Key Terms: igneous rock, extrusive, intrusive, vesicular, silica, felsic, mafic, viscosity

Classifying Igneous Rocks Turn to page 48 in your textbook. Igneous rocks are classified according to their: Origin Where the rock formed (i.e. above or below Earth’s surface). Texture Determined by the size and shape of the mineral grains that make up the rock. Mineral Composition Determine the color and shade of the rock 2

Igneous Rock: Origin Extrusive Rock Igneous rock formed from lava that erupted onto Earth’s surface. Basalt is the most common extrusive igneous rock. Intrusive Rock Igneous rock that formed when magma hardened below Earth’s surface. Granite is the most common intrusive rock. 3

Igneous Rock: Texture The texture of an igneous rock depends on the Size of the mineral grains. Shape of the mineral grains. The grain size created in an igneous rock is determined by the rate of cooling. The faster the cooling the smaller the mineral grains. Fast cooling = Extrusive Rock (fine or no visible grain). The slower the cooling the larger the mineral grains. Slow cooling = Intrusive Rock (coarse grain). 4

Igneous Rock: Texture Data Table TextureRate of CoolingCharacteristicsOrigin and Occurrence Example Glassy (No visible grain) Extremely rapid cooling No visable grains (crystals), concoidal (shell shaped) fracture Extrusive Surface of lava flows, lava entering water, volcanic vent debris Obsidian Fine Grained Rapid cooling Very fine grained crystals (need magnification). Extrusive Interior of lava flows Basalt Coarse Grained Uniform rate of slow cooling from molten rock Visible interlocking crystals of almost equal size Intrusive magma Granite Porphyritic Two stages of cooling initially slow then more rapidly (when extruded) Visible interlocking crystals of two distinct sizes. Large coarse grained and smaller fine grained crystals Both intrusive and extrusive May exist in fine of coarse grained igneous rocks Various fine or coarse grained igneous rocks. Diabase Porphyry Vesicular Rapid Cooling Generally fine grained with many holes and small grains of minerals Extrusive. Lava ejected out of a volcano which cools before it hits the ground. Pumice and Scoria Stop On the “Classifying Igneous Rock Origin and Texture Activities” Worksheet, do numbers 1 through 5 on the front of the sheet. 5

Igneous Rock: Mineral Composition The composition of the igneous rock is determined by the percentages of the different minerals that make up the rock. The color and shade of an igneous rock is determined by the mineral composition of the rock. Stop On the “Classifying Igneous Rock Origin and Texture Activities” Worksheet, do number 6 and the Sharpen your Skills activity on the back of the sheet. Turn in your worksheet when complete, then continue onto the next slide.. 6

Igneous Rock: Color and Shade The color of the igneous rock is determined by the impurities in the magma/lava mixture. More importantly, the shade of the igneous rock is determined by the amount of silica contained in the rock. High in silica = light shade The term used to describe the chemistry of an igneous rock with a high silica content is felsic. Low in silica = dark shade The term used to describe the chemistry of an igneous rock with a low silica content is mafic. 7

Molten Rock: Viscosity All liquids have a viscosity (resistance to flow) The viscosity of any liquid is the how the liquid resists flow. Magma and lava are both molten (liquid) rock. The viscosity of the molten rock is determined by the amount of silica contained in the molten mixture. High in silica content = High Viscosity = thick and resists flow. Low in silica content = Low Viscosity = thin and flows easily Stop On the “Classifying Igneous Rock: Mineral Composition Activities” Worksheet, do numbers 1 through 5. 8

Igneous Rock Identification Petrologists have organized the information we’ve discovered in this power point to create a chart to help identify igneous rocks. You can link to the chart here After linking to and studying the chart: Link to the extrusive igneous rocks page. Read through the page, and view the igneous rock samples.extrusive igneous rocks page Link to the intrusive igneous rocks page. Read through the page, and view the igneous rock samples.intrusive igneous rocks page Stop On the “Classifying Igneous Rock: Mineral Composition Activities” Worksheet, do number 6. Turn in your worksheet when complete. 9