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ASBOG FG PG practice questions

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Question 1General Geology and Earth Materials

Which mineral has a Mohs hardness of 7, so that it scratches orthoclase but is itself scratched by corundum?

  1. Topaz
  2. Apatite
  3. Calcite
  4. Quartz Correct answer.

Explanation

The Mohs scale runs talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10. Quartz therefore scratches orthoclase but is scratched by topaz and corundum.

Question 2General Geology and Earth Materials

At which type of plate boundary does new oceanic lithosphere form?

  1. A continental collision zone
  2. A convergent boundary with subduction
  3. A divergent mid-ocean ridge boundary Correct answer.
  4. A transform boundary with strike-slip motion

Explanation

Divergent boundaries are where plates move apart and mantle upwelling fills the gap, generating new basaltic crust that cools and thickens into lithosphere. Convergent and transform boundaries consume or slide past lithosphere rather than create it.

Question 3General Geology and Earth Materials

Which common sedimentary rock consists dominantly of calcium carbonate and effervesces vigorously in dilute hydrochloric acid?

  1. Shale
  2. Quartz sandstone
  3. Chert
  4. Limestone Correct answer.

Explanation

Limestone is composed mainly of calcite or aragonite, which reacts with acid to release carbon dioxide and produce visible fizzing. Sandstone and chert are silica-dominated, and shale is dominated by clay minerals, so none of these reacts strongly.

Question 4General Geology and Earth Materials

In an undeformed sequence of sedimentary strata, the principle of superposition states that:

  1. Each bed is younger than the one directly below it Correct answer.
  2. Fossils in every layer must belong to the same species
  3. The oldest layers are always found at the top of the sequence
  4. All layers were originally deposited as horizontal beds

Explanation

Superposition holds that in an undisturbed succession, beds were deposited in order from bottom to top, so age decreases upward. The idea that beds were originally horizontal is the principle of original horizontality, a separate principle.

Question 5General Geology and Earth Materials

The seismic boundary separating Earth's crust from the mantle is called the:

  1. Lehmann discontinuity
  2. Conrad discontinuity
  3. Gutenberg discontinuity
  4. Moho discontinuity Correct answer.

Explanation

The crust-mantle boundary is the Mohorovičić (Moho) discontinuity, marked by an abrupt jump in P-wave velocity at roughly 5 to 70 km depth. The Gutenberg marks the core-mantle boundary and the Lehmann lies within the core.

Question 6General Geology and Earth Materials

Which of the following is an example of chemical weathering?

  1. Root wedging along a bedrock fracture
  2. Abrasion of clasts in a river channel
  3. Frost wedging of granite along joints
  4. Dissolution of limestone by acid Correct answer.

Explanation

Chemical weathering involves mineral breakdown by reaction with water, acids, or oxygen; acid dissolution of calcite is a direct chemical reaction. Frost wedging, abrasion, and root wedging are all physical processes that break rock without changing its mineralogy.

Question 7General Geology and Earth Materials

A fault in which the hanging wall has moved down relative to the footwall is called a:

  1. A thrust fault
  2. A strike-slip fault
  3. A normal fault Correct answer.
  4. A reverse fault

Explanation

Normal faults form under extension, with the hanging wall dropping along a steeply dipping surface. Reverse and thrust faults show the opposite sense of slip and form under compression.

Question 8General Geology and Earth Materials

Which magma composition most commonly produces highly explosive eruptions?

  1. Komatiitic
  2. Rhyolitic Correct answer.
  3. Andesitic
  4. Basaltic

Explanation

High-silica rhyolitic magma is polymerized, viscous, and volatile-rich, so gas cannot escape easily and pressure builds until it fragments the magma explosively. Basaltic and komatiitic magmas are fluid and typically erupt effusively.

Question 9General Geology and Earth Materials

According to Bowen's reaction series, which mineral crystallizes first from a cooling basaltic magma?

  1. Olivine Correct answer.
  2. Quartz
  3. Biotite
  4. Muscovite

Explanation

Bowen's series places olivine and calcium-rich plagioclase at the highest-temperature end, crystallizing first from a mafic melt. Quartz and muscovite appear at the low-temperature end, late in crystallization or in more felsic melts.

