Rocks and Minerals
Grade 4 · Science · Study Guide
Explore the three types of rock and learn what minerals are and how scientists identify them.
Rocks are made of one or more minerals. A mineral is a naturally occurring, non-living solid with a definite chemical makeup. There are over 4,000 known minerals, but only a few dozen are very common. Quartz, feldspar, and mica are three of the most common minerals found in rocks.
Geologists classify rocks into three types based on how they form. Igneous rocks form when magma (melted rock underground) or lava (magma that reaches the surface) cools and hardens. Granite and basalt are igneous rocks. Sedimentary rocks form when layers of sediment (sand, mud, shell fragments) are pressed together over time. Sandstone, limestone, and shale are sedimentary rocks — and fossils are found in these! Metamorphic rocks form when heat and pressure deep underground change existing rocks into new ones. Marble and slate are metamorphic rocks.
Scientists identify minerals using several properties. Hardness is how resistant a mineral is to being scratched — measured on the Mohs scale from 1 (softest, talc) to 10 (hardest, diamond). Luster describes how a mineral reflects light — metallic or non-metallic. Streak is the color of the powder a mineral leaves when scratched across a white tile.
| Rock Type | How It Forms | Examples |
|---|---|---|
| Igneous | Cooled magma or lava | Granite, basalt, obsidian |
| Sedimentary | Compressed layers of sediment | Sandstone, limestone, shale |
| Metamorphic | Heat and pressure on existing rock | Marble, slate, quartzite |
Key terms
- Mineral
- A naturally occurring, non-living solid with a definite chemical composition
- Igneous rock
- Rock formed from cooled and hardened magma or lava
- Sedimentary rock
- Rock formed from layers of sediment pressed together over time
- Metamorphic rock
- Rock formed when heat and pressure change existing rock
- Hardness
- A mineral property measuring how resistant it is to scratching
- Mohs scale
- A scale from 1 to 10 used to measure the hardness of minerals
Flashcards
- Igneous rock
- Forms when magma or lava cools and hardens
- Sedimentary rock
- Forms from compressed layers of sediment — fossils found here
- Metamorphic rock
- Forms when heat and pressure change existing rock
- Mineral
- A naturally occurring solid with a specific chemical makeup
- Mohs scale
- Scale 1-10 measuring mineral hardness; diamond is 10
- Luster
- How a mineral reflects light — metallic or non-metallic
Practice questions
Which type of rock forms when lava cools and hardens?
- Sedimentary
- Metamorphic
- Igneous
- Mineral
Show answer
Answer: Igneous
Igneous rocks form when magma (underground) or lava (at the surface) cools and hardens. Granite and basalt are examples.
Where are fossils most likely to be found?
- Igneous rocks
- Sedimentary rocks
- Metamorphic rocks
- Crystals
Show answer
Answer: Sedimentary rocks
Fossils are found in sedimentary rocks because they form in layers. Dead organisms get buried in sediment and preserved over time.
All rocks are made of one or more minerals.
Show answer
Answer: True
All rocks are made of one or more minerals. Granite contains quartz, feldspar, and mica.
Metamorphic rocks form when lava cools on Earth's surface.
Show answer
Answer: False
Metamorphic rocks form deep underground when heat and pressure change existing rocks. Marble forms from limestone under heat and pressure.
The scale from 1 to 10 used to measure the hardness of minerals is called the ___ scale.
Show answer
Answer: Mohs
The Mohs hardness scale rates minerals from 1 (softest — talc) to 10 (hardest — diamond).
Fun facts
- Diamonds are the hardest natural mineral on Earth — rated 10 on the Mohs scale!
- Obsidian is a volcanic glass formed when lava cools so quickly crystals don't have time to form.
- The White House in Washington D.C. is built from sandstone — a sedimentary rock.
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