Geological Structures Questions and Answers

Answer: A

Correct answer: three points of known position and elevation on it

Three non-collinear points define a plane, so if the position and height of a bedding surface are known at three places - typically from boreholes - both the strike and the dip follow by construction. It is the standard means of getting attitude where nothing is exposed at the surface.

Answer: B

Correct answer: elevation on a particular bedding surface

Structural contours describe the shape of a buried surface exactly as topographic contours describe the ground: closely spaced where the bed is steep, widely spaced where it is gentle, and closing round a dome or basin. They are the working tool for predicting the depth to a bed at any point.

Answer: C

Correct answer: bed thickness

Isopachytes map the thickness of a formation rather than its depth, and their pattern shows how the depositional basin behaved - where subsidence was greatest, where the formation thins onto a rising area, and where it dies out altogether. They answer a quite different question from structural contours.

Answer: D

Correct answer: the direction of dip and its amount

The symbol is a short arrow drawn in the direction of true dip with the angle written beside it, and the line at right angles across it gives the strike. So a single symbol conveys the complete attitude of the bed at that point.

Answer: A

Correct answer: 40 m

\[ w = \frac{t}{\sin\delta} = \frac{20}{\sin 30^\circ} = \frac{20}{0.5} = 40\ \mathrm{m}. \] The gentler the dip, the wider the outcrop of a bed of given thickness - and a horizontal bed of any thickness would in principle crop out over the whole area at its level, which is why gently dipping strata dominate a map so disproportionately.

Answer: B

Correct answer: 34 m

\[ t = w\sin\delta = 100 \times \sin 20^\circ = 100 \times 0.342 = 34.2\ \mathrm{m}. \] This is the inverse of the previous calculation and the more useful one in the field, where the outcrop width can be paced out and the dip measured but the thickness cannot be seen directly.

Answer: C

Correct answer: 100 m

\[ d = x\tan\delta = 100 \times \tan 45^\circ = 100\ \mathrm{m}. \] A dip of 45 degrees is the case in which the depth equals the horizontal distance, which makes it a convenient check on the arithmetic. Note the distance must be measured in the dip direction; along strike the bed never gets any deeper.

Answer: D

Correct answer: 115 m

\[ d = 200 \times \tan 30^\circ = 200 \times 0.577 = 115.5\ \mathrm{m}. \] Confusing \( \tan \) with \( \sin \) here would give 100 m, which is the standard error: the depth below a point is a tangent relation, while the perpendicular thickness of a bed is a sine one.

Answer: A

True thickness equals the vertical interval times the cosine of the dip, so the cosine is 15 over 30, that is 0.5, and the dip is 60 degrees. A vertical borehole always overstates the thickness of a dipping bed, and steeply the more so.

Answer: B

Correct answer: less than 40 degrees

Any section oblique to the strike shows an apparent dip smaller than the true one, so it must be under 40 degrees. Specifically \( \tan\delta_{app} = \tan 40^\circ \cos 45^\circ = 0.594 \), giving about 30.7 degrees - not 20, since the relation works on the tangents rather than on the angles themselves.