Question 10General Geology and Earth Materials

Which metamorphic facies, diagnostic of subduction of basaltic crust at high pressure and low temperature, contains garnet and omphacitic pyroxene?

  1. Eclogite Correct answer.
  2. Hornblende hornfels
  3. Greenschist
  4. Zeolite

Explanation

Eclogite is the high-pressure, low-thermal-gradient assemblage of garnet plus omphacite formed from mafic rock in subduction zones. Hornfels, greenschist, and zeolite facies form at much lower pressures.

Question 11General Geology and Earth Materials

In a meandering stream, where is erosion concentrated?

  1. The channel floor at the thalweg
  2. The cut bank on the outer bank Correct answer.
  3. The point bar on the inner bank
  4. The floodplain behind the levee

Explanation

Flow is fastest and deepest on the outside of a meander bend, so the outer (cut) bank is eroded while sediment is deposited on the inner point bar. This lateral migration is what allows meanders to grow and migrate.

Question 12General Geology and Earth Materials

A mineral separate contains 25% of the parent isotope and 75% of its daughter product, and the parent half-life is 100 million years. Assuming no initial daughter, the age is:

  1. 200 Ma Correct answer.
  2. 400 Ma
  3. 100 Ma
  4. 25 Ma

Explanation

A remaining parent fraction of 25% corresponds to two half-lives, because each half-life halves the parent (100% to 50% to 25%). Two half-lives of 100 Ma each give an age of 200 Ma.

Question 13General Geology and Earth Materials

A sandstone composed of well-rounded, well-sorted quartz grains with almost no clay matrix is best described as:

  1. Neither texturally nor compositionally mature
  2. Texturally and compositionally mature Correct answer.
  3. Compositionally immature but texturally mature
  4. Texturally immature but compositionally mature

Explanation

Textural maturity reflects good sorting, rounding, and little fine matrix, while compositional maturity reflects loss of unstable grains so that quartz dominates. This rock satisfies both criteria.

Question 14General Geology and Earth Materials

In an anticline, the oldest exposed rocks occur:

  1. On the limbs of the fold
  2. In the hinge of the adjacent syncline
  3. Along the fold axis at depth only
  4. In the core of the fold Correct answer.

Explanation

Anticlines are convex-upward folds, so erosion exposes progressively older stratigraphic units toward the fold core. Synclines show the reverse pattern, with the youngest rocks in the core.

Question 15General Geology and Earth Materials

Which fossil group is the most useful index fossil for worldwide correlation of marine Jurassic strata?

  1. Stromatolites
  2. Ammonites Correct answer.
  3. Trilobites
  4. Rugose corals

Explanation

Ammonites were rapidly evolving, widely dispersed free-swimming cephalopods with a Jurassic record divided into fine, recognizable zones. Trilobites had largely declined by the Jurassic, and corals and stromatolites are long-ranging.

Question 16General Geology and Earth Materials

S-waves cannot pass through the outer core because:

  1. The outer core has a density too low to support shear
  2. The outer core is too hot to transmit seismic energy
  3. The outer core is liquid and lacks shear strength Correct answer.
  4. The outer core is crystalline and reflects shear waves

Explanation

Shear waves require a material with rigidity, and liquids have zero shear strength, so S-waves cannot propagate through the liquid outer core. This produces the S-wave shadow zone beyond about 103 degrees from the epicenter.

Question 17General Geology and Earth Materials

Porphyry copper deposits are characteristically associated with:

  1. Felsic to intermediate intrusions at arcs Correct answer.
  2. Evaporite basins in arid continental interiors
  3. Carbonatite volcanoes on ancient cratons
  4. Layered mafic intrusions in stable cratons

Explanation

Porphyry systems form above shallow, water-rich granodioritic to quartz monzonitic intrusions in magmatic arcs, where hydrothermal fluids fracture the roof rocks and deposit sulfides. Layered mafic intrusions, evaporites, and carbonatites host different deposit types.

