Galeras has been an active volcano for at
least a million years, withandesite as the dominant product. Two
major caldera-forming eruptions have occurred, the first about
560,000 years ago in an eruption which expelled about 15 cubic kilometres
(3.6 cu mi) of material. The second some time between 40,000 and
150,000 years ago, in a smaller but still sizable eruption of 2 cubic kilometres
(0.48 cu mi) of material. Subsequently, part of the caldera wall has
collapsed, possibly due to instabilities caused by hydrothermal activity, and
later eruptions have
