*2.1.3 Station 4*

 This station is located on the margin of Lago Crater (also known as Laguna Fragatas) as shown in **Figure 3**; according to [1], this structure is the only, fully preserved, explosion crater in the island. The exposed rocks on the lake's margin are basalts altered either by the fluctuating water level (**Figure 8**) or by the chemical action of birds' excrement. The latter mechanism appears more viable since birds are abundant in the island; Islote Pelon, slightly north of the island, is an extreme case of these alterations, being completely covered by bird feces. The white surfaces of the rocks induced saturation in the radiometer making it difficult to obtain valid readings. The spectra from Station 4-1, shown in **Figure 11**, were obtained pointing to dark spots of the chemically altered rock samples. We observed a systematic tendency for reflectance to diminish as the wavelength increases; the large reflectance contribution to the lower part of the spectrum probably arises in dispersed radiation at the white surfaces of the rocks as well as at the lake's surface.

Although distorted by the dispersion effect, two of the signatures clearly show a similar increment in reflectance between 500 and 600 nm as those shown by basalt

*Chemically altered basalts on the margin of Lago Crater. We attribute this alteration to the chemical effect of bird feces.* 

## *Radiometric Mapping of Hydrothermal Alterations in Isla Isabel, Mexico DOI: http://dx.doi.org/10.5772/intechopen.80530*

 in Station 1 and Station 2-1. Between 690 and 900 nm, it also shows the vegetation signature, compare to that of Station 8 (**Figure 12**).
