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Download classifying rocks - St. Michael Catholic School
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CLASSIFYING ROCKS 2.2 HOW DO GEOLOGISTS CLASSIFY ROCKS? • To study a rock sample, geologists observe the rock’s mineral composition, color, and texture. • Rocks are made of mixtures of minerals and other materials. MINERAL COMPOSITION • Some rocks contain only 1 mineral • Others contain several minerals • Ex: Granite = quartz, feldspar, mica, and hornblende • About 20 minerals make up Earth’s crust • Known as rock-forming minerals MINERAL COLOR • Rock’s color = clues to the rock’s mineral composition • Granite: a light-colored rock that has high silica content • Rich in the elements silicon and oxygen • Basalt: a dark-colored rock that has a lower silica content than granite • Has mineral crystals that are too small to be seen with the naked eye • Color alone is NOT enough info to identify a rock! TEXTURE • Most rocks are made of particles of minerals and other rocks = grains • Grains give the rock its texture • Texture: the look and feel of a rock’s surface • Based on size, shape, and patter of the grains GRAIN SIZE • Large and easy to see grains = coarse grained • Small and require a microscope = fine grained • Examples: • Fine = slate • Coarse = diorite • No visible grain = flint GRAIN SHAPE • Can result from the shape of the mineral crystals that form the rock • Can result from rounded or jagged bits of several rocks • Rounded Grain = conglomerate • Jagged Grain = Breccia GRAIN PATTERN • Banded rocks = grains lie in a pattern of flat layers or can form swirls or colored bands • Nonbanded rocks = grains that do not lie in any visible pattern • Banded = gneiss • Nonbanded = quartzite ORIGIN • Use color, texture, and mineral composition to classify rocks • 3 major groups: • Igneous rock • Sedimentary rock • Metamorphic rock 3 TYPES • Igneous Rocks: forms from the cooling of magma or lava • Magma is underground, lava is above ground • Sedimentary Rocks: forms when small particles of rocks or the remains of plants/animals are pressed and cemented together • Forms in layers buried below the surface 3 TYPES – CONTINUED • Metamorphic Rocks: forms when a rock is changed by heat or pressure or by chemical reactions • Most forms deep under ground (heat/pressure)
