Earth Science Quiz: Igneous Rock Classification

What this quiz covers

This quiz focuses on Igneous 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. Aphanitic texture, extrusive origin, mafic composition, contains pyroxene.
2. Phaneritic texture, formed from a lava flow, felsic composition, contains quartz. (correct answer)
3. Glassy texture, extrusive origin, high silica content, conchoidal fracture.
4. Porphyritic texture, two-stage cooling, intermediate composition, contains amphibole.

Explanation: Phaneritic (coarse-grained) texture is the result of slow cooling, which occurs when magma solidifies deep within the Earth (intrusive). A lava flow occurs on the Earth's surface and cools rapidly (extrusive), which would produce an aphanitic or glassy texture. Therefore, a rock cannot simultaneously have a phaneritic texture and have formed from a lava flow.

Question 2

1. Glassy texture indicates an intrusive origin, where color is determined by pressure.
2. The dark color in glassy rocks is often due to trace elements, while obsidian is chemically felsic. (correct answer)
3. All mafic rocks contain large, visible crystals of pyroxene and olivine.
4. The dark color is a result of extremely slow cooling, which concentrates dark minerals.

Explanation: While dark color in crystalline rocks often indicates a mafic composition, this rule does not apply to glassy rocks. Obsidian's dark color is due to the dispersion of tiny magnetite crystals or impurities within the silica-rich glass. Its chemical composition is typically felsic, similar to granite or rhyolite. The glassy texture indicates extremely rapid cooling, not slow cooling.

Question 3

1. A single, continuous phase of very slow cooling deep within the crust.
2. A single, continuous phase of rapid cooling at the Earth's surface.
3. An initial period of slow cooling followed by a subsequent period of rapid cooling. (correct answer)
4. An initial period of rapid cooling followed by a period of slow cooling and reheating.

Explanation: This rock has a porphyritic texture. The large crystals (phenocrysts) formed during an initial phase of slow cooling deep underground, which allowed them to grow to a large size. The fine-grained matrix (groundmass) formed when the magma was erupted or intruded to a shallower depth, causing the remaining liquid to cool rapidly. This two-stage cooling history is characteristic of porphyritic rocks.

Question 4

1. Obsidian fragments
2. Andesite blocks
3. Basaltic cinders
4. Pumice lapilli (correct answer)

Explanation: When analyzing volcanic eruptions, you need to consider three key factors: magma composition, gas content, and cooling conditions. These determine what type of volcanic material forms. The scenario describes felsic magma (high in silica, making it viscous and gas-rich) that's ejected as a frothy spray and cools almost instantly in air. This rapid cooling traps gas bubbles, creating a lightweight, vesicular (bubble-filled) rock texture. The "frothy" description is your biggest clue—this indicates abundant trapped gas bubbles. Pumice lapilli (option D) perfectly matches these conditions. Pumice forms when gas-rich felsic magma cools rapidly, preserving the bubble structure and creating an extremely lightweight rock that can even float on water. "Lapilli" refers to the size classification (2-64mm fragments), which fits the description of spray that falls as unconsolidated deposits. Option A (obsidian fragments) would form from felsic magma, but obsidian requires gas-poor conditions and creates a dense, glassy texture—opposite of the frothy description. Option B (andesite blocks) represents intermediate composition magma and typically forms larger, denser fragments rather than frothy spray. Option C (basaltic cinders) comes from mafic magma, not felsic, and forms under different eruption conditions with lower gas content. Remember this pattern: frothy + felsic + rapid cooling = pumice. The key indicators are always the gas content (frothy, vesicular) and magma composition (felsic vs. mafic). These two factors together determine the final rock type in explosive volcanic eruptions.

Question 5

1. Rock Y will have a coarser-grained texture than Rock X. (correct answer)
2. Rock X will be intrusive, while Rock Y will be extrusive.
3. Rock Y will have a finer-grained texture than Rock X.
4. Rock X will have a mafic composition, while Rock Y will have a felsic composition.

