CHAPTER 4

ROCKS

CHAPTER 4

ROCKS

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Classifying Igneous RocksIgneous rocks are classified by their TEXTURE and their COMPOSITION

TEXTUREINTRUSIVE IGNEOUS ROCKS (Plutonic)

rocks that form below the surface

slowly cooled magma produces individual mineral grains that are large

enough to see

surrounded by layers of Earth-heat cannot escape

EXTRUSIVE IGNEOUS ROCKS (Volcanic)

rocks that form from lava above the surface

lava is exposed to air and water

magma cools quickly producing fine grained rocks (cannot see mineral

grains) atoms do not have time to arrange themselves in repeating patterns

INTRUSIVE OR EXTRUSIVE??

EXTRUSIVE

INTRUSIVE

EXTRUSIVE

INTRUSIVE

EXTRUSIVE

Obsidian

Granite

Scoria

Pumice

Diorite

Porphyry

• A mixture of large and small crystals

• Magma rises slowly through the crust before erupting to the surface– Within the crust, large crystals can grow– On the surface, cooling stops any more

crystals from growing

Vesicular

• Refers to vesicles (holes, pores, or cavities) within the igneous rock.

• Vesicles are the result of gas expansion (bubbles), which often occurs during volcanic eruptions.

• Pumice and scoria are common types of vesicular rocks.

Classifying Igneous RocksCOMPOSITION

type of magma or lava

BASALTIC ROCKS (Mafic)dense dark colored very fluidhigh amount of magnesium & iron

found in the volcanoes of Hawaiilow gas content

How does this explain the black sand beaches of Hawaii?

GRANITIC ROCKS(Felsic/Intermediate)

lower-density light colored

thick & stiff magma

high amount of silica & oxygen

contains a lot of gas causing very explosive eruptions

ANDESETIC ROCKS (Dioritic)

mineral composition between that of basaltic and granitic rockscan have violent eruptions

or quiet eruptionsmedium silica contentmedium thickness

many volcanoes around the Pacific Ring of Fire contain

andesitic magma