| Chapter 4 - Igneous Rocks -formed from the crystallization of magma (a molten state) -cooling liquid forms crystals of certain minerals and form together -most often classified by their texture and mineral composition Magma- completely or partly molten material under Earth's surface. Lava- magma at or above Earth's surface 3 distinct parts to magma 1. Liquid components (aka: melt) -composed of mobile ions of the elements in Earth's crust -made up mostly of Si & O 2. Solid components -silicate minerals that have already crystallized from the melt -as a magma body cools, the size & # of crystals increase 3. Gaseous components -water vapor (H2O), carbon dioxide (CO2), sulfur dioxide (SO2) are the most common. -dissolved within the melt -gas vaporizes (comes out of the melt) when the pressure is low enough or when the magma crystallizes Types of Igneous Rocks * Extrusive (Volcanic)--form from lava * Intrusive (Plutonic)--form from magma Crystallization -magma cools and ions in the melt lose mobility and arrange themselves into orderly crystalline structures -it is generally Si & O atoms that link together first and form silicon-oxygen tetrahedra (building blocks of silicate minerals) -as magma continues to lose heat, the tetrahedra join with each other and other ions Factors affecting crystal size 1. Rate the magma cools (slower means fewer but bigger crystals 2. Amount of silica present 3. Amount of dissolved gasses Igneous Textures * Aphanitic texture -fine grained texture formed where cooling was relatively rapid -tend to be extrusive, don't have time to form big crystals -individual crystals so small that a microscope is needed to see individual minerals * Phaneritic texture -coarse grained texture where large masses of magma slowly solidified below the surface -big chunks, can see from across the room * Porphyritic texture -if magma containing some large crystals change environment (like erupting @ surface), remaining liquid portion cools relatively quickly -big crystals are older than the tiny ones! * Glassy texture -molten rock is ejected into the atmosphere and quenched quickly -unordered ions are "frozen" before they form orderly crystalline structure -no chemical compounds, just ions * Pyroclastic texture (fragmental) -consolidation of shards & fragments ejected during a violent eruption -fragments may be ash, molten blobs, or blocks torn from the walls of the vent Igneous Compositions -mainly composed of silicate minerals -the mineral makeup is determined by the magma from which it crystallizes *8 minerals: most common: Si, O others: Al, Ca, Na, K, Mg, Fe *Study fig 4.7 p 117 and the chart on handout -Felsic (Granitic): light color (pink, white, beige, etc) -Intermediate (Andesitic): light gray, black and white -Mafic (Basaltic): black -Ultramafic: green Naming Igneous Rocks * use chart fig 4.8 p 120 Origin of Magma -magmas form from preexisting rock in the lithosphere (crust and upper portion of upper mantle) 3 ways rocks melt 1. as a result of a decrease in pressure (fig 4.2 p 127) 2. result of the addition of volatiles (things that are easily evaporated) -H2O, CO2 -they lower the melting point 3. Result of heat transfer from rising magma -magma heats rocks, fractures them, and oozes into the fractures -if melting point of the rocks is lower than the temperature of the magma, they melt Composition of Magmas -determine the composition of the igneous rock -viscosity (ch 5) - the more viscous, the greater its resistance to flow (syrup=more viscous than water) 4 Types of magmas 1. Ultramafic-- very high amounts of Fe & Mg, 1100 C, silica content 40% or less 2. Mafic-- relatively high amount of Fe & Mg, silica content less than 50% 3. Intermediate-- moderate amount of Fe & Mg, 55% silica 4. Felsic (silicic)-- very little Fe & Mg, high amounts of Na & K, 70% silica -viscous, very explosive |