance.
The geological history of the transistmic part is very different and while the whole territory north of the isthmus emerged from the end of the Cretaceous period, here instead postcretaceous marine transgressions must have occurred during which Pliocene deposits were formed, raised in very recent times, up to more than 2000 msm
In summary, Mexican soil is essentially made up of three parts: the oldest, which is the least extensive, is represented by the massifs of gneisses and shales recognized in the southern part towards the Pacific, that is, in the Sierra Madre del Sur. This is followed by the series of sedimentary formation which is the most extensive and occupies a large part of the northern plateau, the Sierra Madre Oriental and the southern section, and includes deposits from the various periods with a prevalence of deposits from the Jurassic and Cretaceous periods; the tertiary formations are almost exclusively limited to the Yucatán peninsula and the coastal plains of the Gulf of Mexico, while in the interior of the Chihuahua plateau the Quaternary deposits appear to be of lacustrine origin; the recent Quaternary is also represented along the Atlantic coast in a short coastal zone, and on the shores of the Gulf of California. The third part, which has a development almost equal to the second, is formed by volcanic rocks (andesites, syenites, rhyolites, basalts, tuffs, lavas and ashes) which cover the entire western Cordillera and the Mese del Centro and Anahuac, as well as the southern section of the Sierra Madre Oriental; effusive rocks and post-Cambrian intrusive rocks also form the backbone of the Baja California peninsula and a large part of Chiapas. as well as the southern section of the Sierra Madre Oriental; effusive rocks and post-Cambrian intrusive rocks also form the backbone of the Baja California peninsula and a large part of Chiapas. as well as the southern section of the Sierra Madre Oriental; effusive rocks and post-Cambrian intrusive rocks also form the backbone of the Baja California peninsula and a large part of Chiapas.
