GEOL 001 Lecture Notes - Lecture 6: Plagioclase, Sodium-Potassium Alloy, Lithosphere
20 Dec 2020
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May contain vesicles (holes form gas bubbles) Form in late stages of crystallization of granitic magmas. Large crystals (phenocrysts) are embedded in a matrix of smaller. Fragmental appearance produced by violent volcanic eruptions. Often appear more similar to sedimentary rocks. Surface environment cool, rapid cooling to produce glass or fine-grained lava. Crustal rocks at depth warm; slow cooling to produce coarse-grained plutonic rocks. Basaltic magma- rich in mgo, feo, cao, relatively poor in sio2, rich is alkalis (na2o, k2o), poor in silica- crystallizes olivine, pyroxene, plagioclase (ca) feldspar on cooling. Andesitic magma- somewhere in between the other two (intermediate composition) Rhyolitic magma- relatively rich in sio2, rich is alkalis, poor in mgo, feo, Cao- crystallizes quartz and na-k feldspar on cooling. Different minerals from at different temperatures during cooling, melt changes composition. Crystals removed form the melt as they form (for example, by setting) Basaltic magma can also assimilate crustal material and change composition.
