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Transcript
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 Bohrson and Glen Mattioli who provided
many of the slides.
Magma
Plumbing
System
Melts form in mantle
Pool in magma
chambers
Magma eventually
erupts
Volcanology
Study of generation of magma, transport
of magma, and shallow-level or surface
processes that result from intrusion and
eruption of magma
Volcanology
Physical and chemical behavior of
magmas
Transport and eruption of magma
Formation of volcanic deposits
What do we need for volcanism?
Thermal energy
Material to melt
Ability to erupt
Earth’s Energy Budget
• Solar 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 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 Heating: Earth-Sun-Moon interaction; much smaller than
radioactive decay
• Primordial Heat: Also known as accretionary heat; conversion of
kinetic energy of accumulating planetismals to heat.
• Core 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
within Earth?
Primordial/accretional energy
Radioactive decay
“Natural” Radioactivity
• Elements (determined by Z) typically exist as a mix of
isotopes which have different atomic weights (eg 39K
and 40K, where Z=19).
• Isotopes may be stable, radioactive or radiogenic.
• 39K is stable, 40K is radioactive, 40A and 40Ca radiogenic.
• Decay of radioactive isotopes has a very predictable
rate: N = Noe-t .
• This decay occurs spontaneously everywhere and is not
influenced by changes in T, P or composition!
• Decay reactions of many types occur: 40K-> 40Ca +
electron + heat.
• 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-lives
Radioactive Decay
The Law of Radioactive Decay
dN
N
dt
dN
or = N
dt
time 
D = Net - N = N(et -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 System
40K
 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:
40Ar
=
40Ar
o+
 e 
 
 
40K(e-t -1)
Where e = 0.581 x 10-10 a-1 (electron capture)
and  = 5.543 x 10-10 a-1 (whole process)
• Blocking temperatures for various minerals
differ
• 40Ar-39Ar technique grew from this discovery
Heat Production through Earth History
Earth Structure
How do we know the composition of
the mantle?
Peridotite bodies (e.g., ophiolites)
Xenoliths
Cosmochemical Evidence/Meteorites
Ophiolites
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
Ophiolites
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 Xenoliths
http://www.nhm.ac.uk/mineralogy/petrology/MantleXenoliths.htm
Carbonaceous Chondrites
Left to right: fragments of the Allende, Yukon, and Murchison meteorites
http://www.daviddarling.info/encyclopedia/C/carbchon.html
Mantle vs Model CC
Composition of the Mantle
What is the mineralogy of the mantle?
Olivine +clinopyroxene + orthopyroxene
± plagioclase, garnet, spinel (Al bearing
minerals)
Mineralogy of Mantle
crust
obvious from space that Earth has two fundamentally different
physiographic features: oceans (71%) and continents (29%)
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
global topography
Differentiation of the Earth
Rb>Sr
Nd>Sm
La>Lu
Continental Crust
La
Lu
Rb>Sr
Nd>Sm
La>Lu
Mantle
(After partial
melt extraction)
Rb<Sr
Nd<Sm
La<Lu
La
Lu
• 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 materials
Rock
Type
238U
235U
232Th
40
ppm
ppm
K
ppm
Cont
3.9
crust
Ocean .79
crust
Mantle .01
0.03
18
3.5
96
.006
3
.96
18
7x10-5
0.06
1.2x10-3 0.26
Meteor .01
-ites
7x10-5
0.38
0.1
ppm
Heat
mWkg-1 x
10-8
0.50
Heat production decreases with depth from crust to mantle.
Approximate Pressure (GPa=10 kbar)
Earth’s Geothermal Gradient
Average Heat Flux is
0.09 watt/meter2
Geothermal gradient = DT/ Dz
20-30C/km in orogenic belts;
Cannot remain constant w/depth
At 200 km would be 4000°C
~7°C/km in trenches
Viscosity, which measures
resistance to flow, of mantle
rocks is 1018 times tar at 24°C !
Earth Interior Pressures
P = rVg/A = rgz, if we integrate from the surface to some
depth z and take positive downward we get
DP/Dz = rg
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 pressure
would be 8100 bars = 8.1 kbar = 0.81 GPa
Gravity, Pressure, and the Geobaric Gradient
•
Geobaric gradient defined similarly to geothermal gradient: DP/Dz; 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 r (density) = mass/volume
Heat Flow on Earth
An increment of heat, Dq, transferred into a body produces a
Proportional incremental rise in temperature, DT, given by
Dq = Cp * DT
where Cp is called the molar heat capacity of J/mol-degree
at constant pressure; similar to specific heat, which is based
on mass (J/g-degree).
1 calorie = 4.184 J and is equivalent to the energy necessary
to 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 = DT between
adjacent hotter and cooler masses
Heat Flux = rate at which heat is
conducted over time from a unit
surface area
Thermal Conductivity = K; rocks
have very low values and thus
deep heat has been retained!
