Review Questions and Answers; Igneous Rocks

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1. What is magma?

Magma is a general term that refers to any molten-rock melt on or beneath Earth's surface. Magmas usually include some solid mineral grains and/or dissolved gases in addition to the molten liquid. Basaltic magmas are produced by partial melting in the mantle. More felsic andesite and rhyolite magmas are produced by complex processes that include partial melting of crustal rocks and older rocks of volcanic island arcs, magmatic differentation, and mixing of magmas with different composition, mainly in island arc/continental margin settings.

2. How does lava differ from magma?

Magma is a general term that refers to any molten-rock melt on or beneath Earth's surface. Magmas usually include some solid mineral grains and/or dissolved gases in addition to the molten liquid. Lava is a much more restricted term to describe magma extruded on the surface. Thus all rock melts are magmas, but only those extruded at the surface are lavas.

3. How does the rate of cooling influence the crystallization process?

Crystallization (growth of solid mineral grains from a magma) depends on the rate at which the constituent ionic groups move through the melt and attach to the growing mineral grain. Slow rates of transport and/or short cooling intervals (fast cooling) inhibit in-melt transport and contribute to slow grain growth and small grain size. Natural glasses like obsidian (rhyolite glass) cool so quickly that mineral grains do not have time to grow. Slow cooling allows for a longer period of grain growth, and a high water content in the magma favors higher in-melt transport rates and more rapid grain growth than would occur in a "dry" magma of equivalent composition and temperature.

4. In addition to the rate of cooling, what other factors influence the crystallization process?

As noted above, the in-melt transport properties of the magma have very important effects on crystallization. In general, in-melt transport rates are enhanced by lower magma viscosity and slowed by higher viscosity. Magma viscosities increase (transport rates decrease) with lower temperature and higher silica (SiO2) content. Thus natural rhyolite glasses, formed by rapid cooling of relatively low temperature, silica-rich lavas, are much more common than basaltic glasses formed by rapid cooling of hotter, lower silica content lavas. Large quantities of magmatic volatiles (such as water) can profoundly increase transport rates and crystal growth rates. For this and other reasons, geologists believe that pegmatite (Review Question 7 and Box 3.1) magmas contain very large percentages of water and other volatiles.

5. The classification of igneous rocks is based largely upon two criteria. Name these criteria.

The two are texture and mineral composition. Texture describes the sizes, shapes, and mutual contact relationships of the constituent mineral grains and other physical features of the rock. The mineral composition is also a definitive factor. Names for the common igneous rocks are based mainly on the percentages of three, major minerals; quartz, orthoclase, and plagioclase. For the latter mineral, the ratio Na : Ca basically differentiates diorite from gabbro. In diorite, the plagioclase composition is intermediate (Na Ca) and in gabbro, the plagioclase is dominantly calcic (Ca > Na). Plagioclase in granites is dominantly sodic (Na > Ca).

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6. The statements that follow relate to terms describing igneous rock textures. For each statement, identify the appropriate term.

(a) Openings produced by escaping gases - vesicles

(b) Obsidian exhibits this texture - glassy (not crystalline)

(c) A matrix of fine crystals surrounding phenocrysts - the finer-grained matrix is the groundmass and the texture is porphyritic

(d) Crystals are too small to be seen with the unaided eye - aphanitic texture

(e) A texture characterized by two distinctly different crystal sizes - a porphyritic texture (phenocrysts and groundmass)

(f) Coarse grained, with crystals of roughly equal size - phaneritic texture; the equal-size or equigranular idea helps students understand the difference between phaneritic and porphyritic-phaneritic textures

(g) Exceptionally large crystals exceeding 1 centimeter in diameter - these may be phenocrysts, such as K-feldspar, in phaneritic, porphyritic granite; many crystals in pegmatites exceed 1 centimeter in diameter. In any case, these coarse mineral grains point to a long, sustained period of grain growth in the magma or to specific compositional factors, such as high water content, that greatly facilitate growth of large-size silicate mineral grains from a magma.

7. Why are the crystals in pegmatites so large?

Very large silicate mineral grains (crystals) indicate extremely fast, in-melt transport of the mineral constituents to the growing crystals. We know that pegmatite magmas are small volume, relatively low temperature melts that are extremely rich in water and other dissolved volatiles. The volatiles promote very fast rates of material transfer, thus accounting for rapid growth of very large crystals.

8. What does a porphyritic texture indicate about igneous rock?

Two, distinctively different sizes of mineral grains in the same igneous rock (a porphyritic texture) usually mean that grain growth occurred in two stages. First, the larger grains (phenocrysts) grew over a prolonged period of crystallization at a slow cooling rate in a magma chamber deep below the surface. Then the magma rose nearer to the surface; and the smaller, groundmass grains grew in a second, shorter, crystallization episode during which the cooling rate was much faster. An aphanitic groundmass develops when the second stage of crystallization occurs at or near Earth's surface.

9. What is magmatic differentation? How might this process lead to the formation of several different igneous rocks from a single magma?

