How are igneous rocks formed? Are made from magma that has been cooled down from liquid to solid
Difference between magma and lava?
Answer Magma is molten rock inside the Earth Lava is molten rock on the surface of the Earth
Rock Characteristics Use to tell history, origin of the rock Did it form inside or on the surface of Earth How long did it take to form?
Rock Characteristics Rock Texture: Relates to size of the mineral crystals. The size of the crystal indicates the amount of time it takes to cool down and form a solid object. The larger the crystals, the longer the time of formation Types of textures- Coarse Grained: Large crystals Fine Grained : Small crystals. Will usually not be able to see crystals without microscope
Other texture types Glassy: No crystals, even under a microscope – Indicates very rapid cooling of lava (seconds) Vesicular: Rocks with holes – Holes indicate escape of gases during rapid cooling
Color (2 nd Characteristic) Color indicates composition of the rock Light colored rocks: indicate rocks with high concentrations of Si, O, K and Na White, pinks and gray – These rocks have a Felsic composition – Formed within continents
Color Dark colored: Color dominated by dark greens and black These rocks have high concentrations or Fe, Mg and Ca These rocks have a mafic composition These rocks are formed within oceans
Nothing is black or white… A lot of rocks are combinations of felsic and mafic materials Color will be medium dark (usually dark gray) These have an intermediate composition
Density (3 rd Characteristic) Mafic rocks are generally more dense than Felsic rocks
Rock Names Match the name to your rock sample based on your observations Basalt- fine grained, mafic Gabbro- coarse grained, mafic Granite- coarse grained, felsic Obsidian- glassy, mafic Pumice- vesicular (very small holes), felsic Tuff- fine grained, felsic Scoria – vesicular (larger holes), mafic
Types of volcanic rocks Obsidian: Mafic, Glass, no crystals, instant solid Tuff, pumice: Felsic, Tiny crystals, with holes, quick solidification Basalt: Mafic, tiny crystals, lava solidification Granite, Felsic, large crystals, magma solidification without exposure to air
What do the rocks tell about its history… Basalt, obsidian, pumice, tuff and scoria are examples of extrusive igneous rocks. These rocks were created from the material extruded from an volcanic eruption In general, extrusive rocks have no crystals or very small crystals: short cooling time
Gabbro,and granite are examples of intrusive igneous rocks Formed when magma slowing cools inside the earth before exposed through erosion Formed when magma is given a chance to slowly cool over millions of years, without being exposed to the surface of the earth
Why do intrusive rocks grow such big crystals? Because the rocks in the interior of the Earth are poor conductors of heat, it takes a long time for the hot magma to be cooled. Thus, the intrusive igneous rocks have a coarse- grained texture due to their slow cooling.