How Do We Classify Rocks? p

What do Scientists Look For? When studying a rock sample, geologists observe the rock’s: Color Texture Mineral Composition Then they are able to CLASSIFY it!

Color Color can provide some clues As with minerals, it does not provide enough information Rocks can be similar colors but be completely different kinds of rocks

Texture The look and feel of the rock’s surfaceThe look and feel of the rock’s surface –Smooth and glassy –Rough or chalky Grains – particles of minerals or other rocksGrains – particles of minerals or other rocks Grains give the rock its textureGrains give the rock its texture

Grain Size Coarse grains – large grains, easy to see Fine grains – tiny, can only be seen with a microscope Slate – fine grainsDiorite – coarse grains

Grain Shape Lots of different kinds of shapes –Tiny particles of fine sand –Small seeds –Tiny stars Grain shape could result from shapes of crystals that form the rock Could result from fragments of other rocks –Smooth and rounded –Jagged and sharp

Breccia – jagged grains Conglomerate – rounded grains

Grain Pattern Grains in rocks lie in certain patterns –Stack of pancakes –Wavy, swirling patterns –Rows of multicolored beads –Random patterns of grains throughout rocks marble gneiss schist

No Visible Grain Some rocks have no visible grains, even when viewed under a microscope Crystals grains cool very quickly Rocks look smooth and shiny, like glass obsidian flint chert

Mineral Composition Scientists look more closely at rocks to see the mineral composition They cut the rock very thin so light can shine through the crystals

Classifying Rocks After scientists gather information about the rock, they classify it: –Igneous rocks –Metamorphic rocks –Sedimentary rocks

Look At Your Samples Again… Use your new knowledge to write a description of your rock sample. Describe the following things: Color(s) of the rock Grain size (coarse or fine?) Grain shape (jagged or rounded?) Grain pattern (stripes, waves, bead shapes, stars etc.) Write your descriptions on the index card and turn it in as you leave.