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What is a volcano?What is a volcano?
A hill with a crater?
Does magma need to be involved?
Does it matter?
Lecture material about Introduction to Volcanology, covering, Heat in the earth, where magma comes from and how, earth’s mantle, tectonics and convection, basalt and why it is fundamental, where volcanoes are.
Thanks to Wendy BohrsonWendy Bohrson and Glen MattioliGlen Mattioli who provided many of the slides.
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Magma Plumbing
System
Melts form in mantle
Pool in magma chambers
Magma eventually erupts
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VolcanologyVolcanology
Study of generation of magma, transport Study of generation of magma, transport of magma, and shallow-level or surface of magma, and shallow-level or surface processes that result from intrusion and processes that result from intrusion and
eruption of magmaeruption of magma
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VolcanologyVolcanology
Physical and chemical behavior of Physical and chemical behavior of magmasmagmas
Transport and eruption of magmaTransport and eruption of magma
Formation of volcanic depositsFormation of volcanic deposits
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What do we need for volcanism?What do we need for volcanism?
Thermal energyThermal energy
Material to meltMaterial to melt
Ability to eruptAbility to erupt
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Earth’s Energy BudgetEarth’s Energy Budget• Solar radiationSolar radiation: 50,000 times greater than all other energy sources; primarily
affects the atmosphere and oceans, but can cause changes in the solid earth through momentum transfer from the outer fluid envelope to the interior
• Radioactive decayRadioactive decay: 238U, 235U, 232Th, 40K, and 87Rb all have t1/2 that >109 years and thus continue to produce significant heat in the interior; this may equal 50 to 100% of the total heat production for the Earth. Extinct short-lived radioactive elements such as 26Al were important during the very early Earth.
• Tidal HeatingTidal Heating: Earth-Sun-Moon interaction; much smaller than radioactive decay
• Primordial Heat:Primordial Heat: Also known as accretionary heat; conversion of kinetic energy of accumulating planetismals to heat.
• Core FormationCore Formation: Initial heating from short-lived radioisotopes and accretionary heat caused widespread interior melting (Magma Ocean) and additional heat was released when Fe sank toward the center and formed the core
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What are the sources of heat What are the sources of heat within Earth?within Earth?
Primordial/accretional energyPrimordial/accretional energy
Radioactive decayRadioactive decay
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““Natural” RadioactivityNatural” Radioactivity• Elements (determined by Z) typically exist as a mix of Elements (determined by Z) typically exist as a mix of
isotopes which have different atomic weights (eg isotopes which have different atomic weights (eg 3939K K and and 4040K, where Z=19).K, where Z=19).
• Isotopes may be stable, radioactive or radiogenic.Isotopes may be stable, radioactive or radiogenic.• 3939K is stable, K is stable, 4040K is radioactive, K is radioactive, 4040A and A and 4040Ca radiogenic.Ca radiogenic.• Decay of radioactive isotopes has a very predictable Decay of radioactive isotopes has a very predictable
rate: N = Nrate: N = Nooee--t t ..• This decay occurs spontaneously everywhere and is not This decay occurs spontaneously everywhere and is not
influenced by changes in T, P or composition!influenced by changes in T, P or composition!• Decay reactions of many types occur: Decay reactions of many types occur: 4040K-> K-> 4040Ca + Ca +
electron + heat.electron + heat.• Discovered by Marie Curie.Discovered by Marie Curie.
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Natural Radioactivity is exploited by volcanologists and petrologists.
1. Radiometric dating. System of 40K->40A leads to K/A and A/A dating methodology. These use the age eqn and depend on purging of A at time of eruption.
2. Radioactive Tracing. Use isotopic ratios of elements to tell where the magma came from. Ex: 87Sr/86Sr this is radiogenic/stable, so it can measure the amounts of radioactive parent= 87Rb
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Rates of Heat Production and Half-livesRates of Heat Production and Half-lives
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Radioactive DecayRadioactive Decay
The Law of Radioactive Decay
dN
dtN or
dN
dt= N
# pa
rent
ato
ms
# pa
rent
ato
ms
time time
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D = Net - N = N(et -1)
age of a sample (t) if we know: D the amount of the daughter nuclide produced
N the amount of the original parent nuclide remaining
the decay constant for the system in question
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The K-Ar System40K either 40Ca or 40Ar
– 40Ca is common. Cannot distinguish radiogenic 40Ca from non-radiogenic 40Ca
– 40Ar is an inert gas which can be trapped in
many solid phases as it forms in them
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The appropriate decay equation is:The appropriate decay equation is:
4040Ar = Ar = 4040ArAroo + + 4040K(eK(e--t t -1)-1)
Where Where ee = 0.581 x 10 = 0.581 x 10-10-10 a a-1-1 (electron capture) (electron capture)
and and = 5.543 x 10 = 5.543 x 10-10-10 a a-1 -1 (whole process) (whole process)
e
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• Blocking temperatures for various minerals differ
• 40Ar-39Ar technique grew from this discovery
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Heat Production through Earth HistoryHeat Production through Earth History
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Earth Structure
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How do we know the composition of How do we know the composition of the mantle?the mantle?
