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What is a volcano?. A hill with a crater? Does magma need to be involved? Does it matter?.

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

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

  • 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
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
“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.

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







N or





Radioactive Decay

The Law of Radioactive Decay

# parent atoms

time 

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

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 = 40Aro + 40K(e-lt -1)

Where le = 0.581 x 10-10 a-1 (electron capture)

and l = 5.543 x 10-10 a-1 (whole process)

Blocking temperatures for various minerals differ
  • 40Ar-39Ar technique grew from this discovery
How do we know the composition of the mantle?
  • Peridotite bodies (e.g., ophiolites)
  • Xenoliths
  • Cosmochemical Evidence/Meteorites

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.


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.

Mantle Xenoliths

Carbonaceous Chondrites

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

Composition of the Mantle

What is the mineralogy of the mantle?

Olivine +clinopyroxene + orthopyroxene ± plagioclase, garnet, spinel (Al bearing minerals)


obvious from space that Earth has two fundamentally different

physiographic features: oceans (71%) and continents (29%)


global topography

differentiation of the earth
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”


radioactivity in earth materials
Radioactivity in earth materials

Heat production decreases with depth from crust to mantle.

earth s geothermal gradient
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)

earth interior pressures
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
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
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
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
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
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 in the mantle

observed heat flow

warmer: near ridges

colder: over cratons


examples from western Pacific

blue is high velocity (fast)

…interpreted as slab

note continuity of blue slab

to depths on order of 670 km


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)

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


Processes of Partial Melting

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

earth s geothermal gradient1
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)

mechanisms of melt formation
Mechanisms of melt formation
  • MOR = Adiabatic decompressionIntraplate = adiabatic decompressionConvergent = change in solidus by volatile fluxing
magma chamber structure beneath 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.

magma plumbing system for hawaii
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

characteristics of 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

classification of igneous rocks
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.

alkaline and subalkaline rock suites
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

k 2 o content of subalkaline rocks
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 Basalt Tetrahedron

Yoder & Tilley, 1962; Le Maitre

terrestrial basalt generation summary
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.
apollo 15 basalt sample
Apollo 15 Basalt Sample

Vesicles -


derived from

CO degassing

lunar olivine basalt thinsection
Lunar Olivine Basalt Thinsection

Fe-Ti oxides


Olivine + aligned MIs


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


lunar anorthosite thinsection
Lunar Anorthosite Thinsection


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


earth mars sized impact model for lunar origin
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
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.
lunar interior composition
Lunar Interior Composition

From: BVSP, 1986 and Taylor, 1987

surtsey iceland
Surtsey, Iceland

A new volcanic island formed in 1966


Note hummocky topography from debris avalanche, 1883

ol doinyo lengai
Ol Doinyo Lengai

A sodium carbonatite volcano in the Rift Valley of East Africa

ol doinyo lengai1
Ol Doinyo Lengai

A sodium carbonatite volcano in the Rift Valley of East Africa

olympus mons mars
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.