1. Igneous Rocks

2. Introduction  Igneous Rocks – rocks that appear to have been molten in the past  Molten rock names (based on location)  Magma: molten rock found anywhere beneath the earth’s surface  Lava: molten rock that flows out onto the earth’s surface  Molten rock types (based on where they are formed)  Extrusive rocks: form when lava solidifies above the earth’s surface  Intrusive rocks: form when magma solidifies after squeezing into other rocks, intruding into them (may eventually appear at earth’s surface due to erosion)

3. Texture and Composition  Texture - Sizes of mineral crystals are the main difference between intrusive and extrusive rocks.  Coarse: larger crystals  Fine: small crystals  Glassy: no crystals  Factors determining crystal size:  Number of foreign particles present around which crystal accretion can begin (nucleation sites)  Concentration of various elements in melted rock  Temperature  Pressure  Rate of crystallization

4. Texture and Composition  Intrusive igneous rocks tend to have textures that are coarse to fine because conditions deep underground encourage large crystal formation.  Extrusive rocks tend to have fine to glassy textures.  Composition:  Depends on the mineral content of the molten rock from which they solidify  “Felsic” contain much silica  Examples – light colored rocks like feldspars and quartz  “Mafic” rich in iron and magnesium  Examples – dark colored rocks like augite, olivine, and biotite mica  Igneous rocks may fall anywhere between these extremes

5. Igneous Intrusions  Intrusions = intrusive structures underground  Largest intrusion is called a batholith (hundreds of square kilometers horizontally and several thousands of meters thick!)  Many mountain ranges are built on top.  Stock – a batholith that is less than 100 square kilometers  Pluton – an intrusive rock formation deep in the earth

6. Igneous Intrusions  Laccolith – the solidified formation made when magma intrudes in such a way that it pushes overlying strata into a dome  Multiple laccoliths make low hills.  Sills – sheets of magma that has forced its way between layers of existing rock  Vary from centimeters to meters in thickness and can extend for kilometers  Dikes – hardened magma that intruded into cracks that cut through existing layers of rock  Often associated with volcanic activity

7. Intrusive Igneous Rocks  Granite – “containing grains,” most common, coarse-grained, felsic rock  Colors: pink, white, black  Fractures may be smooth or rough (depending on composition)  Famous building material  Other intrusive igneous rocks include gabbro and diorite (coarse texture)  Porphyry = rock containing large crystals embedded in smaller crystals (probably formed in two different stages of cooling)

8. Extrusive Structures  Lava flow – magma that extrudes onto earth’s surface  Lava plateau – thick rock formation from lava flooding hundreds of square kilometers of land  When a volcano becomes dormant, the lava usually solidifies within it.  The vertical column-like formation that remains after erosion removes overlying volcanic debris is known as a volcanic neck.  Example: Shiprock in New Mexico

9. Extrusive Igneous Rocks  “Lava Rocks” solidify from molten lava.  Basalt = dark, mafic extrusive rock with fine crystal texture  Abundant throughout the world  Colors: dark greenish gray to black  Rapidly solidified near earth’s surface or in the neck of a volcano  Patterns of long fractures from stress of cooling… look like multisided columns

10. Extrusive Igneous Rocks  Felsite is lighter in color and contains silicates.  Scoria is frothy with dissolved gases (cools so fast that gas bubbles are trapped) and looks like dark cinders. Often mistaken for meteorites.  Pumice is similar to scoria, but lighter in color. It can float in water because it contains so many trapped bubbles!  Obsidian is of the granite family, but it forms when degasified lava cools rapidly at near-atmospheric pressures.  “Natural glass”  No crystals  Colors: black, brown, and red most commonly  Conchoidal fracture