Igneous Rocks Igneous Rocks “Ignis” means fire Rocks that are formed from cystallization of magma Magma is molten rock Lava is magma that is on the Earth’s Surface

Igneous Rocks Two Types of Igneous Rocks Extrusive (Exterior) Intrusive (Interior) Igneous rocks that solidify into rock beneath Earth’s surface Coarse Grained Cool Slowly Granite

Extrusive Igneous Rocks Extrusive (Exterior) Igneous rocks that solidify into rock on Earth’s surface Fine Grained Cool Quickly

Intrusive Igneous Rocks Intrusive (Interior) Igneous rocks that solidify into rock beneath Earth’s surface Coarse Grained Cool Slowly Granite

Igneous Rock Formation

Origin of Magma Where does the heat come from that melts rocks? Formation of Earth Heat from the decay of radioactive elements

Factors that Affect Magma Formation Temperature Increases with depth Pressure Water Content Decreases melting point Mineral Composition Different minerals, different melting points

Characteristics of Magma Slushy Mix of molten rock, gases, and mineral crystals Common Elements Oxygen (O) Silicon (Si) Aluminum (Al) Iron (Fe) Magnesium (Mg) Calcium (Ca) Potassium (K) Sodium (Na) Compounds in Magma Silica ( Si02) Most abundant Greatest effect on Magma Characteristics Effects melting temp Viscosity of Magma Types of Magma Based on amount of Silica Basaltic Andesitic Rhyolitic

Type of Magma SiO2 Content Rhyolitic 70% Andesitic 60% Basaltic 50%

Melting Rocks Question Does a rocks melt like an ice cube, all at the same time?

Melting Rocks Rocks melt according to their melting points. Example: Ice cube with wax Which would melt first ice or wax? This example is known as partial melting. Partial Melting: some minerals melt at lower temperatures and other minerals remain solid Think of “stew”

Fractional Crystallization Opposite of Partial Melting Last to melt are first to solidify (crystallize)

Bowen’s Reaction Series N.L. Bowen Canadian 1900’s Stated that “as magma cools, minerals form in predictable patterns” Known as Bowen’s Reaction Series

Bowen’s Reaction Series Two Branches Feldspars Continuous, gradual change of mineral compositions Iron-Rich Minerals An abrupt change of mineral type

Bowen’s Reaction Series

Feldspars Continuous Change First Feldspars are rich in Calcium (Ca) Sodium (Na) increases as cooling continues Last Feldspars to form are Sodium rich (Na)

Iron Rich Minerals Discontinuous Change Magnesium (Mg) cools around 18000C, when olivine crystallizes, this continous up to 15570C. Now Pyroxene begins to form. All olivine that was formed is now turned to pyroxene Quartz is the last to form, because silica and oxygen are the last to crystallize

Why do we find Olivine?

Four main groups of igneous rocks based on magma type/mineral composition Felsic: high silica content, light colored, from thick & slow moving magma, contains low amounts of Ca, Fe, and Mg dominant minerals quartz, potassium rich feldspar Ex: granite, pumice, rhyolite Magma Type: Intermediate: moderate amount of silica, mixture of colors dominant minerals: sodium and calcium rich feldspar Ex: andesite, diorite Mafic: low silica content, dark colored, high levels of Fe & Mg formed from thinner, more fluid, & hotter magma than Felsic rocks dominant minerals hornblende, calcium rich feldspar Ex: basalt, gabbro Ultramafic: very low silica content, dark colors, high levels of Fe & Mg dominant minerals: olivine, pyroxene Ex: peridotite, dunite

Classifying Igneous Rocks Igneous rock textures: Crystal size is dictated by the rate of cooling of the magma body. A slower cooling rate results in larger mineral crystals being formed in the rocks as they cool. Porphyritic texture - Large crystals surrounded by fine grained rock. The rock initially cools slowly to form some large crystals and then cools quickly to form the fine grained rock surrounding them.

The Scheme for Igneous Rock Identification

A Brief Tour

Grain Size Description Crystal size

Igneous rocks have “Intergrown Crystals”

Intergrown

Intergrown

Intergrown

Not Intergrown

Where it was formed Outside the volcano: Extrusive Inside the Earth: Intrusive

Bubbles? Yes= Vesicular

Bubbles? No=Non-vesicular

Color Neither Light nor Dark Very Light NotVery Light Not Very Dark Darkest Lightest

Density Medium Density Very Light Light Dense Very Dense

Finding The Minerals Unless you have other information, work in the middle of the rock’s box. Identify the rock.

This is the amount of Potassium Feldspar in the rock.

This is the amount of Quartz in the rock.

This is the amount of Plagioclase Feldspar in the rock.

Use tick marks on a scrap paper to measure the percentage. Potassium Feldspar 25% Quartz 40%

Name a light-colored, fine-grained rock with no bubbles. Practice

Name a coarse-grained, dense rock.

Igneous Rock Resources Ore Deposits Building Materials Other Uses

Ore Deposits Veins: streaks of valuable metal within a mineral. Created when a metal-rich fluid, such as gold-quartz, goes through fractional crystallization, the mineral (quartz) has a lower crystallization temp and thus solidifies before the gold. The gold remains liquid and settles between the quartz crystals forming “gold veins.” Pegmatites: veins with extremely large grain crystals. Creates some of the world’s most precious gems. Kimberlites: intrusions of magma cooled deep within earth’s crust. Usually find diamonds with kimberlites. Named after location of first discovery, Kimberly, South Africa.

Building Materials Many IR’s are used in building materials because of their interlocking crystals strength IR’s are fairly weather resistant Ex: Granite – building Ex: Basalt – crushed up to make gravel

Other Uses Pumice – cleaning and polishes Obsidian heated to make perlite, a soil additive that keeps soil loose. scalpels – more precise and smoother than steel, but 10x the cost