ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers

Question 1

A geologist is mapping a regional metamorphic terrain and identifies two parallel belts of metamorphic rocks of the same age. Belt A is characterized by blueschist and eclogite facies rocks. Belt B, located continent-ward of Belt A, is characterized by amphibolite and granulite facies rocks. What is the most likely tectonic setting that produced this 'paired metamorphic belt'?

Accepted Answer

A convergent plate boundary with subduction

Answer

Paired metamorphic belts are characteristic of convergent plate boundaries. The high-pressure, low-temperature (HP/LT) belt (e.g., blueschist, eclogite facies) forms in the subducting oceanic slab, which is cool but deeply buried. The high-temperature, low-pressure (HT/LP) belt (e.g., amphibolite, granulite facies) forms in the overriding plate, which is heated by magmatic activity associated with the volcanic arc.

Question 2

According to the discontinuous series of Bowen's reaction series, which of the following sequences represents the correct order of mineral crystallization as a magma cools?

Accepted Answer

Olivine, Pyroxene, Amphibole, Biotite

Answer

Bowen's reaction series describes the sequence of mineral crystallization from a cooling magma. The discontinuous series, which involves mafic minerals, begins with Olivine at the highest temperatures, followed by Pyroxene, then Amphibole, and finally Biotite mica at the lowest temperatures in the series.

Question 3

Which of the following silicate mineral groups is characterized by a framework structure of silica tetrahedra, where all four oxygen atoms in each tetrahedron are shared with adjacent tetrahedra?

Accepted Answer

Quartz and Feldspars (Tectosilicates)

Answer

Tectosilicates, or framework silicates, have a structure where every oxygen ion of a silica tetrahedron is shared with another tetrahedron, creating a complex 3D framework. Quartz (SiO2) and the feldspar group are the most common examples of this structure, which results in a silicon-to-oxygen ratio of 1:2.

Question 4

A coarse-grained igneous rock is composed of 40% quartz, 35% plagioclase feldspar, and 25% alkali feldspar. Using the QAPF classification diagram, this rock would be identified as:

Accepted Answer

Granite

Answer

On the QAPF (Quartz, Alkali Feldspar, Plagioclase, Feldspathoid) diagram, the percentages of Q, A, and P are normalized to 100%. In this case, the total is 40+35+25 = 100. A rock with 40% Quartz and a Plagioclase/Alkali Feldspar ratio of 35:25 (or 58% Plagioclase of total feldspars) plots squarely in the Granite field.

Question 5

A geologist is examining a core of clastic sedimentary rock. They note that the loose sand grains from a similar recent deposit have been converted into a solid rock through compaction and the precipitation of calcite between the grains. Which term best describes the overall processes that have occurred after deposition to turn the sediment into rock?

Accepted Answer

Diagenesis

Answer

Diagenesis refers to all the physical, chemical, and biological changes that sediment undergoes after deposition, but before metamorphism. This includes lithification, which is the process of turning sediment into rock through compaction (reduction of pore space) and cementation (precipitation of minerals in pores). Metamorphism occurs at higher pressures and temperatures.

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 Mineralogy and Petrology Questions and Answers