Chapter 5.2 – Classification of Igneous Rocks Two main types of igneous rock: Intrusive rocks form when magma cools and crystallizes below Earth’s surface. Crystals large enough to see without magnification Extrusive rocks form when magma cools and crystallizes on Earth’s surface (lava flows). Small crystals that need magnification to see

Types of Classification Properties Grain size Texture Mineral composition Basaltic rocks – mafic rocks, dark-colored, low silica contents, and contain mostly plagioclase and pyroxene Granitic rocks – felsic rocks, light-colored, have high silica content, and contain mostly quartz and feldspar

Types of Classification Intermediate rocks – rocks that have mineral compositions in between the two others Ultrabasic – ultramafic, contains rocks with only iron-rich minerals and are always dark Texture refers to size, shape, and distribution of crystals/grains that make up a rock Examples: fine-grained, coarse-grained

Other Properties When lava cools on the surface, it cools so quickly that only small crystals form Sometimes crystals don’t form at all This results in volcanic glass called obsidian

Other Properties When lava cools slowly under the surface, large crystals form – granite, diorite, gabbro

Porphyritic Rocks Porphyritic texture – large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals Complex cooling history (example: slow cooling then rapid cooling) causes different crystal sizes

Vesicular Rocks Vesicular texture – Spongy appearance formed when gas is prevented from escaping. The holes left behind are called vesicles Examples are pumice and vesicular basalt

Thin Sections A Thin Section is a slice of rock 2 x 4 cm and only 0.03 mm thick Light is able to pass through this sample These samples are viewed through a petrographic microscope and geologists look for certain properties Examples of properties: feldspar grains have banding called “twinning,” quartz grains look wavy as stage is rotated, etc.

Igneous Rocks as Resources Useful in many fields such as Construction Energy production Jewelry making Ores can be extracted from veins of lava flows Gold, silver, lead, copper These elements released at end of magma crystallization process

Igneous Rocks as Resources Pegmatites are igneous rocks made of extremely large-grained materials Found as igneous intrusions or veins Rare elements such as Lithium, Beryllium form in these ores They also produce complex crystals

Igneous Rocks as Resources Kimberlites are ultrabasic rocks that were first found in Kimberly, South Africa Only formed under very high pressure Valuable minerals such as diamonds Most diamonds in the world are found in South African mines. Only one diamond mine in the US in Colorado

Igneous Rocks as Resources Igneous rocks are strong building materials Durable Weather-resistant Countertops, floors, statues, etc.