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Lecture 9

Lecture 9 – Igneous Rocks

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Western University
Earth Sciences
Earth Sciences 1083F/G
Cameron Tsujita

Lecture 9Igneous Rocks y Importance of Rocks o Ultimate source of materials that living things requireMineral derived nutrients and materials for skeletal construction supplied to the biosphere through weathering of rock o Most of our knowledge about the Earths processes and history comes from rocks o For example1Rocks are preservation media for fossils in turn reveal patterns of evolutionary change2Rocks provide information on processes that have acted both on and within the Earth3Rocks contain minerals that can be dated allowing for absolute dating of rocks y How are Igneous Rocks Formed o Comes from Latin ignis meaning fire o Hot molten material from which of igneous rocks are formed is called magma y How are Igneous Rocks Classified o Classified based on two major thingsTexture including relative sized of mineral crystals andor distinct features such as bubble and fragmented materials useful for determining how the rock cooledComposition mineral content of the rock usually colour or shade useful for determining the mineral characteristics of the magma y Igneous Textures o PhaneriticCrystals are large enough to be observed with the unaided eyePhaner means visibleMeans magma cooled very slowly deep underground in plutonsOrigin of phaneritic texture y Magma ascends due to being less dense than the surrounding rock y The magma starts to cool allowing minerals to crystallize from chemical components of the melt y If magma remains trapped in a magma chamber it cools very slowly allowing the crystals to grow to large sizes y Rocks formed this way are called plutonic or intrusive igneous rocks o Aphanitic Crystals are too small to be seen with the unaided eyeAnot phanervisible
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