Introduction

“Geodiversity is the natural variety of the Earth's surface, referring to geological and geomorphological aspects, soils, and surface waters, as well as other systems created as a result of both natural processes (endogenous and exogenous) and human activity. Geodiversity and biodiversity are the two elements that determine the possibility of supporting sustainable development” (^{Kozłowski, 2004}).

Recently, this concept has gained international relevance in scientific and political decision-making spheres (^{Gray, 2008}; ^{Gordon et al., 2012}; ^{Erikstad, 2013}; ^{Comer et al., 2015}). Despite a certain initial skepticism about its validity, the concept has demonstrated its usefulness regarding environmental conservation in the current context of climate change (^{Prosser et al., 2010}; ^{Brazier et al., 2012}; ^{Brown et al., 2012}) and in practical economic aspects related to tourism and the promotion of Geoparks as an alternative model for the protection and conservation of landscapes (^{Posada Ayala et al., 2014}).

Geodiversity in specific territories has been a topic addressed by various specialists. Most of these studies correspond to regions of more or less limited extension of hundreds or thousands of km² (^{Pereira et al., 2013}; ^{de Paula et al.; 2015}, ^{Bétard and Peulvast, 2019}; ^{dos Santos et al., 2020}; ^{Dias et al. 2021}); although there are also examples at the national level (^{de Paula Silva, 2021}; ^{Alberico et al., 2023}; ^{Esmaili, 2024}: these three studies refer to Brazil, Italy and Iran, respectively) and even continental (^{Wolniewicz, 2023}, referring to Europe). In all the previous cases, the data consistently include three variables to define geodiversity: geology (rock types), geomorphology (relief units) and soil types. Each of these variables represents a sub-index from which a general Geodiversity index is obtained.

For the purposes of this work, a geodiversity index was developed based on three main components (geology types of rocks, geomorphological units and main soil types). The information sources used correspond to the most up-to-date maps of Geology, Geomorphology and Soils on a 1: 5 000 000 scale, to be published in the National Atlas of Mexico (^{ANM, in press}).

The calculation of the subindices generally uses a grid of square cells within which the number of topics (for example, geological formations) is recorded. Overall diversity is an integer value that indicates how many different topics are observed in each unit area, whether this be a cell, or region or country.

Selection of the cell size will depend on the characteristics of the data and on the purpose of the analysis, whether this be based on lithology and geomorphology (^{Lopes et al., 2023}) or on a variety of physical elements such as geology, geomorphology, hydrology, and soils (Serrano et al., 2007).

The grid system (^{Pereira et al., 2013}) is easy to use, and is the basis of many common tools in Geographic Information Systems (e.g. Raster Calculator in QGIS or ArcGIS).

In this work, a first national map of the Geodiversity of Mexico is created, defined from geological diversity (types of rock), geomorphological diversity (types of landforms) and soil types.

Data used

The data are derived from a set of maps of the geology (^{Ferrari et al., in press}), geomorphology (^{Zamorano et al., in press}) and soils (^{Cruz-Gaistardo et al., in press}) of Mexico, which are included in Section III of the National Atlas of Mexico (^{ANM, in press}). These data were rasterized to obtain ascii type images; the scale of the source maps is 1: 5 000 000 with a pixel spatial resolution of 500 × 500 meters.

a) Geology

The map of the geology of Mexico (^{Ferrari et al., in press}) refers to 75 rock types (see Table 1).

b) Geomorphology

Concerning geomorphology, ^{Zamorano et al. (in press)} refer to 22 topics (Table 2).

c) Soils

^{Cruz-Gaistardo et al. (in press)} reports 28 dominant soils and 1 dominant horizon topics in Mexico.

Methodology

Two specific programs were developed: Map Explor (^{Parrot, 2023a}) calculates the value of the subindices using a grid of square cells, or a moving square or circular window; Diversity_index (^{Parrot, 2023b}) follows the scheme proposed by various geographic information systems, calculates the sum of the three subindices that make up geodiversity (in this case geology, geomorphology and soils) and normalizes the result based on the user-defined number of classes.

An essential notion in the raster world establishes the relationship that exists between the range R and the size m of the side of a square element (cell or moving window). This side corresponds to (R × 2) + 1 and the range R depends on the size m of the pixel side and the desired viewing surface S as follows:

with m in meters, S in km² and S' = S × 1 000 000

In the present case, m is 500 m and the observation area in each cell is 625 km² (25 km × 25 km). The results were compared on the basis of two greater sizes: 2 500 km² (50 km × 50 km) and 10 000 km² (100 km × 100 km). In the case of geology, for example, the maximum number of types that a 625 km² cell can contain is 11 (2 500 pixels, or 0.03% of the surface of the Mexican Republic); for a 2 500 km² cell, the maximum is 12 (30 000 pixels, representing 0.38%) and for a 10 000 km² cell, the maximum is 16 (120 000 pixels, corresponding to 1.54%). As expected, the diversity value in each element of the grid grows in relation to the increase in cell size (see Figure 1), that is, the larger the cell size, the greater the number of topics contained per cell.

