Earth Science Quiz: Magma Generation And Composition

What this quiz covers

This quiz focuses on Magma Generation And Composition, 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. Magmatism would cease because the source of water for flux melting is removed.
2. Magmatism would be dominated by andesites, but produced by decompression melting instead of flux melting.
3. Magmatism would become exclusively felsic due to the intense heating of the overriding continental crust.
4. Magmatism would shift from intermediate compositions to a bimodal suite of mafic and felsic rocks. (correct answer)

Explanation: When you encounter questions about slab windows, think about how the geological setting fundamentally changes the melting processes compared to normal subduction zones. In a typical subduction zone, the descending oceanic plate releases water that lowers the melting point of the overlying mantle wedge, producing intermediate composition magmas (andesites) through flux melting. However, at a slab window, the subducting plate has a gap that allows hot asthenosphere to rise directly and contact the base of the overriding plate. This creates a unique dual melting environment. The hot asthenosphere undergoes decompression melting as it rises, generating mafic magmas with compositions similar to mid-ocean ridge basalts. Simultaneously, this rising hot material provides intense heat to the base of the overriding continental crust, causing partial melting of the crustal rocks and producing felsic magmas (rhyolites/granites). This results in a bimodal suite with both mafic and felsic compositions, making D correct. Option A is wrong because magmatism doesn't cease—it changes character. The asthenosphere itself becomes the primary heat source. Option B incorrectly suggests andesites would still dominate; while decompression melting does occur, it produces mafic compositions, not intermediate ones. Option C is incorrect because magmatism isn't exclusively felsic—the rising asthenosphere contributes significant mafic components. Remember that slab windows create "double duty" melting: decompression melting of the asthenosphere produces mafic rocks, while contact heating of the crust produces felsic rocks, resulting in the characteristic bimodal pattern that distinguishes this setting from normal subduction zones.

Question 2

1. Flux melting of the mantle producing intermediate magmas, and heat-transfer melting of the crust producing felsic magmas.
2. Decompression melting of upwelling mantle producing mafic magmas, and heat-transfer melting of the continental crust producing felsic magmas. (correct answer)
3. Decompression melting of the continental crust producing felsic magmas, and fractional crystallization producing mafic magmas.
4. Heat-transfer melting of the mantle producing mafic magmas, and flux melting of the crust producing intermediate magmas.

Explanation: In a continental rift, the asthenosphere rises to shallower depths, undergoing decompression melting to produce mafic (basaltic) magma. This hot mafic magma then rises and intrudes into the overlying continental crust. The heat from the mafic intrusions causes the felsic crustal rock to melt via heat-transfer melting, producing felsic (rhyolitic) magma. This combination of processes leads to the characteristic bimodal (mafic and felsic) volcanism seen in many continental rifts.

Question 3

1. Basalt
2. Gabbro (correct answer)
3. Diorite
4. Peridotite

Explanation: First, the composition (low $SiO_2$, high Fe/Mg) indicates a mafic magma. Second, the term 'intrusively' means the magma cooled slowly beneath the Earth's surface, forming a coarse-grained plutonic rock. The intrusive equivalent of a mafic magma is gabbro. Basalt is the extrusive equivalent (the most common distractor). Diorite is the intrusive equivalent of an intermediate magma. Peridotite is an ultramafic rock found in the mantle.

Question 4

1. The magma will become more mafic as it assimilates the surrounding high-temperature felsic minerals from the gneiss.
2. The magma's composition will remain basaltic, as the high pressure prevents significant interaction with the crustal rock.
3. The magma will become more silica-rich due to the partial melting and assimilation of the surrounding felsic crustal rocks. (correct answer)
4. The magma will become ultramafic through the preferential loss of silica to the surrounding country rock via diffusion.

Explanation: When mafic (basaltic) magma intrudes into felsic continental crust, heat from the magma can melt the surrounding crustal rock (gneiss). This process, called assimilation, incorporates the silica-rich melt from the crust into the original magma. The result is a hybrid magma that is more felsic (more silica-rich) and compositionally intermediate between the original basalt and the crustal rock.

