**1. Introduction**

 Isla Isabel is an island of volcanic origin located ~29 km west of the coast of Nayarit State and ~70 km east of Islas Tres Marias archipelago in the Mexican Pacific; its geographic location is 21°50.847'N, 105°53.000'W (**Figures 1** and **2**). The geologic composition of the island is basaltic with various explosion craters (maars) of Surtseyan (phreatomagmatic) origin scattered in the surface [1]. The island dimensions are 1.8 km N-S and 1.32 km E-W.

A regional study around Isla Isabel showed that the island represents the emerged portion of a larger body preliminarily identified as a laccolith [2]. Over the average depth of 100 m of the continental platform in which the island is located, a topographic bulge rises to the surface; its dimensions are 20 km in the NW-SE direction and 17 km in the perpendicular direction. This bulge has been assumed to be built by successive magmatic intrusions that are deflected sideways by the sediments; the island represents the only emerged portion of this laccolith. In addition to the regional study, local geophysical surveys have been made on the emerged portion. Interpretation of magnetic and gravity results on the island's surface suggests the existence of diatremes associated with some of the explosion craters, as well as identification of a growth boundary about 800 m deep, from which apparently originated

#### **Figure 1.**

*Location of Isla Isabel (green dot) and exploration well Huichol I (red dot). Tres Marías islands are located SW of Isla Isabel.* 

#### **Figure 2.**

*Landsat 8 image Path/Row 31/45, code LC80310452014002LGN00 obtained on February 13, 2014. The image (RGB composite 4-3-2) is shown after image crop out, and atmospheric and radiometric corrections were made. Spatial resolution is 30 m. The fuzzy appearance is due to the scale used, which begins to show individual pixels.* 

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

one of the latest constructional, basaltic episodes [3]. Alkali basalts have transported mafic and ultramafic peridotite xenoliths to the surface; sequences of dark-red, nearvent scoria deposits, and lava flows in the tidal zone have also been reported [1].

Huichol I exploration well was drilled about 8 km NE of the island (**Figure 1**) penetrating 3157 m, with diabase intersected in the deepest 250 m. Geothermal gradients reached the values of 73°C/km, some of the largest measured in the world [4], highlighting the anomalously high heat flux in the area.

The above observations suggest that the island and its surroundings deserve geological and geophysical studies to evaluate its geothermal potential. In the present study, we focus on hydrothermal alteration detection and mapping in the island, with remote sensing techniques.
