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
Small
crystals
granite
pegmatite
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)
Identification of Igneous Rocks
1. Texture: depends on:
– Size of mineral crystals
– Presence of glass
– Gas bubbles (vesicles)
Vesicular textures are full of bubbles
Pumice
Scoria
Identification of Igneous Rocks
2. Color
Light
or
Dark
Identification of Igneous Rocks
3. Density
Low
Granite is a lower
density igneous rock
or
High
Peridotite is a higher
density igneous rock
Identification of Igneous Rocks
4. Composition
• Mafic : contains Fe (iron) and Mg (magnesium)
• Felsic : contains Al (Aluminum)
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
Practice
Name a light-colored, fine-grained
rock with no bubbles.
Name a coarse-grained, dense rock.
Name a very light-colored, glassy,
extrusive rock with bubbles.
Finding The Minerals
Identify 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%