The Rock Cycle: Igneous Rocks

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

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

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