CHAPTER 5 – IGNEOUS ROCKS

WHAT ARE IGNEOUS ROCKS? Igneous Rock Formation  Igneous rocks form when lava or magma cools and minerals crystallize.  Most rocks need to be heated to degrees Celcius in order to melt, these temperatures are present in the upper mantle/lower crust.  Composition of Magma  Molten rock, many elements, dissolved gases & mineral crystals.  Silica is the most common compound, and has the greatest effect on the type of magma.  Types of Magma – Table 5.1 pg.112  Basaltic – 42-52% silica  Andesitic – 52-66% silica  Rhyolitic – 66+% silica

WHAT ARE IGNEOUS ROCKS? Magma Formation  Four main factors:  Temperature  Pressure  Water content  Mineral content  Partial Melting  Some minerals melt, while others remain solid.

WHAT ARE IGNEOUS ROCKS? Bowen’s Reaction Series  Illustrates that minerals form in predictable patterns.  Left branch represents iron-rich minerals  These minerals undergo abrupt changes, through a series of reactions.  Right branch represents feldspars, rich in calcium.  These minerals undergo continuous changes.  Diagram figure 5.4 on pg.114.

WHAT ARE IGNEOUS ROCKS? Fractional Crystallization  Magma crystallizes in reverse order of partial melting.  The first minerals that crystallize, were the last to melt, during partial melting.  Unlike partial melting, crystals formed are removed from magma, and cannot react with it.

CLASSIFICATION OF IGNEOUS ROCKS Igneous rocks are classified based on:  Mineral Composition  Texture  Crystal Size Mineral Composition  Intrusive vs. Extrusive  Intrusive rocks crystallize below Earth’s surface and form larger crystals.  Extrusive rocks crystallize above Earth’s surface and form smaller crystals.  Basaltic vs. Granitic  Basaltic rocks have a dark color and a lower silica content.  Granitic rocks have a light color and a higher silica content.

CLASSIFICATION OF IGNEOUS ROCKS Igneous Rock Identification  Look at graph on pg.119 Texture  Crystal Size & Cooling Rates  Lava/magma which cools quickly = small crystals.  Lava/magma which cools slowly = large crystals.  Porphyritic Rocks  Combination of small & large crystals.  Vesicular Rocks  Spongy appearance, due to the escape of dissolved gases.

CLASSIFICATION OF IGNEOUS ROCKS Igneous Rocks as Resources – Read pgs  Veins  Pegmatites  Kimberlites