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What is a rock?
• An aggregate of minerals
• 3 types of rock– Igneous– Sedimentary– Metamorphic
• Rock cycle
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
Extrusive Igneous Rocks
• 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
Fine grained basalt
Glassy obsidian
Porphyritic texture
Intrusive Rocks
• 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
Granite: a common intrusive rock
Diorite: another common intrusive rock
“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
More on classification…
• 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”
Rhyolite: felsic
Gabbro: mafic
Diorite: intermediate
More on Classification
• 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
peridotite
dunite
How do magmas form?• Si rich minerals—
feldspar, quartz, etc.. Have low melting points
• Melting point increases with decreasing Si content
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