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Classification of Igneous rocks.

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Classification of Igneous rocks Igneous Rocks are classified is several ways, and methods of classification have evolved a lot over the past 100 years. Each classification is useful for a certain purpose and reflects a particular way of looking at igneous rocks. Early in the days of geology there were few rocks described and classified. In those days each new rock described by a geologist could have shown characteristics different than the rocks that had already been described, so there was a tendency to give the new and different rock a new name. Because such factors as cooling conditions, chemical composition of the original magma, and weathering effects, there is a potential to see an endless variety of igneous rocks, and thus a classification scheme based solely on the description of the rock would eventually lead to a lot of rock names. There are various ways that could be used to classify igneous rocks... Crystal size: - Igneous rocks are formed by the crystallisation of a rock melt or magma. The crystallisation occurs during cooling, as the atoms become organised into crystals. Eventually all the crystals will grow until they meet each other, forming an interlocking three dimensional structure when crystallisation is complete. ...read more.


For example granite consists of lots of quartz and feldspar and is generally light coloured. But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black coloured (i.e. an obsidian). Still we can divide rocks in general into felsic rocks (those with lots of feldspar and quartz) and mafic rocks (those with lots of ferromagnesian minerals). But, this does not allow for a very detailed classification scheme. * Controlled by chemical composition * Looking at % of dark minerals. * Wide variety in colour * Dark grey/ black> light grey/ white There are 3 measurements of % of dark minerals they are... Leucocratic- light coloured, 0-30% dark minerals- Example- Granite Mesocratic- medium coloured, 30-60% dark minerals- Example- Andesite Melanocratic- dark coloured, > 60% dark minerals- Example- Basalt We can only use colour if the rock is crystalline, weathering also affects rock colour. We can divide rock colour into 3 general measurements. 1= Felsic= light coloured- leucocratic, feldspar and Quartz, silica. Major constituents- Granite. 2= Mafic= dark coloured- 60%+- Basalt- melanocratic, Iron and Magnesium rich, with feldspar plus ferromagnesians. 3= Ultra Mafic= Rocks composed almost entirely of ferromagnesians, melanocratic- 90%- Peridotite. ...read more.


Silica under saturated Rocks - In these rocks we should find minerals that, in general, do not occur with quartz. We can relate this to a mineral contents table of well-known igneous rocks, to prove this theory. Rock name Peridotite- Ultra basic Basalt- Gabbro Andesite- Intermediate Granite- Acid Mineral name Quartz 0% 0% 4% 25% Feldspar (plagioclase 8% 40% 58% 15% Feldspar (alkali) 0% 0% 5% 49% Micas 0% 0% 4% 9% Pyroxene 12% 52% 17% 0% Amphibole 0% 0% 12% 2% Olivine 80% 8% 0% 0% Concluding from this table is that Quartz isn't present in ultra basic and basic rocks because there is a low content of SiO2 present. Pyroxene and plagioclase feldspar form first- what's left over forms Quartz. Ultra basic rocks and basic rocks form at very high temps and form quickly. To the lack of timing involved, the amount of SiO2 formed is small- Quartz only formed when there is an excess amount of SiO2. I conclude that using these 4 main ways of classifying igneous rocks, you are able to identify and understand the make up and composition of Igneous rocks. I have also found that each classification relates to another- in a circle so using these together proof can be made of age of rock, type, and the relationship between the rate of cooling and the crystal grain size. Josey Bellamy ...read more.

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