Basalt versus granite in a road cut
A road-widening project exposes a 40 m cutting through a dark, fine-grained rock containing scattered 2–4 mm pale phenocrysts and abundant vesicles in its upper 3 m. Below the vesicles the rock is massive with columnar jointing. A second nearby cutting exposes a coarse-grained rock with roughly equal amounts of pink feldspar, white feldspar, quartz and dark mica, cut by 20 cm-wide vertical fractures spaced about 2 m apart. Engineers must choose blasting and slope-stability designs for each cutting. They also note that the dark rock weathers to a rounded, onion-skin surface in the upper zone, while the coarse rock breaks along planar fractures into blocky slabs.
1. Identify both rocks and justify your identification from the described features.
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Dark fine-grained rock with phenocrysts and vesicles = basalt (volcanic, mafic).
- Fine-grained groundmass indicates rapid cooling at or near the surface.
- Phenocrysts show some crystals grew slowly before eruption (two-stage cooling).
- Vesicles are trapped gas bubbles, confirming a near-surface lava flow.
- Columnar jointing forms as the flow cools and contracts uniformly.
- Dark colour and low quartz content indicate mafic composition.
Coarse rock with pink and white feldspar, quartz and dark mica = granite (plutonic, felsic).
- Coarse, interlocking crystals indicate slow cooling at depth.
- Quartz + two feldspars + mica is the classic granite assemblage.
- Pink feldspar (alkali feldspar) and white plagioclase both present.
- Fracture spacing of ~2 m suggests jointing in a competent, brittle rock mass.
Judgement: The identification rests on texture (grain size, vesicles, phenocrysts) plus mineralogy, not colour alone.
2. Explain how the contrasting weathering styles (onion-skin versus blocky slabs) arise from the rock properties and the joint patterns.
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Basalt: The upper vesicular zone is porous and permeable, so water penetrates and weathering proceeds inward from the surface. Because the rock is relatively uniform and fine-grained, weathering shells peel away roughly parallel to the surface - onion-skin (spheroidal) weathering. Columnar joints also guide moisture, but the vesicular top is the weakest zone.
Granite: The rock is massive and has low porosity, so water mainly enters along the widely spaced vertical fractures. Weathering is concentrated on fracture surfaces, producing blocky slabs bounded by joints. The 2 m spacing means large intact blocks remain between fractures.
Engineering implication: The basalt's weathered top is a weak, rounded, low-strength layer that may fail as soil-like debris, while the granite's stability depends on the orientation and persistence of its fractures. The two cuttings therefore need different designs: the basalt needs removal of the weathered cap and drainage control; the granite needs rock-bolt design keyed to fracture orientation.
3. Which cutting poses the greater risk of planar sliding, and what additional information would you request before finalising the design?
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Greater risk: the granite cutting. Its stability is controlled by through-going planar fractures, and a 2 m-spaced set of vertical joints can define discrete blocks that slide if the joints dip out of the slope. The basalt is more massive below the vesicular zone, so failure is more likely to be controlled by the weathered cap than by large planar slides.
Information to request:
- Fracture orientation (dip and dip direction) relative to the cut face - critical for kinematic analysis.
- Fracture persistence, aperture, infill and roughness.
- Rock strength (point-load or UCS tests) for both units.
- Depth and extent of the weathered zone in the basalt.
- Groundwater levels and permeability.
- Presence of any faults or shear zones.
Judgement: Without fracture orientation data, the conclusion is provisional - a joint set parallel to the slope is far more dangerous than one perpendicular to it.