short time after, and possibly even before, the ignimbrites. They are thus old volcanic centres and deeply eroded, displaying moraine deposits from glaciation and rocks which have been subject to hydrothermal alteration from fumarolic activity. Such hydrothermal alteration processes, together with desublimation of fumarolic sulfur, are also the origin of the sulfur deposits at Purico. Aspero, Cerro El Chascon, Cerros El Negro and Putas are younger and show no evidence of glaciation. El Chascon especially may be only tens of thousands of years old, seeing as it displays both a summit crater and pristine lava flow structures. Aspero was once considered to be of Holocene age in light of it and Chascon overlying moraines; later, dates of 180,000 ± 20,000 years ago were obtained on Aspero and Chascon. Apart from these, there are no radiometric dates for post-ignimbrite volcanic structures at Purico. The Alitar volcano is considered to be of Plio-Pleistocene age. The eruptive episode that formed these centres is thus more recent than the Purico ignimbrite and may have been triggered by mafic magma being injected into the Purico system. It is also much smaller, with volumes ranging . This change in the pattern of eruptive activity from large ignimbrites to smaller domes reflects a change in the nature of the magma supply, from large volume flow that heavily interacted with the crust and gave rise to the ignimbrites to smaller volume flows in a colder and thus brittler crust and did not accumulate or interact with it in a significant way. Thus the later eruption products appear to be more primitive and less affected by crustal contamination. Holocene and fumarolic activity Macon stratovolcano is considered to be of Holocene age, and Alitar maar displays active fumaroles and hot springs. There are no know historical eruptions of Alitar and there is no indication of seismic activity in the Purico area. Renewed activity at Alitar would likely be in the form of phreatic eruptions of only local significance. The fumaroles of Alitar are concentrated in the northern and eastern parts of Alitar, while the hot springs occur in the Quepiaco creek area about southwest of Alitar and consist of six separate small vents. The temperatures of the Alitar vents range between . Fumarolic gases are mostly water vapour, with lesser amounts of carbon dioxide, and sulfur deposition takes place. They appear to originate from both magmatic and precipitation water, with