Rocks and minerals

Rocks are made of minerals, and the minerals largely decide how a rock behaves. Quartz is hard and resists weathering; feldspars alter to clay; mica forms weak flat planes; calcite dissolves slowly in acidic water.

Geologists group rocks by how they formed. Igneous rocks such as granite and basalt cooled from molten material; they are usually strong, though basalt flows can contain joints and weathered layers between flows, as in the Deccan Traps. Sedimentary rocks such as sandstone, shale and limestone settled in layers; their bedding planes are natural planes of weakness, and limestone can hide solution cavities. Metamorphic rocks such as gneiss, schist, quartzite and marble were changed by heat and pressure; schists split easily along their foliation.

For a civil engineer the questions are practical. How strong is the rock, and how does that strength change with weathering? Which way do the layers and joints dip relative to a cut slope or a tunnel? Will water pass through or dissolve it? Is it suitable as aggregate?

The answers come from mapping, boreholes and lab tests, and they decide dam sites, tunnel alignments, foundation levels and quarry choices.

Even within one rock type, behaviour changes with weathering grade. Fresh granite may be excellent, while completely weathered granite behaves like soil. Engineers therefore describe both the rock type and its weathering state on every borehole log.