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

INTRUSIVE IGNEOUS ROCKS Batholiths- spread over at least 100 km2 (Half Dome, Yosemite) Stocks- spread over less than 100 km2 (Wilson Peak, CO)

Laccoliths – as magma flows between rock layers and spreads upwards, it sometimes pushes the overlying rocks into a dome Sills and Dikes - 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