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Igneous Rocks Pages 129-134 Origin of igneous rocks • Igneous rocks are formed by molten material from a volcano or molten material within Earth • Temps are 1600 at 60-200 km beneath the surface Why is it so hot? • The deep rocks are under pressure from the rocks on top • Also … radioactive elements produce heat • Leftover heat from formation of the planet Formation of magma Three factors affect whether rock melts • Temperature • Pressure • Presence of fluids Rocks have different melting points based upon the chemical composition of their minerals This means they crystallize (and melt) at different times Partial Melting • Different minerals have different melting points, and minerals with lower melting points are the first to melt • This creates lava with a specific composition • As the magma heats up, other minerals melt and the magma’s composition changes Where do Igneous Rocks form? Magma—Inside Earth Lava—Outside Earth Cooling time=crystal size • Magma trapped underground is insulated so it cools slowly • As it cools, the atoms arrange themselves in patterns to form crystals • If the magma cools slowly, the atoms have time to arrange large crystals. • These crystals are called mineral grains. Rocks form when these mineral grains grow together. Intrusive Igneous Rocks • Rocks that form beneath Earth's surface are called intrusive igneous rocks. • They have crystals large enough to see. • We call this texture/grain size: phaneritic How am I ever going to remember phaneritic? Think of large “ph”annies Granite Granite is an intrusive igneous rock It cooled slowly inside the earth It has a phaneritic texture Diorite—salt and pepper look Typical Minerals Na/Ca PLAGIOCLASE & MAFICS (amphibole) about 50/50 Quartz - absent to trace Gabbro • A mafic, phaneritic, igneous rock from the top of Bowen's Reaction Series. Extrusive Igneous Rocks • When rocks form from molten lava above Earth's surface we call them extrusive igneous rocks. • They cool quickly so the atoms don't have time to arrange themselves into patterns to form crystals. They have a fine-grained texture. • This texture is called aphanitic. Rhyolite Rhyolite is an extrusive igneous rock It cooled quickly,It has small grains This is called an aphanitic texture Andesite Porphyry • This is an andesite porphyry • A porphyry has big chunks in a matrix • Matrix—finegrained background Basalt • Basalt makes up the ocean floors • It is a mafic igneous extrusive rock Common Igneous rock terms Type of lava Felsic Intrusive Granite Extrusive Intermediate Mafic Texture term Diorite Gabbro Phaneritic Basalt Aphanitic Rhyolite Andesite Vesicular Pumice -Frothy Scoria Obsidian HyalineGlassy Pairs Granite Cooled slowly Rhyolite cooled quickly Others-- Scoria formed when lava cooled quickly above ground. You can see where little pockets of air had been. Scoria is actually a kind of glass and not a mixture of minerals. Vesicular—bubbly texture Pumice • Vesicular texture • Uses – lava soap – Stone-washed jeans • Glass. Pumice has cooled too quickly for minerals to form. • Chemical composition, however, will be from the bottom of Bowen's Reaction series (i.e. felsic). Obsidian • Hyaline texture is also called a glassy texture • Obsidian--Volcanic glass • Notice the conchoidal fracture Pegmatite Rock cools extremely slowly, so very big mineral grains form. Classification of Magmas • Mafic—high in iron and magnesium • Felsic—high in feldspar and silica (quartz) • Mafic—50% silica – Colors-- black, green, red • Intermediate—60% silica • Felsic—70% silica – Colors-- pink, white, light gray Composition of Igneous Rocks • Felsic—high in silica (quartz and feldspar) – Light colored • Mafic—high in magnesium and iron – Dark colored • Intermediate – A mixture of the two – In between colored Review Igneous Rocks Melting point is also affected by two factors • The pressure – As pressure melting point • The water content – High water content=lower melting point – Low water content=higher melting point Remember a rock is made of different minerals • When rocks melt, those with lowest melting points melt first – Partial melting • When rock crystallize, those with highest melting points crystallize first as the magma cools – Fractional crystallization Bowen’s Reaction Series Bowen’s Reaction Series Igneous Rock formation beneath the Earth's surface • Pluton-large intrusive igneous rock body • Dike—cuts across rock layers-vertical • Sill—horizontal – parallel to existing rock layers Caldera Pillow Basalt Igneous Rock Structures