Primary deposits occur in acidic granitic magmas and pegmatites. Felsic magmas of andesitic and rhyolitic composition tend to erupt explosively. Magmas and lavas are the original unlithified source of all igneous rocks. Solubility is lower for more mafic magmas. There are also peritectic and residual minerals in granitic magmas. Porphyry deposits are formed in arc-related settings and are associated with subduction zone magmas. This is caused by the addition of seawater into magmas formed at subduction zones. Likewise, peridotite is a much more important source rock of common magmas. Eruptions of low-viscosity magmas (e.g., basalt) typically form droplet shaped particles. On Earth, most basalt magmas have formed by decompression melting of the mantle. Where these magmas reach the surface they create volcanic arcs. Temperatures of the postglacial magmas have been estimated at . Felsic magmas can erupt at temperatures as low as . Some Ajata magmas bypassed the shallow reservoirs completely. Viscosity is a key melt property in understanding the behaviour of magmas.