IGNEOUS ROCKS
Formation and
Characteristics
IGNEOUS ROCKS
 Igneous rocks are formed from the crystallization of magma.
 Molten rock can occur in three dif ferent ways:
 Magma: molten rock below Earth’s surface
 Lava: magma on the Earth’s surface
 Pyroclastic material: (pyro=fire, clastic=debris) Airborne lava
that cools at it falls.
MAGMA
 When we talked about Earth’s crust we said that the most
common elements were:
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Oxygen
Silicon
Aluminum
Iron
Magnesium
Calcium
Potassium
Sodium
 We said that the crust was mostly Silicate (SiO 2 ) rocks,
magma is classified by its Silica content.
MAGMA
 Mafic magma (Basaltic)
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Thin and runny
LOW silica content (<50%)
LOW gas content
HIGH in Ca, Fe, and Mg
 Intermediate magma (Andesitic)
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Medium Consistancy
MODERATE silica content (~60%)
MODERATE gas content
MODERATE amount of all crustal elements
MAGMA
 Felsic magma (Rhyolitic)
 Thick and blocky
 HIGH silica content
 HIGH gas content
 LOW in Ca, Fe, and Mg
 HIGH in K and Al
VISCOSIT Y OF MAGMA/ LAVA
 Viscosity: the resistance of a liquid to flow.
 Very important for volcanic activity
 MAFIC- low silica, low viscosity, runny, less violent eruptions
 FELSIC- high silica, high viscosity, thick, more violent eruptions
IGNEOUS ROCK COMPOSITION
 98.5% of earths crust is made of only 8 elements!
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Oxygen and Silicon make up 75%
Aluminum=8.1%
Iron=5%
Calcium=3.6%
Sodium=2.8%
Potassium=2.6%
Magnesium=2.1%
 Sooooooo, the type of rock created is mainly determined by:
 What elements are present during cooling
 How rapidly does the rock cool
 Neatly, illustrated by Bowen’s reaction series.
BOWEN’S REACTION SERIES
IGNEOUS TEXTURES
 Igneous rocks are classified in two ways
 Texture
 Composition
 Textures (How big are the mineral grains)
 Phaneritic: can be seen with naked eye different colored and
interlocking shapes
 Aphanitic: mineral grains too small to be seen, microscopic
 Porphyritic: two distinct sizes of mineral
PHANERITIC TEXTURE
 Intrusive rock
 Cooled beneath Earth’s surface
 Cooled very slowly
 Higher temperature and pressure
APHANITIC TEXTURE
 Extrusive Igneous Rock
 Cooled on Earth’s surface
 Cooled relatively fast
 Lower temperature and pressure
PORPHYRITIC TEXTURE
 Complex cooling
 Partially cools below and above Earth’s surface
 Distinct crystal sizes
COMPOSITION
 Igneous rocks are named by texture and mineral composition
SUMMARY
 Magmas have three main types
 Mafic
 Intermediate
 Felsic
 Composition and cooling rate determine what minerals will
form
 Three types of igneous textures
 Phaneritic Texture
 Aphanitic Texture
 Porphyritic Texture
 Put it all together and you can identify your igneous rock