Rocks and Minerals — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Pumice is a very light rock that can float on water. How does it form?
A) Sediment compresses slowly on the ocean floor
B) Lava cools quickly trapping gas bubbles inside
C) Limestone is squeezed by intense pressure
D) Minerals crystallize deep underground over time
Pumice forms from frothy, gas-rich lava that cools so rapidly the gas bubbles are trapped, creating a highly porous, lightweight rock.
2. Which sequence correctly shows one path through the rock cycle?
A) Sedimentary → melting → metamorphic → cooling → igneous
B) Igneous → weathering and erosion → sedimentary → heat and pressure → metamorphic
C) Metamorphic → compaction → sedimentary → weathering → igneous
D) Sedimentary → cooling → igneous → erosion → metamorphic
Igneous rocks can be weathered into sediment → compacted into sedimentary rock → transformed by heat/pressure into metamorphic rock.
3. A geologist tests a mineral and finds it scratches glass but cannot scratch quartz. What is its hardness on the Mohs scale?
A) between 1 and 3
B) between 3 and 5
C) between 5 and 7
D) between 7 and 10
Glass hardness ≈ 5.5; quartz hardness = 7. A mineral that scratches glass (>5.5) but not quartz (<7) has hardness between 5 and 7.
4. Why is marble preferred over limestone for sculptures?
A) Marble is softer and easier to shape
B) Marble has a finer texture and takes a better polish
C) Limestone contains too many fossils
D) Marble is lighter in weight
The metamorphic process recrystallizes limestone into marble, giving it a finer grain and uniform texture that holds detail and polishes beautifully.
Part B: Fill in the Blank
Write the correct answer on each line.
1. Igneous rock that forms on the surface from lava is called extrusive.
Extrusive (volcanic) igneous rocks form at or near the surface when lava cools quickly. Examples: basalt, obsidian, pumice.
2. Igneous rock that forms underground from magma is called intrusive.
Intrusive (plutonic) igneous rocks form deep underground when magma cools slowly, producing large crystals. Example: granite.
3. Coal is a sedimentary rock formed from the remains of ancient plants.
Coal forms over millions of years from compressed and heated remains of ancient plants, primarily in swampy environments.
4. The process of sediment being pressed together by weight above is called compaction.
Compaction is the first step in lithification — the weight of overlying sediment presses lower layers together, squeezing out water and air.
5. Diamonds form deep in Earth's mantle under extreme heat and pressure.
Diamonds form at depths of 100+ miles where extreme temperature and pressure transform carbon atoms into the crystal structure of diamond.