Earth Science Quiz: Metamorphic Rock Classification

What this quiz covers

This quiz focuses on Metamorphic Rock Classification, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. X: gneiss with distinct banding; Y: phyllite with a silky sheen; Z: schist with visible mica flakes
2. X: phyllite with a silky sheen; Y: schist with visible mica flakes; Z: gneiss with distinct banding (correct answer)
3. X: schist with visible mica flakes; Y: gneiss with distinct banding; Z: phyllite with a silky sheen
4. X: phyllite with a silky sheen; Y: gneiss with distinct banding; Z: schist with visible mica flakes

Explanation: The standard prograde metamorphic sequence for a shale protolith is slate -> phyllite -> schist -> gneiss. Given that sample W is slate and the grade increases from W to Z, sample X must be phyllite (characterized by a silky sheen from microscopic micas), sample Y must be schist (characterized by visible, aligned mica flakes), and sample Z must be gneiss (characterized by compositional banding).

Question 2

1. The metamorphic grade decreases towards the east, away from the core of a metamorphic belt.
2. A large, shallow igneous intrusion is located to the west, causing contact metamorphism that decreases in intensity eastward.
3. The protolith changed from a clay-rich rock in the west to a sand-rich rock in the east, causing the textural differences.
4. The metamorphic grade increases towards the east, towards the core of a metamorphic belt. (correct answer)

Explanation: The sequence of rocks observed—slate to phyllite to schist—is a classic prograde metamorphic sequence, meaning it represents a progressive increase in metamorphic grade. The appearance of garnet in the schist further confirms this, as garnet is an index mineral for medium-grade metamorphism. This spatial progression suggests the student is walking from the lower-grade outer edge of a regional metamorphic belt toward its higher-grade core.

Question 3

1. X: gneiss with distinct banding; Y: phyllite with a silky sheen; Z: schist with visible mica flakes
2. X: phyllite with a silky sheen; Y: schist with visible mica flakes; Z: gneiss with distinct banding (correct answer)
3. X: schist with visible mica flakes; Y: gneiss with distinct banding; Z: phyllite with a silky sheen
4. X: phyllite with a silky sheen; Y: gneiss with distinct banding; Z: schist with visible mica flakes

Explanation: The standard prograde metamorphic sequence for a shale protolith is slate -> phyllite -> schist -> gneiss. Given that sample W is slate and the grade increases from W to Z, sample X must be phyllite (characterized by a silky sheen from microscopic micas), sample Y must be schist (characterized by visible, aligned mica flakes), and sample Z must be gneiss (characterized by compositional banding).

Question 4

1. Schist; the protolith was likely a mudstone or shale. (correct answer)
2. Gneiss; the protolith was likely a granite or a sandstone.
3. Phyllite; the protolith was likely a volcanic ash deposit.
4. Marble; the protolith was likely a fossiliferous limestone.

Explanation: When identifying metamorphic rocks, you need to examine three key characteristics: grain size, texture/fabric, and mineral composition. This question tests your ability to distinguish between foliated metamorphic rocks based on these features. The rock described has coarse grains with visible mica crystals aligned parallel to each other, creating a foliated texture. The presence of mica, quartz, and feldspar in distinct layers (micas separate from granular quartz-feldspar lenses) without strong compositional banding points to a schist. Schists are characterized by medium to coarse grain size, strong foliation from aligned platy minerals like mica, and typically form from clay-rich sedimentary rocks under moderate metamorphic conditions. Choice A is correct because schist matches all the described features, and mudstone/shale contains the clay minerals that transform into mica during metamorphism, providing the necessary composition. Choice B is wrong because gneiss, while coarse-grained and containing similar minerals, displays distinct compositional banding of alternating light and dark layers, which this rock specifically lacks. Choice C is incorrect because phyllite has a much finer grain size where individual mica crystals aren't visible to the naked eye, contrary to the "visible" micas described. Choice D is wrong because marble forms from carbonate rocks and consists primarily of calcite or dolomite, not the mica-quartz-feldspar assemblage described. Remember: grain size and the visibility of foliation minerals are crucial distinguishing features between schist (coarse, visible micas) and phyllite (fine, microscopic micas), while banding presence separates schist from gneiss.

Question 5

1. The gneiss formed under lower temperatures and pressures, preserving fine-grained textures.
2. The gneiss formed under higher temperatures and pressures, causing extensive mineral recrystallization and segregation. (correct answer)
3. The gneiss formed through rapid heating during contact metamorphism, while the slate formed during slow regional metamorphism.
4. The gneiss formed from a more quartz-rich protolith, while the slate formed from a more clay-rich protolith.