Explanation: When you encounter questions about igneous rock formation, focus on how cooling conditions affect crystal size. The depth at which magma solidifies directly controls the cooling rate, which determines the final texture of the rock. Magma that cools at greater depths experiences slower cooling because it's surrounded by warmer rock that acts as insulation. This slower cooling allows more time for crystals to grow, resulting in larger, coarser-grained textures. Conversely, magma cooling at shallower depths loses heat more rapidly to the cooler surrounding environment, producing smaller crystals and finer-grained textures. Since magma body Y solidifies at 20 km depth compared to magma body X at 2 km depth, rock Y will cool much more slowly and develop larger crystals, making it coarser-grained than rock X. This confirms that answer A is correct. Answer B is incorrect because both rocks are intrusive (formed below Earth's surface) - extrusive rocks form when magma reaches the surface. Answer C reverses the relationship by suggesting the deeper rock would be finer-grained, which contradicts the cooling rate principle. Answer D is wrong because the question explicitly states both magma bodies have identical felsic composition - composition doesn't change based on cooling depth. Remember this key relationship: deeper intrusion = slower cooling = coarser crystals. This principle applies consistently across igneous petrology questions, so whenever you see varying depths of magma emplacement, immediately think about how cooling rates will affect crystal size and final rock texture.

Question 6

1. Identifying the presence of quartz phenocrysts using a hand lens. (correct answer)
2. Noting that one rock is slightly darker gray than the other.
3. Observing the presence of vesicles, as only one of the rock types can have them.
4. Measuring the average grain size of the groundmass to see which is finer.

Explanation: When distinguishing between similar igneous rocks in the field, you need to focus on compositional differences rather than textural similarities. Both rhyolite and andesite are volcanic rocks with aphanitic (fine-grained) textures, but they have distinctly different silica contents and mineral compositions. A is correct because quartz phenocrysts (large crystals visible to the naked eye or hand lens) are diagnostic of high-silica rocks. Rhyolite is felsic with 70-77% silica content, making quartz a common phenocryst mineral. Andesite is intermediate with 57-63% silica content—too low for quartz to crystallize as phenocrysts. Finding quartz phenocrysts definitively identifies the rhyolite sample. B is wrong because both rocks can vary in color within overlapping ranges. Slight color differences are subjective and unreliable for classification, especially since both are described as light-colored. C is wrong because vesicles (gas bubble holes) can form in any volcanic rock regardless of composition. Both rhyolite and andesite can be vesicular or non-vesicular depending on gas content during eruption, not silica content. D is wrong because both rocks have aphanitic textures by definition, meaning their groundmass grain sizes are similarly fine and difficult to distinguish meaningfully in the field. Study tip: For igneous rock identification, remember that mineral composition reflects magma chemistry and is more diagnostic than texture. Learn which minerals are diagnostic of different silica contents—quartz indicates felsic composition, while olivine indicates mafic composition.

Question 7

1. Rock X cooled at a greater depth and more slowly than Rock Y. (correct answer)
2. Rock Y was subjected to higher pressure during formation than Rock X.
3. Rock X formed from a magma with a higher water content than Rock Y.
4. Rock Y cooled within the crust, while Rock X cooled on the surface.

Explanation: Gabbro and basalt are the intrusive and extrusive equivalents of a mafic magma. The primary difference between them is their texture, which is determined by cooling rate. Gabbro (intrusive) has a phaneritic (coarse-grained) texture due to slow cooling deep within the crust. Basalt (extrusive) has an aphanitic (fine-grained) texture due to rapid cooling at or near the surface. Therefore, Rock X (gabbro) cooled slower and deeper than Rock Y (basalt).

Question 8

1. Extrusive, with a felsic composition (rhyolite).
2. Intrusive, with a felsic composition (granite). (correct answer)
3. Extrusive, with a mafic composition (basalt).
4. Intrusive, with a mafic composition (gabbro).

Explanation: The rock's texture is described by large (5 mm), interlocking crystals, which is a phaneritic texture. Phaneritic texture indicates slow cooling, characteristic of an intrusive origin. The mineral assemblage—potassium feldspar, quartz, and mica—is characteristic of a felsic (silica-rich) composition. An intrusive, felsic rock is classified as granite.

Question 9

1. Phaneritic texture and a felsic composition.
2. Aphanitic texture and a mafic composition. (correct answer)
3. Phaneritic texture and a mafic composition.
4. Aphanitic texture and a felsic composition.

Explanation: Shield volcanoes with fluid, low-viscosity lava are characteristic of mafic magmas. When this lava erupts onto the surface, it cools rapidly. Rapid cooling produces an aphanitic (fine-grained) texture. Therefore, the rock found would be aphanitic and mafic, which is basalt.

