Faults Folds and Joints Questions and Answers

Answer: A

Correct answer: fault movement

Slickensides are the polished, striated surfaces produced when two blocks grind past each other along a fault plane, and the striations record the direction of the last movement. They are direct evidence of faulting, and their orientation is used to determine the slip direction. Glacial abrasion produces striations too, but on an exposed land surface rather than on a fracture plane within the rock.

Answer: B

Correct answer: finely ground, clay-like material

Gouge and breccia are both products of crushing along a fault, differing in grain size. Gouge is ground to a clay-like paste and is often impermeable, so it can seal a fault; breccia is coarse angular fragments and is usually highly permeable, so it conducts water. Which of the two occupies a fault matters greatly to a tunnel or a dam foundation crossing it.

Answer: C

Correct answer: a thrust or reverse fault

A reverse or thrust fault stacks the hanging wall on top of the foot wall, so a vertical borehole passes down through a bed, crosses the fault, and meets the same bed again in the block beneath. Repetition is therefore the signature of shortening. Omission of part of the sequence is the signature of the opposite, a normal fault, and the pair of tests is the standard way of identifying a fault at depth.

Answer: D

Correct answer: a normal fault

A normal fault drops the hanging wall, so beds slide past one another and part of the sequence is cut out where the hole crosses the plane. The amount missing gives a minimum estimate of the throw. The converse test applies to reverse and thrust faults, which repeat beds rather than omitting them.

Answer: A

Correct answer: no appreciable displacement

A joint is a fracture across which the rock has parted without the two sides sliding relative to each other. It may be open, closed or mineral-filled, and mineral growth in it does not make it a fault; only displacement does. Joints matter engineering because they divide the rock mass into blocks and provide the paths along which water flows and failure occurs.

Answer: B

Correct answer: contraction on cooling

A cooling lava flow or sill contracts, and the tension that develops is relieved by fractures that propagate inward from the cooling surface. Where cooling is uniform the fractures organise themselves into a polygonal, commonly hexagonal, pattern perpendicular to the isotherms, giving columns normal to the cooling surface - as at the Giant’s Causeway and Devils Tower. It is a primary igneous feature, not a tectonic one.

Answer: C

Correct answer: release of confining pressure as overburden is removed

Rock formed at depth is under great confining pressure, and as erosion removes the overburden it expands upward. The tension that results opens fractures parallel to the ground surface, closely spaced near the top and further apart with depth. The effect is most obvious in granite, where it produces the onion-like sheets of exfoliation domes, and it is why a quarry floor may heave after excavation.

Answer: D

Correct answer: parallel to one another

A joint set is a family of parallel or nearly parallel joints of common origin, and two or more sets together make a joint system. It is the sets that matter to an engineer: their orientations, spacings and persistence determine the size and shape of the blocks the rock mass is divided into, and therefore how it will behave in a slope or a tunnel.

Answer: A

Correct answer: intact pieces longer than 100 mm

Rock quality designation is

measured along the core axis. The 100 mm threshold is arbitrary but standard, and the index is a simple measure of how closely fractured the rock mass is. It ignores everything else - joint orientation, roughness, infill - which is why it is only one input to the fuller rock mass classifications.

Answer: B

It measures the spacing and continuity of the discontinuities rather than the strength of the intact material between them. A strong rock cut by close joints scores badly and behaves badly, which is the point of the index.