Question 5

1. The magma becomes more felsic; olivine will be among the last minerals to crystallize.
2. The magma becomes more mafic; quartz will be among the last minerals to crystallize.
3. The magma becomes more felsic; quartz will be among the last minerals to crystallize. (correct answer)
4. The magma becomes more mafic; olivine will be among the last minerals to crystallize.

Explanation: According to Bowen's Reaction Series, as a mafic magma cools, mafic minerals (like olivine and pyroxene) crystallize first. This process, known as fractional crystallization, removes iron and magnesium from the melt, causing the remaining liquid to become progressively enriched in silica, sodium, and potassium, thus more felsic. Quartz is one of the last minerals to crystallize from a cooling magma, forming at the lowest temperatures from a silica-rich melt.

Question 6

1. At both locations, magma is generated by an increase in temperature, but the heat source is radioactive decay at ridges and friction at subduction zones.
2. At ridges, magma is generated by flux melting due to seawater infiltration, while at subduction zones, it is generated by decompression of the overriding plate.
3. At ridges, magma generation is driven by decreased pressure on the upwelling asthenosphere, while at subduction zones, it is driven by volatiles lowering the mantle's melting point. (correct answer)
4. At both locations, magma generation is driven by decompression, but the rate of plate spreading is the key variable controlling melt volume.

Explanation: The primary mechanism for magma generation at mid-ocean ridges is decompression melting, where hot asthenospheric mantle rises to fill the gap created by diverging plates, and the decrease in pressure lowers its melting point. At subduction zones, the primary mechanism is flux melting, where water and other volatiles are released from the subducting oceanic plate, lowering the melting temperature of the overlying mantle wedge.

Question 7

1. The melt will be more mafic than the residue because iron and magnesium have the lowest melting points.
2. The melt will be more felsic than the residue because silica-rich minerals have lower melting temperatures. (correct answer)
3. The melt and the residue will have identical compositions, reflecting the original peridotite.
4. The melt will be enriched in calcium and aluminum, while the residue will be enriched in sodium and potassium.

Explanation: During partial melting, minerals with lower melting points melt first. In an ultramafic rock like peridotite, the more felsic mineral components (those richer in silica, aluminum, sodium, and potassium) have lower melting temperatures than the more mafic components (richer in iron and magnesium). Therefore, the initial liquid produced is always more felsic (silica-rich) than the source rock, leaving behind a solid residue that is more mafic (depleted in silica).

Question 8

1. Basalt
2. Gabbro
3. Granite
4. Rhyolite (correct answer)

Explanation: The chemical composition (high $SiO_2$, low Fe/Mg, high K/Na) is characteristic of a felsic magma. The question specifies that this magma erupts onto the surface and cools quickly, which results in an extrusive (volcanic) rock with a fine-grained or glassy texture. Rhyolite is the extrusive, felsic volcanic rock. Granite is the intrusive equivalent, Basalt is extrusive but mafic, and Gabbro is intrusive and mafic.

Question 9

1. The geothermal gradient is less than the adiabatic rate of cooling, preventing the mantle from reaching its melting point.
2. The mantle lacks sufficient radioactive isotopes to generate the additional heat required to initiate widespread melting.
3. The immense pressure at depth increases the melting point of mantle rock to a temperature higher than the ambient mantle temperature. (correct answer)
4. Convective circulation is too rapid to allow any single parcel of rock to remain in a high-temperature zone long enough to melt.

Explanation: Both temperature and pressure increase with depth in the Earth. While the temperature is very high in the mantle, the extreme pressure compacts the mineral structures, making it more difficult for them to transition to a less-dense liquid state. This means the melting temperature (the solidus) of the rock also increases with pressure. Under most stable parts of the Earth, the actual temperature profile (geothermal gradient) does not cross the solidus, so the rock remains solid.

