Igneous Rocks

What are Igneous Rocks?• Igneous rocks: form when molten (liquid) rock

material cools into a solid.

How do Igneous Rocks Form?• All igneous rocks form during solidification.– Crystallization: when magma/lava cools and forms a

solid composed of intergrown mineral crystals.

Animation

Compare the crystal sizes of these igneous rocks.

basalt pegmatitegranite

Small crystals

Large crystals

Crystal Sizes• Crystal size depends on the length of time it takes

the rock to cool.– Longer cooling = larger crystals

Crystal size

Time to Cool

Large crystals indicate a slow cooling (days to 1000’s of years).

Small crystals indicate a fast cooling (seconds to hours).

What is the difference between extrusive and intrusive igneous

rocks?• Extrusive – lava cools ON or ABOVE Earth’s surface– rocks have small or no crystals.

• Intrusive – magma cools BELOW Earth’s surface–Rocks have large crystals.

Devils Tower National Monument, Wyoming. An igneous intrusive body exposed by erosion.

Palisades sill: A sill is a horizontal igneous intrusion. The Palisades sill is located on the west side of the Hudson river in NJ and lower NYS. Exposed by erosion, the Palisades are vertical cliffs seen in the photos (right)

Extrusive (Volcanic) Textures

Glassy (no crystals)(Obsidian)

Fine crystals(Basalt)

Intrusive (Plutonic) Textures

Coarse crystals(Granite)

Very Coarse Crystals(Pegmatite)

1. Texture: depends on:– Size of mineral crystals

–Presence of glass

–Gas bubbles (vesicles)

Identification of Igneous Rocks

Vesicular textures are full of bubbles

Pumice Scoria

2. ColorLight or Dark

Identification of Igneous Rocks

3. Density Low or High

Granite is a lower density igneous rock

Peridotite is a higher density igneous rock

Identification of Igneous Rocks

4. Composition• Mafic : contains Fe (iron) and Mg (magnesium)

• Felsic : contains Al (Aluminum)

Identification of Igneous Rocks

Key Identifying Features of Igneous Rocks

1. Glassy Texture: No crystals

2. Intergrown mineral crystals: Interlocking (like puzzle pieces)

3. Vesicular: gas bubbles

Intergrown Crystals

Intergrown Crystals

Intergrown Crystals

NOT Intergrown Crystals

PracticeName a light-colored, fine-grainedrock with no bubbles.

Name a coarse-grained, dense rock.

Name a very light-colored, glassy, extrusive rock with bubbles.

Finding The MineralsIdentify the rock. Unless you have other information,

work in the middle of the rock’s box.

This is the amount of Potassium Feldspar in the rock.

This is the amount of Quartz in the rock.

This is the amount of Plagioclase Feldspar in the rock.

Use tick marks on a scrap paper to measure the percentage.Potassium Feldspar 25%Quartz 40%