Question 18General Geology and Earth Materials

Which soil horizon is typically the zone of accumulation of clay, iron oxides, and other materials leached from overlying horizons?

  1. The O horizon
  2. The A horizon
  3. The B horizon Correct answer.
  4. The C horizon

Explanation

The B horizon is the subsoil zone of illuviation, where clays, iron and aluminum oxides, and carbonates accumulate after being leached downward. The A horizon is the zone of eluviation and organic enrichment.

Question 19General Geology and Earth Materials

The pattern of magnetic stripes on the seafloor is symmetrical about mid-ocean ridges because:

  1. New crust records reversals as it spreads symmetrically Correct answer.
  2. Compacted sediment records the field direction
  3. Field intensity varies systematically with latitude
  4. Seawater uniformly magnetizes the oceanic crust

Explanation

As basalt cools below the Curie temperature at the ridge, it acquires a remanent magnetization matching the field at that time; continued spreading carries the normally and reversely magnetized strips away on both sides. This produces the mirror-image anomaly pattern that supported seafloor spreading.

Question 20General Geology and Earth Materials

Which landform is diagnostic of continental glaciation?

  1. Alluvial fans at range fronts
  2. V-shaped stream valleys
  3. Point bars on floodplains
  4. U-shaped valleys with truncated spurs Correct answer.

Explanation

Glaciers erode broad U-shaped troughs and behead or truncate the interfluves (spurs) of pre-existing valleys. V-shaped valleys, point bars, and alluvial fans are produced by running water.

Question 21General Geology and Earth Materials

In the structural classification of silicates, single-chain minerals such as pyroxenes have a silicon-to-oxygen ratio of:

  1. 1:3 Correct answer.
  2. 1:4
  3. 2:7
  4. 1:2

Explanation

Pyroxenes have the formula unit Si2O6, giving Si:O of 1:3 because each tetrahedron shares two of its four oxygens with neighbors along the chain. Framework silicates approach 1:2 and isolated tetrahedra give 1:4.

Question 22General Geology and Earth Materials

A bed strikes east-west and dips 30 degrees to the south. In a vertical cross-section oriented N45E, the apparent dip is closest to:

  1. 30 degrees
  2. 45 degrees
  3. 15 degrees
  4. 22 degrees Correct answer.

Explanation

Apparent dip follows tan(apparent dip) = tan(true dip) x sin(beta), where beta is the angle between the section line and strike. Here beta is 45 degrees, so tan(30) x sin(45) = 0.577 x 0.707 = 0.408, giving about 22 degrees.

Question 23General Geology and Earth Materials

A granitic rock is strongly enriched in large-ion lithophile elements such as Rb, Ba, and K, strongly depleted in Nb and Ta, and shows a negative Eu anomaly. Which interpretation best fits these data?

  1. Arc magma with plagioclase fractionation Correct answer.
  2. A mantle plume with no crystal fractionation
  3. A mid-ocean-ridge source with no crustal input
  4. An ultramafic magma formed by closed-system melting

Explanation

Enrichment in large-ion lithophile elements with negative Nb-Ta anomalies is the classic subduction-related (arc) signature, reflecting fluids from the descending slab. A negative Eu anomaly requires removal of plagioclase, in which Eu2+ is compatible, so fractionation is also indicated.

Question 24General Geology and Earth Materials

A sandstone plots in the recycled orogen field of a Qt-F-L provenance diagram, with abundant quartz and sedimentary lithic fragments. This most likely indicates derivation from:

  1. A continental interior craton source
  2. A magmatic-arc source
  3. An oceanic island-arc source
  4. An uplifted fold-thrust belt of sedimentary rocks Correct answer.

Explanation

The recycled orogen field reflects quartz-rich detritus shed from uplifted and deformed sedimentary or metasedimentary successions, typically in fold-thrust belts. Craton sources plot toward the quartz pole with few lithics, and arc sources are feldspar- and volcanic-lithic-rich.

