Rocks: what are they?
How do they form?
 Rocks are aggregates of minerals…that is, they are
made up of minerals but not in the same form as a
collectible mineral specimen would be.
 There are three simple types:
 Igneous: crystallized from magmatic flows
 Volcanic or extrusive: above ground
 Intrusive: below ground
 Sedimentary: deposited from air and water
 Metamorphic: changed by heat and pressure from the
original material; caution: if re-melting occurs, no
longer metamorphic but a new igneous rock.
 Discussion points: look back at rock cycle:
 All three classes of rocks can weather and become
sediments, which may form new sedimentary rocks with
sufficient time.
 All three classes of rocks can melt and become magma,
which may form new igneous rocks.
 All three classes of rocks can undergo metamorphosis
and change into new metamorphic rocks, even if already
metamorphosed before.
 These rocks form when magma or lava cools and hardens.
 Intrusive Igneous rocks: these form when magma cools under the
surface of the Earth…they intrude into existing rock forms,
regardless of type. They are also called Plutonic. They are typically
exposed by erosion of overlaying material.
 Examples: Granite, Pegmatite, Gabbro, Diorite, Peridotite
 Extrusive Igneous rocks: these form when magma flows upon the
surface as lava, or, if underwater, as Basalt.
 Examples: Basalt, Andesite, Rhyolite
 All of these rocks follow a range of chemical make-up, which is
called Bowen’s Reaction Series…it allows us to realize how the rocks
crystallize, and also tells us what minerals we can typically expect in
a given exposure.
 As the temperature of the magma decreases, the
minerals that crystallize out change, dependent upon
their composition.
 Typically, those minerals with the highest amounts of
“mafic” metals, Magnesium and Iron, tend to
crystallize out first. As the magma/lava cools further,
the minerals with less Mg, Fe, and more Ca start to
crystallize, eventually shading into sodium, potassium,
and aluminum bearing minerals. Also, as the
temperature drops, the amount of silica increases in
the minerals that are crystallizing out.
 Gravitational Differentiation: this is the process
whereby the minerals that are crystallizing out settle
out of the parent magma, which allows minerals to
develop into zones in the final rock formation.
 This is how geologists and miners know how and where
to look for various minerals and ores in rock formations.
 Fine-grained: aphanitic, where the mineral grains are
too small to see without strong magnification.
 Coarse-grained: phaneritic, with clearly visible mineral
grains.
 Aphanitic are typically extrusive/volcanic, phaneritic
are typically intrusive/plutonic, but these textures can
follow either type of igneous rock depending upon its
individual location of formation…they are truly ranges
which the magma/lava can vary widely.
 Phenocrysts: larger mineral grains, readily visible.
 Ultramafic: <45% silica, largely ferromagnesian silicates; generally dark
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green to black. Almost always plutonic.
Basalt-Gabbro: 45-52% silica, typically darker rocks, Basalt is aphanitic,
whereas Gabbro is phaneritic. Both are common at mid-oceanic spreading
ridges.
Andesite-Diorite: 53-65% silica, with Diorite showing a salt/pepper
appearance of its crystals, while Andesite may be a more generic gray
shade, with phenocrysts.
Rhyolite-Granite: >65% silica, usually light colored except for dark,
“accent” minerals, such as Biotite mica. Rhyolite (extrusive) is much rarer
than the more mafic extrusive rocks (Basalt and Andesite). Rhyolite tends
to be more often seen as a pyroclastic rock.
Pegmatite: a variation of Granite (usually)where the crystals are quite
large. Some are called complex pegmatites when elements are present
which create less common minerals, frequently producing gem minerals of
great value and beauty…Tourmaline, Beryl, Spodumene are examples.
 From the name, we can tell that sedimentary rocks are
made from sediments…which can be weather derived
particles from preexisting rocks, minerals from
seawater-borne solutions, or minerals extracted from
biological organisms. Regardless, sediments are loose
aggregations of solids, which, when bound together
become sedimentary rocks.
 As you might guess, many sedimentary rocks are
named after the sediments that they are made from:
sandstone from sand, siltstone from silt, mudstone
from mud, limestone from lime-containing materials.