Peridotite bodies (e.g., ophiolites)Peridotite bodies (e.g., ophiolites)
XenolithsXenoliths
Cosmochemical Evidence/MeteoritesCosmochemical Evidence/Meteorites
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OphiolitesOphiolites
Seismic velocity is plotted on the horizontal axis versus depth below the seafloor on the vertical axis. The different seismic layers are marked on the plot with geologic interpretations of the rock units. The layers are defined by velocities and velocity gradients. Cross section through a typical ophiolite sequence is shown to the right.
http://www.womenoceanographers.org/doc/KGillis/Lesson/gillis_lesson.htm
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OphiolitesOphiolites
Picture of a hillside in Cyprus. The vertical slabs of rock are dikes intruding into lavas that erupted on the seafloor. This section represents the transition from lavas to sheeted dikes and is thought to correspond to seismic Layer 2B as seen in Figure 5. Taken from the RIDGE field school in Cyprus.
http://www.womenoceanographers.org/doc/KGillis/Lesson/gillis_lesson.htm
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Mantle XenolithsMantle Xenoliths
http://www.nhm.ac.uk/mineralogy/petrology/MantleXenoliths.htm
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Carbonaceous ChondritesCarbonaceous Chondrites
http://www.daviddarling.info/encyclopedia/C/carbchon.html
Left to right: fragments of the Allende, Yukon, and Murchison meteorites
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Mantle vs Model CCMantle vs Model CC
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Composition of the MantleComposition of the Mantle
What is the mineralogy of the mantle?What is the mineralogy of the mantle?
Olivine +clinopyroxene + orthopyroxene Olivine +clinopyroxene + orthopyroxene ± plagioclase, garnet, spinel (Al bearing ± plagioclase, garnet, spinel (Al bearing minerals)minerals)
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Mineralogy of MantleMineralogy of Mantle
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obvious from space that Earth has two fundamentally differentphysiographic features: oceans (71%) and continents (29%)
global topography
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
crust
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Differentiation of the Earth
Mantle
Continental Crust
Rb>SrNd>Sm
La Lu
La>Lu
La LuRb<SrNd<SmLa<Lu
Rb>SrNd>SmLa>Lu
(After partialmelt extraction)
• Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements– Continental crust becomes “incompatible element enriched”– Mantle becomes “incompatible element depleted”
From: http://www.geo.cornell.edu/geology/classes/geo302
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Radioactivity in earth materialsRadioactivity in earth materialsRock Type
238U ppm
235U ppm
232Th ppm
40 K ppm
Heat mWkg-1 x 10-8
Cont crust
3.9 0.03 18 3.5 96
Ocean crust
.79 .006 3 .96 18
Mantle .01 7x10-5 0.06 1.2x10-3 0.26
Meteor-ites
.01 7x10-5 0.38 0.1 0.50
Heat production decreases with depth from crust to mantleHeat production decreases with depth from crust to mantle..
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Earth’s Geothermal GradientEarth’s Geothermal GradientA
ppro
xim
ate
Pre
ssur
e (G
Pa=
10
kbar
)
Average Heat Flux isAverage Heat Flux is0.09 watt/meter0.09 watt/meter22
Geothermal gradient = Geothermal gradient = / / zz
C/km in orogenic belts;C/km in orogenic belts;Cannot remain constant w/depthCannot remain constant w/depthAt 200 km would be 4000°CAt 200 km would be 4000°C
~7°C/km in trenches~7°C/km in trenches
Viscosity, which measuresViscosity, which measuresresistance to flow, of mantleresistance to flow, of mantlerocks is 10rocks is 101818 times tar at 24°C ! times tar at 24°C !
