Introduction to Earth Science
Week 2
I. Chapter 2 -- Rocks
• Why study rocks?
• Economic value of minerals
• Understand Earth history (environment of deposition and
creation)
A. Rock cycle
Fig. 2.2
• Relations among rocks -- complex, endless
• Whot first proposed cycle? When?
• Start @ top (12 o’clock, go clockwise)
• Magma - molten rock that crystallizes into
• Igneous rocks, either at surface
(“extrusive”)
or at depth (“intrusive”) that eventually
weathers/erodes to
• Sediment, which becomes cemented,
compacted,
lithified to become
• Sedimentary rock. Both sedimentary &
igneous
rock are subjected to heat, pressure to
become
• Metamorphic rock, which can be partially or
totally
melted back to
• Magma
• Note complexities, shortcuts in cycle
B. Igneous rocks
• “formed by fire” (“ign” like in ignite)
• primarily composed of silicate minerals
• works toward surface/less dense than surface
rocks) -Fig.2.3
if it makes it to surface, is extruded through
volcanoes
-volcanoes variable explosive due to change in
chem.,
H20 content, so,
• Magma cools into either lava (extruded on surface)
or
plutons (intruded into other rocks)
• cooling results in crystallization
-sequence: nuclei of crystals begin to form in
rapid cooling - bunch of small
the “melt”
crystals
slow cooling - crystals can grow larger
and larger
as crystal nuclei expand
• special case of very rapid cooling-volcanic glass “obsidian”
cools so quickly no time for ordered
structure to develop
• Classification of igneous rocks based on
1. texture
2. mineralogy
1. Texture is size and arrangement of crystals
• “fine grained”, crystals v. small, can’t see with
eye
naked
WEEK 2
naked
• often see “vesicles” due to trapped gas
• “coarse-grained”, crystals large enough to see with
eye
• “porphyritic” - large crystals in fine-grained
groundmass
• “glassy” - disordered structure due to most rapid
cooling
which rock type can actually
float?
2. Mineralogy of igneous rock --depends on chemistry of
melt, and
sequence of crystallization
e.g., diff stages of eruption of some volcanoes yield
diff chem
• Bowen’s reaction series Fig. 2.8
• use this in conjunction with Fig. 1.11 to make sense
of the
series ...note increasing complexity of crystal
structure
higher temp ....to.....lower temp
Left branch: olivine...to...pyroxene...to
...amphibole...to
biotite mica, etc.
Right branch: Ca rich...to...plagioclase
fspar...to...Na-rich
• minerals precipitate out of melt, then either
maintain their
identity or can react with melt to
form other
minerals
• minerals that form in same general temperature
regime are
found in the same rock
• note that chemistry of melt changes as crystals
precipitate out
-Mg, Fe pulled out of melt, locked into olivine,
pyroxene
- often, these dense crystals settle to base of
magma
chamber
- remaining melt leads to crystals richer in Ca,
Na, Al, K
3. Naming igneous rocks
High Mg, Fe
Low SiO2 “ultramafic”
“mafic”
Course Gr.
Fine Gr.
Peridotite
Kematiite
Gabro
Dionite
High K, Al
High SiO2 “felsic”
Table 2.1
Basalt
Granite
Andesite
Rhyolite
• Remember to look and classify first
• Interpret second
WEEK 2
• Note that the Bowen’s reactions series is a guide
for
evaluation of weathering; wthrig tend to occur
opposite to
Bowen’s reaction series. Makes sense
in terms of mineral’s
stability field.
e.g. olivine very unstable in Earth surface envir, quartz most
stable
P,T Diagram
C. Weathering of rocks to form sediment
• all things weather --concrete sidewalk example
• Why weathering?
-”natural response to a changing
environment”...
-constant drive to equilibrate with new
environment
-example of igneous rock being exposed to
new P, T
environment once exposed to
surface
• 2 main types of weathering:
• Mechanical weathering- physical breakup
• chemical - chemical alteration to diff
substance
1. Mechanical weathering
• break into pieces, but chem same as original
• but more surf area expose to chem attack
–sugar example - rock candy vs
granular sugar
• 3 important physical processes break up
rocks:
- frost wedging
- expansion due to unloading
-biological activity
a. frost wedging
-H20 expands 9% when it freezes Fig
2.11
- cracked engine block is example of the
power
of wedging
-freeze-thaw cycle is the key progressively more
open fractures as water
fills, freezes, fills, freezes,
etc.
b. unloading
-sheeting (“exfoliation”) like layers of
onion due
to release of pressure as overlying rocks
removed
by erosion
- granite produces exfoliation domes
c. biological activity
-plants, burrowing animals, humans
WEEK 2
2. Chemical weathering
• alters internal structure by removal/addition of
elements
• trend toward substances more stable in surface
environment
• Water very important weathering agent
-O2 in water is an oxidizer
(iron nail changing to rust example)
so Fe-rich minerals show signs of oxidation
-CO2 in water can make H2Co3 (carbonic acid - the
fizz in
sodas)
example of effect on granite:
-H2CO3 attacks K-feldspar, K atoms displace,
breaking
up the crystal structure. Result of
weathering is more
stable “clay minerals”
-some SiO2 liberated, carried away in
groundwater
- quartz component of granite very resistant to
chemical
weathering so as f-spar weathers to
clay, the
quartz grains fall out of the
granite, transported to
other sites (rivers,
beaches, oceans)
- weathering products are the raw materials for
sedimentary rx
D. Sedimentary rocks
• weathering, transport create and carry sediment to
new
locations
• sediment becomes “lithified” through
-compaction (by weight of overlying sediments)
- cementation (by water percolating through
pore space
and precipitating cement.