Explanation: Gneiss is a high-grade metamorphic rock, while slate is a low-grade metamorphic rock. To transform a shale into a gneiss requires significantly higher temperatures and pressures than those needed to form slate. These high-grade conditions lead to extensive recrystallization, the growth of larger mineral grains, and the segregation of minerals into the characteristic gneissic bands.

Question 6

1. conglomerate, because it contains fragments of two different rock types.
2. breccia, because it consists of angular clasts cemented together.
3. porphyry, because it contains large crystals within a finer matrix.
4. migmatite, because it represents a rock that has undergone partial melting. (correct answer)

Explanation: When you encounter a rock description that mentions both metamorphic features and granite-like characteristics formed under extreme conditions, you're likely dealing with high-grade metamorphism where temperatures approached the melting point. The correct answer is D because migmatite forms when metamorphic rocks undergo partial melting at extremely high temperatures (typically 650-900°C). During this process, some minerals melt and recrystallize into granite-like material (the lighter zones called "leucosome"), while other more resistant minerals remain as dark, foliated metamorphic rock (the "melanosome"). This creates the distinctive banded appearance of alternating light and dark zones described in the question. Answer A is incorrect because conglomerates are sedimentary rocks composed of rounded clasts cemented together, not intergrown metamorphic and igneous-looking materials. Answer B is wrong because breccias are also sedimentary rocks, but with angular fragments rather than rounded ones - neither describes the intimate mixing of metamorphic and granite-like zones. Answer C misidentifies the texture because porphyry refers to igneous rocks with large crystals (phenocrysts) in a fine-grained matrix, not the zone-like separation described here. Remember that migmatites represent the extreme end of metamorphism - they're essentially "hybrid" rocks caught between metamorphic and igneous processes. When you see descriptions of high-grade metamorphic rocks mixed with granite-like material in distinct zones or bands, think migmatite. This rock type is key evidence that crustal rocks can partially melt without becoming fully igneous.

Question 7

1. schistosity, indicating medium-grade metamorphism.
2. gneissic banding, indicating high-grade metamorphism. (correct answer)
3. slaty cleavage, indicating low-grade metamorphism.
4. porphyroblastic texture, indicating complex mineral growth.

Explanation: The segregation of light and dark minerals into distinct layers or bands is the definition of gneissic banding (or gneissosity). This texture is characteristic of high-grade metamorphism, where temperatures and pressures are high enough for significant mineral migration and recrystallization. Schistosity is the alignment of platy minerals, not compositional banding. Slaty cleavage is found in low-grade rocks and is microscopic. Porphyroblastic texture refers to large crystals in a finer matrix.

Question 8

1. Gneiss; it lacks foliation because the pressure was not uniformly directed.
2. Quartzite; it lacks foliation because the parent rock's quartz grains are equant. (correct answer)
3. Marble; it lacks foliation because the temperature was too low to align minerals.
4. Schist; it lacks compositional banding because the parent rock was chemically uniform.

Explanation: The metamorphism of pure quartz sandstone produces quartzite. This rock is non-foliated because its primary mineral, quartz, has an equant (or granular) crystal shape. Foliation requires platy or elongated minerals (like micas or amphiboles) that can align themselves perpendicular to the direction of maximum stress. Equant minerals do not align in this manner, regardless of pressure conditions.

Question 9

1. lower than slate, as the grains have not yet begun to align.
2. higher than slate but lower than schist. (correct answer)
3. higher than schist but lower than gneiss.
4. the highest possible, bordering on partial melting.

Explanation: The description perfectly matches phyllite. Phyllite is a low- to medium-grade metamorphic rock that represents a progression from slate. The microscopic mica crystals have grown larger than in slate, giving the rock a characteristic silky or satiny sheen (phyllitic luster), but are still too small to be seen individually, unlike in schist. Therefore, its grade is between that of slate and schist.

Question 10

1. Quartz, the primary mineral, has an equant crystal habit that does not align into layers under directed pressure. (correct answer)
2. The rock formed under hydrostatic pressure, which does not cause mineral alignment.
3. The rock formed at temperatures so high that any pre-existing foliation was destroyed by annealing.
4. The protolith was a chemical limestone, which recrystallizes into randomly oriented calcite crystals.