Question 10

1. The edge would have larger crystals due to assimilation of the country rock.
2. The texture would be uniform throughout the pluton due to consistent magma composition.
3. The edge would have smaller crystals due to more rapid cooling against the country rock. (correct answer)
4. The edge would be glassy, while the center would be coarse-grained phaneritic.

Explanation: The magma at the edge of the pluton is in direct contact with the much cooler surrounding rock. This causes the edge to cool more rapidly than the insulated interior of the pluton. Faster cooling leads to the formation of smaller crystals. Therefore, the chill margin will be finer-grained (though still likely phaneritic) than the center. A glassy texture is unlikely for a large, slowly cooling body like a pluton.

Question 11

1. Aphanitic texture, extrusive origin, mafic composition, contains pyroxene.
2. Phaneritic texture, formed from a lava flow, felsic composition, contains quartz. (correct answer)
3. Glassy texture, extrusive origin, high silica content, conchoidal fracture.
4. Porphyritic texture, two-stage cooling, intermediate composition, contains amphibole.

Explanation: Phaneritic (coarse-grained) texture is the result of slow cooling, which occurs when magma solidifies deep within the Earth (intrusive). A lava flow occurs on the Earth's surface and cools rapidly (extrusive), which would produce an aphanitic or glassy texture. Therefore, a rock cannot simultaneously have a phaneritic texture and have formed from a lava flow.

Question 12

1. A granitic sill
2. A basaltic dike (correct answer)
3. A gabbroic batholith
4. A rhyolitic lava flow

Explanation: Let's break down the description: 'dark-colored' suggests a mafic composition (like basalt or gabbro); 'aphanitic' (fine-grained) indicates rapid cooling, typical of extrusive or shallow intrusive settings; 'tabular' is a sheet-like shape; 'cuts across layers' defines a dike. Combining these, the rock body is a dike composed of a mafic, aphanitic rock, which is basalt. Therefore, it is a basaltic dike.

Question 13

1. Intrusive with a felsic composition.
2. Extrusive with a felsic composition.
3. Intrusive with a mafic or ultramafic composition. (correct answer)
4. Extrusive with an intermediate composition.

Explanation: The large, interlocking crystals indicate a phaneritic texture, which means the rock is intrusive. The mineral assemblage is key to determining composition. Olivine and pyroxene are defining minerals for mafic and ultramafic rocks. The absence of quartz and potassium feldspar rules out a felsic or even intermediate composition. Therefore, the rock is intrusive and has a mafic or ultramafic composition (e.g., gabbro or peridotite).

Question 14

1. A basaltic magma cooled slowly underground to form a granitic xenolith before erupting.
2. Granitic and basaltic magmas mixed completely to form an intermediate rock, which then separated.
3. A fragment of rhyolite fell into a slowly cooling gabbro pluton deep within the crust.
4. Ascending basaltic magma broke off a piece of a pre-existing granite body in the crust before erupting. (correct answer)

Explanation: When you encounter questions about xenoliths (foreign rock fragments found within igneous rocks), focus on the timing relationships and cooling histories revealed by crystal size and rock composition. The key evidence here is a coarse-grained granite xenolith within fine-grained basalt. Crystal size tells us cooling history: coarse grains form from slow cooling deep underground, while fine grains form from rapid cooling at or near the surface. Since granite is coarse-grained, it must have formed as a plutonic rock deep in the crust long before the basaltic eruption occurred. Answer D correctly explains this sequence: ascending basaltic magma encountered a pre-existing granite body during its journey to the surface, broke off a fragment, and carried it upward before erupting as fine-grained basalt. The granite xenolith preserves its original coarse texture because it was already solidified when incorporated. Answer A is impossible because basaltic magma cannot cool to form granite—these are completely different compositions with different source materials. Answer B incorrectly suggests complete mixing followed by separation, but xenoliths represent incomplete incorporation, not mixing and separation. The presence of distinct granite and basalt indicates no mixing occurred. Answer C reverses the actual rock types and cooling environments—rhyolite is fine-grained (not what we observe), and gabbro forms from slow cooling (contradicting the fine-grained basalt we see). Remember: xenoliths are "hitchhikers" picked up during magma ascent. Always match crystal size to cooling environment, and consider which rock must be older based on the inclusion relationship.

