fault intersections. The volcanic rocks produced by these volcanoes have similar compositions, and historical seismic and volcanic activity at Ubinas and Ticsani indicate that they share a magma reservoir. A magma reservoir may underpin this volcanic system. Composition The eruption products of the 1600 eruption are dacites, which define a calc-alkaline, potassium-rich suite sometimes described as adakitic. The 1600 rocks also contain rhyolite inclusions and a rhyolite matrix. Andesite has also been found at Huaynaputina. Phenocrysts include biotite, chalcopyrite, hornblende, ilmenite, magnetite and plagioclase; amphibole, apatite and pyroxene have been reported as well. Aside from newly formed volcanic rocks, Huaynaputina in 1600 also erupted material that is derived from rocks underlying the volcano, including sediments and older volcanic rocks, both of which were hydrothermally altered. Pumices from Huaynaputina are white. The amount of volatiles in the magma appears to have decreased during the 1600 eruption, indicating that it originated either in two separate magma chambers or from one zoned chamber. This may explain changes in the eruption phenomena during the 1600 activity as the "Dacite 1" rocks erupted early during the 1600 event were more buoyant and contained more gas and thus drove a Plinian eruption, while the latter "Dacite 2" rocks were more viscous and only generated Vulcanian eruptions. Interactions with the crust and crystal fractionation processes were involved in the genesis of the magmas as well, with the so-called "Dacite 1" geochemical suite forming deep in the crust, while the "Dacite 2" geochemical suite appears to have interacted with the upper crust. The rocks had a temperature of about when they were erupted, with the "Dacite 1" being hotter than the "Dacite 2". Their formation may have been stimulated by the entry of mafic magmas into the magmatic system; such an entry of new magma in a volcanic system is often the trigger for explosive eruptions. The magmas erupted early during the 1600 event (in the first stage of the eruption) appear to have originated from depths of more than ; petrological analysis indicates that some magmas came from depths greater than and others from about . An older hypothesis by de Silva and Francis held that the entry of water into the magmatic system may have triggered the eruption. A 2006 study argues that the entry of new dacitic magma into an already existing dacitic magma system triggered the 1600 eruption; furthermore movement of deep andesitic magmas