Figure 1 Influence of cell size on the number of topics per unit area (cell) used in maps of Mexico. In general, the larger the cell the greater the number of topics contained (the diversity index, In_div). 

The values presented above come from the application of the following variables:

R = 25 (with m = 500 and S = 625); and c = (R × 2) + 1 = (25 × 2) + 1; and according to the cell size c (in pixels), the surface (in pixels) = R ² = 51 × 51 = 2 601, that is to say a cell surface in km² = 650.250 with a pixel of 500 × 500 meters.

Thus, the recalculated area is close to the value defined by the user, namely 625 km² (25 km × 25 km).

The number of topics (nc) that characterize a variable (i.e., 7 classes [segments] for the slope) allows defining the number of histograms (hist[nc]) needed to calculate the diversity subindex In _div of this variable. Before entering a cell, all the hist[nc] histograms are initialized to zero. Then, by scanning the image within each cell, a pixel nc value produces an increment of 1 of the corresponding hist[nc] histogram. Thus, a distribution of the nc values within the cell is obtained and according to the number of histograms whose content is greater than zero, the value of the diversity subindex In _div is obtained; this index corresponds directly to the number of not empty hist[nc] histograms.

Regarding geology, application of this calculation to the 75 rock types (Table 1) results in 8 groups or classes that correspond to the number of topics per cell (Table 4).

Regarding geomorphology, application to the 22 relief units (Table 2), results in 6 classes (Table 5).

Regarding soils, application to the 29 types (Table 3) results in 12 classes; classes 11 and 12 that only contain one soil type per cell are not reported in the Table 6.

The most frequent number of topics per cell was 3 for geology, 2 for geomorphology and 4 for soils (27.38% for geology, 42.62% for geomorphology and 29.40% for soils) (Figure 2).

Figure 2 Number of topics per cell for each subindex. 

The Map_Explor module generates, for each variable, a map of the number of topics per cell according to the cell size. It is possible to superimpose the grid of cells if necessary.

The Diversity_index module that sums the subindices also draws the grid of square cells and generates a detailed report on the treatment and results.

Finally, the treatment distributes the cells into 5 levels of diversity (very low, low, medium, high, very high); for each cell k, this process uses the following normalization:

where Val _(k) is the normalization value of the cell k, Val _Sum(k) is the value of the sum of the ranks reached by all the variable in the cell k, Val _Max is the value of the sum of the maximum rank reached for each variable in the whole image and n is the diversity levels, here 5.

It is possible to weight the three diversity subindices to know if of these subindices plays a major role in defining the geodiversity.

Results

As mentioned above, subindices were obtained for the diversity of rocks, relief units and soil types, derived from the corresponding maps contained in the National Atlas of Mexico (^{Ferrari et al., in press}; ^{Zamorano et al., in press}; ^{Cruz-Gaistardo et al., in press}).

Geological diversity (Figure 3) is greatest in the northwest and south of the country. The first region includes parts of the states of Baja California, Sonora, and Sinaloa, while the second includes portions of the Sierra Madre del Sur and the Sierra de Chiapas (states of Guerrero, Oaxaca and Chiapas). The diversity is lower in the Sierra Madre Occidental, the Mexican Altiplano, the Trans-Mexican Volcanic Belt and part of the Gulf Coastal Plain and is markedly low in the Yucatán Peninsula. Lithology and tectonics determine much of the geological diversity.

Figure 3 Geological diversity across Mexico. 

Geomorphological diversity (Figure 4) is very low in the Baja California Peninsula, the Sierra Madre Occidental, the Trans-Mexican Volcanic Belt and the plain of the Yucatán Peninsula. The areas with low diversity are in the Mexican highlands, the hilly areas in the south of the Yucatán Peninsula, and in some of the coastal plains of the Gulf of Mexico such as the low plain of the Coatzacoalcos and Grijalva rivers. Intermediate and high values of geomorphological diversity are mainly in the Sierra Madre del Sur and portions of the Sierra Madre Oriental. In general, the geomorphological variable introduces a pattern of heterogeneity in most of the country's physiographic provinces.

Figure 4 Geomorphological diversity across Mexico. 

