So, I've had requests on the blog to help to do some defining of volcanologic terms on the blog, so I thought I'd try a new column called Eruptions Word of the Day. I'm not sure how often it will run, but let's give it a try.
Eruptions Word of the Day for July 5, 2010: Dacite Dacite is a magma type defined by silica (SiO2) content between 63-68 (or 69) weight percent. That is the textbook definition, but some other typical characteristics of dacite lavas (or magmas) is the presence of certain minerals: plagioclase feldspar and hydrous minerals (containing water in their mineral structure) such as amphibole (typically hornblende) or biotite mica. If the dacite is hot and dry (lacking in water), you would expect to find pyroxene in the lava as well, although it is common in almost all dacites that even have hydrous minerals. Typically, dacite erupt anywhere from 800 to 1000 degrees Celsius. The intrusive equivalent for a dacite is granodiorite.
Dacite lava hand sample, with abundant amphibole and plagioclase feldspar.
Dacite is found in a wide variety of tectonic settings but is most common in continental subduction zone/arc settings, such as the Andes or the Cascades. They tend to be products of magma mixing or crustal assimilation (by another magma) to form the dacite. The types of eruptions that dacite magma produces can vary from lava flows and domes (effusive, passive eruptions) to explosive, plinian-style eruptions - but they are most famous for the explosive eruptions like Mount St. Helens, Lassen Peak and Unzen in Japan (see below).
Pyroclastic flows from Mt. Unzen in Japan. A flow like this killed the Kraffts in 1991.
However, there are impressive effusive dacite lavas flows, such as the Chao Dacite and Volcan Aucanquilcha (see below) in Chile and Llao Rock at Crater Lake, where minor explosive deposits preceded the lava flows that stretch upwards of 5-10 km from the vent. These lava flows are usually steep-sided with well-formed levees on the sides and pressure ridges along the flow tops (see below). It is thought that dacite lava flows or domes form when the magma is allowed to degas before erupting, allowing for a passive rather than explosive eruption.
Dacite tend to be a "garbage bag" of minerals - they have collected a variety of crystals from different sources. These sources include the active magma of the eruption in question (phenocrysts), crystals from previous magmatism at the volcano (antecrysts) and wholly unrelated crystals (xenocrysts). This mixing creates disequilibrium, where minerals only stable in one condition find themselves in another, creating impressive reaction textures (see below). This variety of crystals supports the ideas that many dacites are the product of magma mixing.
Amphibole crystal breaking down in a mixed dacite from Volcan Aucanquilcha, Chile. The interior is cored with biotite mica, iron-titanium oxide (such as magnetite), and quartz, while the outer roughly hexagonal shape is amphibole. The clear crystals along the edge of the grain are formed from the breakdown of the larger amphibole crystal when it is in disequilibrium. Click on the image to see a larger version. Image by Erik Klemetti.