IGNEOUS ROCKS How does igneous rock form From

IGNEOUS ROCKS How does igneous rock form? From magma which cools and hardens.

How does magma form? • When temperature increases above the melting point of the minerals in the rock • Rocks melt at lower temps under lower pressure (excess pressure removed) • When fluid such as water is added (decreases the melting point)

Partial melting • Different minerals have different melting points • The composition of the magma changes as different minerals melt at different temperatures • This process is called partial melting

Fractional crystallization • Opposite process of partial melting • Chemicals in magma combine to form minerals, each mineral has a different freezing point. • Highest freezing points (freeze at the highest temperature) crystallize first. • As minerals crystallize they remove specific chemicals from the magma changing its composition

TEXTURES • Determined by size of their crystals and grains • Size of crystals and grains determined by the cooling rate of the magma

Intrusive Igneous Rock Extrusive Igneous Rock • Cools deep inside the crust • Slowly cools and hardens • Hint: think INterior • Cools at Earth’s surface • Hint: think EXterior

Coarse-Grained Igneous Rock Example: QUARTZ-DIORITE • Composed of large mineral crystals. • Cools slowly allowing minerals to form large well developed crystals.

Fine- Grained Igneous Rock Example: BASALT • Composed of small mineral grains that you can’t see with the naked eye. • Magma cools too rapidly to form large crystals.

Glassy Igneous Rock Example: OBSIDIAN • Highly viscous magma that cools too quickly for crystals to grow.

Porphyritic Igneous Rock Example: VOLCANIC BRECCIA • This texture forms when magma cools slowly at first, then cools more rapidly as it nears Earth’s surface. • Larger crystals are imbedded within the smaller ones.

Vesicular Texture Igneous Rocks Example : PUMICE • When magma contains a large amount of gases and cools rapidly, the gases become trapped as bubbles in the rock. • This produces a rock full of holes called vesicles

Composition of Igneous Rocks Mafic Felsic Intermediate

FELSIC • Light colored • Large proportion of Silica • Main mineral components- potassium feldspar, and quartz • INCLUDES granite, rhyolite, obsidian and pumice

Mafic • Dark in color • Lower silica; Rich in Fe and Mg • Main mineral components: Plagioclase feldspar and Pyroxene minerals. • INCLUDES basalt and gabbro

INTERMEDIATE • May contain lower proportion of silica then felsic rocks but higher proportions than mafic • Made up of Plagioclase feldspar, hornblende, pyroxene, biotite mica • Includes diorite andesite

INTRUSIVE IGNEOUS ROCKS Batholiths- spread over at least 100 km 2 (Half Dome, Yosemite) Stocks- spread over less than 100 km 2 (Wilson Peak, CO)

Laccoliths – as magma flows Sills and Dikes between rock layers and spreads upwards, it sometimes pushes the overlying rocks into a dome When magma flows between layers of rock and hardens a sill forms parallel to the rock layers When magma forces itself through cracks in existing rock layers or create new ones, a dike forms

EXTRUSIVE IGNEOUS ROCKS Volcanic neck Solidified central vent of a volcano