The diverse visages of Geoheritage in Mexico City: Knowing your place, knowing your risks

Geological elements in cities are usually damaged and invisible to their inhabitants, despite the wide range of benefits they provide to them. In particular, they offer them an opportunity to learn about their geological setting (= sense of place) and be aware of associated risks (= sense of risk). Such geoheritage comes in different forms that ought to be identified and described precisely in order to be preserved and used. The gigantic capital city of Mexico is settled in a paleolake basin embedded in a diverse volcanic landscape. City inhabitants face numerous hazards (floods, landslides, debris flows, subsidence, earthquakes, drought, fires) that create high risks with combined natural and anthropogenic causes due to uncontrolled city growth. The geoheritage of the city is mostly formed by monogenetic (one-event) volcanoes that are an important resource for geoconservation and geoeducation. We present two case studies that were investigated by students near their homes, which was a strategy employed during the COVID-19 pandemic to allow them to undertake field work. The first study area is a popular neighborhood (Lomas de Padierna) that was rooted in the 1970s on basaltic lavas from the ca. AD200 Xitle cone. In this highly urbanized area, the geological nature is only visible as sparse vertical outcrops along streets and small surface exposures in private gardens and road central reservations. Such sites are seldom maintained by the locals who rarely recognize their value. Debris flows during rain storms cause a chronic hazard. The second area consists of a chain of several cones, a dome and a tuff ring (Sierra Santa Catarina) located in the southeastern part of the basin, that conserves elements of the native fauna and flora. Depicted in the great landscape paintings of the Mexico basin made in the 19^th century, mining activities and illegal settlements have severely degraded this site since the 1970s, even after the creation of a protected area in 1994 and a conservation area in 2003. Enhanced cone erosion causes frequent landslides affecting settlements. Data on the age and eruptive style of these volcanoes is surprisingly limited, given their importance for hazard assessment.

This work reveals the diverse visages of geoheritage in a large city, showing their low level of scientific knowledge and public appreciation, which account for their pronounced degree of degradation. Hence, the memory of past geological events, awareness of impending hazards, and vital mineral and organic resources are disappearing quickly, further increasing the risks faced by the city.  The vulnerability of people to hazards can be linked to their poor knowledge of their environment. The dissemination of information on local geosites may be highly valuable to raise environmental awareness and reduce risks. In this respect, we plan to make and distribute leaflets to local schools and community centers.