Download Rocks - Earth and Atmospheric Sciences

EAS 2200
The Earth System
Spring 2012
Lecture 4
Earth Materials and their Properties II:
(Rocks, Water, Seismic Waves)
Why minerals occur where they do
Whether a mineral will occur in a system* depends on 3 parameters:
The overall composition of the system
Temperature
Pressure
All minerals are stable over only a restricted range of these parameters.
(However, kinetic factors may inhibit either formation or breakdown of a mineral).
Examples
Diamond-Graphite
Only 1 form of carbon can be stable under a given set of conditions.
Because the atoms are more tightly packed and hence occupy less volume, diamond is favored by higher
pressure.
A diagram illustrating the stability of minerals or assemblages of minerals as a function of temperature (T),
pressure (P), or composition (X) is called a phase diagram .
Diamond is not stable at the surface of the Earth, but is stable in Earth’s interior.
Its persistence at the surface illustrates how kinetic factors can inhibit a transition to the stable phase.
*
We can define “system” is a region of space where chemical components are free to
interact. In the lab, this might be an experimental container, such as a beaker. In nature,
a system may be a rock.
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EAS 2200
The Earth System
Spring 2012
Lecture 4
Quartz - Olivine System
Stability of minerals is also limited by composition. The stability relationships can be determined by
experiments in the laboratory and illustrated with phase diagrams.
Olivine, enstatite, and quartz are common igneous minerals, but olivine and quartz will not occur in the
same rock because they react to form enstatite.
Kyanite-Andalusite-Sillimanite
The three minerals kyanite, andalusite, and sillimanite all have the same composition: Al2SiO5, but differ in
lattice structure.
Consequently, they differ in appearance and in their properties.
They typically occur in metamorphosed sedimentary rocks.
Which occurs depends on temperature and pressure of equilibration.
The Bottom Line
Since we can determine the range of stability of each mineral through laboratory
experiments, this allows us to deduce the conditions under which a rock formed, and
more fundamentally, the nature of processes shaping the Earth.
Mineral Properties Affecting Behavior of the Earth
Thermal conductivity
Magnetic susceptibility
Density
Compressibility (Bulk Modulus)
Shear Modulus
Rocks
Assemblages of minerals (and non-minerals) form rocks and partly determine the
properties of those rocks.
In addition to the minerals present, the size, shape, and arrangement of minerals are
important. Bedding and biological structures (i.e., fossils) are also important.
Nature of a rock reflects the way it formed and its history.
Rock Cycle
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EAS 2200
The Earth System
Spring 2012
Lecture 4
Rocks form by:
Crystallization from melt– Igneous
Recrystallization – Metamorphic
Deposition from a fluid (water, wind, ice) – Sedimentary
Cycle emphasizes one forms from the other. In actuality, there are short-circuits and it is
sometimes a one-way trip.
Igneous Rocks
Igneous rocks form by crystallization (or solidification in the case of glasses) from a melt.
We can broadly divide them into:
Intrusive: crystallize from magma at depth
Extrusive: crystallize from lava erupted onto the surface
Magma is molten rock (usually only partially)
Usually contains crystals and gas bubbles
Lava is magma that has erupted onto the surface.
Extrusive vs. Intrusive Rocks
Intrusive igneous rocks cool slowly, allowing crystals to grow to larger size. Thus they are
coarse-grained.
Some intrusive rocks show layering and sedimentary structures resulting from the minerals settling out of
the magma.
Extrusive igneous rocks crystallize quickly, so mineral grains are typically too small to see
with naked eye.
They may contain large crystals, phenocrysts, that grew in magma chamber before eruption.
Types of Magmas
Most magmas are silicates.
Carbonate (called carbonatite), sulfur, and sulfide magmas occur, but are very rare.
Igneous rocks are generally classified by the amount of SiO2 they contain.
Amounts of other elements generally vary in relation to this.
Al2O3 and alkalis generally increase with increasing SiO2, MgO, FeO, and CaO generally decrease.
Origin of Magmas
Igneous rocks form by crystallization of magmas, but how do the magmas form?
Essentially all mafic magmas (basaltic) form by partial melting of the Earth’s mantle (typically at depths of
30 to 200 km). Usually this results from decompression in upwelling convection cells or from water fluxing
in subduction zones.
Felsic or silicic magmas (granitic) form either by melting of crustal rocks or by fractional crystallization of
mafic magmas.
