Petrology

INTRODUCTION TO PETROLOGY

WHAT IS PETROLOGY???

WHAT IS PETROLOGY??? Study of rocks (“petros”) igneous & metamorphic chiefly in the lithosphere

WHAT IS PETROLOGY??? Study of rocks (“petros”) igneous & metamorphic chiefly in the lithosphere We will be dealing with hot rocks tell us about composition & history of lithosphere origin of rocks involves: transfer of heat (energy) movement of material

WHAT IS PETROLOGY??? Study of rocks (“petros”) igneous & metamorphic chiefly in the lithosphere We will be dealing with hot rocks tell us about composition & history of lithosphere origin of rocks involves: transfer of heat (energy) movement of material

LITHOSPHERE

THINK LIKE A PETROLOGIST

what criteria do we use to distinguish rocks? what do we want to know? how do we answer these questions?

WHAT DO WE WANT TO KNOW? how do we make melts? what is melted, and where? what is the role of water? how do melts behave during solidification? what causes metamorphism? how are metamorphism & deformation related? how to rocks flow in the interior of mountain belts? how do tectonic rates compare to heat conduction rates? in what tectonic settings do these rocks form?

BASIS FOR UNDERSTANDING

field methods & sample study (observation) theory, experiment & modeling (analytical)

THINGS TO CONSIDER

THINGS TO CONSIDER materials of earth

THINGS TO CONSIDER materials of earth physical conditions energy pressure temperature & heat

THINGS TO CONSIDER materials of earth physical conditions energy pressure temperature & heat relationship to tectonics

THINGS TO CONSIDER materials of earth physical conditions energy pressure temperature & heat relationship to tectonics

EgyPT

Fe-rich

Si-rich

EARTH INTERIOR

Fe-rich

Structure of Earth:

Si-rich

EARTH INTERIOR

Structure of Earth:

Si-rich

EARTH INTERIOR

core, mantle & crust

Fe-rich

chemical divisions

Structure of Earth:

Si-rich

EARTH INTERIOR

core, mantle & crust mechanical divisions mesosphere, asthenosphere & lithosphere

Fe-rich

chemical divisions

Fe-rich

Si-rich

EARTH INTERIOR

Fe-rich

Core:

Si-rich

EARTH INTERIOR

Si-rich

EARTH INTERIOR

Core: Fe-Ni metallic alloy

inner core is solid

Fe-rich

outer core is liquid (no S-waves)

Si-rich

EARTH INTERIOR

Core: Fe-Ni metallic alloy

inner core is solid differentiation at work! compositional separation within the planet (fractionation)

Fe-rich

outer core is liquid (no S-waves)

Fe-rich

Si-rich

EARTH INTERIOR

Fe-rich

Mantle:

Si-rich

EARTH INTERIOR

peridotite (ultramafic) greatest V, m & E (moves & carries heat)

Fe-rich

Mantle:

Si-rich

EARTH INTERIOR

Mantle: peridotite (ultramafic) greatest V, m & E (moves & carries heat)

Si-rich

EARTH INTERIOR

contains low velocity layer 60-220 km

Fe-rich

upper layer to 410 km (olivine to spinel)

Mantle: peridotite (ultramafic) greatest V, m & E (moves & carries heat)

Si-rich

EARTH INTERIOR

contains low velocity layer 60-220 km transition zone between 410-660 km (spinel to perovskite) SiIV to SiVI

Fe-rich

upper layer to 410 km (olivine to spinel)

Mantle: peridotite (ultramafic) greatest V, m & E (moves & carries heat)

Si-rich

EARTH INTERIOR

contains low velocity layer 60-220 km transition zone between 410-660 km (spinel to perovskite) SiIV to SiVI lower mantle has more gradual velocity increase

Fe-rich

upper layer to 410 km (olivine to spinel)

Fe-rich

Si-rich

EARTH INTERIOR

Fe-rich

Crust:

Si-rich

EARTH INTERIOR

mafic (magnesium + ferric) to felsic (feldspar + silica) rich in Si, Al, K, Na, Ca

Fe-rich

Crust:

Si-rich

EARTH INTERIOR

mafic (magnesium + ferric) to felsic (feldspar + silica) rich in Si, Al, K, Na, Ca two main types: oceanic continental + “transitional”

Fe-rich

Crust:

Si-rich

EARTH INTERIOR

EARTH INTERIOR

EARTH INTERIOR Oceanic crust:

EARTH INTERIOR Oceanic crust: thin: ~10 km on average

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite)

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments pillow basalt

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments pillow basalt sheeted dikes

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments pillow basalt sheeted dikes massive gabbro

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments pillow basalt sheeted dikes massive gabbro ultramafic rocks (mantle)

EARTH INTERIOR Oceanic crust: thin: ~10 km on average dense: ρavg = 3.0 g/cm3 relatively uniform stratigraphy (= ophiolite suite) sediments pillow basalt sheeted dikes massive gabbro ultramafic rocks (mantle)

mafic rocks

CONTINENTAL

OCEANIC

EARTH INTERIOR

EARTH INTERIOR Continental crust: thicker: 20-90 km (avg = 35 km) less dense: ρavg = 2.7 g/cm3 highly variable composition average = granodiorite

CHEMICAL DIVISIONS

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process how do we know these things???

