echoesof.science · Ages 15–16
Rock textures, density and porosity
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Rocks are classified by how they formed. Igneous rocks form when magma or lava cools and solidifies; sedimentary rocks when pieces of weathered and eroded rock are cemented together; metamorphic rocks when existing rocks are altered by heat and/or pressure.
Texture records that history. Igneous and metamorphic rocks are crystalline, with interlocking minerals; sedimentary rocks are mainly clastic, with grains separated by cement. Igneous crystals are randomly oriented, while most metamorphic rocks have grains with a preferred orientation.
Cooling rate controls crystal size. Intrusive rocks such as granite cool slowly underground and grow large crystals; extrusive rocks cool much faster and have small crystals. Lava that cools exceptionally fast can form glass, and escaping gas leaves holes (a vesicular texture).
Experiment: Density and porosity of rock samples
- Measure the mass of each dry sample, m_dry.
- Find its volume V by displacement: lower it on a thread into a measuring cylinder partly filled with water, and read the rise in level. Read quickly, before much water soaks in.
- Calculate the bulk density ρ = m_dry / V in g/cm³, and compare with typical values (granite 2.66, sandstone 2.22 g/cm³).
- Soak the samples in water overnight. Pat the surface dry and measure the wet mass, m_wet.
- Pore volume ≈ (m_wet − m_dry) ÷ 1.00 g/cm³. Porosity = pore volume ÷ V × 100%.
- Estimate the uncertainty in V from the scale (for example ±1 cm³) and work out the percentage uncertainty in your density.
| Sample | m_dry (g) | V (cm³) | ρ (g/cm³) | m_wet (g) | Porosity (%) |
|---|---|---|---|---|---|
| Sample 1 | |||||
| Sample 2 | |||||
| Sample 3 |
Questions
Use your results to calculate the density of each sample with a percentage uncertainty. Which sample is most porous?
Explain why sandstone usually has a lower bulk density and a higher porosity than granite.
Explain, using cooling rate, why granite and basalt can contain similar elements but look so different.
Draw a labelled cross-section of a magma intrusion, showing the zone of contact metamorphism around it.
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Sources for every fact on this sheet: echoesof.science/school/rocks/sources/