The first minerals to crystallize are generally more mafic than the magma, hence as they crystallize
and the magma cools, the magma composition changes. This process is called fractional
crystallization.
Impact melting is an additional, but much rarer, way in which magmas form.
Sedimentary Rocks
Sedimentary rocks can form in two ways:
Settling of pre-existing particles out of a fluid (water, wind, ice).
Such rocks are often called detrital sedimentary rocks.
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EAS 2200
The Earth System
Spring 2012
Lecture 4
Examples: sandstone, shale.
Precipitation from aqueous solution.
Such rocks are often called chemical sedimentary rocks.
Examples: limestone, chert, rock salt
Often, the precipitation is biologically mediated.
In actuality, formation of most sediments involves both
Particles in detrital sedimentary rocks are held together by a cement or matrix that has precipitated from
solution (usually after deposition).
Many chemical sediments are formed by (1) biological precipitation (usually of shells) in the water column
followed by (2) settling of the particle following death of the organism.
Metamorphic Rocks
Metamorphic rocks are ones that have undergone significant change as a result of heat
and pressure. This is often, but not always, accompanied by deformation.
Metamorphic rocks are typically classified primarily by their texture, but also by origin
Metasediments
Meta-igneous rocks
Texture is related to the intensity of metamorphism, which depends on temperature and
pressure.
Metamorphism can involve recrystallization of existing minerals or crystallization of new
ones.
Metamorphism usually takes place in the presence of water, which facilitates crystallization
by transport of chemicals.
Causes of Metamorphism
Metamorphism is usually a result of burial.
Because temperature and pressure increase with depth, burial exposes rocks to higher
pressure and temperature.
Burial metamorphism happens on broad scales, so this is called regional metamorphism.
Metamorphism can also be caused by intrusion of magma, which heats surrounding rock.
This kind of metamorphism is limited in extent (< 1 km from magma body), and is called contact
metamorphism.
Changes occurring with progressive metamorphism
Extent of metamorphism is called the metamorphic grade
Nearly synonymous with temperature
Grain size increases
Fabric develops as minerals take on preferred orientations that reflect the distribution of
stresses
In rocks dominated by minerals with platy or needle-like habit such as micas, this results in a foliation.
At high pressures and temperatures, minerals may segregate into distinct bands
As metamorphism progresses, anhydrous minerals such as feldspars, garnet, and
pyroxene replace hydrous minerals such as micas and amphiboles.
The Role of Water
If there is any one substance that makes the Earth unique, it is water.
Dominates the hydrosphere, but also plays a HUGE role in atmosphere and solid Earth,
even though it is present in only minor amounts.
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EAS 2200
The Earth System
Spring 2012
Lecture 4
HUGE role in climate.
HUGE role in rock cycle.
Important role in plate tectonics.
Makes life possible.
Role of Water in Climate
It is the major greenhouse gas.
Clouds greatly increase the albedo (reflectivity) of the Earth.
Redistributes energy vertically in atmosphere.
Redistributed energy around planet through ocean circulation.
Evaporation and precipitation of water is THE cause of storms.
Role in the Solid Earth
Primarily responsible for the breakdown (weathering) of rock.
Transports components (both particles and dissolved) of sediments.
Mediates metamorphic reactions.
Is the principal cause of explosive volcanic eruptions.
Addition of water to rock lowers the melting temperature of rock. This is probably
responsible for some volcanism, particularly in “island arcs”.
In the deep Earth, even small quantities of water (50 ppm) greatly reduce the strength of
minerals, thereby decreasing viscosity and enhancing flow and convection.
Properties of Water
Highest heat capacity of all substances except liquid ammonia.
Highest latent heat of fusion of all substances except liquid ammonia.
Highest latent heat of evaporation.
Dissolves more substances in greater quantity than any other substance.
Solid is less dense that the liquid; liquid increasingly stable with increasing pressure.
Thermal expansion:
Maximum density occurs at 4˚C, with decreasing density at higher and lower temperature (in pure water).
The Water Molecule
Many of the properties of water result from the structure of the molecule.
Two hydrogens covalently bonded to one oxygen.
However, the electrons are not shared equally, giving the oxygen a partial (δ) net negative
charge and each hydrogen a partial net positive charge. Consequently, the molecular is
polar.
The Hydrogen Bond
As a result of the partial charges, a weak bond, called the hydrogen bond, forms between
a hydrogen of one molecule and an oxygen of another.