Explanation: When you encounter questions about metamorphic rock textures, focus on how mineral properties and metamorphic conditions interact to create either foliated or non-foliated textures. Quartzite's non-foliated texture results from quartz's unique crystal structure and habit. Quartz forms equant (roughly equal-dimensional) crystals that lack a preferred cleavage direction. During metamorphism under directed pressure, minerals with platy or elongated habits (like mica or amphibole) easily align perpendicular to the stress direction, creating foliation. However, quartz's equant crystals resist this alignment—they simply recrystallize into interlocking grains without developing preferred orientation, producing the characteristic non-foliated texture you observe. Choice A correctly identifies this fundamental relationship between mineral habit and metamorphic texture. Choice B incorrectly assumes hydrostatic pressure conditions—most regional metamorphism involves directed pressure, yet quartzite still remains non-foliated. Choice C misunderstands the process; high temperatures don't destroy foliation through annealing in quartzite formation, and the rock's hardness indicates it wasn't subjected to extreme temperatures that would cause such effects. Choice D contains a major factual error—quartzite forms from sandstone (a silicate protolith), not limestone, and the question states the rock doesn't react with acid, ruling out any carbonate composition. Remember this key principle: a metamorphic rock's texture depends heavily on the crystal habits of its constituent minerals. Rocks dominated by equant minerals (like quartz) tend to be non-foliated, while those with platy or elongated minerals typically develop foliation under directed stress.

Question 11

1. Amphibolite, a foliated or lineated rock dominated by hornblende and plagioclase. (correct answer)
2. Marble, a non-foliated rock rich in calcite and dolomite.
3. Greenschist, a low-grade foliated rock rich in chlorite and actinolite.
4. Quartzite, a non-foliated rock composed primarily of recrystallized quartz.

Explanation: When you encounter questions about metamorphic rocks forming during subduction, focus on matching the original rock type, metamorphic grade, and resulting mineral assemblages. Subduction zones create specific pressure-temperature conditions that transform oceanic basalt in predictable ways. Basaltic oceanic crust contains minerals like pyroxene, plagioclase, and olivine. Under medium-grade metamorphic conditions (amphibolite facies), these minerals become unstable and recrystallize into new assemblages. The key phrase "alignment of prismatic crystals" points to hornblende, which forms elongated, needle-like crystals that align under directed pressure during metamorphism. Answer A is correct because amphibolite forms when basaltic rocks undergo medium-grade metamorphism. The original pyroxenes transform into hornblende (a prismatic amphibole mineral), while plagioclase remains stable but recrystallizes. This creates the characteristic foliated or lineated texture from aligned hornblende crystals. Answer B is wrong because marble forms from limestone or dolomite, not basalt, and lacks the prismatic crystal alignment described. Answer C represents greenschist, which forms under lower-grade conditions than specified—chlorite and actinolite characterize low-grade metamorphism, not the medium-grade amphibolite facies mentioned. Answer D is incorrect because quartzite forms from sandstone, not basalt, and is dominated by quartz rather than the mafic minerals present in oceanic crust. Remember: metamorphic facies names often match their characteristic rock types. Amphibolite facies produces amphibolite from mafic protoliths like basalt, while the alignment of prismatic minerals indicates directed pressure typical of subduction environments.

Question 12

1. The claim is correct; high-grade conditions over geological time always produce coarse-grained rocks like gneiss.
2. A rock metamorphosed at high pressure but low temperature (blueschist facies) can be high-grade but is often fine-grained.
3. A rock undergoing contact metamorphism can reach high temperatures but remain fine-grained if the duration of heating is very short. (correct answer)
4. A non-foliated rock like marble can be high-grade but will always be fine-grained because its calcite crystals resist growth.

Explanation: While grain size generally increases with metamorphic grade, time is also a critical factor for crystal growth. In contact metamorphism, especially near small, rapidly cooled intrusions like dikes or sills, the country rock can be heated to very high temperatures (high grade) for a relatively short time. This duration may be insufficient for large crystals to grow, resulting in a fine-grained but high-grade rock like hornfels. This provides a clear exception to the general rule.

Question 13

1. Chlorite
2. Muscovite
3. Quartz
4. Garnet (correct answer)

Explanation: Certain minerals, known as index minerals, are stable only under specific pressure and temperature conditions and can be used to define metamorphic zones and grades. For a shale (pelitic) protolith, garnet is a key index mineral whose first appearance typically marks the transition into medium-grade metamorphism (the garnet zone). Chlorite is indicative of low grade. Muscovite and quartz are stable over a very wide range of metamorphic conditions and are therefore not good indicators of a specific grade.

Question 14

1. Marble; its crystalline texture implies metamorphism of a carbonate rock. (correct answer)
2. Chemical limestone; its texture implies precipitation from seawater.
3. Quartzite; its interlocking crystals imply high-pressure metamorphism.
4. Anorthosite; its crystalline texture implies slow cooling of a magma.

