Teacher sheet · Ages 15–16 · about 45–50 minutes
Rock textures, density and porosity
Lesson page: echoesof.science/school/rocks/ages-15-16/ · Pupil worksheet: echoesof.science/school/rocks/ages-15-16/print/
Learning aims
- Classify rocks using texture as evidence of their formation.
- Relate cooling rate to crystal size and texture.
- Describe the sedimentary pathway and the two main kinds of metamorphism.
- Measure density and porosity, and estimate uncertainty.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: hand lenses on granite, basalt and sandstone. What is the evidence of how each formed? |
| 10 min | Read the text; worked example of ρ = m/V and of porosity. |
| 20 min | Density measurements (porosity readings after overnight soaking, or with pre-soaked samples). |
| 5 min | Play Read the evidence. |
| 5 min | Quiz and discussion of uncertainties. |
Materials for the experiment
- 3 dry, fist-sized-or-smaller rock samples (for example granite, sandstone, limestone or slate)
- Electronic balance (0.1 g)
- Measuring cylinder (100 or 250 cm³) or a displacement can
- Water and a beaker for soaking
- Paper towels
- Thread to lower the samples
Safety: Use samples that are small enough to handle and lower them gently on a thread so the cylinder does not crack. Wipe up spills. Do not break rocks with hammers without eye protection and teacher supervision.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| All rocks of one name have the same density. | Density varies with mineral content and porosity; book values are averages or ranges. |
| Metamorphic rocks form from melted rock. | Metamorphism happens without melting entirely; melting leads to igneous rock. |
| A rock with holes must be sedimentary. | Vesicles in basalt are gas-bubble holes in an igneous rock. |
Worksheet answers
- ρ = m_dry ÷ V; % uncertainty ≈ (uncertainty in V ÷ V) × 100% plus that in m; the sample with the largest (m_wet − m_dry) per cm³ is most porous, usually the sandstone.
- Sandstone is made of grains held by cement, with pore spaces between them (porosity can be over 30%); granite is interlocking crystals with few pores, so more of its volume is solid mineral.
- Granite cooled slowly underground, so large crystals grew; basalt cooled fast from lava, so its crystals are small (or glassy); the cooling history, not just the chemistry, controls texture.
- A body of magma inside older rock, with a surrounding rim labelled 'contact metamorphism (baked, not melted)'.
Quiz answers
- A sample has a mass of 53.2 g and a volume of 20.0 cm³. What is its bulk density, and which rock does it most resemble? 2.66 g/cm³, like granite. ρ = 53.2 ÷ 20.0 = 2.66 g/cm³, the mean value for granite.[6]
- A 20.0 cm³ sample takes up 6.0 g of water when soaked. What is its porosity? 30%. Pore volume 6.0 cm³ ÷ 20.0 cm³ × 100% = 30%.[6]
- Why is a rock's bulk density lower than the density of its grains? The bulk volume includes empty pore space. Pores add volume but almost no mass.[6]
- Which texture shows that lava cooled exceptionally fast? Glassy. Very rapid cooling leaves no time for crystals: glass forms.[4]
- A rock has grains separated by cement (a clastic texture). Which type is it most likely to be? Sedimentary. Clastic texture is typical of sedimentary rocks.[5]
- What causes contact metamorphism? Heat from nearby magma. Rock next to hot magma is 'baked' without melting.[4][1]
Going further
Calculate the mass of a 1 m³ block of granite and of sandstone using the mean densities, and discuss why builders and engineers care about porosity (for example frost damage, as water freezes in pores and cracks).
Sources used in this lesson
- [1] Rocks and minerals (British Geological Survey (BGS)): reliability High
- [2] Geological processes (British Geological Survey (BGS)): reliability High
- [3] Weathering (British Geological Survey (BGS)): reliability High
- [4] The Rock Cycle (Physical Geography and Natural Disasters, section 3.5) (R. Adam Dastrup, open textbook (Salt Lake Community College Pressbooks)): reliability Medium-high
- [5] Chapter 5: The Rock Cycle (Investigating the Earth: Exercises for Physical Geology) (D. Hauptvogel, V. Sisson and M. Comas, open lab textbook (University of Houston Libraries Pressbooks)): reliability Medium-high
- [6] Rock: physical properties (Encyclopaedia Britannica): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/rocks/sources/
Privacy: the lesson pages have no ads, no tracking and no cookies. Quiz answers and game scores stay in the pupil's browser. A short video for each age is coming.