Igneous Rocks Basic Principles

Igneous Rocks Igneous means “fire formed” Igneous rocks originate at high temperatures Temperatures are hot enough to melt rock Heat comes from radioactive decay of unstable elements such as U 238 and K 40

Igneous Rocks Molten (melted) rock is Magma Magma on Earth’s surface is Lava Magma may form igneous rocks: –On the surface = volcanic or extrusive rocks –Within the Earth = plutonic or intrusive rocks

Hawaiian lava flow

Igneous Rocks Igneous rocks form when magma –Cools –Crystallizes (forms crystals) –Crystals interlock

Cooled Lava becomes Igneous Rock

Texture of Igneous Rocks Texture means the size of the crystals Crystal size is a result of the rate of cooling Slow cooling produces large crystals Rapid cooling produces small crystals

Coarse and Fine Igneous Textures Intrusive Extrusive

Texture of Igneous Rocks Where is slow cooling likely to happen? –Within the Earth = INTRUSIVE Where is rapid cooling likely to happen? –On Earth’s surface = EXTRUSIVE

Thin Sections showing Coarse and Fine Igneous Texture Intrusive Extrusive

Explosive eruption from cooler magma

Texture of Igneous Rocks Texture is a clue to rate of cooling and therefore place of cooling

Composition of Igneous Rocks Composition is determined by: What materials are present to be melted Temperature (different minerals have different melting points) Where the melting happens (related to temperature and type of material available)

Composition of Igneous Rocks Mafic igneous rocks are enriched in: –Magnesium and Iron (Ferrum) Felsic igneous rocks are enriched in: –Feldspar (especially Na and K) and Silica (quartz) Intermediate igneous rocks are: –Intermediate in terms of Mg, Fe, Na, K, Si

Composition of Igneous Rocks Mafic Felsic

Classification of Igneous Rocks Igneous rocks are classified on the basis of their: TEXTURE COMPOSITION