Heat Flux = Thermal Conductivity * DT
Types of Thermal Energy
Transfer
Models of Earth’s interior converge on core Ts
of 4000°C ± 500 °C
Thermal energy moves from hot to cold-->
thus, modes of energy transport within
Earth:
• Conduction
• Convection
• Radiation
Earth Structure
How do we know that convection
is important?
Thought experiment:
Distance heat transported by conduction =
sqrt (thermal diffusivity * age of Earth)
• Thermal diffusivity = 10-6 m2/s
• 3.2 x 107 sec/yr
How do we know that convection
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
from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
models
convection in the mantle
observed heat flow
warmer: near ridges
colder: over cratons
from: http://www-personal.umich.edu/~vdpluijm/gs205.html
examples from western Pacific
blue is high velocity (fast)
…interpreted as slab
note continuity of blue slab
to depths on order of 670 km
from: http://www.pmel.noaa.gov/vents/coax/coax.html
Earth’s Plates
Where Volcanoes Occur
Volcano geography
1. Divergent margins
2. Convergent margins
3. Intraplate 4. Hotspots
Plate 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
which magmatism occurs?
Processes of Partial Melting
Precursor to all igneous rocks is magma
or melt (liquid rock)
How does melting occur?
Processes of Partial Melting
Let’s first look at a phase diagram (P-T)
diagram of mantle
Processes of
Partial
Melting
A simpler phase
diagram (P-T)
diagram of mantle
Processes of Partial Melting
What causes partial melting in the
mantle?
Two processes:
 Lowering of solidus by volatile addition
 Adiabatic Decompression
Processes of Partial Melting
Lowering solidus by volatile addition
Temperature
Processes of Partial Melting
Pressure
Adiabatic Decompression
The Mantle
Why is melting in the mantle important?
Because most of the melts that make
extrusive rocks on Earth originate in the
mantle
Approximate Pressure (GPa=10 kbar)
Earth’s Geothermal Gradient
Average Heat Flux is
0.09 watt/meter2
Geothermal gradient = DT/ Dz
20-30C/km in orogenic belts;
Cannot remain constant w/depth
At 200 km would be 4000°C
~7°C/km in trenches
Viscosity, which measures
resistance to flow, of mantle
rocks is 1018 times tar at 24°C !
Mechanisms of melt formation
1.
MOR = Adiabatic
decompression
Intraplate =
adiabatic
decompression
Convergent =
change in solidus
by volatile fluxing
Divergent settings: The Midocean Ridge
Bathymetry of the East Pacific Rise
Magma Chamber Structure beneath East
Pacific 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 Genesis
Intraplate settings: Mantle
Plumes
Proposed Hot Spot Traces
Magma Plumbing System for Hawaii
Zone of partial melting at depth (>100 km)
Magma ascends through conduit system
Presence of summit reservoir and rift zones
Shallow Magma Plumbing System
Geometry of Magma Reservoir beneath
Kilauea
Convergent settings:
Subduction Zone Magmatism
Characteristics of Subduction Zone Magmatism
Down-going, hydrated slab undergoes metamorphism and dehydration
Fluids infiltrate overlying mantle “wedge”
Reduces solidus and melting can occur
Produces arc magmatism
Relative Volumes
What are the relative volumes of eruption
and intrusion?
What are the relative volumes of eruption
and intrusion?
Volumes of Igneous Rocks on Earth
Convergent 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 Variation
Alkaline and Subalkaline Rock Suites
15,164 samples
Irregular solid line defines the boundary between Ne-norm rocks
Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988
K2O content of subalkaline rocks
K2O content
may broadly
correlate with
crustal thickness.
Low-K 12 km
Med-K 35 km
High-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 Probably
derived from
CO degassing
Lunar Olivine Basalt Thinsection
Fe-Ti oxides
Plagioclase
Olivine + aligned MIs
Pyroxenes
Plane Polarized Light
Sample collected from the SE end of
Mare Procellarum by the Apollo 12 mission.
Interpreted as a Lava Lake basalt.
Cross Polarized Light
From: http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/12005.htm
Lunar Anorthosite Thinsection
Pyroxenes
Fractured Plagioclase Feldspar
Rock is 98% fsp,
An95 to An97
Plane Polarized Light
Highly brecciated lunar anorthosite was
collected by the Apollo 16 mission to the
lunar highlands SW of Mare Tranquillitatis.
It has been dated at 4.44 Ga.
Cross Polarized Light
From: http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/moon_rocks/12005.htm
Earth Mars-sized Impact Model for Lunar Origin
Impact + 0.5 hr
Impact + 5hr
From: Kipp & Melosh, 1986 (above) and W. Hartmann paintings of Cameron, Benz, & Melosh models (right)
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 (rMoon = 3.3 g/cc << rEarth = 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. EarthMoon 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
Phase 1: Pu’u O’o
Curtain of lava
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 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 Africa
Ol Doinyo Lengai
A 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.ht
m
• http://pubs.usgs.gov/gip/volc/types.html
• http://hvo.wr.usgs.gov/
• http://cvo.wr.usgs.gov/