Rock textures, density and porosity
Reading a rock's history from its texture; cooling rate and crystal size; the sedimentary pathway; regional and contact metamorphism; and measuring density and porosity of real rock samples with uncertainties.
Read
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.[5]
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.[5]
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).[4][1]
The sedimentary pathway: weathering (physical, chemical, biological) breaks rock; carbon dioxide dissolved in rain makes it slightly acidic, so it reacts with minerals. Erosion transports the fragments, deposition lays them down, then compaction by overlying sediment and cementation by minerals crystallising from fluids in the pore spaces turn sediment into rock.[3][4]
Metamorphism changes rock without melting it entirely. Regional metamorphism comes from high pressure over a wide area; contact metamorphism happens when rock is 'baked' by nearby magma. Limestone becomes marble; shale can become slate.[4][1]
Density is ρ = m / V. A rock's bulk volume includes its pores, so its bulk density is lower than the density of its grains; with zero porosity they would be equal. Mean dry bulk densities are about 2.66 g/cm³ for granite, 2.74 g/cm³ for basalt and 2.22 g/cm³ for sandstone.[6]
Porosity is the fraction of a rock's volume that is pore space. Granite has low porosity; sandstone ranges from low to more than 30%; basalt can contain many gas-bubble holes. Over millions of years, plate tectonics and surface processes move rock through the whole cycle.[6][5][2]
Try it: Density and porosity of rock samples
You need
- 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.
What to do
- 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.
What happens, and why?
Rocks with large, well-connected pore spaces such as many sandstones soak up much more water and have a lower bulk density than tightly crystalline rocks such as granite. Your values will differ from book values because real samples vary and the soaking method only fills the pores water can reach.[6]
Play: Read the evidence
Which rock type does each piece of evidence point to?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11