CVE20004 - Geomechanics Unit 1:Geology Laboratory 2: Identification of Igneous Rocks Geology: The Geological Rock Cycle Compaction, Cementation or Crystalisation Sedimentary Rocks Sediments Metamorphism Weathering and Transportation Metamorphic Rocks Melting Magma or Lava Igneous Rocks CVE20004 Geomechanics 1
Magma Magma is molten rock, which is a silicate based composition that is formed deep in the earth by the fusion of pre-existing solid masses. Magma is typically 45 to 75% silica (SiO2) with the other portion consisting of: · aluminium, iron, · magnesium, · calcium, sodium, . potassium or . oxygen. Depending on chemical composition, will depend on what minerals are formed. And the silica percentage will depend whether the mineral is either basic or acidic. Magma chamber, Hawaii CVE20004 Geomechanics Laboratory Exercise No. 2 - Identification of Igneous Rocks Extrusive (Pyroclastic) Extrusive (Volcanic) Minor Intrusive (Hypabyssal) Major Intrusive (Plutonic) Acidic CVE20004 Geomechanics Intermediate Basic Ultrabasic 2
Acidic Rating of Igneous Rocks The acidic rating of an igneous rock is affected by the SiO2 content as well as the other minerals within the rock. If an igneous rock is rich in quartz, orthoclase, or sodium plagioclase then the igneous rock is likely to be light in colour and have a high acidic rating. 27 100 80 orthoclase feldspar 60 Composition (%) 40 quartz plagioclase feldspar Ca-rich Granite ferromagnesian minerals 20 Na-rich biotite 0 syenite granite Acid olivine hornblende - pyroxene granodiorite diorite gabbro peridotite dunite Ultrabasic Granite CVE20004 Geomechanics KAIS? 3Os Orthoclase and Microcline Light in Colour High Silica Content Acidic Rating Light in Colour Feldspar Series Sanidine .....- Alkali Feldspars -----********** Anorthocias e Intermediate Silica Content Intermediate Rating Intermediate in Colour /0-10 An% 10-30 An% Albite NaAlSi 3 Os Oligoclase 30-50 An% Andesine 50-70 An% 70-90 An% 90-100 An% Low Silica Content Labradorite Bytownite Anorthite Basic Rating CaAL Si2O Dark in Colour Plagioclase CVE20004 Geomechanics Darker in Colour Darkest in Colour 3