GEOL 1114
Physical Geology
Fall 2013
Lecture 6
Igneous Rocks I
September 9, 2013
General characteristics of magma
• Igneous rocks form as molten rock cools and solidifies
• General characteristics of magma
• Parent material of igneous rocks
• Forms from partial melting of rocks
• Magma at surface is called lava
General characteristics of magma
• General characteristic of magma
• Rocks formed from lava = extrusive, or volcanic rocks
• Rocks formed from magma at depth = intrusive, or plutonic rocks
General characteristics of magma
• The nature of magma
• Consists of three components:
– Liquid portion = melt
– Solids, if any, are silicate minerals
– Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
General characteristics of magma
• Crystallization of magma
• Cooling of magma results in the systematic arrangement of ions into orderly patterns
• Silicate minerals result from crystallization in a predictable order
• Texture - size and arrangement of mineral grains Igneous textures
• Texture - overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals
• Factors affecting crystal size
• Rate of cooling
– Slow rate = fewer but larger crystals
– Fast rate = many small crystals
– Very fast rate forms glass
Igneous textures
• Types of igneous textures
• Porphyritic texture
– Minerals form at different temperatures
– Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass)
• Glassy texture
– Very rapid cooling of lava
– Rock is called obsidian
Igneous textures
• Types of igneous textures
• Pyroclastic texture
– Fragmental appearance produced by violent volcanic eruptions
– Often appear more similar to sedimentary rocks
• Pegmatitic texture
– Exceptionally coarse grained
– Form in late stages of crystallization of granitic magmas Porphyritic texture
Figure 4.4 D
Glassy texture
Figure 4.4 C
Igneous compositions
• Igneous rocks are composed primarily of silicate minerals
• Dark (or ferromagnesian) silicates
– Olivine, pyroxene, amphibole, and biotite mica
Igneous compositions
• Granitic versus basaltic compositions
• Granitic composition
– Light-colored silicates
– Termed felsic (feldspar and silica) in composition
– High silica (SiO2) content
– Major constituent of continental crust
Igneous compositions
• Granitic versus basaltic compositions
• Basaltic composition
– Dark silicates and Ca-rich feldspar
– Termed mafic (magnesium and ferrum, for iron) in composition
– Higher density than granitic rocks
– Comprise the ocean floor and many volcanic islands Igneous compositions
• Other compositional groups
• Intermediate (or andesitic) composition
– Contain 25% or more dark silicate minerals
– Associated with explosive volcanic activity
• Ultramafic composition
– Rare composition that is high in magnesium and iron – Composed entirely of ferromagnesian silicates
