Igneous Rocks Basic Principles Igneous Rocks Igneous means

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