Galeras has been an active volcano for at least a million years. Its dominant product is andesite, an igneous rock . Two major caldera-forming eruptions have occurred: the first was about 560,000 years ago in an eruption which expelled about of material. The second was some time between 40,000 and 150,000 years ago, in a smaller but still sizable eruption of of material. Since that second eruption, part of the caldera wall has collapsed, possibly due to instabilities caused by hydrothermal (high-temperature water) activity, and later eruptions have built up a smaller cone inside the now horseshoe-shaped caldera.