Question 10

1. A mid-ocean ridge
2. A continental rift valley
3. An oceanic-continental subduction zone (correct answer)
4. A mantle plume beneath an oceanic plate

Explanation: Andesite, dacite, and rhyolite are intermediate to felsic volcanic rocks. This compositional suite is the hallmark of volcanic arcs formed at oceanic-continental subduction zones (e.g., the Andes Mountains). Here, flux melting of the mantle wedge produces mafic-to-intermediate magmas, which then ascend through thick continental crust, undergoing fractional crystallization and assimilation to generate the full range of intermediate and felsic compositions.

Question 11

1. The mafic magma from the Yellowstone plume interacts with and melts thick felsic continental crust, while the Hawaiian plume erupts through thin mafic oceanic crust. (correct answer)
2. The Yellowstone hotspot involves flux melting due to ancient trapped water, whereas the Hawaiian hotspot does not.
3. The mantle plume beneath Yellowstone is felsic, while the plume beneath Hawaii is mafic.
4. The rate of plate motion is much faster over the Yellowstone hotspot, leading to more advanced fractional crystallization.

Explanation: When you encounter questions about volcanic composition differences between hotspots, think about how magma composition changes as it travels from the mantle to the surface, especially when it interacts with different types of crustal rock. Both the Yellowstone and Hawaiian hotspots originate from similar mafic (magnesium and iron-rich) mantle plumes that produce basaltic magma initially. The key difference lies in what happens to this magma on its journey upward. At Yellowstone, the rising mafic magma encounters and melts the thick continental crust above it. Continental crust is predominantly felsic (rich in silica, aluminum, potassium, and sodium), so when the hot mafic magma assimilates this crustal material, it becomes increasingly felsic and rhyolitic. In contrast, Hawaiian volcanoes erupt through thin oceanic crust that is already mafic in composition, so there's minimal change to the original basaltic magma composition. Option A correctly describes this crustal assimilation process. Option B is incorrect because flux melting refers to melting caused by water lowering melting points, which isn't the primary mechanism distinguishing these hotspots. Option C is wrong because mantle plumes are fundamentally mafic in nature—the difference isn't in the source but in the crustal interaction. Option D incorrectly suggests that plate motion speed affects fractional crystallization enough to explain the dramatic compositional difference, when crustal interaction is the dominant factor. Remember this pattern: when comparing volcanic compositions at different hotspots, always consider what type of crust the magma travels through—continental versus oceanic crust can dramatically alter final magma composition through assimilation processes.

Question 12

1. The magma becomes more felsic; olivine will be among the last minerals to crystallize.
2. The magma becomes more mafic; quartz will be among the last minerals to crystallize.
3. The magma becomes more felsic; quartz will be among the last minerals to crystallize. (correct answer)
4. The magma becomes more mafic; olivine will be among the last minerals to crystallize.

Explanation: According to Bowen's Reaction Series, as a mafic magma cools, mafic minerals (like olivine and pyroxene) crystallize first. This process, known as fractional crystallization, removes iron and magnesium from the melt, causing the remaining liquid to become progressively enriched in silica, sodium, and potassium, thus more felsic. Quartz is one of the last minerals to crystallize from a cooling magma, forming at the lowest temperatures from a silica-rich melt.

Question 13

1. Flux melting of the mantle producing intermediate magmas, and heat-transfer melting of the crust producing felsic magmas.
2. Decompression melting of upwelling mantle producing mafic magmas, and heat-transfer melting of the continental crust producing felsic magmas. (correct answer)
3. Decompression melting of the continental crust producing felsic magmas, and fractional crystallization producing mafic magmas.
4. Heat-transfer melting of the mantle producing mafic magmas, and flux melting of the crust producing intermediate magmas.

Explanation: In a continental rift, the asthenosphere rises to shallower depths, undergoing decompression melting to produce mafic (basaltic) magma. This hot mafic magma then rises and intrudes into the overlying continental crust. The heat from the mafic intrusions causes the felsic crustal rock to melt via heat-transfer melting, producing felsic (rhyolitic) magma. This combination of processes leads to the characteristic bimodal (mafic and felsic) volcanism seen in many continental rifts.

