Rocks, Minerals & the Rock Cycle — Science

  1. Grade 9 — Minerals & Their Properties
  2. Grade 9 — The Rock Cycle
  3. Grade 9 — The Three Rock Types
  4. Grade 9 — Weathering, Erosion & Deposition

Apply it in a new setting

Problem 1

In the field, a geologist tests an unknown mineral. It scratches a copper penny (hardness ≈ 3.5) but is itself scratched by a steel knife blade (hardness ≈ 5.5). What can she conclude about its Mohs hardness?
  1. It is less than 3.5, since it touched the penny
  2. It is between about 3.5 and 5.5
  3. It is greater than 5.5, since a knife was involved
  4. It must be exactly 7 (quartz)
Hints

The rule: a mineral scratches anything softer than itself and is scratched by anything harder.

If it scratches the penny (3.5), how does its hardness compare to 3.5?

If the knife (5.5) scratches it, how does its hardness compare to 5.5?

Answer and explanation

It is between about 3.5 and 5.5

It scratches the penny (3.5), so the mineral is harder than 3.5.

The knife (5.5) scratches it, so the mineral is softer than 5.5.

Its hardness therefore lies between about 3.5 and 5.5 — B.

Problem 2

Two mineral samples both look shiny and silvery-gray. On an unglazed porcelain plate, Sample A leaves a reddish-brown streak and Sample B leaves a gray-black streak. What is the best conclusion?
  1. They are the same mineral, since they look identical
  2. They are different minerals, because streak is reliable and theirs differ
  3. Nothing can be concluded, because color is the only valid test
  4. They are the same mineral, because both left a streak
Hints

Color and outer luster can be misleading — surfaces tarnish and impurities vary.

Streak (the color of the powdered mineral) stays true even when the surface is stained.

If two reliable streak colors differ, what does that say about the minerals?

Answer and explanation

They are different minerals, because streak is reliable and theirs differ

Outer color and luster are unreliable — different minerals can look alike, and tarnish can fool you.

Streak is far more reliable because a mineral's powder color doesn't change with surface staining.

Different streak colors (reddish-brown vs gray-black) mean different minerals — B.

Problem 3

A layered rock full of fossil shells (a sedimentary rock) is buried very deep, where intense heat and pressure act on it — but it never gets hot enough to melt. What does it most likely become?
  1. Magma, then a new igneous rock
  2. A metamorphic rock, changed by heat and pressure without melting
  3. Loose sediment again
  4. It cannot change unless it melts first
Hints

Metamorphism requires heat and/or pressure — does it require melting?

The rock is squeezed and heated but stays below its melting point.

Melting completely would instead make magma (and then igneous rock).

Answer and explanation

A metamorphic rock, changed by heat and pressure without melting

Metamorphism changes a rock using heat and/or pressure, without melting it.

Since this rock is heated and squeezed but never melts, it transforms directly into metamorphic rock (for example, limestone → marble).

Only if it melted completely would it become magma and then igneous rock — so the answer is B.

Problem 4

Two igneous rocks came from the same magma. Rock 1 has large, easily visible interlocking crystals; Rock 2 is fine-grained, its crystals too small to see. What best explains the difference?
  1. Rock 1 cooled slowly deep underground; Rock 2 cooled quickly at or near the surface
  2. Rock 1 cooled quickly at the surface; Rock 2 cooled slowly underground
  3. Rock 1 is sedimentary and Rock 2 is metamorphic
  4. Crystal size is random and tells you nothing about cooling
Hints

Crystal size in igneous rock records how fast the melt cooled.

Slow cooling gives atoms time to build large crystals; fast cooling freezes tiny ones.

Where does magma cool slowly — deep underground or at the surface?

Answer and explanation

Rock 1 cooled slowly deep underground; Rock 2 cooled quickly at or near the surface

In igneous rock, slow cooling lets crystals grow large, while fast cooling produces tiny or no crystals.

Large crystals (Rock 1) mean slow cooling deep underground (intrusive); fine grains (Rock 2) mean fast cooling at or near the surface (extrusive).

That matches choice A.

Problem 5

Two identical marble statues are placed outdoors: one in a hot, rainy tropical city, the other in a cold, dry desert. After 100 years, which will likely show more chemical weathering, and why?
  1. The desert statue, because dry air is more corrosive
  2. The tropical statue, because warmth and abundant water speed up chemical weathering
  3. Both exactly the same, since climate doesn't affect weathering
  4. Neither, because marble cannot weather chemically
Hints

Chemical weathering involves water and reactions like acids dissolving minerals.

Do warm, wet conditions speed those reactions up or slow them down?

Marble (from limestone) is dissolved by slightly acidic rainwater.

Answer and explanation

The tropical statue, because warmth and abundant water speed up chemical weathering

Chemical weathering (like slightly acidic rain dissolving marble) speeds up with heat and water.

The hot, rainy tropical site provides both, while the cold, dry desert slows the reactions.

So the tropical statue weathers more — B.

Problem 6

Water expands by about 9% when it freezes into ice. A crack in a rock is filled with 200 mL of water. When that water freezes, about what volume of ice does it become?
Hints

Freezing adds about 9% to the water's volume — that expansion is what pries cracks apart (frost wedging).

New volume = original + 9% of the original.

Compute 200 + 0.09 × 200.

Answer and explanation

218 mL

Freezing expands the water by 9% of its volume.

Extra volume = 0.09 × 200 mL = 18 mL.

New volume = 200 + 18 = 218 mL (this expansion is what drives frost wedging).

Problem 7

A lake-bottom sedimentary deposit shows 250 thin annual layers (varves) that together took 12,500 years to accumulate. On average, how many years does each layer represent?
Hints

The total time is shared evenly among all the layers.

Years per layer = total years ÷ number of layers.

Divide 12,500 by 250.

Answer and explanation

50 years/layer

Divide the total time by the number of layers: 12,500 years ÷ 250 layers.

12,500 ÷ 250 = 50.

Each layer represents about 50 years.

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