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Earth Interior PressuresEarth Interior Pressures
P = P = Vg/A = Vg/A = gz, if we integrate from the surface to somegz, if we integrate from the surface to somedepth z and take positive downward we getdepth z and take positive downward we get
P/P/z = z = gg
Rock densities range from 2.7 (crust) to 3.3 g/cm3 (mantle)270 bar/km for the crust and 330 bar/km for the mantle
At the base of the crust, say at 30 km depth, the lithostatic pressurewould be 8100 bars = 8.1 kbar = 0.81 GPa
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Gravity, Pressure, and the Geobaric Gradient
• Geobaric gradient defined similarly to geothermal gradient: P/z; in the interior this is related to the overburden of the overlying rocks and is referred to as lithostatic pressure gradient.
• SI unit of pressure is the pascal, Pa and 1 bar (~1 atmosphere) = 105 Pa
Pressure = Force / Area and Force = mass * acceleration
P = F/A = (m*g)/A and (density) =mass/volume
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Heat Flow on Earth
An increment of heat, q, transferred into a body produces aProportional incremental rise in temperature, T, given by
q = Cp * T
where Cp is called the molar heat capacity of J/mol-degreeat constant pressure; similar to specific heat, which is basedon mass (J/g-degree).
1 calorie = 4.184 J and is equivalent to the energy necessaryto raise 1 gram of of water 1 degree centigrade. Specific heat of water is 1 cal/g°C, where rocks are ~0.3 cal/g°C.
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Heat Transfer Mechanisms
• Radiation: involves emission of EM energy from the surface of hot body into the transparent cooler surroundings. Not important in cool rocks, but increasingly important at T’s >1200°C
• Advection: involves flow of a liquid through openings in a rock whose T is different from the fluid (mass flux). Important near Earth’s surface due to fractured nature of crust.
• Conduction: transfer of kinetic energy by atomic vibration. Cannot occur in a vacuum. For a given volume, heat is conducted away faster if the enclosing surface area is larger.
• Convection: movement of material having contrasting T’s from one place to another. T differences give rise to density differences. In a gravitational field, higher density (generally colder) materials sink.
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Magmatic Examples of Heat Transfer
Thermal Gradient T betweenadjacent hotter and cooler masses
Heat Flux = rate at which heat isconducted over time from a unitsurface area
Heat Flux = Thermal Conductivity * T
Thermal Conductivity = K; rockshave very low values and thusdeep heat has been retained!
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• Conduction
Types of Thermal Energy Types of Thermal Energy Transfer Transfer
• Convection
Models of Earth’s interior converge on core Ts of Models of Earth’s interior converge on core Ts of 4000°C ± 500 °C4000°C ± 500 °C
Thermal energy moves from hot to cold--> thus, Thermal energy moves from hot to cold--> thus, modes of energy transport within Earth:modes of energy transport within Earth:
• Radiation
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Earth Structure
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How do we know that convection How do we know that convection is important? is important?
Thought experiment:Thought experiment:
Distance heat transported by conduction = Distance heat transported by conduction =
sqrt (thermal diffusivity * age of Earth)sqrt (thermal diffusivity * age of Earth)
• Thermal diffusivity = 10-6 m2/s
• 3.2 x 107 sec/yr
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How do we know that convection How do we know that convection is important? is important?
10-6 m2/s * 4.5 x 109 yr * 3.2 x 107 sec/yr =
380 km
Radius of Earth = 6371 km
Conclusion: barely any heat transported by conduction. Requires a convective mechanism.
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Convection Examples
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Rayleigh-Bernard Convection
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Convection in the Mantle
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convection in the mantle
models
observed heat flowwarmer: near ridgescolder: over cratons
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
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note continuity of blue slab to depths on order of 670 km
blue is high velocity (fast) …interpreted as slab
from: http://www.pmel.noaa.gov/vents/coax/coax.html
examples from western Pacific
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Earth’s Plates
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Where Volcanoes OccurWhere Volcanoes Occur
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Volcano geographyVolcano geography
1. Divergent margins 2. Convergent margins3. Intraplate 4. Hotspots
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Plate tectonics and magma compositionPlate tectonics and magma composition
1. Divergent margins: Plate separation and decompression melting -> low volatile abundance, low SiO2 (~50%), low viscosity basaltic magmas (e.g. Krafla,
Iceland)2. Convergent margins : Mixtures of basalt from the mantle, remelted continental crust and material from the subducted slab. High volatile abundance, intermediate SiO2 (60-70%), high viscosity andesites and dacites (e.g.
Montserrat, West Indies) 3. Intraplate `Hot-spot` settings:
A. Oceanic: Mantle plumes melt thin oceanic crust producing low viscosity basaltic magmas (e.g. Kilauea, Hawaii)
B. Continental: Mantle plumes melt thicker, silicic continental crust producing highly silicic (>70% SiO2)
rhyolites (e.g. Yellowstone, USA)
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What are the plate tectonic settings in What are the plate tectonic settings in which magmatism occurs?which magmatism occurs?