Question 25General Geology and Earth Materials

Two minerals in the same rock give different K-Ar ages, with hornblende older than biotite. The best explanation is that:

  1. The rock cooled almost instantaneously
  2. Biotite incorporated excess argon
  3. Hornblende has a higher closure temperature Correct answer.
  4. Hornblende lost argon during weathering

Explanation

K-Ar and Ar-Ar ages record cooling through a mineral-specific closure temperature, and hornblende retains argon at higher temperature than biotite. Slow cooling therefore yields an older hornblende age and a younger biotite age from the same rock.

Question 26Mineralogy, Petrology, and Geochemistry

Which mineral has a hardness of 9 on the Mohs scale?

  1. Talc
  2. Corundum Correct answer.
  3. Calcite
  4. Quartz

Explanation

The Mohs scale ranks minerals by relative scratch hardness, with talc at 1 and diamond at 10. Corundum defines hardness 9, while quartz is 7 and calcite is 3.

Question 27Mineralogy, Petrology, and Geochemistry

Olivine belongs to which silicate structural class?

  1. Isolated tetrahedra Correct answer.
  2. Double-chain silicate
  3. Sheet silicate
  4. Framework silicate

Explanation

Olivine is a nesosilicate, consisting of independent SiO4 tetrahedra linked by Mg and Fe cations. Framework silicates include quartz and feldspar, double-chain silicates include amphibole, and sheet silicates include mica and clay.

Question 28Mineralogy, Petrology, and Geochemistry

Which mineral typically exhibits perfect basal cleavage?

  1. Quartz
  2. Muscovite Correct answer.
  3. Pyroxene
  4. Calcite

Explanation

Muscovite is a sheet silicate whose weak interlayer bonds cause it to split into thin flexible sheets parallel to the basal plane. Quartz lacks cleavage, calcite has rhombohedral cleavage, and pyroxene has two cleavages near 90 degrees.

Question 29Mineralogy, Petrology, and Geochemistry

According to Bowen's reaction series, which mineral crystallizes first from a cooling mafic magma?

  1. Olivine Correct answer.
  2. Muscovite
  3. Biotite
  4. Quartz

Explanation

The discontinuous branch of Bowen's reaction series begins with olivine at the highest temperature and progresses through pyroxene, amphibole, and biotite. Quartz and muscovite crystallize at much lower temperatures.

Question 30Mineralogy, Petrology, and Geochemistry

Which clastic sedimentary rock consists dominantly of rounded pebble-size clasts?

  1. Fossiliferous limestone
  2. Conglomerate Correct answer.
  3. Calcareous shale
  4. Quartz arenite

Explanation

Conglomerate is defined by rounded gravel-size clasts, whereas breccia is similar but has angular clasts. Quartz arenite is a sandstone, and limestone and shale are not gravel-dominated clastic rocks.

Question 31Mineralogy, Petrology, and Geochemistry

Which metamorphic rock forms primarily from the metamorphism of limestone?

  1. Schist
  2. Marble Correct answer.
  3. Quartzite
  4. Slate

Explanation

Marble is the metamorphic product of limestone or dolostone and is composed mainly of recrystallized calcite or dolomite. Quartzite forms from quartz sandstone, whereas slate and schist typically form from shale or mudstone.

Question 32Mineralogy, Petrology, and Geochemistry

Which mineral group is most abundant in Earth's continental crust?

  1. Oxides
  2. Feldspars Correct answer.
  3. Carbonates
  4. Sulfides

Explanation

Feldspars make up roughly half of continental crustal volume, and silicates as a whole dominate crustal mineralogy. Carbonates, sulfides, and oxides are important but much less abundant in the bulk crust.

Question 33Mineralogy, Petrology, and Geochemistry

Which statement best defines a mineral?

  1. A synthetic crystalline compound
  2. A natural crystalline inorganic solid Correct answer.
  3. An organic solid with variable composition
  4. Any solid material found in rocks

Explanation

A mineral is naturally occurring, inorganic, and crystalline, with a definite chemical composition that may vary within limits through solid solution. Synthetic compounds, organic materials, and noncrystalline solids do not satisfy the definition.

Question 34Mineralogy, Petrology, and Geochemistry

An igneous rock with an aphanitic texture most likely formed by which process?

