Rocks and Minerals — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Obsidian is a sedimentary rock formed from layers of dark mud at the bottom of the ocean.
Corrected: Obsidian is an igneous rock that forms when lava cools very quickly on Earth's surface, creating a smooth, glassy texture.
Obsidian is actually an igneous rock, not sedimentary. It forms when lava cools so rapidly on the surface that no crystals have time to grow, giving it a smooth, glassy look.
2. Fix the sentence:
Weathering is the process of gluing sediment together to form new rocks.
Corrected: Weathering is the process of breaking rocks into smaller pieces by wind, water, ice, or plant roots. Cementation is the process of gluing sediment together.
Weathering breaks rocks apart, while cementation glues sediment together. The original mixed up these two opposite processes.
3. Fix the sentence:
Slate is an igneous rock that forms when a volcano erupts underwater.
Corrected: Slate is a metamorphic rock that forms when shale is changed by heat and pressure deep inside the Earth.
Slate is metamorphic, not igneous. It forms when the sedimentary rock shale gets squeezed and heated deep underground, changing its structure.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Basalt is an igneous rock that forms when lava cools quickly on Earth's surface.
Basalt forms when lava cools quickly on Earth's surface, so its mineral crystals stay very small compared to rocks like granite that cool slowly underground.
2. Wind, water, and ice break rocks apart through a process called weathering.
Weathering is the natural process where wind, water, and ice slowly break rocks into smaller and smaller pieces over time.
3. Fossils of ancient sea creatures found on a mountaintop tell us the rock was once at the bottom of an ocean.
Sea creature fossils on a mountaintop prove that rock formed underwater in an ocean, then was pushed up by forces inside the Earth over millions of years.
4. Pyrite is called fool's gold because its yellow color looks like real gold.
Pyrite earned the nickname "fool's gold" because its bright yellow, metallic color tricked miners into thinking they had found real gold.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Explain one way a metamorphic rock can become a sedimentary rock.
Sample answer: A metamorphic rock can be pushed to Earth's surface where weathering and erosion break it into small pieces. These pieces, called sediment, are carried by water and deposited in layers that are pressed and cemented into sedimentary rock over time.
In the rock cycle, a metamorphic rock must first be broken down by weathering and erosion into sediment. That sediment then settles in layers and gets pressed and cemented together to become sedimentary rock.
2. Why can two minerals that look the same color still be different minerals?
Sample answer: Color alone is not reliable for identifying minerals because many different minerals share the same color. Scientists also test hardness, streak, and luster to tell minerals apart.
Many different minerals can share the same color, so color alone is unreliable. Scientists use additional tests like hardness, streak, and luster to tell look-alike minerals apart.