Geologists classify rocks based on their mineral composition, color, and texture. When studying a rock sample, they observe which minerals it contains, its color, and properties like grain size and shape. There are three main types of rocks: igneous rocks formed from cooled lava or magma, either intrusively underground or extruded onto the surface; sedimentary rocks formed by compaction or cementation of sediments or chemical/organic processes; and metamorphic rocks formed from changes to existing rocks through heat and pressure.

1. CLASSIFYI
NG ROCKS

2. 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.

3. 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.

4. 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.

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

6. 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?

7. • 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.

8. 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.

9. 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

10. The 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.

11. The 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

12. Patterns can often by seen in rocks
• Flat horizontal layers
• Swirling patterns
• Banded colors
• Random throughout

13. 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.

14. 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