igneous rocks. what are igneous rocks? igneous rocks: form when molten (liquid) rock material cools...
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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.
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)
1. Texture: depends on:– Size of mineral crystals
–Presence of glass
–Gas bubbles (vesicles)
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
Finding The MineralsIdentify the rock. Unless you have other information,
work in the middle of the rock’s box.