 Let’s think about sediments…these are transported
materials
Running water
Wind
Glaciers
Waves
Near shore currents
Each of these generates their own type of deposited
sediments, with its own typical characteristics…these
characteristics are what enable us to determine the
nature of the rock, and when it was formed in its
environment, frequently what that environment was.
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 Possibilities:
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Glacial: mountainous or plains
Lakes
Streams/rivers
Beach
Delta
Tidal flat
Sand dunes
Alluvial fan
Submarine fan
Barrier island
Deep marine
 Three options
 Compaction
 Cementation
 Both compaction and cementation together
 Compaction: the process of reducing the volume of the
material…pore spaces are reduced, then eliminated, can
have notable adhesion depending upon grain size.
 Compaction is enough for lithification for mud and silt, as
their grain size is so small that adhesion will be achieved by
this alone.
 Cementation: when a mineral or minerals crystallize in the
pore spaces and serve as a binder on the sedimentary
particles present.
 Both: self-explanatory, I hope.
 Common cementing minerals:
 Calcium carbonate: CaCO3
 Silicon dioxide: SiO2
 Iron oxides and hydroxides: Hematite: Fe2O3,
Limonite: FeO(OH)-nH2O
 Two broad categories: detrital and chemical
 Detrital: made up of solid particles derived from existing
rocks by chemical and mechanical weathering; they
have a clastic texture, which means that they are
composed of clasts, which are particles or fragments.
 Examples: conglomerate, sedimentary breccia, sandstone,
mudrocks: siltstone, mudstone, claystone, shale.
 C. 40% of all sedimentary rocks are mudrocks.
 Chemical: resulting from inorganic processes or the
chemical activities of organisms…sometimes called
biochemical sedimentary rocks.
 May be either crystalline or clastic in texture.
 Carbonate rocks
 Limestone: CaCO3
 Variations:
 Travertine: finely crystalline variant, found near hot springs
 Oolitic: formed from layers of ooids, spherical grains
 Coquina: entirely shell fragments
 Chalk: entirely microscopic shells
 Dolostone: CaMg(CO3)2
 Thought to have occurred when the magnesium
concentration in seawater increased due to calcium
deposition.
 Evaporites: rocks formed by evaporation and the
subsequent deposition of minerals
 Rock Gypsum: CaSO4·2H2O
 Rock Salt: Halite: NaCl
 Chert: SiO2, thought to be from chemical replacement
of limestone by precipitation of silica from silicaceous
organisms.
 Coal: a biochemical sedimentary rock: starts out as
peat deposits (50%C), further altered into lignite
(70%C), then bituminous coal (80%C), finally to
anthracite (98%C).
 Metamorphism is from the ancient Greek for change
of form/shape. This usually is exhibited by the
recrystallization of minerals within a preexisting rock.
This process occurs with heat, pressure, fluid activity,
and time. Metamorphism occurs when there is a
change in the physical or chemical environment of a
rock, which then results in a “changed” rock, the result
of which is a new rock, more stable for the conditions.
 Metamorphic fluids are under high temperature and
pressure, so these are frequently only a few molecules
thick as they move in rock where there is little
porosity.
 Contact/thermal: occurs when a body of magma alters
the surrounding country rock, so extent of
metamorphism will be dependent upon the size of the
igneous intrusion. This area is called an aureole and
can range from centimeters to kilometers.
 Metasomatism: when the bulk composition of the rock
changes due to sufficient high temperature fluid
interchange.
 Dynamic: pressure-dominated metamorphism,
commonly associated with fault zones and the
adjacent rocks. Minerals frequently have a smeared
appearance.
 Shock: metamorphism due to meteorite impact.
 Regional/dynathermal: large area zones of
metamorphism, usually due to converging plate
boundaries, almost always showing a gradation of
effect as distance from the convergence increases.
 Foliation: from Encyclopedia Brittanica: planar
arrangement of structural or textural features in any
rock type, but particularly that resulting from the
alignment of constituent mineral grains of a
metamorphic rock of the regional variety along
straight or wavy planes. Foliation often occurs parallel
to original bedding, but it may not be ostensibly
related to any other structural direction. Foliation is
exhibited most prominently by sheety minerals, such
as mica or chlorite.
 Nonfoliation: self-explanatory
 Visit these sites:
http://www.physicalgeography.net/fundamentals/10g.
html
http://www.rocksandminerals4u.com/metamorphic.h
tml