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Processes of Partial MeltingProcesses of Partial Melting
Precursor to all igneous rocks is magma Precursor to all igneous rocks is magma or melt (liquid rock)or melt (liquid rock)
How does melting occur?How does melting occur?
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Processes of Partial MeltingProcesses of Partial MeltingLet’s first look at a phase diagram (P-T) Let’s first look at a phase diagram (P-T)
diagram of mantlediagram of mantle
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Processes of Processes of Partial Partial MeltingMelting
A simpler phase A simpler phase diagram (P-T) diagram (P-T)
diagram of mantlediagram of mantle
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Processes of Partial MeltingProcesses of Partial Melting
What causes partial melting in the What causes partial melting in the mantle?mantle?
Two processes:Two processes: Lowering of solidus by volatile additionLowering of solidus by volatile addition Adiabatic DecompressionAdiabatic Decompression
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Processes of Partial MeltingProcesses of Partial MeltingLowering solidus by volatile additionLowering solidus by volatile addition
Temperature
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Processes of Partial MeltingProcesses of Partial MeltingAdiabatic DecompressionAdiabatic Decompression
Pre
ssu
re
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The MantleThe Mantle
Why is melting in the mantle important?Why is melting in the mantle important?
Because most of the melts that make Because most of the melts that make extrusive rocks on Earth originate in the extrusive rocks on Earth originate in the
mantle mantle
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Earth’s Geothermal GradientEarth’s Geothermal GradientA
ppro
xim
ate
Pre
ssur
e (G
Pa=
10
kbar
)
Average Heat Flux isAverage Heat Flux is0.09 watt/meter0.09 watt/meter22
Geothermal gradient = Geothermal gradient = / / zz
C/km in orogenic belts;C/km in orogenic belts;Cannot remain constant w/depthCannot remain constant w/depthAt 200 km would be 4000°CAt 200 km would be 4000°C
~7°C/km in trenches~7°C/km in trenches
Viscosity, which measuresViscosity, which measuresresistance to flow, of mantleresistance to flow, of mantlerocks is 10rocks is 101818 times tar at 24°C ! times tar at 24°C !
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Mechanisms of melt formationMechanisms of melt formation
1.1. MOR = Adiabatic MOR = Adiabatic decompressiondecompression
Intraplate = Intraplate = adiabatic adiabatic
decompressiondecompression
Convergent = Convergent = change in solidus change in solidus by volatile fluxingby volatile fluxing
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Divergent settings: The Mid-Divergent settings: The Mid-ocean Ridgeocean Ridge
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Bathymetry of the East Pacific RiseBathymetry of the East Pacific Rise
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Magma Chamber Structure beneath East Magma Chamber Structure beneath East Pacific RisePacific Rise
Volcanic layer transitions into sheeted dike zone, which represents feeder zone from magma chamber.Below is a sill-like magma body (1-2 km depth) that transitions to crystal mush (partially solidified zone >50% crystals).Transitional zone is solidified but hot gabbro.
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MORB GenesisMORB Genesis
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Intraplate settings: Mantle Intraplate settings: Mantle PlumesPlumes
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Proposed Hot Spot TracesProposed Hot Spot Traces
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Magma Plumbing System for HawaiiMagma Plumbing System for Hawaii
Zone of partial melting at depth (>100 km)Magma ascends through conduit systemPresence of summit reservoir and rift zones
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Shallow Magma Plumbing SystemShallow Magma Plumbing System
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Geometry of Magma Reservoir beneath Geometry of Magma Reservoir beneath KilaueaKilauea
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Convergent settings: Convergent settings: Subduction Zone MagmatismSubduction Zone Magmatism
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Characteristics of Subduction Zone MagmatismCharacteristics of Subduction Zone Magmatism
Down-going, hydrated slab undergoes metamorphism and dehydrationFluids infiltrate overlying mantle “wedge”Reduces solidus and melting can occurProduces arc magmatism
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Relative VolumesRelative Volumes
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What are the relative volumes of eruption What are the relative volumes of eruption and intrusion?and intrusion?
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What are the relative volumes of eruption What are the relative volumes of eruption and intrusion?and intrusion?
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Volumes of Igneous Rocks on EarthVolumes of Igneous Rocks on Earth
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Convergent Margin Magma GenesisConvergent Margin Magma Genesis
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Classification of Igneous Rocks
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al. (1986) J. Petrol., 27, 745-750. Oxford University Press.
