Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Geology 151j: Volcanic Rocks ... Page 1 ... AN EASY GUIDE TO THE COMMON VOLCANIC ROCKS MAGMA TYPE SILICA CONTENT (Variable) (variable) (most commonly intermediate to felsic) VOLCANIC ROCK(S) COLOR COMMON MINERALS ACCESSORY MINERALS Obsidian Pumice black, brown no minerals - all or mostly glass; massive - looks like glass (it IS!) light gray no minerals - frothy glass, very light; often floats on water Felsic > 70% Rhyolite pink to light gray Intermediate 65-70% Dacite dark pink to Sanidine, Na-plagioclase (white) medium gray pyroxenes (greenish black), quartz magnetite (Fe3O4) 55-65% Andesite medium to dark gray biotite Mafic 45-55% Sanidine (K-feldspar)(white) Na-rich plagioclase (white to pale gray) Biotite and/or hornblende (black) Quartz (clear - will look gray) Ca-plagioclase phenocrysts (gray) Na-plagioclase in groundmass olivine (green), pyroxene (greenish black) and/or hornblende (black) NO quartz Basalt very dark Olivine (bright green) Scoria gray to black Pyroxene (greenish black) (spongy appearance) Ca-rich plagioclase (dark gray) zircon magnetite ilmenite (FeTiO3) magnetite ilmenite General rules that will help put order in the system: Silica content and mineral content: 1. Volcanic rocks low in silica will consist mostly of minerals at or near the top of Bowen's Reaction Series. 2. Volcanic rocks high in silica will consist mostly of minerals near the bottom of Bowen's Reaction Series. 3. Intermediate volcanic rocks will consist of minerals in the middle of Bowen's Reaction Series. Crystal size (texture) and origins: (See reverse for illustrations and more detailed discussion.) 1. Igneous rocks with a coarse-grained texture (generally, all or almost all grains at least sand-sized) are going to be plutonic in origin - having crystallized underground. Larger crystals imply more time to crystallize. 2. Igneous rocks with a fine-grained texture (grains generally tiny or too small to see) are volcanic in origin. 3. Igneous rocks with a porphyritic texture (large grains - phenocrysts - surrounded by a finer-grained groundmass) cooled in a two-stage process, first slowly and then more rapidly. Occurs in both plutonic and volcanic rocks; very common in volcanic rocks in particular. Geology 151j: Volcanic Rocks ... Page 2 ... TEXTURES IN VOLCANIC ROCKS Aphanitic Fine-Grained Porphyritic Pyroclastic Glassy textures result from rapid cooling. There are few if any mineral grains; the rock is chiefly or entirely comprised of natural volcanic glass, or obsidian. Most obsidian is felsic (high in silica), but it also forms from other magma types as well. (For example, in Hawai'i, basaltic lavas flow into the ocean and chill into basaltic glass.) Frequently, there are tiny phenocrysts, called microlites, in glasses. Over time, these glasses are unstable and will recrystallize into aphanitic volcanic rocks. Volcanic glass that is frothy as well, from trapped gas bubbles, is called pumice. Aphanitic textures characterize volcanic rocks that have cooled rapidly but nonetheless have allowed for mineral crystals to form. However, the individual crystals are too small to be seen or identified with the naked eye. These can be differentiated from glassy textures by the fact that they are far less "glassy" or shiny in appearance; the surface is dull. Fine-Grained textures are very common in volcanic rocks, particularly basalts. In these, all the rock is crystallized, but the individual grains are very small (typically in the range of 0.25-1.0 mm across, the size of fine to medium sand, and commonly even smaller). Porphyritic textures may be the most common volcanic texture, especially for volcanic rocks on the continents. In these, large grains, called phenocrysts are embedded in a finer groundmass. Most commonly, the phenocrysts are grains that began crystallizing while the magma was still underground, then became entrapped in the fine-grained groundmass that formed following eruption and rapid cooling. Phenocrysts are commonly minerals higher in Bowen's Reaction Series than groundmass minerals, and may include minerals such as hornblende or biotite, which will not occur in the groundmass. Plagioclase feldspar phenocrysts will commonly be more calcium-rich than the plagioclase feldspars in the groundmass. Pyroclastic textures result from the fusion of solid volcanic ejecta that have been welded into a solid, coherent mass. These often include shards of glass and many irregular rock fragments, ash, etc.