Lastarria displays vigorous fumarolic activity on the summit and down the northwestern slopes. Such activity has been observed since the European discovery of Lastarria, in the late 19thcentury. Lastarria is the only volcano in the area with ongoing fumarolic activity. It manifests in fumaroles forming high chimneys, small cones reaching heights of , and fractures, as well as degassing through cracks and fissures on surfaces. Individual fumaroles have been found in the craters, on the crater edges, and on the slopes. A northwest-southeast striking fracture is associated with some of the fumaroles. Four different fumarole fields have been found, one along this fissure on the northwestern slope at altitudes of about , two on the rims of the fourth crater, and one in the fifth crater. The fissure field is the largest, covering a surface area of .