The Rock Cycle- Minerals form rocks All rocks can be transformed into other rock types Rocks are divided into 3 categories Igneous- crystalline- forms as liquid cools Metamorphic- crystalline-forms as rocks are heated and squeezed Sedimentary- non-crystalline- smaller pieces or chemicals from other rocks

L Igneous formed from Magma and Lava Magma • molten rock below Earth's surface. L ava • magma on the Earth's surface. Pyroclastic material • ( pyro = fire, clastic = debris) • Airborne lava — cools as it falls

Composition of the magma • Analogous to what makes up the “stew" • What chemical elements are present • What material has the magma moved through Temperature of the melt • Not only how hot, but how long it stays that hot • also relates to pressure of the molten rock Cooling environment • fast vs slow • Internal vs External Water content

Bowen's Reaction Series- IMPORTANT! Plagioclase Olivine (Ca-feldspar) Pyroxene Continuous Discontinuous Amphibole Plagioclase Biotite (Na-feldspar) Orthoclase (K-feldspar) Muscovite Quartz

What things might you describe when looking at an igneous rock?

between the two extremes in Si content and other atoms. Mafic magma • Composition of Igneous rocks Felsic or Sialic magma • Si-rich (> 65%) • rich in K, and Al • little Ca, Fe, and Mg. Intermediate magma • between the two extremes in Si content and other atoms. Mafic magma • Si - poor (< 35%) richer in Ca, Fe, and Mg. •

Ways of Changing Magma Composition

Viscosity of Magma/ Lava Viscosity of Magma/ Lava Viscosity- important for volcanic activity • • the resistance of a liquid to flow the resistance of a liquid to flow — — high viscosity = thick and stiff — — low viscosity = thin and "runny". Related to: Related to: • • amount of water (H2O) in magma • • amount of silica (Si) in magma • • Mafic Mafic — — thin, low viscosity • • Felsic Felsic — — thick, high viscosity

Igneous Rock Textures- how big are the Phaneritic Texture minerals? Phaneritic Texture • Visible Mineral Grains distinguishable as different colored interlocking shapes Aphanitic Texture Mineral grains too small to be seen — microscopic Porphyritic Texture Two distinct sizes of mineral grains Large and Small Large = Phenocrysts Small = Groundmass or Matrix Groundmass Grains may be either visible or not

Porphyritic texture Aphanitic basalt (mafic composition) constitutes the groundmass

Cooling Histories Minerals need time and space to grow • More time = Bigger crystals — visible mineral grains P & T control cooling rates of magma Temp Earth is a good insulator » holds in heat keeps out cool Atmosphere is a relatively bad insulator transfers heat easily Earth has pressure Weight of overlying rocks Magma trying to push up (density) water vapor (steam), wants to expand

Categories of Igneous EOF Granite Intrusive rocks • cool beneath Earth's surface • cool very slowly • higher P & T — Phaneritic textures Rhyolite Extrusive rocks • cool on the Earth's Surface • cool relatively fast • lower T & P — Aphanitic textures Basalt porphyry — Pyroclastic textures Complex • Partially cools below and above — Porphyritic textures

Igneous rock names • determined by texture • — size and arrangement of mineral grains • AND by mineral composition — minerals affect rock color and indicate temperature of creation

Pegmatite A Pegmatite is a very coarse-grained igneous rock. Crystals are >2 cm, often larger. Most are granitic, although mafic pegmatites can form. Biotite mineral grain Feldspar mineral grain Quartz mineral grain

Granite & Rhyolite What are the textures in these two rocks ? Phaneritic Texture Felsic magma Aphanitic Texture What are the textures in these two rocks ?

Andesite & Diorite What are the textures in these two rocks ? Phaneritic texture -forms from intermediate composition magma Aphanitic texture forms from intermediate composition magma What are the textures in these two rocks ?

Basalt & Gabbro phaneritic texture aphanitic texture forms from mafic magma forms from mafic magma Basalt Gabbro »

Peridotite- the abundance of Olivine crystals (more than 40%) makes the rock take on a green appearance

Obsidian- a popular rock among paleopeople for tool making Volcanic Glass

Tuffs & Breccias- look like sedimentary rock Tuffs & Breccias But they are not….it is volcanic ash that is lithified

How do we tell where the igneous rocks formed and what can we derive from the rocks about the conditions of formation?

Pyroclastics Lava flow Volcanic neck Igneous dike Igneous Sill Batholith

Sierra Nevada Batholith- Home to Yosemite National Park and a lot of Granites and Grano-diorites

Sierra Nevada Batholith

Areal extent of the Idaho Batholith- a huge (15,400 square miles) intrusive body of primarily felsic composition (granites) igneous rocks

An igneous dike- Discordant with surrounding rock It cuts across other rocks

An igneous sill- Concordant with surrounding rock It runs parallel to other rocks

Necks & Ig Dikes Neck Dike igneous sill Igneous sill