The Three Rock Types — Drills · Grade 9 Science

Chapter: Rocks, Landscapes & Water

What these drills covers

By the end of The Three Rock Types you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: igneous rock, sedimentary rock, metamorphic rock, foliation.

Problem 1

Which rock type forms when magma cools slowly deep underground, producing large, easy-to-see crystals?
  1. Extrusive igneous rock
  2. Intrusive igneous rock
  3. Sedimentary rock
  4. Foliated metamorphic rock
Answer and explanation

Intrusive igneous rock

Slow cooling underground gives atoms time to form large crystals — that's intrusive (plutonic) igneous rock, like granite.

Problem 2

Which term describes a planar metamorphic fabric, such as aligned minerals or compositional bands?
  1. Foliated
  2. Non-foliated
  3. Extrusive
  4. Clastic
Answer and explanation

Foliated

Foliation describes a planar fabric, not only visible bands.

Problem 3

A rock formation has 40 visible sedimentary layers that took an estimated 8,000 years to accumulate. On average, how many years does each layer represent?
Answer and explanation

200 years/layer

8,000 ÷ 40 = 200 years per layer.

Problem 4

A rock sample shows wavy, colorful banding, no fossils, and no randomly scattered crystals. What rock type is this most likely?
  1. Igneous
  2. Sedimentary
  3. Foliated metamorphic
  4. Non-foliated metamorphic
Answer and explanation

Foliated metamorphic

Wavy banding (foliation) is the signature of foliated metamorphic rock, like gneiss or schist.

Problem 5

Explain why obsidian (volcanic glass) has essentially no visible crystals at all, even though it's an igneous rock.
Answer and explanation

Obsidian forms when lava cools extremely fast at Earth's surface, so its atoms have almost no time to arrange into ordered crystal structures at all.

Problem 6

A different rock formation has 250 visible layers and formed over about 100,000 years. On average, how many years does each layer represent?
Answer and explanation

400 years/layer

100,000 ÷ 250 = 400 years per layer.

Browse all courses