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.

Magma Plumbing

System

Melts form in mantle

Pool in magma chambers

Magma eventually erupts

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

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

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

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

What are the sources of heat What are the sources of heat within Earth?within Earth?

Primordial/accretional energyPrimordial/accretional energy

Radioactive decayRadioactive decay

““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.

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

Rates of Heat Production and Half-livesRates of Heat Production and Half-lives

Radioactive DecayRadioactive Decay

The Law of Radioactive Decay

dN

dtN or

dN

dt= N

# pa

rent

ato

ms

# pa

rent

ato

ms

time time

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

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

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

• Blocking temperatures for various minerals differ

• 40Ar-39Ar technique grew from this discovery

Heat Production through Earth HistoryHeat Production through Earth History

Earth Structure

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

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

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

Mantle XenolithsMantle Xenoliths

http://www.nhm.ac.uk/mineralogy/petrology/MantleXenoliths.htm

Carbonaceous ChondritesCarbonaceous Chondrites

http://www.daviddarling.info/encyclopedia/C/carbchon.html

Left to right: fragments of the Allende, Yukon, and Murchison meteorites

Mantle vs Model CCMantle vs Model CC

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)

Mineralogy of MantleMineralogy of Mantle

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

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

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..

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 !

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

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

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.

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.

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!

• 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

Earth Structure

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

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.

Convection Examples

Rayleigh-Bernard Convection

Convection in the Mantle

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

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

Earth’s Plates

Where Volcanoes OccurWhere Volcanoes Occur

Volcano geographyVolcano geography

1. Divergent margins 2. Convergent margins3. Intraplate 4. Hotspots

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)

What are the plate tectonic settings in What are the plate tectonic settings in which magmatism occurs?which magmatism occurs?

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?

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

Processes of Processes of Partial Partial MeltingMelting

A simpler phase A simpler phase diagram (P-T) diagram (P-T)

diagram of mantlediagram of mantle

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

Processes of Partial MeltingProcesses of Partial MeltingLowering solidus by volatile additionLowering solidus by volatile addition

Temperature

Processes of Partial MeltingProcesses of Partial MeltingAdiabatic DecompressionAdiabatic Decompression

Pre

ssu

re

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

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 !

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

Divergent settings: The Mid-Divergent settings: The Mid-ocean Ridgeocean Ridge

Bathymetry of the East Pacific RiseBathymetry of the East Pacific Rise

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.

MORB GenesisMORB Genesis

Intraplate settings: Mantle Intraplate settings: Mantle PlumesPlumes

Proposed Hot Spot TracesProposed Hot Spot Traces

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

Shallow Magma Plumbing SystemShallow Magma Plumbing System

Geometry of Magma Reservoir beneath Geometry of Magma Reservoir beneath KilaueaKilauea

Convergent settings: Convergent settings: Subduction Zone MagmatismSubduction Zone Magmatism

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

Relative VolumesRelative Volumes

What are the relative volumes of eruption What are the relative volumes of eruption and intrusion?and intrusion?

What are the relative volumes of eruption What are the relative volumes of eruption and intrusion?and intrusion?

Volumes of Igneous Rocks on EarthVolumes of Igneous Rocks on Earth

Convergent Margin Magma GenesisConvergent Margin Magma Genesis

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.

Basalt Types-Major Element VariationBasalt Types-Major Element Variation

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

K2O content of subalkaline rocks

K2O contentmay broadlycorrelate withcrustal thickness.

Low-K 12 kmMed-K 35 kmHigh-K 45 km

Ewart, 1982

Yoder & Tilley Basalt Tetrahedron

Yoder & Tilley, 1962; Le Maitre

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.

Lunar Surface

Apollo 15 Basalt Sample

Vesicles -Probablyderived fromCO degassing

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

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

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

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.

Lunar Interior Composition

From: BVSP, 1986 and Taylor, 1987

1984 Mauna Loa Eruption

Curtain of lava

Phase 1: Pu’u O’o

Phase 1: Pu’u O’o

Fire Fountain

Pu’u O’o Vent with pahoehoe

flows

Pahoehoe flow, Kilauea

Tree Molds, ~1983

Halemaumau, Kilauea

Surtsey, Iceland

A new volcanic island formed in 1966A new volcanic island formed in 1966

Cerro Negro, Nicaragua

Stromboli Volcano, Italy

Paricutin, Mexico

1943-1954

Mt. Augustine, Alaska

Augustine

Note hummocky topography from debris avalanche, 1883

Eruption of Mt. Augustine, 1986

Crater Lake

Crater Lake

Ol Doinyo Lengai

A sodium carbonatite volcano in the Rift Valley of East AfricaA sodium carbonatite volcano in the Rift Valley of East Africa

Ol Doinyo Lengai

A sodium carbonatite volcano in the Rift Valley of East AfricaA sodium carbonatite volcano in the Rift Valley of East Africa

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.

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/