Magmas contain many, different, chemical constituents. Mineral grains that crystallize from a magma almost always have different compositions from the magma; and, during any given portion of the crystallization history, only a fraction of the magma crystallizes to mineral grains. Physical separation of melt and crystals can produce rocks enriched in the early-formed minerals and a magma enriched in those components excluded from the early-formed grains. Rocks different in composition from the original parent magma can then crystallize from the remaining, compositionally changed (compositionally differentiated) magma. This process operates throughout the crystallization history of a parent magma and derivative magmas. Thus accumulations of early-formed mineral grains and crystallization of later-stage derivative magmas can result in different igneous rocks being derived from a single batch of an original parent magma.

10. Relate the classification of igneous rocks to Bowen's reaction series.

Bowen's reaction series depicts the order in which major minerals crystallize at low (crustal) pressures from a hot, basaltic magma and how that magma changes composition (differentiates) as it gradually cools. In terms of rock classification, the reaction series predicts that Ca-rich plagioclase, olivine, and pyroxenes will crystallize first (basalt or gabbro), followed by hornblende and plagioclase with Na : Ca of about one (diorite and andesite); at lower temperatures, quartz and orthoclase (granite and rhyolite) crystallize from fractionated magmas strongly enriched in silica and potassium feldspar components.

11. How are granite and rhyolite different? In what way are they similar?

Both are igneous rocks with quartz and orthoclase feldspar as major minerals. Granite is the phaneritic-textured rock crystallized slowly at depth from intrusive, granitic magma. Rhyolite is the aphanitic, rapidly cooled, volcanic rock that forms when granitic magma is extruded during a volcanic eruption. Both have similar chemical and mineralogical compositions. All granites have phaneritic crystalline textures; rhyolites may have glassy textures (obsidian), fragmental textures (tuffs and welded tuffs), and aphanitic crystalline textures.

12. Compare and contrast each of the following pairs of rocks:

(a) Granite and diorite - Both are phaneritic igneous rocks. Granite has quartz and orthoclase feldspar as dominant minerals and is light in color. Diorite has plagioclase (sodium and calcium contents about equal) as the definitive mineral and is darker than granite in color. Biotite, hornblende, and augite are the most common ferromagnesian minerals.

(b) Basalt and gabbro - Both rocks are dark in color and have the same mineral compositions. Calcium-rich plagioclase is the definitive feldspar and quartz is absent; olivine and augite are the main ferromagnesian minerals. Basalt is an aphanitic volcanic rock and gabbro has a phaneritic texture, reflecting its origin at depth from a slow-cooling intrusive magma.

(c) Andesite and rhyolite - Both are aphanitic-textured rocks, usually of volcanic origin. Rhyolite has the same dominant minerals (quartz and orthoclase) as granite; andesite has the same mineral composition as diorite. Typically, rhyolites are light in color and andesites are somewhat darker. Whereas biotite is the only common ferromagnesian mineral in rhyolite, andesite often contains one or more of the mafic minerals biotite, hornblende, and augite.

13. How do tuff and volcanic breccia differ from other igneous rocks such as granite and basalt?

Tuff and breccia have pyroclastic textures as opposed to the crystalline textures of granite and basalt. In pyroclastic textures, the rock is composed of solidified magma fragments and/or fragments broken from other volcanic rocks. In crystalline textures, the minerals grow from the magma into mutually interlocking grains. Clastic means fragmental; "pyro" means fire. Tuffs and volcanic breccias are products of explosive volcanism. Granites form from intrusive magmas and basalts crystallize from shallow sills, dikes, and lava flows.

14. What is the geothermal gradient?

General knowledge gained from deep mines and drill holes tells us that rock temperatures gradually increase with depth below a relatively shallow zone wherein rock temperatures are dominated by circulating groundwaters and surface climatic conditions. The change in rock temperature with depth, a temperature difference/vertical distance, is called the temperature gradient. Thus at each point along a vertical drill hole, the incremental quantity, dT/dx, where T is temperature and x is depth measured positively downward, defines the gradient.

Heat flowing by conduction from Earth's interior is directly proportional to the geothermal gradient. Areas of active volcanism and/or shallow, still-hot intrusive masses exhibit high geothermal gradients; thus rock temperatures increase rapidly with depth below the surface. Tectonically stable areas devoid of late Tertiary or Quaternary volcanism, such as most of the United States east of the Rockies, show much lower gradients, 30° C/km being a representative value. Some of this heat is produced by radioactive decay, mainly in the crust, and some must be coming from great depth, probably from the gradual crystallization of iron alloy at the surface of the inner core.

15. Describe the three conditions that are thought to cause rocks to melt.

Melting temperature ranges for silicate rocks are most strongly affected by three factors; total pressure (depth), chemical composition, and fluid pressures (essentially the quantity of water involved in the partial melting process). Melting temperatures increase with increasing pressure and as compositions change from felsic to ultramafic. Increased fluid pressures significantly decrease melting temperature ranges of most silicate rocks. Water released by dehydration of sinking, subducted oceanic crustal slabs can move upward and promote partial melting in overlying mantle rocks where temperatures are too low for dry-rock melting but high enough for "wet-rock" partial melting.

