ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers — 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'?
- A continental rift zone
- A transform plate boundary
- A convergent plate boundary with subduction (Correct answer)
- A mid-ocean ridge spreading center
Correct answer: A convergent plate boundary with subduction
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?
- Pyroxene, Olivine, Amphibole, Biotite
- Olivine, Pyroxene, Amphibole, Biotite (Correct answer)
- Biotite, Amphibole, Pyroxene, Olivine
- Amphibole, Pyroxene, Biotite, Olivine
Correct answer: Olivine, Pyroxene, Amphibole, Biotite
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?
- Pyroxenes (Inosilicates)
- Micas (Phyllosilicates)
- Olivines (Nesosilicates)
- Quartz and Feldspars (Tectosilicates) (Correct answer)
Correct answer: Quartz and Feldspars (Tectosilicates)
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:
- Granodiorite
- Tonalite
- Granite (Correct answer)
- Quartz Monzonite
Correct answer: Granite
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?
- Metamorphism
- Diagenesis (Correct answer)
- Weathering
- Erosion
Correct answer: Diagenesis
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.
Question 6: Which of the following physical properties is LEAST reliable for identifying most minerals, especially varieties of quartz?
- Hardness
- Cleavage
- Color (Correct answer)
- Specific Gravity
Correct answer: Color
Color is often the most ambiguous and least reliable physical property for mineral identification. Many minerals, like quartz, can exhibit a wide range of colors due to minor impurities (e.g., amethyst is purple quartz, citrine is yellow quartz). Other properties like hardness (7 for quartz), cleavage (absent in quartz, showing conchoidal fracture), and specific gravity are much more consistent and diagnostic.
ASBOG - Association of State Boards of Geology Mineralogy and Petrology Questions and Answers
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'?