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Basalt Types-Major Element VariationBasalt Types-Major Element Variation
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Alkaline and Subalkaline Rock Suites
Irregular solid line defines the boundary between Ne-norm rocks
15,164 samples
Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988
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K2O content of subalkaline rocks
K2O contentmay broadlycorrelate withcrustal thickness.
Low-K 12 kmMed-K 35 kmHigh-K 45 km
Ewart, 1982
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Yoder & Tilley Basalt Tetrahedron
Yoder & Tilley, 1962; Le Maitre
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Terrestrial Basalt Generation Summary
• MORBs are derived from the partial melting of a previously depleted upper mantle under largely anhydrous conditions at relatively shallow depths.
• True primary mantle melts are rare, although the most primitive alkali basalts are thought to represent the best samples of direct mantle melts.
• The trace element and isotopic ratio differences among N-MORB (normal), E-MORB (enriched), IAB, and OIB indicate that the Earth’s upper mantle has long-lived and physically distinct source regions.
• Ancient komatiites (>2.5 Ga) indicate that the Earth’s upper mantle was hotter in the Archean, but already depleted of continental crustal components.
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Lunar Surface
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Apollo 15 Basalt Sample
Vesicles -Probablyderived fromCO degassing
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Lunar Olivine Basalt Thinsection
From: http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/12005.htm
Sample collected from the SE end of Mare Procellarum by the Apollo 12 mission.
Interpreted as a Lava Lake basalt.
Olivine + aligned MIs
Pyroxenes
Plane Polarized Light
Cross Polarized Light
Fe-Ti oxides
Plagioclase
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Lunar Anorthosite Thinsection
From: http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/12005.htm
Cross Polarized Light
Plane Polarized Light
Highly brecciated lunar anorthosite wascollected by the Apollo 16 mission to the lunar highlands SW of Mare Tranquillitatis. It has been dated at 4.44 Ga.
Fractured Plagioclase Feldspar
Pyroxenes
Rock is 98% fsp,An95 to An97
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Earth Mars-sized Impact Model for Lunar Origin
From: Kipp & Melosh, 1986 (above) and W. Hartmann paintings of Cameron, Benz, & Melosh models (right)
Impact + 5hr
Impact + 0.5 hr
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Features of the Giant Impact Hypothesis• Original idea paper by Hartmann & Davis, 1975; additional
geochemical research by Michael Drake and computer models by Jay Melosh and colleagues.
• Impact occurs soon after Earth’s core formation event because of the small lunar Fe core and difference in bulk density (Moon = 3.3 g/cc << Earth = 5.5 g/cc).
• Impact event must occur before formation of the lunar highlands at 4.4 Ga, which formed as a result of the crystallization of the lunar magma ocean. Lunar differentiation continues w/ basalt genesis (3.95 to 3.15 Ga).
• Oxygen isotope compositions of lunar and terrestrial rocks are similar, but different from Mars and meteorites. Earth-Moon must be made of the same stuff.
• Volatiles are depleted in the proto-moon during impact event. This is consistent with geochemistry and petrology of lunar samples.
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Lunar Interior Composition
From: BVSP, 1986 and Taylor, 1987
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1984 Mauna Loa Eruption
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Curtain of lava
Phase 1: Pu’u O’o
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Phase 1: Pu’u O’o
Fire Fountain
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Pu’u O’o Vent with pahoehoe
flows
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Pahoehoe flow, Kilauea
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Tree Molds, ~1983
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Halemaumau, Kilauea
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Surtsey, Iceland
A new volcanic island formed in 1966A new volcanic island formed in 1966
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Cerro Negro, Nicaragua
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Stromboli Volcano, Italy
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Paricutin, Mexico
1943-1954
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Mt. Augustine, Alaska
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Augustine
Note hummocky topography from debris avalanche, 1883
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Eruption of Mt. Augustine, 1986
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Crater Lake
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Crater Lake
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Ol Doinyo Lengai
A sodium carbonatite volcano in the Rift Valley of East AfricaA sodium carbonatite volcano in the Rift Valley of East Africa
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Ol Doinyo Lengai
A sodium carbonatite volcano in the Rift Valley of East AfricaA sodium carbonatite volcano in the Rift Valley of East Africa
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Olympus Mons, Mars
A giant Martian volcano 25 km high and 700 km wide. The Island of Maui in Hawaii would fit inside the huge caldera of Olympus Mons.
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Sources• http://www.doubledeckerpress.com/archive.htm• http://pubs.usgs.gov/gip/volc/types.html• http://hvo.wr.usgs.gov/• http://cvo.wr.usgs.gov/
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