Explanation: When identifying rocks, you need to consider three key characteristics: composition, texture, and any special properties like reaction to acid. This systematic approach helps you distinguish between igneous, sedimentary, and metamorphic rocks that might look similar. The rock described has several diagnostic features: it's crystalline and non-foliated, reacts with dilute HCl, and consists of interlocking calcite crystals. The acid reaction immediately tells you this is a carbonate rock, since calcite (CaCO₃) fizzes when it contacts hydrochloric acid. The crystalline, interlocking texture indicates the original rock has been recrystallized under heat and pressure, which is the hallmark of metamorphism. This combination points to marble, which forms when limestone or other carbonate rocks undergo metamorphic processes. Looking at the wrong answers: Choice B (chemical limestone) is incorrect because while limestone does react with acid and contains calcite, sedimentary limestone typically shows layered or granular textures, not the coarse, interlocking crystals described. Choice C (quartzite) is wrong because quartzite is composed of quartz (SiO₂), not calcite, and wouldn't react with acid. Choice D (anorthosite) is incorrect because this igneous rock is composed primarily of plagioclase feldspar, not calcite, and also wouldn't react with acid. For rock identification questions, always work through composition first (use clues like acid reactions), then texture (crystalline vs. granular vs. glassy), and finally consider the formation environment these characteristics suggest. The acid test is particularly useful for quickly identifying carbonate minerals.

Question 15

1. schistosity, indicating medium-grade metamorphism.
2. gneissic banding, indicating high-grade metamorphism. (correct answer)
3. slaty cleavage, indicating low-grade metamorphism.
4. porphyroblastic texture, indicating complex mineral growth.

Explanation: The segregation of light and dark minerals into distinct layers or bands is the definition of gneissic banding (or gneissosity). This texture is characteristic of high-grade metamorphism, where temperatures and pressures are high enough for significant mineral migration and recrystallization. Schistosity is the alignment of platy minerals, not compositional banding. Slaty cleavage is found in low-grade rocks and is microscopic. Porphyroblastic texture refers to large crystals in a finer matrix.

Question 16

1. The gneiss formed under lower temperatures and pressures, preserving fine-grained textures.
2. The gneiss formed under higher temperatures and pressures, causing extensive mineral recrystallization and segregation. (correct answer)
3. The gneiss formed through rapid heating during contact metamorphism, while the slate formed during slow regional metamorphism.
4. The gneiss formed from a more quartz-rich protolith, while the slate formed from a more clay-rich protolith.

Explanation: Gneiss is a high-grade metamorphic rock, while slate is a low-grade metamorphic rock. To transform a shale into a gneiss requires significantly higher temperatures and pressures than those needed to form slate. These high-grade conditions lead to extensive recrystallization, the growth of larger mineral grains, and the segregation of minerals into the characteristic gneissic bands.

Question 17

1. The rock contains large, well-formed crystals (porphyroblasts) embedded in a finer-grained matrix.
2. The rock shows distinct layers of different colors that are parallel to the original sedimentary bedding of the protolith.
3. The rock is composed of interlocking crystals of a single mineral with a uniform, granular texture.
4. The rock breaks along parallel planes, and platy minerals like mica appear to be oriented in the same direction. (correct answer)

Explanation: When identifying foliation in metamorphic rocks, you need to look for evidence of preferred mineral orientation and directional weakness created by metamorphic processes. Foliation results from pressure and temperature causing platy minerals to align and creating planes of weakness in the rock. Option D correctly identifies the key characteristics of foliation: the rock breaking along parallel planes combined with oriented platy minerals like mica. This planar weakness develops because sheet silicate minerals align perpendicular to the direction of maximum stress during metamorphism. When you can split a rock along these planes and see aligned minerals, you're observing true foliation. Option A describes porphyroblasts, which are large crystals that can occur in both foliated and non-foliated metamorphic rocks. While porphyroblasts might be present in foliated rocks, their mere existence doesn't prove foliation. Option B is tricky because it mentions parallel layers, but these are relict sedimentary structures (bedding), not metamorphic foliation. The key phrase "parallel to the original sedimentary bedding" indicates these are inherited features from the protolith, not new metamorphic structures. Option C describes a granular texture with interlocking crystals of uniform size, which is characteristic of non-foliated metamorphic rocks like marble or quartzite. This uniform texture specifically indicates the absence of foliation. Remember that foliation requires both structural evidence (planar weakness) and mineralogical evidence (oriented minerals). Look for rocks that split easily along planes with visible mineral alignment—this combination definitively indicates metamorphic foliation rather than other rock features.

