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 Fine grained basalt – Cools quickly, crystals have little time to cool • Porphyry—large crystals formed at depth prior to emplacement on surface Porphyritic texture Glassy obsidian Intrusive Rocks 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 “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 Course grained: • Texture will help us granite differentiate between the two Fine grained: rhyolite More on classification… • Felsic – Light colored—generally composed of light colored minerals • mafic Rhyolite: felsic Gabbro: mafic – Generally composed of Fe/Mg—dark colored minerals • Intermediate rocks— composition between felsic and mafic – “Salt and pepper” Diorite: intermediate 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 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