Rocks and Minerals — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. Basalt is the most common igneous rock on Earth's surface.
Basalt is an igneous rock formed from cooled lava. It covers most of the ocean floor and is the most widespread igneous rock on Earth's surface.
2. Gneiss is a metamorphic rock that has light and dark bands of minerals.
Heat and pressure sort gneiss's minerals into alternating light and dark bands, giving it a striped appearance that helps scientists identify it.
3. Sedimentary rocks form in layers called strata.
Strata are the visible horizontal layers in sedimentary rock. Each layer was deposited at a different time, so the oldest strata are on the bottom.
4. When dissolved minerals fill in spaces between sediment grains, the process is called cementation.
Cementation happens when dissolved minerals seep between loose sediment grains and act like glue, binding them together into solid sedimentary rock.
5. A fingernail has a hardness of about 2.5 on the Mohs scale.
A fingernail rates about 2.5 on the Mohs scale, which means it can scratch talc (1) and gypsum (2) but cannot scratch calcite (3) or anything harder.
6. Mica is a mineral that splits into thin, flat sheets.
Mica has perfect cleavage in one direction, so it peels apart into thin, flexible sheets. This property makes it easy to identify.
7. Fossils in sedimentary rock layers help scientists figure out the age of the rock.
Scientists compare fossils found in rock layers to organisms from known time periods. This lets them estimate when the sedimentary rock formed.
8. When any type of rock melts completely, it becomes magma.
No matter what type of rock it started as, once it melts deep underground it becomes magma. If that magma cools again, it will form a brand-new igneous rock.
9. A copper penny can scratch a mineral with a hardness less than 3.
Copper has a Mohs hardness of about 3, so a penny can scratch any mineral softer than 3 but will not leave a mark on harder minerals like fluorite (4).
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each rock to its correct type.
granite
→ igneous rock with large crystals from slow cooling
igneous rock with small crystals from fast cooling
limestone
→ sedimentary rock made from shells and marine remains
sedimentary rock made from shells and marine remains
marble
→ metamorphic rock formed from heated limestone
igneous rock with large crystals from slow cooling
basalt
→ igneous rock with small crystals from fast cooling
metamorphic rock formed from heated limestone
Correct matches: granite → igneous rock with large crystals from slow cooling; limestone → sedimentary rock made from shells and marine remains; marble → metamorphic rock formed from heated limestone; basalt → igneous rock with small crystals from fast cooling.