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Unformatted text preview: 1 Lab 7 Acidic Volcanic and Plutonic Rocks As you are aware, "volcanic" (s.l.) rocks are either lava flows or pyroclastics. Whereas lava flows have a wide range in compositions (from SiO 2 undersaturated rocks as leucitophyres and phonolites, through basalts and andesites to the SiO 2 oversaturated dacites and rhyolites), pyroclastic rocks are predominantly acidic or intermediate in composition (although some are basic). In this lab, we will focus on the acidic "volcanic" rocks, and their plutonic equivalents. Many of the samples that you will see this week are from the St. Francois Mountains, MO, which we will visit in our next field trip. Acidic volcanic rocks belong to one of three groups: 1- Lava flows: e.g. rhyolites, dacites and rhyodacites. 2- Ash flow tuffs: are also known as "ignimbrites" that form by the flow of a cloud of ash and other ejecta downslope to their depositional sites after some violent volcanic eruption. They are therefore poorly sorted and unbedded . 3- Ash fall tuffs: result from the settling of ash thrown up in the air by a volcanic eruption, and carried away by wind for long distances before its final deposition. They are therefore well sorted and bedded . Rocks forming from acidic lava flows are described in the same way used for other volcanic rocks. On the other hand, pyroclastic rocks, and in particular ash flow tuffs, have their own characteristic textures and structures. Characteristics of ash flow tuffs: 1- Occurrence: They occur only in continental settings. 2- Association: They are associated with granitic and intermediate batholiths. 3- Constituents: They consist of: (a) glass shards, (b) phenocrysts, (c) pumice lumps, and (d) lithic fragments. The glass shards are bits and pieces of glass with a variety of shapes and thicknesses (Fig. 1), which depend to some extent on the composition and viscosity of the magma prior to eruption. The phenocrysts are usually broken up or fractured by the eruption. The pumice lumps show various degrees of flattening, depending on their temperature upon settling, and the amount of overburden. Flattened pumice lumps are called " fiamme ". The lithic fragments are fragments of any rock constituting the volcanic cone or occurring in its vicinity, which were broken up by the volcanic eruption and incorporated in the glowing avalanche. 2 4- Types: Based on the relative abundance of the glass, phenocrysts and lithic fragments, tuffs are classified into three groups: (a) Vitric tuffs: predominated by glass or glass shards. (b) Crystal tuffs: predominated by phenocrysts (c) Lithic tuffs: predominated by lithic fragments. Crystal tuffs are generally the most common type, as glass shards tend to be blown away over long distances, leaving behind the fragmented crystals to settle. This explains why crystal tuffs have a much larger concentration of phenocrysts than corresponding lava flows, which also makes them poor representatives of the original magma composition....
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- Fall '11