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.