Question 15

1. Intrusive and mafic (Gabbro).
2. Extrusive and intermediate (Andesite).
3. Intrusive and ultramafic (Peridotite).
4. Extrusive and mafic (Basalt). (correct answer)

Explanation: First, classify the texture. Crystals that are too small to be seen with the naked eye define an aphanitic texture, which indicates an extrusive origin. Second, classify the composition. A mineralogy dominated by pyroxene and plagioclase feldspar, with some olivine, is characteristic of a mafic composition. An extrusive, mafic rock is basalt.

Question 16

1. Rock Y will have a coarser-grained texture than Rock X. (correct answer)
2. Rock X will be intrusive, while Rock Y will be extrusive.
3. Rock Y will have a finer-grained texture than Rock X.
4. Rock X will have a mafic composition, while Rock Y will have a felsic composition.

Explanation: When you encounter questions about igneous rock formation, focus on how cooling conditions affect crystal size. The depth at which magma solidifies directly controls the cooling rate, which determines the final texture of the rock. Magma that cools at greater depths experiences slower cooling because it's surrounded by warmer rock that acts as insulation. This slower cooling allows more time for crystals to grow, resulting in larger, coarser-grained textures. Conversely, magma cooling at shallower depths loses heat more rapidly to the cooler surrounding environment, producing smaller crystals and finer-grained textures. Since magma body Y solidifies at 20 km depth compared to magma body X at 2 km depth, rock Y will cool much more slowly and develop larger crystals, making it coarser-grained than rock X. This confirms that answer A is correct. Answer B is incorrect because both rocks are intrusive (formed below Earth's surface) - extrusive rocks form when magma reaches the surface. Answer C reverses the relationship by suggesting the deeper rock would be finer-grained, which contradicts the cooling rate principle. Answer D is wrong because the question explicitly states both magma bodies have identical felsic composition - composition doesn't change based on cooling depth. Remember this key relationship: deeper intrusion = slower cooling = coarser crystals. This principle applies consistently across igneous petrology questions, so whenever you see varying depths of magma emplacement, immediately think about how cooling rates will affect crystal size and final rock texture.

Question 17

1. A single, continuous phase of very slow cooling deep within the crust.
2. A single, continuous phase of rapid cooling at the Earth's surface.
3. An initial period of slow cooling followed by a subsequent period of rapid cooling. (correct answer)
4. An initial period of rapid cooling followed by a period of slow cooling and reheating.

Explanation: This rock has a porphyritic texture. The large crystals (phenocrysts) formed during an initial phase of slow cooling deep underground, which allowed them to grow to a large size. The fine-grained matrix (groundmass) formed when the magma was erupted or intruded to a shallower depth, causing the remaining liquid to cool rapidly. This two-stage cooling history is characteristic of porphyritic rocks.

Question 18

1. Extrusive, with a felsic composition (rhyolite).
2. Intrusive, with a felsic composition (granite). (correct answer)
3. Extrusive, with a mafic composition (basalt).
4. Intrusive, with a mafic composition (gabbro).

Explanation: The rock's texture is described by large (5 mm), interlocking crystals, which is a phaneritic texture. Phaneritic texture indicates slow cooling, characteristic of an intrusive origin. The mineral assemblage—potassium feldspar, quartz, and mica—is characteristic of a felsic (silica-rich) composition. An intrusive, felsic rock is classified as granite.

Question 19

1. Glassy texture indicates an intrusive origin, where color is determined by pressure.
2. The dark color in glassy rocks is often due to trace elements, while obsidian is chemically felsic. (correct answer)
3. All mafic rocks contain large, visible crystals of pyroxene and olivine.
4. The dark color is a result of extremely slow cooling, which concentrates dark minerals.

Explanation: While dark color in crystalline rocks often indicates a mafic composition, this rule does not apply to glassy rocks. Obsidian's dark color is due to the dispersion of tiny magnetite crystals or impurities within the silica-rich glass. Its chemical composition is typically felsic, similar to granite or rhyolite. The glassy texture indicates extremely rapid cooling, not slow cooling.

Question 20

1. Phaneritic texture and a felsic composition.
2. Aphanitic texture and a mafic composition. (correct answer)
3. Phaneritic texture and a mafic composition.
4. Aphanitic texture and a felsic composition.

Explanation: Shield volcanoes with fluid, low-viscosity lava are characteristic of mafic magmas. When this lava erupts onto the surface, it cools rapidly. Rapid cooling produces an aphanitic (fine-grained) texture. Therefore, the rock found would be aphanitic and mafic, which is basalt.