CLASSIFYING ROCKS

What characteristics do geologists use to

identify rocks?The rock of Earth’s crust forms mountains, hills, valleys, beaches, and the ocean floor.

When studying a rock sample, geologists observe the rock’s mineral composition, color, and texture.

Rocks What are rocks?

Rocks are combinations of minerals, rock fragments, and other material stuck together.

Minerals (Rice Krispy's) Rocks (Rice Krispy's Treats)

Think of minerals as ingredients…you have different minerals, you have different rocks.

Rocks There are many different types of

rocks… All rocks are classified based on what

they are made from and how they are made. Like potatoes: mashed, baked, scalloped,

etc. Each come from a plain potato, but were

made differently.

Rocks are made of mixtures of minerals and other materials, although some rocks may contain only a single mineral.

1.Mineral Composition – The type of minerals that compose the rock.

• There is a list of 20 common minerals that form most rocks…these minerals are called rock forming minerals.

What do we examine when looking at rocks?

• Geologists also observe the shape and color of crystals in a rock to identify its minerals.

• They may perform other tests, such as testing the surface of a rock with acid, to determine the presence of certain compounds.

2. Color - A rock’s color provides clues to the rock’s mineral composition. • Granite is generally a light colored

rock that has high silica content. • Basalt is a dark colored rock that is

low in silica.

3. Texture - A rock’s texture, the look and feel of the rock’s surface, is very useful in identifying a rock. • Most rocks are made up of grains,

particles of minerals or other rocks.

• Grains give the rock its texture.Rough

Smooth

Grain SizeThe size of the grain indicates how smooth or rough the rock will be.• Fine Grain – Feel small

crystals.• Coarse Grain – Feel rough• No Grain – Smooth, like

glass.

Grain ShapeThe grain shape can vary

widely in rocks, but can indicate clues to its formation.• Particles of Sand• Small Seeds / Exploding

Stars• Shapes of mineral crystals• Rock Fragments

Grain PatternPatterns can often by seen in rocks

• Flat horizontal layers• Swirling patterns• Banded colors• Random throughout

Just like there are different types of potatoes, there are different types of rocks.

The 3 main types of rocks are:1.Igneous: Made from cooled lava or magma.2.Sedimentary: When sediment is squeezed

together.3.Metamorphic: Rocks change under high

heat and pressure.

IGNEOUS ROCK

INTRUSIVE

SEDIMENTARY ROCK

CLASTIC

CHEMICAL

EXTRUSIVE

ORGANIC

METAMORPHIC ROCK

FOLIATED NON-FOLIATED

ROCK TYPES

INSIDE EARTH

OUTSIDE EARTH

Mineral Bands NO BANDS

Compaction /

Cementation

Chemical Reactions

From Living Organisms