Silicic Magmatism and the Volcanic–Plutonic Connection

The relationships between silicic volcanic and plutonic rocks have long puzzled geologists because the rich set of observations from petrology, geochronology, thermal modeling, geophysical techniques, and geochemistry have led to contradictory interpretations. Although compositional evolutionary trends leading to granite and rhyolite are congruent, it is not clear if rhyolites are formed by the extraction of melt from shallow crystal mushes that otherwise solidify to form granite plutons, or are derived from a greater depth in parallel with granite plutons, or are formed by processes separate from those which form granite plutons. Finding a consistent explanation for the silicic volcanic–plutonic relationship bears on important Earth science questions, including, “How is silicic continental crust formed?” and, “Can we predict supereruptions?” * Dacite : Relatively high-silica volcanic rock containing ~65–70 wt% SiO2. Granite, granitoids : Plutonic rocks composed predominantly of quartz, alkali feldspar, and plagioclase feldspar, typically in subequal amounts. Ignimbrite : A deposit formed from a pyroclastic flow. Magma : Although the definition of this fundamental geologic substance is disturbingly vague, the American Geosciences Institute Glossary of Geology definition is appropriate here: “naturally occurring molten or partially molten rock material, generated within the Earth and capable of intrusion and extrusion.” Plutonic : Referring to magma that crystallized at depth and is, therefore, relatively coarse-grained. Granite is a plutonic rock. Pyroclastic : Said of material ejected into the air during eruption, such as volcanic ash. Pyroclastic fall : Fine material from an explosive eruption that falls like snow. Pyroclastic fall material can circle the globe if fine enough and thrown high enough. Pyroclastic flow : A hot, fluidized, ground-hugging mass of fragmental material from an explosive eruption that moves across the ground surface at high speed. Pyroclastic flows are dominated by fine (~70 wt% SiO2. Silica : The chemical component SiO2. Silicic : Denoting a high concentration of silica (typically >66 wt%). Silicic melts are far more viscous than less-silicic ones.