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Chapt 4 HW
1. 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.
2. 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. 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
(rhyolitic glass) cool so quickly that mineral grains do not have time to grow. Slower cooling
allows for a longer period of crystal growth and therefore larger grain sizes. Also, 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. 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 rhyolitic 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 magmas that form pegmatites contain
very large percentages of water and other volatiles.
5. The two criteria 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. The names used for the common
igneous rocks are based mainly on the percentages of three major minerals; quartz, potassium
feldspar, and plagioclase feldspar. For the
latter mineral, the ratio of sodium (Na) to calcium (Ca) basically differentiates diorite from
gabbro. In diorite, the plagioclase composition is intermediate (sub-equal amounts of Na and
Ca) and in gabbro, the plagioclase is dominantly calcic (Ca > Na). Plagioclase in granites is
dominantly sodic (Na > Ca).
6. (a) vesicles (b) glassy (not crystalline) (c) porphyritic (d) aphanitic texture (e) porphyritic
texture (f) phaneritic texture (g) pegmatite
8. Two distinctively different sizes of mineral grains in the same igneous rock (a porphyritic
texture) usually indicate that crystal 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 at some depth below the surface of the Earth. Then the magma rose closer to
the surface; and the smaller, groundmass grains grew in a second, shorter, crystallization
episode during which the cooling rate was much faster. If this second stage of crystallization
is extremely rapid, than an aphanitic texture may develop in the groundmass surrounding the
larger phenocrysts.
9. Both are igneous rocks with quartz and potassium 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.
10. (a) Granite and diorite - both are phaneritic igneous rocks. Granite has quartz and potassium
feldspar as dominant minerals and is light in color. Diorite has plagioclase (sub-equal
amounts of sodium and calcium) as the definitive mineral and is darker than granite in color.
Biotite, hornblende, and augite are ferromagnesian minerals that are commonly found in
diorite.
(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 in both rocks. 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 potassium feldspar) as granite while
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 hornblende or augite in addition to biotite.
15. 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 pyroxene will crystallize
first (basalt or gabbro), followed by hornblende and plagioclase with Na to Ca ratios
of about one (diorite and andesite). At lower temperatures, quartz and potassium
feldspar (granite and rhyolite) crystallize from fractionated magmas strongly enriched
in silica and potassium.
16. Partial melting denotes the fusion behavior of multi-component solids (rocks are mixtures of
minerals 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.