Formation and Characteristics IGNEOUS ROCKS. Igneous rocks are formed from the crystallization of magma. Molten rock can occur in three different.

Formation and Characteristics

IGNEOUS ROCKS

Igneous rocks are formed from the crystallization of magma.

Molten rock can occur in three diff erent 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.

IGNEOUS ROCKS

When we talked about Earth’s crust we said that the most common elements were: Oxygen Silicon Aluminum Iron Magnesium Calcium Potassium Sodium

We said that the crust was mostly Silicate (SiO2) rocks, magma is classified by its Silica content.

MAGMA

Mafic magma (Basaltic) Thin and runny LOW silica content (<50%) LOW gas content HIGH in Ca, Fe, and Mg

Intermediate magma (Andesitic) Medium Consistancy MODERATE silica content (~60%) MODERATE gas content MODERATE amount of all crustal elements

MAGMA

MAGMA

Felsic magma (Rhyolitic) Thick and blocky HIGH silica content HIGH gas content LOW in Ca, Fe, and Mg HIGH in K and Al

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

VISCOSITY OF MAGMA/ LAVA

98.5% of earths crust is made of only 8 elements! 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.

IGNEOUS ROCK COMPOSITION

BOWEN’S REACTION SERIES

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

IGNEOUS TEXTURES

Intrusive rock Cooled beneath Earth’s surface Cooled very slowly Higher temperature and pressure

PHANERITIC TEXTURE

Extrusive Igneous Rock Cooled on Earth’s surface Cooled relatively fast Lower temperature and pressure

APHANITIC TEXTURE

Complex coolingPartially cools below and above Earth’s surfaceDistinct crystal sizes

PORPHYRITIC TEXTURE

Igneous rocks are named by texture and mineral composition

COMPOSITION

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

SUMMARY