**Abstract**

 Isla Isabel is a small island of volcanic origin about 29 km west of Nayarit's coast, Mexico, exhibiting phreatomagmatic activity (maars). In a nearby (8 km), ocean located, exploration well reaching 3157 m geothermal gradients of 73°C/km were recorded, which are among the largest in the world. These features, plus gravimetric anomalies in the area, suggest that the region may become an important source of geothermal energy. Direct hydrothermal alteration mapping in the island is difficult since it is a National Park and a bird sanctuary where various bird species thrive. We used remote sensing techniques to identify hydrothermally altered sections in the island, including radiometric spectrum determinations in the 350–1050 nm wavelength interval and Landsat 8 satellite imagery. The island is mainly composed of basalt but large portions contain vegetation, posing a challenge to satellite hydrothermal alteration detection. Four pixel-classification schemes are used for oxides, hydroxyls, and vegetation; shallow underwater basalt flow detection was possible.

**Keywords:** Isla Isabel, Mexico, hydrothermal alteration, phreatomagmatic activity, oxide and hydroxyl detection, radiometric spectra