Soil diversity (Figure 5) is more generally dispersed. In general, it is very high and high in the central-eastern zone of the Trans-Mexican Volcanic Belt, in the Sierra de Chiapas and in the north of Chihuahua. Diversity is low or very low in the Baja California Peninsula, northern Nuevo León and Coahuila, the Mesa del Centro, areas of the Balsas Depression and the Gulf Coastal Plain and the Yucatán Peninsula.

Figure 5 Soil diversity across Mexico. 

A geodiversity map of Mexico was derived from the sum of the three subindices (Figure 6). It is very high across 1.44% of the land and high across 19.45% and reflects a wide variety of rock types (sedimentary, igneous, and metamorphic of diverse ages) in the northwest and in the south (generally corresponding to the Sierra Madre del Sur in the states of Guerrero and Oaxaca).

Figure 6 Geodiversity across Mexico, derived from the subindices for geology, geomorphology, and soils. 

Areas of medium geodiversity are scattered across 57.56% of the land area. When this is considered together with the 21.19% of the land with low geodiversity, this ~79% forms a continuum that includes most of the mountainous areas of the country, as well as the Mexican Altiplano and a considerable area of the two peninsulas (Baja California and Yucatán).

Geodiversity is very low across only 0.36% of the land, all of it on the Yucatán Peninsula, a low-altitude platform lacking notable geomorphological contrasts and with a lithological composition dominated by marine sedimentary rocks.

Geodiversity in Mexico

The geological diversity of Mexico results from a long and complex history associated with the interaction between tectonic plates and diverse geological environments. A mosaic of tectonostratigraphic terranes assembled during the Paleozoic and Mesozoic as a result of the complex interaction between Laurentia, Gondwana and the Paleo-Pacific plate (^{Centeno-García, 2017}). Strong activity continues through the interaction of five main plates: North American, Pacific, Cocos, Caribbean and Rivera.

Because of this long geological history, there are rocks of diverse characteristics and ages (see Geology maps of Mexico in Section III of the National Atlas of Mexico, in press). In general, the most abundant are those of sedimentary origin (continental and marine), followed by magmatic (intrusive and extrusive) and metamorphic (see Table 1), and their ages range from the Precambrian to the present.

The oldest rock outcrops in Mexico, metamorphic rocks, are few and are mainly in the states of Sonora and Oaxaca (^{Ferrari et al., in press}), two regions with the high and very high values of geodiversity referred to above. Precambrian metamorphic outcrops of the Caborca area of Sonora are represented by igneous and sedimentary rocks metamorphosed to greenschist and amphibolite facies (^{Anderson et al., 1978}; ^{Anderson and Silver, 1978}). Precambrian rocks are also present in the State of Oaxaca.

The sedimentary rocks of the Cretaceous and to a lesser extent the Jurassic (mainly limestones, marls, shales) are distributed throughout the Sierra Madre Oriental, Sierra Madre del Sur, and the Yucatán Peninsula. Finally, igneous rocks (both intrusive and volcanic) are preferentially distributed along the Sierra Madre Occidental (extensive Paleogene pyroclastic deposits) and along the Trans-Mexican Volcanic Belt (Neogene to the present).

As a consequence of geological evolution, the diversity of rock types and the exogenous processes that shape the relief (see Geomorphologic Map of the National Atlas of Mexico, in press), the geomorphological diversity of Mexico includes units that are grouped into endogenous relief forms (e.g. mountains of folded, blocky or volcanic rocks, generally young, relatively well preserved) and exogenous relief forms (e.g. various forms derived from fluvial, aeolian, marine and, to a lesser extent, glacial activity), karst, relief and mixed landforms (see Table 2).

Lithology and relief are two of the most important factors involved in the formation of different types of soil. Here, the diversity of rocks and relief units partly explains the diversity of soils across Mexico.

Conclusion

The Geodiversity map of Mexico, based on rock types, landforms and soils, constitutes a first approach to the country's abiotic natural diversity. The methodology is adjustable and capable of being applied in specific, smaller areas to achieve greater detail. A further characterization of the country's geodiversity will depend on the availability of data regarding other variables such as climate, morphometry, and underground water resources. Results must be contextualized according to the scale of origin of the data used, in this case national level (1:5,000,000). The relatively high degree of geodiversity across a large part of the Mexican territory indicates a high potential for geoconservation and consequently the need for appropriate management. The results and methods addressed are of interest in various basic and applied fields of Earth Sciences and Geography, not least in geoconservation, the promotion of natural heritage for various purposes and the creation of Geoparks.

It should be noted that, in order to define comprehensive conservation of resources, a subsequent analysis would need to consider not only geodiversity and, additionally, morphodiversity, which requires an accurate Digital Elevation Model of the entire Mexican territory, but also the associated biodiversity, which implies an analysis of vegetation cover, the level of environmental protection, land use and its degree of degradation, as well as the impact of human activity.