- A pluton of magma intrudes into a bed of pure quartz sandstone. The heat from the magma causes metamorphism in the surrounding sandstone. Which of the following correctly identifies the resulting metamorphic rock and explains why it lacks foliation?
Answer: Quartzite; it lacks foliation because the parent rock's quartz grains are equant.
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.
- Two rocks, a shale and a limestone, are subjected to the same conditions of high directed pressure and temperature during regional metamorphism. The metamorphosed shale develops a strong foliation, while the metamorphosed limestone does not. What is the primary reason for this difference in texture?
Answer: The shale contained abundant platy clay minerals that recrystallized into micas, while the limestone was composed of equant calcite crystals.
Foliation is the parallel alignment of platy or elongated minerals. The development of foliation is primarily controlled by the mineralogy of the parent rock (protolith). Shale is rich in clay minerals, which are platy. During metamorphism, they recrystallize into other platy minerals like mica, which align under directed pressure. Limestone is composed of calcite, which has an equant (blocky) crystal shape and therefore does not produce a foliated texture upon metamorphism, instead forming marble.
- A geologist discovers a rock that contains distinct zones of dark-colored, foliated, high-grade metamorphic rock intermixed with lighter-colored, non-foliated, crystalline rock that resembles granite. This complex rock is best classified as a:
Answer: migmatite, because it represents a rock that has undergone partial melting.
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.
- The discovery of a large region of outcropping garnet-mica schist is most suggestive of which past geological environment?
Answer: The core of an ancient mountain belt formed by the collision of tectonic plates, resulting in regional metamorphism.
Garnet-mica schist is a medium-grade, foliated metamorphic rock. The combination of foliation (schistosity) and medium-grade index minerals (garnet) is characteristic of regional metamorphism. This process occurs over large areas subjected to intense heat and directed pressure, which is typical of the cores of mountain belts formed at convergent plate boundaries.
- A geologist finds a crystalline, non-foliated rock that readily reacts with dilute hydrochloric acid. The rock is composed almost entirely of interlocking calcite crystals. How should this rock be classified, and what does its texture imply about its formation?
Answer: Marble; its crystalline texture implies metamorphism of a carbonate rock.
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.
- If a body of granite is subjected to intense directed pressure and high temperatures during a continental collision, it will undergo metamorphism. What is the most probable resulting metamorphic rock and its classification?
Answer: Granite Gneiss, a foliated rock, because the felsic and mafic minerals will segregate into bands.
Granite is a coarse-grained igneous rock containing quartz, feldspar, and mafic minerals (like biotite or hornblende). When subjected to high-grade regional metamorphism, these minerals will align and segregate into layers of light-colored (felsic) and dark-colored (mafic) minerals, creating a gneissic banding. The resulting rock is a gneiss, often called a granite gneiss or orthogneiss to indicate its igneous protolith. While some new micas might form, the overall texture will be gneissic, not schistose.
- A student is trying to determine if a metamorphic rock sample is foliated. Which of the following observations would provide the most definitive evidence of foliation?
Answer: The rock breaks along parallel planes, and platy minerals like mica appear to be oriented in the same direction.
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.
- An analysis of mineral assemblages from four rock samples (W, X, Y, Z) taken from this region reveals a progressive increase in metamorphic grade from W to Z. Sample W is identified as slate. Which descriptions most accurately correspond to samples X, Y, and Z?
Answer: X: phyllite with a silky sheen; Y: schist with visible mica flakes; Z: gneiss with distinct banding
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).
- A coarse-grained metamorphic rock is described as having a fabric defined by the parallel alignment of visible, platy mica crystals, between which are lenses of granular quartz and feldspar. The rock lacks the distinct compositional banding of light and dark layers. How would this rock be classified, and what was the likely protolith?
Answer: Schist; the protolith was likely a mudstone or shale.
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.
- Which statement best distinguishes between slaty cleavage and schistosity in foliated metamorphic rocks?
Answer: Slaty cleavage results from the microscopic alignment of platy minerals, while schistosity involves visible, parallel-aligned mica crystals.
The key difference between slaty cleavage and schistosity is the size of the aligned mineral grains. In slate, the clay minerals have recrystallized into microscopic micas (like chlorite and sericite) that are too small to see, creating very flat, smooth cleavage planes. In schist, the mica crystals have grown large enough to be visible to the naked eye, creating a sparkly, often wavy foliation known as schistosity.
- A geologist examines a coarse-grained metamorphic rock sample. The sample exhibits a distinct textural feature where light-colored minerals (feldspar and quartz) and dark-colored minerals (biotite and amphibole) have segregated into separate, alternating layers. This texture is best described as:
Answer: gneissic banding, indicating high-grade metamorphism.
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.
- A geologist is assessing the metamorphic grade of a series of rocks derived from a shale protolith. The first appearance of which of the following minerals in the rock provides the strongest evidence that the rocks have reached at least a medium metamorphic grade?
Answer: Garnet
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.
- A metamorphic rock is exceptionally hard, will scratch steel, and shows no reaction to acid. It is composed of interlocking, equigranular crystals. Under a microscope, the original grain boundaries of the protolith are obliterated. This rock is a quartzite. Which statement best explains its typically non-foliated texture?
Answer: Quartz, the primary mineral, has an equant crystal habit that does not align into layers under directed pressure.
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.
- A metamorphic rock sample has a distinct silky luster and a finely wrinkled or wavy surface. Individual mineral grains are too small to be seen with the naked eye. This rock represents a metamorphic grade that is:
Answer: higher than slate but lower than schist.
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.
- A geologist is comparing two metamorphic rock samples, a slate and a gneiss, that both originated from the same shale protolith. Which statement accurately contrasts the formation conditions of the gneiss relative to the slate?
Answer: The gneiss formed under higher temperatures and pressures, causing extensive mineral recrystallization and segregation.
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.
- During the subduction of an oceanic plate, a piece of basaltic crust is subjected to increasing pressure and temperature. At medium-grade metamorphic conditions (amphibolite facies), which of the following rock types, characterized by an alignment of prismatic crystals, is most likely to form?
Answer: Amphibolite, a foliated or lineated rock dominated by hornblende and plagioclase.
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.
- A geology student is mapping a metamorphic terrane. They start in the west, where they identify outcrops of slate. As they travel several kilometers eastward, they observe that the rocks transition into phyllite, and then further east, into garnet-bearing schist. What is the most valid interpretation of these field observations?
Answer: The metamorphic grade increases towards the east, towards the core of a metamorphic belt.
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.
- A student claims that because metamorphic grade and crystal size generally increase together, it is impossible to have a high-grade, fine-grained metamorphic rock. Which scenario provides a valid counterexample to this claim?
Answer: A rock undergoing contact metamorphism can reach high temperatures but remain fine-grained if the duration of heating is very short.
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.