• Latin derivation from “sedimentum” (means
“settling” as in
settling out a fluid, which is
primary mode of deposition
of seds)
• sed rocks account for only 5% volume of Earth’s
outer 10
miles, but they are very abundant at
surface
- contain fossils, help us understand past life,
evolution
-very important economically
-coal (50% of all electric power from
coal)
- petroleum (oil/natural gas)
- Iron, fertilizer, aluminum
• Classification
-2 types
° Detrital sedimentary rocks ( from
“detritus”, solid
particles)
° Chemical sedimentary rocks precipitated from
water
1. Detrital Sedimentary Rocks
-primary constituents are clay minerals
& quartz
WEEK 2
classification
or
rounded)
° quartz is durable
° clay minerals are an end product
- particle size very important for
Table 2.3
° very coarse grained are either breccia
conglomerate (angular or
° sand-size are sandstone
° silt and clay-size are shale (or
mudstone)
determining particle
from source area
-particle shape important for
distance
-angular--close
- rounded -far away
-particle size very important for determining
environment
of deposition
° water and air velocity carry and sort particles
by size
° clay-size carried by slightest current or water
motion
° sand-size by moderate velocity
° cobbles, boulders only by very high velocity
2. Chemical Sedimentary Rocks
-precipitates, usually out of sea water
-direct precip - example - halite (salt)
precipitates out as
H2O evaporates
-indirect precipitation, by accumulation of time
CaCO3
and SiO2 shells from invertebrate
animals “biochemical” sediment
example - limestone
° 2 rocks composed of shells
1. Coquina - large shell fragments
2. Chalk - microscopic shell fragments
-inorganic CaCO3 is travertine ( found in caverns)
- silica precipitates to form
° chert
° flint
° jasper
° agate
-evaporties includes halites gypsum
° usually associated with restricted ancient
seas ( like
Red Sea today)
° Death Valley
° Great Salt Lake
-Coal unusual
° sed deposit, but primarily organic matter
(leaves bark,
wood)
° “fossil fuel” because it is made up of fossil
material
° reducing conditions allow preservation, not
decay by
oxidation
WEEK 2
° Stages of compaction
1. Lignite
2. bituminous
3. anthracite
Fig 2.19
• Lithification
-compaction
° pore space reduced ( by 40% or more in
some cases
with clays)
° sands compress less
-cementation
° cementing material carried by water through
pore
space, precipitates, glues
particles together
° 2 main cements
1. calcite - CaCO3
2. Silica - SiO2
3. Iron oxide - FeO
• Features of sedimentary rocks
-main feature - layering, recording diff episodes
of
deposition
- bedding planes, separate strata, usually mark
a period
of non-deposition
-lots of deductions can be made from
sedimentary rocks,
especially from fossils
land or ocean
hot or cold
shallow or deep
clear or murky
E. Metamorphic Rocks
• means “change form”
-size
-shape
-texture
-minerals
° agents of change
-pressure
-temperature
-fluids
° Variable degrees
-low grad vs high grade
-slight change vs huge change
Fig
2.22
WEEK 2
-if T goes too high, melting begins and
the
igneous environment is entered
• Metamorphosis causes rocks to be places in new
state to
which they try to equilibrate (not unlike
the example of
weathering). But with
metamorphosis, pressure and
temperature are
usually higher in new environment, opposite
that from
weathering case
• 2 general types
1. regional m-m large scale deformation both
important
-contact m-m local high T, “bakes” rocks in
contact with
magma body
• mm rocks often assoc with mt. belts (Appalatians,
Alps,
Rockies)
• Look at the 3 agents of change
1. Heat may be most important
-energy to drive chem rxns to change
mineralogy
-different effects depending on material
being
heated - eg, near-surface clay
minerals
more affected than ign.
rx.
- T usually increases with depth
2. Pressure also increases with depth, but can
also
occur from lateral compression
(continents
ramming into each other)
-rocks can be quite ductile at high T, P,
fold
dramatically
P, T
3. “chemically active fluids”
-mostly water
-facilitates ion exchange
• Textural changes
Fig 2.23
-foliation develops with re crystallization
-mins grow perpendicular to dected
stress
quartzites that
platy minerals as a
shaly rock
-non foliated rocks
exist as well; some marbles,
don’t have as many
granite or more
-new minerals often introduced through
“hydrothermal” (meaning_______?)
soln’s
WEEK 2
-lots important ore deposits (gold,
copper,
uranium) occur in
hydrothermal veins
• classification
-whole host of changes happen during
metamorphosis
° incr. density
° crystal growth
° foliation
° change in mineralogy
° classify based on texture difference - foliated vs
non-foliated
1. Foliated m-m rocks
-slate - v. fine grained, parent rock
usually shale;
usually good cleavage.
Slate good for tile, chalk
boards, pool tables
-schist - more strongly foliated, often
shiny due to
lots of mica, this case in partic
know as “mica
schist”
-”gneiss (“Nice”) - banded, elongate, but
more
granular than the platy slates 7
schists
gneiss usually composed
of quarts, fspar.
Gneisses are often
m-m granite
2. Non-foliated rocks
-marble-coarsely recrystalized limestone
° impurity can cause colors other than
white
-quartzite - recrystalized, very cemented
sandstone
around
so strong
° diagnostic tool - in a ss, rock breaks
grains
-in quartzite, rock breaks across
grains, cause cement is