What factors can affect the texture of an igneous rock? What type of texture(s)
ID: 231817 • Letter: W
Question
What factors can affect the texture of an igneous rock? What type of texture(s) would you expect to find in an igneous rock that was formed by a high silica, gas-rich magma that erupted explosively?
Why are the crystals so small or non-existent in some extrusive igneous rocks?
Explain the importance of Bowen’s reaction series to our understanding of how magmas differentiate (change composition from mafic felsic) (hint: fractional crystallization, partial melting). Be able to label the minerals, temperature change, and change in composition (mafic felsic) on Bowen’s reaction series.
If a rock partially melts, what is the composition of the first melt to form (mafic or felsic?)? Think about what minerals have the lowest melting/crystallization temperature on Bowen’s reaction series! Can you give an example of one of the first minerals that would melt?
If a magma fractionally crystallizes, what is the composition of the first crystals to form (mafic or felsic?)? Can you give an example of one of the first minerals to crystallize? How does the composition of the magma change as it crystallizes (i.e., does it get more mafic or more felsic)?
Be able to give examples of mafic, intermediate, and felsic intrusive and extrusive rocks.
How are a gabbro and a basalt different from one another? How are a granite and a rhyolite different from one another? (chemistry? crystal size? etc.)
Explanation / Answer
An igneous rock is any crystalline rock that forms from cooling of magma. Magma consists mostly of liquid rock material, may contain crystals of various minerals, and contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase. Magma can cool to form an igneous rock either on the surface of the Earth in which it produces a volcanic or extrusive igneous rock, or beneath the surface of the Earth in which it produces a plutonic or intrusive igneous rock. Different igneous rocks have different type of chemical and mineralogical composition. And they are formed under various plate tectonic environments.
An important factor that determines the texture of an igneous rock is the rate of coolinge. Other factors are-(1)The diffusion rate which the rate at which atoms or molecules can move (diffuse) through the liquid. (2) The rate of nucleation of new crystals, which is the rate at which enough of the chemical constituents of a crystal can come together in one place without dissolving. (3) The rate of growth of crystals, which is the rate at which new constituents can arrive at the surface of the growing crystal. This depends largely on the diffusion rate of the molecules of concern.
Rhyolite rocks are formed by high silica, gas-rich magma that erupted explosively.
Extrusive or volcanic igneous rocks form at the Earth's surface. At the surface the molten rock is exposed to temperatures much colder than at depth, and so the lava cools rapidly and crystals do not have time to grow very large.
The temperature of magma its chemical composition determine the crystallization of minerals and the type of igneous rock. Different minerals crystallize at different temperatures and so the mineral composition of an igneous rock shows about the cooling history of that rock. The awareness that types and modes of occurrences of igneous rocks can be related to a common history of cooling, and this was invented by the petrologist bowen, by a theoretical pattern known as bowen's reaction series.
Partial melting always produces a magma that is less mafic than the parent rock.
Felsic has the lowest melting/crystallization temperature on Bowen’s reaction series. One of the first minerals to crystallize are quartz and muscovite.
The composition of the magma change as it crystallizes from mafic magma into a felsic magma.
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