Igneous Rock Formation

• Molten rock comes from depth (less dense so works its way to surface) - full of gases (H20, C02, S02) plus elements in silicates

• Eruption - gases escape as pressure lessons• Magma cools and hardens beneath the surface

(intrusive; plutonic) • Crystallization - process of cooling and solidifying • Lava cools and hardens on surface (extrusive) • In volcanic eruptions on or under the earth’s surface

Intrusive Igneous Rocks

Form when magma hardens beneath Earth’s surface

Coarse grained due to slow cooling

Examples- Granite, Diorite, Gabbro, Periodotite

(image on left is diorite)

Extrusive Igneous Rocks

Form when lava hardens above the Earth’s surface, when most of the gases have escaped

Fine grained due to rapid cooling

Examples- Basalt, Rhyolite, Scoria

(image on right is basalt)

Classification of Igneous Rocks

• Texture- size, shape, arrangement of crystals

• Composition- proportions of light and dark minerals in the rock

(images from top to bottom: obsidian, diorite, basalt)

Coarse Grained = Large Crystals

• Intrusive rocks• Magma cools slowly• Ions have time to

move large distances within magma

• Few centers of crystal growth develop

Fine Grained: Fast Cooling

• Extrusive rocks• Magma or lava cools

rapidly resulting in small, interconnected mineral grains

• The ions in the melted material lose their motion and quickly combine

Glassy Texture

• Fast Cooling, Extrusive

• No time for the ions in the lava to arrange themselves into a network of crystals

• Obsidian (top); pumice (bottom)

Porphyritic Texture: Different Sized Crystals

• Minerals that do not crystallize at the same rate or time in magma --different sized crystals

• Inside volcano some magma never reaches the surface--two waves of crystallization

• Large crystals called phenocrysts (visible-crystals) in a matrix of fine grained crystals

Granitic Composition

• 0%-25% dark• Felsic• Light colored• Quartz, feldspar• 10% dark silicate

materials-- with magnesium, iron

• 70% silica (light silicates)

• Example- Rhyolite(also granite)

Basaltic Composition

• 45%-85% dark minerals• Plagioclase feldspar• Rich in magnesium and

iron• Darker and denser

(because of iron) • Mafic• Ocean floor = basaltExample- Basalt(also gabbro)

Andesitic Composition

• Between granitic and basaltic

• 25%-45% ‘dark’• 25% dark silicate

minerals-- amphibole, pyroxene, biotite mica

• Other dominant mineral: Plagioclase feldsparExample- Andesite(also diorite)

Ultramafic Composition

• 85%-100% dark• Olivine and pyroxene• Almost entirely dark

silicate minerals

• Peridotite rock. Rare at E’s surface but composition of much of mantle