Igneous Rocks
Rock vs. Mineral
•Rock- a naturally formed consolidated materialcomposed of grains of one or more minerals.
•Mineral- a naturally occurring element orcompound with a relatively constant chemical andstructural composition.
Three Rock Types
1.Igneous: Rock formed from the cooling of magma
2.Sedimentary: Rock formed from the accumulationand lithification (cementing) of weathered material.
3.Metamorphic: Rocks formed by the altering existingrock by heat and/or pressure.
The Rock Cycle
How do igneous rocks form?
Intrusive
Extrusive
Identification of Igneous Rocks
Texture: the size of the grains thatmake up the rock
Mineralogy: the minerals present inthe rock
Textures:
Phaneritic (Fan-er-itic): all large grains
Aphanitic (A-fan-itic): all small grains
Porphyritic (Poor-for-itic): large crystals surrounded by fine-grained matrix
Terms to explain to chemistry of igneous rocks:
•Mafic: (May-fik) Silica deficient igneous rockswith a high content of Magnesium, Iron, andCalcium. More dark minerals than light minerals.
•Felsic: (fell-sick) Silica rich igneous rock with ahigh percentage of potassium and sodium. Usuallylots of quartz and feldspar.
Mineral content of Igneous Rocks
Intrusive Structures: Rock formations caused by the coolingof magma beneath the Earth’s Surface.
Shiprock, New Mexico
Formed when a volcano goes extinct (loses its heat source). Themagma that was inside the volcano cools and hardens into a rockmuch harder than that on the surface.
Eventually the volcano is weathered away, exposing the rock.
Devil’s Tower, WY
Dikes: Magma from below is pushed up into cracks inthe existing rock.
Dikes from Intrusion
Magma Formation
•molten rock created near the subduction zone.
•granitic magma ~ 650o C
•geothermal gradient – 3o C/100 m
•melting point increases with pressure; pressureincreases with depth.
•mineral melting point is lowered by water underpressure.
Bowen’s Reaction Series
•sequence in which minerals crystallize incooling magma.
Volcanism and Extrusive Rocks
•pyroclastic (pyro = “fire”; clast = “broken”)
•pyroclastic flow- mixture of gas andpyroclastic debris.
•caldera-
•pumice-
•vesicles-
•cinder cone-
Extrusive Rocks
pyroclastic flow
(click the picture below for a short video
pumice with vesicles
Pyroclastic Flows
volcanos
cinder cone
caldera