Question 14

1. Basalt
2. Gabbro
3. Granite
4. Rhyolite (correct answer)

Explanation: The chemical composition (high $SiO_2$, low Fe/Mg, high K/Na) is characteristic of a felsic magma. The question specifies that this magma erupts onto the surface and cools quickly, which results in an extrusive (volcanic) rock with a fine-grained or glassy texture. Rhyolite is the extrusive, felsic volcanic rock. Granite is the intrusive equivalent, Basalt is extrusive but mafic, and Gabbro is intrusive and mafic.

Question 15

1. Basalt
2. Gabbro (correct answer)
3. Diorite
4. Peridotite

Explanation: First, the composition (low $SiO_2$, high Fe/Mg) indicates a mafic magma. Second, the term 'intrusively' means the magma cooled slowly beneath the Earth's surface, forming a coarse-grained plutonic rock. The intrusive equivalent of a mafic magma is gabbro. Basalt is the extrusive equivalent (the most common distractor). Diorite is the intrusive equivalent of an intermediate magma. Peridotite is an ultramafic rock found in the mantle.

Question 16

1. Magmatism would cease because the source of water for flux melting is removed.
2. Magmatism would be dominated by andesites, but produced by decompression melting instead of flux melting.
3. Magmatism would become exclusively felsic due to the intense heating of the overriding continental crust.
4. Magmatism would shift from intermediate compositions to a bimodal suite of mafic and felsic rocks. (correct answer)

Explanation: When you encounter questions about slab windows, think about how the geological setting fundamentally changes the melting processes compared to normal subduction zones. In a typical subduction zone, the descending oceanic plate releases water that lowers the melting point of the overlying mantle wedge, producing intermediate composition magmas (andesites) through flux melting. However, at a slab window, the subducting plate has a gap that allows hot asthenosphere to rise directly and contact the base of the overriding plate. This creates a unique dual melting environment. The hot asthenosphere undergoes decompression melting as it rises, generating mafic magmas with compositions similar to mid-ocean ridge basalts. Simultaneously, this rising hot material provides intense heat to the base of the overriding continental crust, causing partial melting of the crustal rocks and producing felsic magmas (rhyolites/granites). This results in a bimodal suite with both mafic and felsic compositions, making D correct. Option A is wrong because magmatism doesn't cease—it changes character. The asthenosphere itself becomes the primary heat source. Option B incorrectly suggests andesites would still dominate; while decompression melting does occur, it produces mafic compositions, not intermediate ones. Option C is incorrect because magmatism isn't exclusively felsic—the rising asthenosphere contributes significant mafic components. Remember that slab windows create "double duty" melting: decompression melting of the asthenosphere produces mafic rocks, while contact heating of the crust produces felsic rocks, resulting in the characteristic bimodal pattern that distinguishes this setting from normal subduction zones.

Question 17

1. The mafic magma from the Yellowstone plume interacts with and melts thick felsic continental crust, while the Hawaiian plume erupts through thin mafic oceanic crust. (correct answer)
2. The Yellowstone hotspot involves flux melting due to ancient trapped water, whereas the Hawaiian hotspot does not.
3. The mantle plume beneath Yellowstone is felsic, while the plume beneath Hawaii is mafic.
4. The rate of plate motion is much faster over the Yellowstone hotspot, leading to more advanced fractional crystallization.

Explanation: When you encounter questions about volcanic composition differences between hotspots, think about how magma composition changes as it travels from the mantle to the surface, especially when it interacts with different types of crustal rock. Both the Yellowstone and Hawaiian hotspots originate from similar mafic (magnesium and iron-rich) mantle plumes that produce basaltic magma initially. The key difference lies in what happens to this magma on its journey upward. At Yellowstone, the rising mafic magma encounters and melts the thick continental crust above it. Continental crust is predominantly felsic (rich in silica, aluminum, potassium, and sodium), so when the hot mafic magma assimilates this crustal material, it becomes increasingly felsic and rhyolitic. In contrast, Hawaiian volcanoes erupt through thin oceanic crust that is already mafic in composition, so there's minimal change to the original basaltic magma composition. Option A correctly describes this crustal assimilation process. Option B is incorrect because flux melting refers to melting caused by water lowering melting points, which isn't the primary mechanism distinguishing these hotspots. Option C is wrong because mantle plumes are fundamentally mafic in nature—the difference isn't in the source but in the crustal interaction. Option D incorrectly suggests that plate motion speed affects fractional crystallization enough to explain the dramatic compositional difference, when crustal interaction is the dominant factor. Remember this pattern: when comparing volcanic compositions at different hotspots, always consider what type of crust the magma travels through—continental versus oceanic crust can dramatically alter final magma composition through assimilation processes.