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process how do we know these things??? seismic velocity structure

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process how do we know these things??? seismic velocity structure meteorites

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process how do we know these things??? seismic velocity structure meteorites xenoliths in volcanics

CHEMICAL DIVISIONS divisions separate Earth into Si-rich and Fe-rich spheres, largely a result of early chemical differentiation based on a redistribution of matter, prior to major solidification, by density later, continued differentiation was (and is) mostly a result of melting and igneous process how do we know these things??? seismic velocity structure meteorites xenoliths in volcanics experimental petrology

MECHANICAL DIVISIONS

Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS Velocity structure (v)

Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS Velocity structure (v) v increases with density (ρ) v = f (ρ)

Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS Velocity structure (v) v increases with density (ρ) v = f (ρ) ρ dependent on physical properties & compositions ρ = f (X, T)

Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS Velocity structure (v) v increases with density (ρ) v = f (ρ) ρ dependent on physical properties & compositions ρ = f (X, T) v increases with depth (z) — mostly!

Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS Velocity structure (v) v increases with density (ρ) v = f (ρ) ρ dependent on physical properties & compositions ρ = f (X, T) v increases with depth (z) — mostly! v discontinuities indicate a change in material composition ± properties Figure 1-3.Variation in P and S wave velocities with depth. Compositional subdivisions of the Earth are on the left, rheological subdivisions on the right. After Kearey and Vine (1990), Global Tectonics. © Blackwell Scientific. Oxford.

MECHANICAL DIVISIONS composition

property LVZ

CMB

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

CMB

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ?

CMB

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ? warmer? liquid?

CMB

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ? warmer? liquid? source of CMB?

CMB

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ? warmer? liquid? source of CMB?

CMB

change in composition

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ? warmer? liquid? source of CMB?

CMB

change in composition source of OC-IC?

OC-IC

MECHANICAL DIVISIONS composition

property

Velocity boundaries

LVZ

source of LVZ? warmer? liquid? source of CMB?

CMB

change in composition source of OC-IC? phase change (S to L)

OC-IC

MECHANICAL DIVISIONS composition Velocity boundaries source of LVZ? warmer? liquid? source of CMB?

property

LVZ LET’S LOOK IN MORE DETAIL CMB

change in composition source of OC-IC? phase change (S to L)

OC-IC

PETROLOGY & TECTONICS major mineral transformations occur at ~410 and ~660 km result from isochemical phase changes due to increased P marked by seismic velocity discontinuities

Mineral

Structure

olivine

tetrahedral

spinel perovskite

dense tetrahedral octahedral

Depth

Density low

~410 km ~660 km

high

VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE

LO

HI

VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE

LO

HI

LO

HI

VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE

LO

HI

LO

HI

MG-SILICATES VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE

LO

LO

. H SP

O N HE

T S A

T.Z.

HI

H P S

HI

E R E

O S E

M

MG-SILICATES VARIATION IN SEISMIC VELOCITY, DENSITY & MINERAL STRUCTURE

PETROLOGY & TECTONICS Resulting divisions lithosphere asthenosphere mesosphere

PETROLOGY & TECTONICS Resulting divisions lithosphere asthenosphere mesosphere

properties?

PETROLOGY & TECTONICS Resulting divisions lithosphere asthenosphere mesosphere

properties? connections?

PETROLOGY & TECTONICS Resulting divisions lithosphere asthenosphere mesosphere

properties? connections? is the mantle solid everywhere?

MANTLE GEOTHERM

MANTLE GEOTHERM

compare continental, oceanic & ridge

MANTLE GEOTHERM

compare continental, oceanic & ridge typical continental geotherm = 25 °C/km

MANTLE GEOTHERM

compare continental, oceanic & ridge typical continental geotherm = 25 °C/km T @ 100 km = 1000 °C (enough to melt rocks!)

MANTLE GEOTHERM

compare continental, oceanic & ridge typical continental geotherm = 25 °C/km T @ 100 km = 1000 °C (enough to melt rocks!) are they molten? P too high? where?

