ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers

Question 1

On a geologic map, a series of parallel ridges and valleys are observed. Rock units on the map show a repeating sequence of older rocks at the core of the ridges and younger rocks in the valleys. The beds on both sides of the ridges dip away from the ridge crests. What structural feature does this pattern represent?

Accepted Answer

A series of plunging anticlines

Answer

This map pattern is characteristic of a series of plunging anticlines. Anticlines are up-folds where the oldest rocks are in the core, and the limbs dip away from the axial plane. When these folds plunge and are eroded, they create a V-shaped outcrop pattern on the map where the 'V' points in the direction of plunge. The ridges represent the more resistant rock layers within the anticlines.

Question 2

According to Anderson's theory of faulting, which stress regime is responsible for the formation of a normal fault?

Accepted Answer

The maximum principal stress (σ1) is vertical and the minimum principal stress (σ3) is horizontal.

Answer

Anderson's theory classifies fault types based on the orientation of the three principal stresses (σ1 > σ2 > σ3). For normal faults, which accommodate crustal extension, the maximum principal stress (σ1) is vertical (due to gravity), and the minimum principal stress (σ3) is horizontal, allowing the crust to stretch and pull apart.

Question 3

A structural geologist plots the poles to bedding from numerous outcrops across a folded region onto a stereonet. The poles form a great-circle girdle. What does the pole to this great-circle girdle represent?

Accepted Answer

The trend and plunge of the fold axis

Answer

For a cylindrical fold, the poles to the bedding planes measured along the fold lie on a great circle (a pi-circle or girdle). The pole to this great circle represents the orientation (trend and plunge) of the fold axis itself.

Question 4

A geologist is working in a continental rift zone, such as the East African Rift. Which of the following combinations of structural and volcanic features would they most likely encounter?

Accepted Answer

Normal faults, horsts and grabens, and bimodal (basalt-rhyolite) volcanism.

Answer

Continental rift zones are characterized by extensional tectonics. This leads to the formation of normal faults, which create down-dropped blocks (grabens) and uplifted blocks (horsts). The thinning crust allows for magma to ascend, often resulting in bimodal volcanism, with both mafic (basalt) and felsic (rhyolite) lavas erupting.

Question 5

Which of the following conditions would most favor ductile deformation of a rock over brittle failure?

Accepted Answer

High temperature, high confining pressure, and slow strain rate.

Answer

Ductile deformation, or flowing/bending of rock, is favored by conditions found deeper in the crust. These include high temperatures, which allow minerals to recrystallize and flow; high confining pressure, which inhibits the formation of fractures; and slow strain rates, which provide time for the material to deform plastically rather than break.

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Practice Test

ASBOG - Association of State Boards of Geology Structural Geology and Tectonics Questions and Answers