1. 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.

2. Magma Plumbing System Melts form in mantle Pool in magma chambers
Magma eventually erupts

3. Volcanology
Study of generation of magma, transport of magma, and shallow-level or surface processes that result from intrusion and eruption of magma

4. Volcanology Physical and chemical behavior of magmas
Transport and eruption of magma
Formation of volcanic deposits

5. What do we need for volcanism?
Thermal energy
Material to melt
Ability to erupt

6. 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

7. What are the sources of heat within Earth?
Primordial/accretional energy
Radioactive decay

8. Rb follows K & conc. in Ksp, mica, & late melt
Ni follows Mg & conc in olivine

9. “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.

10. Natural Radioactivity is exploited by volcanologists and petrologists.
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.
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

11. Rates of Heat Production and Half-lives

12. Radioactive Decay The Law of Radioactive Decay - µ dN dt N or = N l
Note half-life is a constant (as is decay constant)
# parent atoms
time 

13. D = Nelt - N = N(elt -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
l the decay constant for the system in question
Practical limitations on age range to which apply:
Very young rocks: cannot measure tiny amount of daughter accurately
Very old rocks: cannot measure tiny amounts of parent left accurately
Range depends on lambda
How can we distinguish radiogenic daughter isotopes from identical stable isotopes that were in a rock at its formation?

14. 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
When a rock is hot all of the Ar escapes, “resetting the radiometric clock” - all of the daughter is removed
When a volcanic rock forms, the clock is reset, and 40Ar that accum. after must be daughter from 40K

15. The appropriate decay equation is: 40Ar = 40Aro + 40K(e-lt -1)
Where le = x a-1 (electron capture)
and l = x a-1 (whole process) l e æ è ç ö ø ÷
Normally 40Aro = 0 for igneous rocks
Thus can get an age from a single rock by measuring 40K and 40Ar in it (only unknowns)

16. Blocking temperatures for various minerals differ
40Ar-39Ar technique grew from this discovery
A recent discovery- blocking temperatures differ
Stepwise heating of sample releases Ar from different minerals -> several ages reflecting thermal history of rock
We can use it to -> uplift rates in eroded orogenic belts

17. Heat Production through Earth History

18. Earth Structure

19. How do we know the composition of the mantle?
Peridotite bodies (e.g., ophiolites)
Xenoliths
Cosmochemical Evidence/Meteorites

20. 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.

21. 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.

22. Mantle Xenoliths

23. Carbonaceous Chondrites
Left to right: fragments of the Allende, Yukon, and Murchison meteorites

24. Mantle vs Model CC

25. Composition of the Mantle What is the mineralogy of the mantle?
Olivine +clinopyroxene + orthopyroxene ± plagioclase, garnet, spinel (Al bearing minerals)

26. Mineralogy of Mantle

27. obvious from space that Earth has two fundamentally different
crust
obvious from space that Earth has two fundamentally different
physiographic features: oceans (71%) and continents (29%)
from:
global topography

28. Differentiation of the Earth
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:

29. Radioactivity in earth materials
Rock 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
Mantle
.01
7x10-5
0.06
1.2x10-3
0.26
Meteor-ites
0.38
0.1
0.50
Heat production decreases with depth from crust to mantle.

30. 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 !
Approximate Pressure (GPa=10 kbar)

31. 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

32. 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

33. Heat Flow on Earth Dq = Cp * DT
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 = 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.

34. 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.

35. 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

36. 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

37. Earth Structure

38. 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

39. 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.

40. Convection Examples

41. Rayleigh-Bernard Convection

42. Convection in the Mantle

43. convection in the mantle
models
from:
convection in the mantle
observed heat flow
warmer: near ridges
colder: over cratons
from:

44. 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:

45. Earth’s Plates

46. Where Volcanoes Occur

48. 1. Divergent margins 2. Convergent margins
Volcano geography
1. Divergent margins 2. Convergent margins
3. Intraplate 4. Hotspots

49. 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)

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

51. Processes of Partial Melting
Precursor to all igneous rocks is magma or melt (liquid rock)
How does melting occur?

52. Processes of Partial Melting
Let’s first look at a phase diagram (P-T) diagram of mantle

53. Processes of Partial Melting
A simpler phase diagram (P-T) diagram of mantle

54. Processes of Partial Melting
What causes partial melting in the mantle?
Two processes:
Lowering of solidus by volatile addition
Adiabatic Decompression

55. Processes of Partial Melting Lowering solidus by volatile addition
Temperature

56. Processes of Partial Melting Adiabatic Decompression
Pressure

57. Why is melting in the mantle important?
Because most of the melts that make extrusive rocks on Earth originate in the mantle

58. 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 !
Approximate Pressure (GPa=10 kbar)

59. Mechanisms of melt formation
MOR = Adiabatic decompression Intraplate = adiabatic decompression Convergent = change in solidus by volatile fluxing

60. Divergent settings: The Mid-ocean Ridge

61. Bathymetry of the East Pacific Rise

62. 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.

63. MORB Genesis

64. Intraplate settings: Mantle Plumes

65. Proposed Hot Spot Traces

66. 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

67. Shallow Magma Plumbing System

68. Geometry of Magma Reservoir beneath Kilauea

69. Convergent settings: Subduction Zone Magmatism

70. 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

71. Relative Volumes

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

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

74. Volumes of Igneous Rocks on Earth

75. Convergent Margin Magma Genesis

76. 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, Oxford University Press.

77. Basalt Types-Major Element Variation

78. 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

79. K2O content of subalkaline rocks
may broadly
correlate with
crustal thickness.
Low-K 12 km
Med-K 35 km
High-K 45 km
Ewart, 1982

80. Yoder & Tilley Basalt Tetrahedron
Yoder & Tilley, 1962; Le Maitre

81. 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.

82. Lunar Surface

83. Apollo 15 Basalt Sample
Vesicles -
Probably
derived from
CO degassing

84. 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:

85. 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:

86. Earth Mars-sized Impact Model for Lunar Origin
Impact hr
Impact + 5hr
From: Kipp & Melosh, 1986 (above) and W. Hartmann paintings of Cameron, Benz, & Melosh models (right)

87. 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. 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.

88. Lunar Interior Composition
From: BVSP, 1986 and Taylor, 1987

89. 1984 Mauna Loa Eruption

90. Phase 1: Pu’u O’o
Curtain of lava

91. Phase 1: Pu’u O’o
Fire Fountain

92. Pu’u O’o Vent with pahoehoe flows

93. Pahoehoe flow, Kilauea

94. Tree Molds, ~1983

95. Halemaumau, Kilauea

96. Surtsey, Iceland
A new volcanic island formed in 1966

97. Cerro Negro, Nicaragua

98. Stromboli Volcano, Italy

99. Paricutin, Mexico

100. Mt. Augustine, Alaska

101. Augustine
Note hummocky topography from debris avalanche, 1883

102. Eruption of Mt. Augustine, 1986

103. Crater Lake

104. Crater Lake

105. Ol Doinyo Lengai
A sodium carbonatite volcano in the Rift Valley of East Africa

106. Ol Doinyo Lengai
A sodium carbonatite volcano in the Rift Valley of East Africa

107. 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.

108. Sources http://www.doubledeckerpress.com/archive.htm