Rock Types and the Rock Cycle

Why Geology Matters in Engineering

Geology defines the ground conditions engineers build upon. Rock type determines:

• Foundation bearing capacity
• Tunneling difficulty
• Slope stability
• Seismic amplification
• Groundwater presence

The Three Rock Types

Igneous Rocks

Formed from solidification of magma or lava.

Intrusive (plutonic): cooled slowly underground → coarse-grained

• Example: Granite, Gabbro, Diorite

Extrusive (volcanic): cooled rapidly at surface → fine-grained

• Example: Basalt, Rhyolite, Obsidian

Engineering: Generally strong, low porosity. Excellent foundation material.

Sedimentary Rocks

Formed from compaction and cementation of sediment.

Clastic: fragments of existing rock

• Sandstone (sand), Shale (clay), Conglomerate (gravel)

Chemical/Organic: precipitation or organic material

• Limestone, Dolomite, Coal

Engineering: Variable strength; limestone may dissolve (karst); shale may swell.

Metamorphic Rocks

Formed when existing rocks are transformed by heat and/or pressure.

Foliated: layered structure

• Slate, Phyllite, Schist, Gneiss

Non-foliated: no layering

• Marble (from limestone), Quartzite (from sandstone)

Engineering: Foliation creates planes of weakness; may affect slope stability.

The Rock Cycle

All rock types can transform into one another:

Igneous → weathering → Sediment → compaction → Sedimentary
Sedimentary → heat/pressure → Metamorphic
Metamorphic → melting → Magma → Igneous

Rock Identification in the Field

• Color (dark = mafic/iron-rich; light = felsic/silica-rich)
• Texture (grain size, foliation)
• Hardness (Mohs scale 1–10)
• Cleavage/Fracture
• Reaction to acid (HCl fizzes on carbonate rocks)

Summary

Igneous rocks are the strongest, sedimentary rocks most variable, metamorphic rocks anisotropic. Always know your rock type before foundation design or tunneling — it drives everything else.