In liquid water, 75-90% of water molecules are hydrogen bonded to one and other
(depending on temperature).
Progressive breaking of hydrogen bonds accounts for the high heat capacity of water.
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EAS 2200
The Earth System
Spring 2012
Lecture 4
Iceπ
In ice, 100% of water molecules are hydrogen bonded.
When ice melts, only 11% of these bonds break.
More bonds break with increasing temperature.
All remaining bonds break upon evaporation.
The solvation shell
Dissolving power of water is due to polar nature of the molecule
Water molecules surround ions and orient themselves so as to neutralize the charge of the
ion.
The surrounding molecules, which are weakly bound to the ion, are called the solvation
shell.
Dissociation
At 25˚ C, one in every 10 7 water molecules will dissociate to form hydrogen ion &
hydroxide ion
The product of the hydrogen & hydroxide ion concentrations is always 10 -14.
In other words [H+] x [OH–] = 10-14.
Presence of other substances can cause either the hydrogen or hydroxide ion to
dominate, resulting in acidic or alkaline water respectively.
Hydrogen ions play a critical role in the weathering and breakdown of rock.
pH and Acidity
pH is defined as the negative logarithm of the hydrogen ion concentration
pH = - log [H-]
In pure water [H+] = [OH-] and [H+][OH-] = 10–14, so [H+] = 10-7 and pH = 7.
Hydrogen ions dominate when pH < 7: acidic, hydroxyl ions dominate when pH > 7:
alkaline.
In natural water in contact with the atmosphere, some CO2 will dissolve in water:
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3Thus pure rain water would be slightly acidic. Hydrogen ions produced in this way are responsible for
breaking down rock in the process called “weathering”.
Light Transmission through Water
Light, and other forms of electromagnetic radiation, do not penetrate well through water.
Blue light propagates better through water than other wavelengths.
(infrared propagation particularly poor).
Sunlight penetrates to a maximum of 200 m through ocean water (usually less).
Consequently
Ocean heat gained and lost at the surface
Photosynthesis restricted to surface waters.
Seismic Waves
Motion produces waves that propagate through material.
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EAS 2200
The Earth System
Spring 2012
Lecture 4
In gases and liquids, only compressional and surface waves are possible
Compressional (p) waves are familiar sound waves in which pressure varies in wave-like manner. Vibration is
parallel to direction of wave propagation.
Surface waves are familiar as wind-driven ocean waves, in which rotary motion occurs at the interface
between materials of different density.
In solids, an additional wave is possible, a shear (s) wave, in which the material vibrates
perpendicular to the direction of wave motion.
Wave Velocities
General expression for wave velocity is:
For the p-wave:
For the s-wave:
VP =
Velocity =
elastic modulus
density
4
µ
3
ρ
κ+
VS =
µ
ρ
Where ρ is density (m/v) (mass/volume)
κ is the compressibility or bulk modulus: κ = -v(dP/dv)
This is the resistance of the material to compression
µ is the rigidity modulus - resistance to change in shape
µ=
shear stress
shear strain
Earth Materials and Seismic Waves
We can use seismic waves somewhat like X-rays to probe the Earth’s interior. To make
sense of this, we need to know how seismic properties relate to materials.
Seismic wave velocities depend on material properties: density and the elastic moduli.
These depend on the nature of the minerals: their structure and composition.
Density
Density depends on:
The atoms present in the mineral (i.e., composition). For example, iron bearing minerals are more dense
than other ones:
Forsterite Olivine (Mg 2SiO4): 3227 kg/m 3

Fayalite Olivine (Fe 2SiO4): 4402 kg/m 3
The difference reflects the difference in atomic weight between magnesium (atomic weight 24.3)
and iron (55.8)
How tightly packed the atoms are in the crystal. For example, diamond and graphite both consist of C
(atomic weight 12).
Diamond: 3516 kg/m 3
Graphite: 2281 kg/m 3
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EAS 2200
The Earth System
Spring 2012
Lecture 4
Elastic Moduli
Seismic velocities also depend on two elastic moduli, the compressibility (or bulk modulus)
and the rigidity modulus.
These depend on how strong the bonds are between atoms and how tightly they are
packed.
For anisotropic minerals, the elastic moduli will vary depending on the crystalographic
direction
Thus seismic velocities can be faster in one direction than another.
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