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Hawai’i Minerals and Rocks The Composition of the Hawai’i Topics • Silicates • Other Minerals • Rocks • Igneous Rocks Topics • Silicates • Other Minerals • Rocks • Igneous Rocks Igneous Rocks Rocks that crystallize from a magma Igneous Rock Classification Igneous Rock Classification • Two types of igneous rocks: Igneous Rock Classification • Two types of igneous rocks: 1. Extrusive Igneous Rock Classification • Two types of igneous rocks: 1. Extrusive 2. Intrusive • Extrusive igneous rocks cool and solidify on the surface • Intrusive igneous rocks cool and solidify in the crust • Extrusive igneous rocks cool quickly • Intrusive igneous rocks cool slowly Igneous Rock Textures Igneous Rock Textures • The different cooling rates result in different rock textures Igneous Rock Textures • The different cooling rates result in different rock textures • Geologists use rock textures to determine where and how rocks formed Phaneritic Texture Phaneritic Texture • A slow cooling rate results in fewer, larger crystals Phaneritic Texture • A slow cooling rate results in fewer, larger crystals • Large crystals have more time to grow Phaneritic Texture • A slow cooling rate results in fewer, larger crystals • Large crystals have more time to grow • Intrusive igneous rocks generally have macroscopic crystals Phaneritic Texture • A slow cooling rate results in fewer, larger crystals • Large crystals have more time to grow • Intrusive igneous rocks generally have macroscopic crystals • Also called phaneritic or coarse-grained Aphanitic Texture Aphanitic Texture • A rapid cooling rate results in more, smaller crystals Aphanitic Texture • A rapid cooling rate results in more, smaller crystals • Smaller crystals have less time to grow Aphanitic Texture • A rapid cooling rate results in more, smaller crystals • Smaller crystals have less time to grow • Extrusive igneous rocks generally have microscopic crystals Aphanitic Texture • A rapid cooling rate results in more, smaller crystals • Smaller crystals have less time to grow • Extrusive igneous rocks generally have microscopic crystals • Also called aphanitic or fine-grained Phaneritic & Aphanitic Phaneritic & Aphanitic Glassy Texture Glassy Texture • If magmas cool too rapidly, crystals do not have time to form Glassy Texture • If magmas cool too rapidly, crystals do not have time to form • A very rapid or instantaneous cooling rate forms volcanic glass Glassy Texture • If magmas cool too rapidly, crystals do not have time to form • A very rapid or instantaneous cooling rate forms volcanic glass • Very rapid or instantaneous cooling occurs when magma is eject into the air or flows into water Glassy Texture • If magmas cool too rapidly, crystals do not have time to form • A very rapid or instantaneous cooling rate forms volcanic glass • Very rapid or instantaneous cooling occurs when magma is eject into the air or flows into water • e.g. obsidian or ash Volcanic Glass • Igneous rocks often have two distinct sizes of crystals • Igneous rocks often have two distinct sizes of crystals • Porphyritic texture Porphyritic Texture Porphyritic Texture • Porphyritic rocks cool in two stages Porphyritic Texture • Porphyritic rocks cool in two stages • The larger crystals form as magma cools below the surface - intrusive stage Porphyritic Texture • Porphyritic rocks cool in two stages • The larger crystals form as magma cools below the surface - intrusive stage • Mixture of magma and large crystals Porphyritic Texture Porphyritic Texture • Magma with crystals is extruded as lava Porphyritic Texture • Magma with crystals is extruded as lava • The remaining liquid cools rapidly forming numerous, smaller crystals - extrusive stage Olivine Basalt • Porphyritic texture Next Lava and Volcanoes
