The study of ephemeral outcrops is an important tool to reconstruct the geology of highly urbanised areas in which increasing construction and occupation of soil difficult direct observations. During the last decade, the Natural Park of the Garrotxa Volcanic Field (NE Spain) has undertaken a pioneering activity aimed to study and catalogue all new ephemeral outcrops in order to improve geological mapping and volcano-stratigraphy of the area. An inventory of the geological information obtained from these temporary outcrops in construction sites, that rapidly disappear, as well as water wells and geotechnical boreholes, has been collected, analysed, classified, and recorder in an open GIS database for public use.
This field study consisted in the collection of data registered as standardized field forms. The data necessary to build the ephemeral outcrops catalogue are: geographical location, geological description stratigraphic columns, geological sections and photographs of the ephemeral outcrops. Each outcrop was sampled for further analysis. This detailed geological information is registered in a database.
As a case study we applied this methodology in the Natural Park of the Garrotxa Volcanic Zone. During 8 years we have described and catalogued up to 85 ephemeral outcrops in this area. With key outcrops we have been able to make a detailed mapping of the city of Olot around the Montsacopa volcano. This mapping has allowed us reinterpreting the eruptive events in the area and to describe new eruptive vents. Furthermore, this methodology has provided new data on the volcanic alignments in the Olot city evidencing the significance of the structural controls. This study has provided new data for geological research in the volcanic area and it has improved significantly our knowledge on the relative geochronology and extension of the most recent volcanic deposits. This method is also a useful tool to improve the preservation of the geological heritage in the area and to assess volcanic risk

1. VUELCO conference