1. The Rock Cycle: Igneous Rocks

2. What is a rock? An aggregate of minerals 3 types of rock Rock cycle
Igneous
Sedimentary
Metamorphic
Rock cycle

4. Igneous Rocks Rocks created when molten rock cools and crystallizes
Two types
Extrusive (volcanic)
Erupted and cooled quickly at the surface
Intrusive (plutonic)
Cooled slowly beneath the surface of the earth

5. Extrusive Igneous Rocks

6. Extrusive Igneous Rocks
Fine grained basalt
Occur when lava cools quickly on the surface
Fine-grained textures common
Cools quickly, crystals have little time to cool
Porphyry—large crystals formed at depth prior to emplacement on surface
Glassy obsidian
Porphyritic texture

7. Intrusive Rocks Occur when magma cools within the crust
Granite: a common intrusive rock
Occur when magma cools within the crust
Medium to course grained texture
Crystals can generally be identified without the aid of a magnifier
Large crystals are the result of slow cooling over thousands/millions of years
Diorite: another common intrusive rock

8. “Can I see your I.D., please?”
Rocks are identified by
Textures
Mineral compositions
Ex: We have a rock that is composed of feldspar, quartz and hornblende—could be granite or rhyolite
Texture will help us differentiate between the two
Fine grained:
rhyolite
Course grained:
granite

10. More on classification…
Rhyolite: felsic
Felsic
Light colored—generally composed of light colored minerals
mafic
Generally composed of Fe/Mg—dark colored minerals
Intermediate rocks—composition between felsic and mafic
“Salt and pepper”
Gabbro: mafic
Diorite: intermediate

11. More on Classification
peridotite
Ultramafic rocks:
Very low Si
Dark colored, and heavy for their size
Often contain dark pyroxene, and large amounts of olivine—a granular-looking, green mineral
dunite

12. How do magmas form?
Si rich minerals—feldspar, quartz, etc.. Have low melting points
Melting point increases with decreasing Si content

13. Partial Melting
Melts of different compositions can be generated by melting the same rock several times at different temperatures
This is why not all igneous rocks look the same
Can also occur as magmas cool

14. How does rock melt? Increase temp
Temp increases 30°C for every kilometer of depth
At depths of 100 to 350 km, the temp is so high that some areas of melt begin to form
There are other
ways; well talk
about them later