How Do We Classify How Do We Classify Rocks?Rocks?

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What do Scientists Look What do Scientists Look For?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

TextureTexture

• The The looklook and and feelfeel of the rock’s of the rock’s surfacesurface– Smooth and glassySmooth and glassy– Rough or chalkyRough or chalky

• Grains – particles of minerals or other Grains – particles of minerals or other rocksrocks

• Grains give the rock its textureGrains give the rock its texture

Grain SizeGrain Size

• Coarse grains – large grains, easy to see

• Fine grains – tiny, can only be seen with a microscope

Slate – fine grains Diorite – 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 PatternGrain 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 GrainNo 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 CompositionMineral 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 RocksClassifying 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 descriptionof your rock sample. Describe the followingthings:• 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.