  1. Rapid cooling at or near the surface Correct answer.
  2. Complete remelting of country rock
  3. Solid-state recrystallization
  4. Very slow cooling at depth

Explanation

Aphanitic textures are fine-grained because many small crystals nucleate during rapid cooling, commonly in volcanic or shallow intrusive settings. Slow cooling at depth produces coarse-grained phaneritic textures instead.

Question 35Mineralogy, Petrology, and Geochemistry

Blueschist-facies metamorphism is most characteristic of which setting?

  1. High-pressure, high-temperature lower crust
  2. High-pressure, low-temperature subduction zone Correct answer.
  3. Low-pressure, high-temperature contact aureole
  4. Medium-pressure, medium-temperature regional setting

Explanation

Blueschist facies is marked by glaucophane and lawsonite, which require high pressure and low temperature, conditions found along subduction zones. Contact metamorphism produces hornblende hornfels or sanidinite facies at low pressure and high temperature.

Question 36Mineralogy, Petrology, and Geochemistry

Which element is most incompatible during partial melting of the mantle?

  1. Ni
  2. Mg
  3. Cr
  4. Rb Correct answer.

Explanation

Rb is a large-ion lithophile element that preferentially enters the melt, so it is highly incompatible in mantle minerals. Ni, Cr, and Mg are compatible and concentrate in olivine, pyroxene, and spinel in the residue.

Question 37Mineralogy, Petrology, and Geochemistry

In foraminiferal calcite, a higher delta-18O value generally indicates which condition?

  1. Cooler water or more ice Correct answer.
  2. Lower seawater pH from dissolved CO2
  3. Higher salinity with no temperature change
  4. Warmer seawater at constant ice volume

Explanation

Oxygen isotope fractionation between calcite and seawater is temperature dependent, with colder water producing higher delta-18O in calcite. Growth of continental ice sheets also removes light oxygen from the ocean, raising seawater and carbonate delta-18O.

Question 38Mineralogy, Petrology, and Geochemistry

A rock with a high Rb/Sr ratio will develop what Sr isotope composition over time?

  1. High 87Rb/86Sr only after melting
  2. No change in Sr isotopes
  3. Low 87Sr/86Sr over time
  4. High 87Sr/86Sr over time Correct answer.

Explanation

Rubidium-87 decays to strontium-87, so a high parent-to-daughter ratio produces greater radiogenic 87Sr ingrowth with time. This is the basis for Rb-Sr isochron dating and for using initial Sr ratios as petrogenetic tracers.

Question 39Mineralogy, Petrology, and Geochemistry

A positive europium anomaly in an igneous rock most likely indicates what process?

  1. Removal of plagioclase by fractionation
  2. Derivation from a garnet-rich source
  3. Olivine accumulation
  4. Plagioclase accumulation Correct answer.

Explanation

Eu can occur as Eu2+ and substitute for Ca2+ in plagioclase, so plagioclase-rich cumulates develop positive Eu anomalies. Fractional crystallization that removes plagioclase leaves the residual melt with a negative Eu anomaly.

Question 40Mineralogy, Petrology, and Geochemistry

At the eutectic point of a binary system at fixed pressure, which situation applies?

  1. Complete solid solution between phases
  2. Two solids plus liquid coexist Correct answer.
  3. No liquid is present
  4. Only one phase is stable

Explanation

The eutectic is the lowest-temperature melt point in a binary system, where two solid phases and liquid coexist at a fixed temperature and melt composition for a given pressure. The phase rule gives zero degrees of freedom at that point when pressure is fixed.

Question 41Mineralogy, Petrology, and Geochemistry

What is the coordination number of silicon in the silicate tetrahedron?

  1. 3
  2. 2
  3. 4 Correct answer.
  4. 6

Explanation

Each silicon atom in a silicate tetrahedron is bonded to four oxygen atoms, giving a coordination number of 4. This SiO4 tetrahedron is the fundamental building block of all silicate minerals.

Question 42Mineralogy, Petrology, and Geochemistry

Which clay mineral generally has the highest cation exchange capacity?

