1. Igneous Rocks

2. Igneous Rock Formation
Igneous rock: forms when lava or magma cools and minerals crystalize
Magma: molten rock below the Earth’s surface
Lava: is magma that flows out onto the Earth’s surface
The type of igneous rock that forms depends on the composition of the magma.
Of all of the compounds present in magma, silica is the most abundant and has the greatest effect on the magma characteristics

3. TYPES of MAGMA Three types of Magma: 1. Basaltic 2. Andesitic
3. Rhyolitic
Type
Silica Content
Example Location
Basaltic
42-52%
Hawaiian Islands
Andesitic
52-66%
Cascade Mountains
Rhyolitic
More than 66%
Yellowstone National park

4. Bowen Reaction Series Low Silica High Silica
Left Branch: Iron rich minerals that undergo abrupt changes as magma cools and crystalizes
Right Branch: Feldspar undergoes a continuous change of composition. Start out as calcium
rich and become sodium rich

5. Igneous rock--mineral composition
Felsic
forms light-colored rocks
less dense
lots of aluminum, potassium, silicon, and sodium
ex: granite
Mafic
forms dark-colored rocks
lots of calcium, iron, and magnesium (low in silicon)
ex: basalt

6. Classification of Igneous Rocks
Classification of igneous rock is based on mineral composition, crystal size, and texture.
Two Classifications of Igneous Rocks
Intrusive: when magma cools and crystalizes below the Earth’s surface
Extrusive: Magma that cools and crystalizes on Earth’s surface

7. Classification of Igneous Rocks
Texture:
Size
Shape
Distribution of the crystals or grains that make up the rock
Rhyolite is fine grained while granite is coarse grained
The difference in crystal size is because one is intrusive and the other extrusive
Rhyolite
Granite
Obsidian

8. Classification of Igneous Rocks
Crystal size and cooling rates
Cools quickly- small crystals
Example: rhyolite- extrusive
Cools slowly- larger crystals
Example: Granite- intrusive

9. Classification of Igneous Rocks
Rock textures provide information about rock’s formation.
Porphyritic Rocks: large, well formed crystals surrounded by finger grained crystals of the same mineral or different minerals
Vesicular Rocks: Magma contains dissolved gases. If lava is thick enough it traps the gases and holes called vesicles are formed
pumice

10. Igneous Rocks as Resources
Veins: a vein is a distinct sheet-like body of crystalized minerals within a rock. Veins form when mineral are deposited through precipitation.
Pegmatites: Ores of rare elements, such as lithium and beryllium.

11. Igneous Rocks as Resources
Kimberlites: minerals formed deep in the crust or in the mantle. Can only from under high pressure.
Example: Diamonds
Igneous Rocks in Construction:
Interlocking grain texture makes them strong
Resistant to weather

13. Review Questions 1.What is the most common igneous rock? Granite
2.Why does PUMICE float in water?
It came from an explosive volcano, so it had a lot of trapped air bubbles (so density less than 1)
3.Why does lava cool more quickly than magma?
exposed to the air or water

14. Review Questions 4. What determines the texture of an igneous rock?
>>how fast the lava/magma cools (and if the volcano is explosive)
5. A batholith forms from a large magma chamber that slowly cools deep beneath the earth’s surface. What type of texture is this igneous rock formation most likely to have?
>>coarse-grained