Rocks and Minerals — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. Obsidian is an igneous rock that forms when lava cools quickly.
Obsidian forms from rapid cooling of lava, resulting in a smooth, glassy texture with no visible crystals.
2. Sandstone is a sedimentary rock made mostly of grains of sand.
Sandstone forms when sand grains are compacted and cemented together over long periods.
3. Marble is a metamorphic rock formed from limestone.
Limestone (sedimentary, made of calcium carbonate) is transformed into marble under heat and pressure.
4. Fossils are most commonly found in sedimentary rocks.
Fossils form when organisms are buried in sediment that later becomes sedimentary rock. Heat/pressure of other rock types destroys fossils.
5. The Mohs scale ranks minerals from 1 to 10 by their hardness.
The Mohs Hardness Scale ranks minerals from 1 (softest, talc) to 10 (hardest, diamond).
6. Talc has a hardness of 1 and diamond has a hardness of 10 on the Mohs scale.
Diamond is the hardest natural substance, rating 10 on the Mohs scale. Talc (soapstone) is the softest at 1.
7. Pumice is a light igneous rock that floats on water because it is full of tiny holes.
Pumice forms when lava cools rapidly with trapped gas bubbles, leaving tiny pores (holes) that make it very lightweight.
8. Slate is a metamorphic rock formed from the sedimentary rock shale.
Shale (sedimentary, formed from clay and silt) becomes slate under low-grade metamorphic conditions.
9. Conglomerate is a sedimentary rock made of rounded pebbles cemented together.
Conglomerate contains rounded pebbles, gravel, or other coarse fragments cemented in a finer matrix.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
igneous
→ formed from cooled magma or lava
single natural substance with a crystal structure
sedimentary
→ formed from compressed layers of sediment
formed from cooled magma or lava
metamorphic
→ changed by heat and pressure
changed by heat and pressure
mineral
→ single natural substance with a crystal structure
formed from compressed layers of sediment
Igneous → cooled magma/lava; sedimentary → compressed layers; metamorphic → changed by heat/pressure; mineral → single natural substance.