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 Increases with depth Decreases melting point
Pressure
Water Content
Mineral Composition
Different minerals,
Characteristics of Magma
Slushy Mix of molten Compounds in Magma rock, gases, and Silica ( Si02) mineral crystals Most abundant Common Elements Greatest effect on
Oxygen (O) Silicon (Si) Aluminum (Al) Iron (Fe) Magnesium (Mg) Calcium (Ca) Potassium (K) Sodium (Na)
Magma Characteristics Effects melting temp Viscosity of Magma
Types of Magma
Based on amount of Silica
Basaltic Andesitic Rhyolitic
Type of Magma Rhyolitic Andesitic Basaltic
SiO2 Content 70% 60% 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 abrupt change of mineral type
Iron-Rich Minerals
An
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
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 Magma Type:
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 Magma Type:
Ultramafic: very low silica content, dark colors, high levels of Fe & Mg dominant minerals: olivine, pyroxene
Ex: peridotite, dunite Magma Type:
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
Crystal size
Grain Size
Description
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
Very Light Color NotVery Light
Light
Neither Light nor Dark Not Very Dark Very Dark Dark
Darkest
Lightest
Density
Very Light
Medium Density Dense Very Dense
Light
Finding The Minerals
Identify the rock.
Unless you have other information, work in the middle of the rock’s box.
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%
Practice
Name a light-colored, fine-grained rock with no bubbles.
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 goldquartz, 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