Question 18

1. Quartz, the primary mineral, has an equant crystal habit that does not align into layers under directed pressure. (correct answer)
2. The rock formed under hydrostatic pressure, which does not cause mineral alignment.
3. The rock formed at temperatures so high that any pre-existing foliation was destroyed by annealing.
4. The protolith was a chemical limestone, which recrystallizes into randomly oriented calcite crystals.

Explanation: When you encounter questions about metamorphic rock textures, focus on how mineral properties and metamorphic conditions interact to create either foliated or non-foliated textures. Quartzite's non-foliated texture results from quartz's unique crystal structure and habit. Quartz forms equant (roughly equal-dimensional) crystals that lack a preferred cleavage direction. During metamorphism under directed pressure, minerals with platy or elongated habits (like mica or amphibole) easily align perpendicular to the stress direction, creating foliation. However, quartz's equant crystals resist this alignment—they simply recrystallize into interlocking grains without developing preferred orientation, producing the characteristic non-foliated texture you observe. Choice A correctly identifies this fundamental relationship between mineral habit and metamorphic texture. Choice B incorrectly assumes hydrostatic pressure conditions—most regional metamorphism involves directed pressure, yet quartzite still remains non-foliated. Choice C misunderstands the process; high temperatures don't destroy foliation through annealing in quartzite formation, and the rock's hardness indicates it wasn't subjected to extreme temperatures that would cause such effects. Choice D contains a major factual error—quartzite forms from sandstone (a silicate protolith), not limestone, and the question states the rock doesn't react with acid, ruling out any carbonate composition. Remember this key principle: a metamorphic rock's texture depends heavily on the crystal habits of its constituent minerals. Rocks dominated by equant minerals (like quartz) tend to be non-foliated, while those with platy or elongated minerals typically develop foliation under directed stress.

Question 19

1. Amphibolite, a foliated or lineated rock dominated by hornblende and plagioclase. (correct answer)
2. Marble, a non-foliated rock rich in calcite and dolomite.
3. Greenschist, a low-grade foliated rock rich in chlorite and actinolite.
4. Quartzite, a non-foliated rock composed primarily of recrystallized quartz.

Explanation: When you encounter questions about metamorphic rocks forming during subduction, focus on matching the original rock type, metamorphic grade, and resulting mineral assemblages. Subduction zones create specific pressure-temperature conditions that transform oceanic basalt in predictable ways. Basaltic oceanic crust contains minerals like pyroxene, plagioclase, and olivine. Under medium-grade metamorphic conditions (amphibolite facies), these minerals become unstable and recrystallize into new assemblages. The key phrase "alignment of prismatic crystals" points to hornblende, which forms elongated, needle-like crystals that align under directed pressure during metamorphism. Answer A is correct because amphibolite forms when basaltic rocks undergo medium-grade metamorphism. The original pyroxenes transform into hornblende (a prismatic amphibole mineral), while plagioclase remains stable but recrystallizes. This creates the characteristic foliated or lineated texture from aligned hornblende crystals. Answer B is wrong because marble forms from limestone or dolomite, not basalt, and lacks the prismatic crystal alignment described. Answer C represents greenschist, which forms under lower-grade conditions than specified—chlorite and actinolite characterize low-grade metamorphism, not the medium-grade amphibolite facies mentioned. Answer D is incorrect because quartzite forms from sandstone, not basalt, and is dominated by quartz rather than the mafic minerals present in oceanic crust. Remember: metamorphic facies names often match their characteristic rock types. Amphibolite facies produces amphibolite from mafic protoliths like basalt, while the alignment of prismatic minerals indicates directed pressure typical of subduction environments.

Question 20

1. The claim is correct; high-grade conditions over geological time always produce coarse-grained rocks like gneiss.
2. A rock metamorphosed at high pressure but low temperature (blueschist facies) can be high-grade but is often fine-grained.
3. A rock undergoing contact metamorphism can reach high temperatures but remain fine-grained if the duration of heating is very short. (correct answer)
4. A non-foliated rock like marble can be high-grade but will always be fine-grained because its calcite crystals resist growth.

Explanation: While grain size generally increases with metamorphic grade, time is also a critical factor for crystal growth. In contact metamorphism, especially near small, rapidly cooled intrusions like dikes or sills, the country rock can be heated to very high temperatures (high grade) for a relatively short time. This duration may be insufficient for large crystals to grow, resulting in a fine-grained but high-grade rock like hornfels. This provides a clear exception to the general rule.