Upward flow of solid, dry mantle rock accounts for most basaltic magma production. A rising mantle plume has a slightly higher temperature and lower viscosity than the slightly cooler, more rigid peridotite outside the plume. Given the small temperature contrast between the two masses of peridotite, the plume rock loses little heat as it rises. The plume-rock temperatures remain more or less constant as the melting temperature range of the rock is decreasing due to the lower pressure. Eventually plume-rock temperatures exceed the lower limit of the melting temperature range and partial melting is initiated at the top or leading edge of the plume.

16. What is partial melting?

Partial melting denotes the fusion behavior of multicomponent solids (rocks are mixtures of mineral grains of different compositions) that melt over a range of temperatures. Melt fractions produced at lower temperatures are enriched in the more fusible components and unmelted, residual solids are enriched in refractory components. Just think of the reverse of Bowen's Reaction Series. Low temperature, small volume, partial melts of basaltic rocks would be enriched in K- and Na-rich feldspar components and silica. At higher temperatures, pyroxenes and olivine would be the last minerals to melt. In general, the compositions of partial melt fractions are more felsic than the solid parent and the residual solid rocks are more mafic. Partial melt liquid fractions and residual, unmelted solid fractions are generally different in composition from the parental solid rock.

17. How does the composition of a melt produced by partial melting compare with the composition of the parent rock?

As a generalization, partial melts are enriched in chemical components from minerals with lower melting temperature ranges and depleted in components from the more refractory minerals. Thus basalt partial melts are enriched in alkalis and silica and depleted in MgO compared to the parental mantle peridotite. Partial melting of mafic rocks in the lower crust is expected to yield more felsic liquids such as andesite and rhyolite, depending on the extent or degree to which the basaltic parent is melted. The smaller the percentage of the parent rocks that melts, the more felsic the derived liquid. Of course, complete melting without any fractionation produces a liquid with the same composition as the solid rock.

18. How are most basaltic magmas generated?

Basalt magmas are generated in slowly rising, mantle-rock plumes. In this case, melting temperatures decrease as the plume rock decompresses. Deep in the mantle, plume-rock temperatures are slightly above ambient temperatures but well below those necessary for partial melting to begin. However, as the plume rises, little heat is lost and its temperature remains essentially constant; however its melting temperature range gradually decreases due to lowered pressures. Eventually, partial melting begins and continues for as long as the plume keeps rising without significant heat loss. Hot spot, flood basalt, and mid ocean-ridge basaltic volcanism are thought to be driven by partial melting of rising, mantle-peridotite plumes. In this case, no heat is added; the internal heat of the plume and lowered melting temperatures due to lowered pressure result in partial melting.

19. Basaltic magma forms at great depths. Why doesn't most of it crystallize as it rises through the relatively cool crust?

Because melting temperatures of dry rock increase with depth, basaltic magmas generated deep in the mantle have temperatures well above their range of crystallization temperatures at the surface. A rising, dry (water-poor), basalt magma crystallizes through heat loss and falling temperature; its solidification temperatures decrease with shallow depths and lowered pressures. Heat losses in the mantle and lower crust are very slight because silicate rocks are poor heat conductors (insulators) and temperatures of rock in contact with a magma quickly rise, reducing thermal gradients and slowing down conductive losses. Heats of fusion released by fractional crystallization of olivine and other refractory phases offset heat losses to some extent and help maintain the magma at high temperatures. Heat losses rise significantly when the magma contacts cooler rocks of the middle and upper crust. The magma will erupt at the surface if it rises fast enough and in large enough volumes to compensate for increased rates of heat loss at shallow depths. Although basaltic volcanism is very important on the continents and especially in the ocean basins, large volumes of basaltic magma never erupt, but are intruded as subhorizontal sheets in the lower and middle crust.

20. Why are rocks of intermediate (andesitic) and felsic (granitic) composition generally not found in the ocean basins?

Basaltic magma is the main product of partial melting at depth in the mantle below the ocean basins. Andesite is the common magma type of island and continental margin volcanic arcs, and rhyolite is an important magma of the continental lithosphere. Much if not all rhyolite is derived by partial melting of middle and upper crustal rocks that are more felsic than basalt, These include older intrusive rocks, gneisses, and granulites of intermediate to felsic composition. Basalt magma carries the heat necessary for melting into the lower and middle crust, but partial melting and magmatic differentation combine to favor production of magmas more felsic than basalt. In the ocean basins, basaltic magma only encounters a thin mafic crust on its way to the surface. Thus, it induces little partial melting and undergoes little change in composition due to assimilation and or mixing with other more felsic rocks and magmas. Magmatic differentation processes are active and account for small quantities of more evolved magmas at specific oceanic volcanic centers, but the amounts are orders of magnitude less than in continental and continental margin environments.