1. The rock cycle relates the three major rock types, igneous, sedimentary
and metamorphic, and the processes by which they are formed. These processes
reflect a lack of equilibrium caused by external forces (weathering and erosion)
or internal forces (tectonism). The major rock types are also related to one
another at convergent plate boundaries.
2. Intrusive rocks crystallize from magmas emplaced into country rock. They
possess mineralogies identical to volcanic rocks, but coarse-grained (= slow
cooling) textures. Intrusions exhibit both "baked" and chill zone
contacts, and they may contain xenoliths.
3. Names of plutonic rocks are the counterparts of extrusive rocks, sharing
their mineralogy, but distinguished by their coarse-grained textures. Mineralogically
equivalent granite-rhyolite, diorite-andesite, gabbro-basalt. The gabbro-basalt
pair is dominated by ferromagnesian minerals and plagioclase feldspar. The
granite-rhyolite pair is dominated by feldspars and quartz. The diorite-andesite
pair is composed of feldspars and significant ferromagnesian minerals (30%-50%).
4. Classification systems are arbitrary and there is considerable variation
in the composition of granite and rhyolite.
5. Silica content varies significantly among rock types and influences the
minerals comprising various rock types. Mafic rocks contain 50% or less silica
by weight. They are silica-deficient and have high magnesium, iron, and calcium
content. Silicic (Felsic) rocks are silica-rich (greater than 65%), and have
significant content of aluminium, sodium, and potassium. Intermediate rocks
fall between mafic and silicic (felsic), and Ultramafic rocks, of which peridotite
is the most abundant, are composed of pyroxene and olivine and have less than
45% silica. They have no fine-grained counterparts.
6. Intrusive bodies are defined by size, shape and relationship to country
rock. Volcanic necks are the solidified throats of volcanoes, dikes are discordant,
tabular intrusions, while sills, are concordant, tabular intrusions. Plutons
crystallize at great depth, and most are granite. Batholiths are large and
discordant, while stocks are small and discordant. Detached bodies of magma
that moved to shallow depths are called diapirs.
7. Granite comprises the bulk of continents. Basalt and to a lesser extent
gabbro underlay the oceans, while andesite forms most volcanoes along continental
margins. Ultramafic rocks are thought to form the mantle.
8. Magmas are melted by a combination of the effects of the geothermal gradient,
mantle plumes, water under pressure, pressure release, and mixed mineralogies.
9. Bowen's Reaction Series (Fig. 11.18) explains the variation in rock composition
that can be produced from a single magma. Crystallization proceeds simultaneously
along two branches: a discontinuous branch for ferromagnesian minerals that
remain reactive with the magma, and a continuous branch for plagioclase feldspars
that exhibit zoning from changes in calcium and sodium content. These minerals
are formed by silicon-oxygen tetrahedra that control their silica content.
Any magma left after the discontinuous and continuous branches are complete
is enriched in silica, and the last minerals to form are potassium feldspar,
muscovite and quartz. Differentiation, crystal-settling, partial melting,
assimilation, and magma mixing also account for compositional differences
in magmas.
10. Basaltic magmas are produced at diverging plate boundaries from partial
melting of the asthenosphere and build oceanic crust. Mantle plumes produce
intraplate volcanism that is basaltic under oceanic crust, and rhyolitic under
continental crust. Converging plate boundaries produce andesite by partial
melting, and magmatic underplating that promotes melting of the lower continental
crust for granite production.
