Igneous Rock Characteristics and Classification

This lecture by Tor Lacy introduces igneous rocks, explaining their formation through crystallization and the rock cycle. It covers the textures of igneous rocks, such as fine-grained and coarse-grained, and special textures like glassy and vesicular. The origin of the rock affects its texture, with extrusive rocks cooling quickly and intrusive rocks cooling slowly. The composition of igneous rocks is categorized by silica content into felsic, intermediate, mafic, and ultramafic. The lecture concludes with how to classify and name igneous rocks using texture and composition.

What process is responsible for the formation of igneous rocks?

• Metamorphism

• Sedimentation

• Crystallization

• Erosion

What term is used to describe igneous rocks with large crystals?

• Aphanitic

• Phaneritic

• Vesicular

• Porphyritic

Which texture is formed when lava cools rapidly and traps gas bubbles?

• Glassy

• Vesicular

• Porphyritic

• Pegmatitic

What is the term for igneous rocks with a mixture of fine and coarse grains?

• Porphyritic

• Phaneritic

• Pegmatitic

• Aphanitic

What type of origin is associated with igneous rocks that cool slowly underground?

• Extrusive

• Intrusive

• Sedimentary

• Volcanic

What is the primary factor that controls the texture of igneous rocks?

• Weight

• Crystal size

• Color

• Density

Which type of igneous rock has the highest silica content?

• Intermediate

• Felsic

• Ultramafic

• Mafic

How is the silica content of a rock approximated?

• By its color

• By its density

• By the ratio of light to dark minerals

• By its hardness

What is the classification of a rock with a fine, fragmented texture and intermediate composition?

• Basalt

• Rhyolite

• Andesite Tuff

• Granite

Which category of igneous rocks contains less than 45% silica?

• Felsic

• Intermediate

• Mafic

• Ultramafic