Question 18

1. It is composed of the earliest, highest-temperature crystals that settled out from a large, cooling mafic magma body. (correct answer)
2. It formed from the complete melting of a section of continental crust followed by slow cooling.
3. It represents a felsic lava that cooled very rapidly, preventing the growth of feldspar or quartz.
4. It is an extrusive rock formed from an ultramafic lava that erupted directly from the deep mantle.

Explanation: When you encounter questions about rock composition and formation, focus on the relationship between mineral crystallization sequences and magma cooling processes. The key here is understanding Bowen's reaction series and how cumulate rocks form. A cumulate rock composed almost entirely of olivine and pyroxene crystals tells a specific story about magma crystallization. These are mafic minerals that crystallize first from cooling basaltic magma at the highest temperatures (around 1200-1400°C). In a large magma chamber, these dense, early-formed crystals settle to the bottom due to gravity, accumulating as a layered deposit while the remaining liquid magma becomes progressively more evolved. Option A correctly identifies this process - these are the earliest, highest-temperature crystals that settled from a cooling mafic magma body, forming what geologists call a cumulate deposit. Option B is incorrect because complete melting of continental crust would produce felsic magma rich in silica, leading to quartz and feldspar crystals, not olivine and pyroxene. Option C misidentifies this as a felsic rock and incorrectly suggests rapid cooling prevented feldspar growth - but the rock contains no felsic minerals at all. Option D describes direct mantle eruption, but such ultramafic lavas are extremely rare and wouldn't form the layered cumulate texture described. Remember: when you see olivine + pyroxene cumulates, think "early crystallization from mafic magma with crystal settling." The mineral composition directly reflects the crystallization sequence predicted by Bowen's reaction series.

Question 19

1. The magma that formed Sample B cooled more rapidly and was less viscous than the magma that formed Sample A.
2. The magma that formed Sample A was likely hotter and less viscous than the magma that formed Sample B. (correct answer)
3. Sample A is granite that formed at a subduction zone, and Sample B is basalt that formed at a mid-ocean ridge.
4. Both magmas were likely generated by flux melting of the asthenosphere and erupted in the same volcanic event.

Explanation: Sample A is mafic (dark, low silica) and extrusive (fine-grained), so it is basalt. Sample B is felsic (light, high silica) and intrusive (coarse-grained), so it is granite. Mafic magmas are hotter (1000-1200°C) and have lower viscosity than felsic magmas (650-800°C), which are highly viscous. Therefore, the magma for Sample A was hotter and less viscous than the magma for Sample B. Distractor A incorrectly links coarse grains (slow cooling) with rapid cooling. Distractor C misidentifies the rocks and their typical settings.

Question 20

1. The magma will become more mafic as it assimilates the surrounding high-temperature felsic minerals from the gneiss.
2. The magma's composition will remain basaltic, as the high pressure prevents significant interaction with the crustal rock.
3. The magma will become more silica-rich due to the partial melting and assimilation of the surrounding felsic crustal rocks. (correct answer)
4. The magma will become ultramafic through the preferential loss of silica to the surrounding country rock via diffusion.

Explanation: When mafic (basaltic) magma intrudes into felsic continental crust, heat from the magma can melt the surrounding crustal rock (gneiss). This process, called assimilation, incorporates the silica-rich melt from the crust into the original magma. The result is a hybrid magma that is more felsic (more silica-rich) and compositionally intermediate between the original basalt and the crustal rock.