  1. Kaolinite
  2. Chlorite
  3. Smectite Correct answer.
  4. Illite

Explanation

Smectite has a high layer charge and an expandable interlayer that adsorbs exchangeable cations, giving it a high cation exchange capacity. Kaolinite has little substitution and low capacity, whereas illite and chlorite are intermediate or lower.

Question 43Mineralogy, Petrology, and Geochemistry

Hydrolysis of potassium feldspar during weathering mainly produces which materials?

  1. Clay minerals and ions Correct answer.
  2. Pyrite and hematite
  3. Dolomite and gypsum
  4. Calcite and quartz

Explanation

Hydrolysis breaks down feldspar by reaction with water and carbonic acid, producing clay minerals plus dissolved potassium, silica, and other ions. This process is a major control on soil clay formation and natural water chemistry.

Question 44Mineralogy, Petrology, and Geochemistry

A porphyritic igneous texture most directly indicates what cooling history?

  1. Shock metamorphism
  2. Slow cooling throughout
  3. Complete melting
  4. Two-stage cooling history Correct answer.

Explanation

Porphyritic texture records early slow growth of large phenocrysts followed by more rapid cooling that forms a finer-grained groundmass. This two-stage history is common in magmas that ascend and crystallize at shallower levels.

Question 45Mineralogy, Petrology, and Geochemistry

Which mineral is a typical index mineral for amphibolite-facies pelitic rocks?

  1. Garnet Correct answer.
  2. Lawsonite
  3. Zeolite
  4. Chlorite

Explanation

In pelitic rocks, garnet appears with increasing metamorphic grade and is characteristic of amphibolite-facies assemblages along with staurolite, kyanite, or sillimanite. Chlorite indicates greenschist facies, lawsonite indicates blueschist facies, and zeolites indicate very low-grade conditions.

Question 46Mineralogy, Petrology, and Geochemistry

Which sulfide mineral is the principal ore of lead?

  1. Magnetite
  2. Sphalerite
  3. Chalcopyrite
  4. Galena Correct answer.

Explanation

Galena has the formula PbS and is the main lead ore mineral, commonly associated with sphalerite in hydrothermal deposits. Sphalerite is the main zinc ore, and chalcopyrite is the main copper ore.

Question 47Mineralogy, Petrology, and Geochemistry

In optical mineralogy, pleochroism refers to which behavior?

  1. Simple twinning
  2. Color change during stage rotation Correct answer.
  3. Relief change during rotation
  4. Extinction at 90 degrees

Explanation

Pleochroism is the change in color or absorption when a mineral is rotated under plane-polarized light, reflecting anisotropic absorption in different crystallographic directions. It is common in minerals such as biotite, hornblende, and tourmaline.

Question 48Mineralogy, Petrology, and Geochemistry

A magma containing about 65 wt% SiO2 is best classified as what composition?

  1. Felsic Correct answer.
  2. Mafic
  3. Intermediate
  4. Ultramafic

Explanation

Igneous compositions are commonly divided by silica content: ultramafic below about 45 wt%, mafic at 45-52 wt%, intermediate at 52-63 wt%, and felsic above about 63 wt%. A value near 65 wt% SiO2 therefore falls in the felsic field.

Question 49Mineralogy, Petrology, and Geochemistry

A trace-element partition coefficient greater than 1 means what?

  1. The element prefers solid Correct answer.
  2. The element is volatile
  3. The element prefers melt
  4. The element is radioactive

Explanation

The partition coefficient is the concentration in the solid divided by the concentration in the melt. A value greater than 1 indicates compatible behavior, meaning the element preferentially enters the solid phase.

Question 50Mineralogy, Petrology, and Geochemistry

Which fine-grained, foliated metamorphic rock forms from low-grade metamorphism of shale?

  1. Hornfels
  2. Quartzite
  3. Slate Correct answer.
  4. Marble

Explanation

Slate is a low-grade foliated metamorphic rock derived mainly from shale and characterized by slaty cleavage. Marble and quartzite are nonfoliated metamorphic rocks from limestone and quartz sandstone, respectively.