CAN MELTING OCCUR?

compare geotherm to petrologic solidus for mantle rocks (peridotite) if “dry” conditions, no melting possible (geotherm below solidus) solid lherzolite is stable at T above geotherm

CAN MELTING OCCUR? under “wet” conditions (with H2O or CO2), solidus shifts to lower T melting can occur where T > solidus low seismic velocities indicate partial melting between 100-250 km (the LVZ) the LVZ marks the base of “plates” formed by rigid lithosphere

CAN MELTING OCCUR? under “wet” conditions (with H2O or CO2), solidus shifts to lower T melting can occur where T > solidus low seismic velocities indicate partial melting between 100-250 km (the LVZ) the LVZ marks the base of “plates” formed by rigid lithosphere

LITHOSPHERE

LITHOSPHERE

LITHOSPHERE lithosphere includes crust + upper mantle

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km)

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates)

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates) lowest densities (2.7-3.0 g/cm3)

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates) lowest densities (2.7-3.0 g/cm3) slowest seismic velocities (6-8 km/sec)

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates) lowest densities (2.7-3.0 g/cm3) slowest seismic velocities (6-8 km/sec) internal boundary is the Moho (density boundary)

LITHOSPHERE lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates) lowest densities (2.7-3.0 g/cm3) slowest seismic velocities (6-8 km/sec) internal boundary is the Moho (density boundary) base of lithosphere is the low-velocity zone (LVZ)

LITHOSPHERE

lithosphere = plate

lithosphere includes crust + upper mantle can be oceanic, continental, or both typical lithosphere is 100 km (70-125 km) largely solid material (silicates) lowest densities (2.7-3.0 g/cm3) slowest seismic velocities (6-8 km/sec) internal boundary is the Moho (density boundary) base of lithosphere is the low-velocity zone (LVZ)

EgyPT

Physical conditions of Earth necessary to understand petrologic process: 1. pressure 2. temperature 3. energy & heat

PRESSURE GRADIENT

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT P = ρgh

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT P = ρgh P increases with depth

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT P = ρgh P increases with depth

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT P = ρgh P increases with depth Mantle: nearly linear through mantle ~ 30 MPa/km ≈ 1 GPa at base of avg crust

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT P = ρgh P increases with depth Mantle: nearly linear through mantle ~ 30 MPa/km ≈ 1 GPa at base of avg crust slope (ΔP/Δz) depends on density (composition & compressibility) of material

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE GRADIENT H IG H Y

IT

S

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

N

slope (ΔP/Δz) depends on density (composition & compressibility) of material

DE

~ 30 MPa/km ≈ 1 GPa at base of avg crust

Y SIT

Mantle: nearly linear through mantle

DEN

P increases with depth

LOW

P = ρgh

PRESSURE GRADIENT

P = ρgh P increases with depth Core: ρ increases more rapidly since alloy is more dense smaller increase in P with depth suggests inner core is more uniform, solid, and has decreasing compressibility

Figure 1-8. Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science.

PRESSURE IN THE CRUST

h

ρgh

PRESSURE IN THE CRUST in the crust, rocks “feel” a lithostatic pressure like a submarine feels hydrostatic pressure on its hull implies equal pressure in all directions when materials can flow

h

ρgh

PRESSURE IN THE CRUST in the crust, rocks “feel” a lithostatic pressure like a submarine feels hydrostatic pressure on its hull implies equal pressure in all directions when materials can flow

h

for shallow rocks, can have differential pressure (where horizontal stress ≠ vertical stress), causing rocks to deform ρgh

PRESSURE IN THE CRUST in the crust, rocks “feel” a lithostatic pressure like a submarine feels hydrostatic pressure on its hull implies equal pressure in all directions when materials can flow

h

for shallow rocks, can have differential pressure (where horizontal stress ≠ vertical stress), causing rocks to deform ρgh

PRESSURE IN THE CRUST in the crust, rocks “feel” a lithostatic pressure like a submarine feels hydrostatic pressure on its hull implies equal pressure in all directions when materials can flow

h

for shallow rocks, can have differential pressure (where horizontal stress ≠ vertical stress), causing rocks to deform based on normal crustal densities, 1 kbar = 3.3 km

ρgh

ENERGY

ENERGY Energy is the capacity to do work (subatomic, to mountain, to mantle scale)

ENERGY Energy is the capacity to do work (subatomic, to mountain, to mantle scale) 1. kinetic energy: EK = 1/2mv2

•

motion of a body

ENERGY Energy is the capacity to do work (subatomic, to mountain, to mantle scale) 1. kinetic energy: EK = 1/2mv2

•

motion of a body

2. potential energy: EP = mgz

•

energy of position; can be converted to Ek

ENERGY Energy is the capacity to do work (subatomic, to mountain, to mantle scale) 1. kinetic energy: EK = 1/2mv2

•

motion of a body

2. potential energy: EP = mgz

•

energy of position; can be converted to Ek

3. thermal energy: ET = EK + EP

•

motions & attractions in a body (subatomic and larger)

•

ET ≠ heat (transferred energy)

HEAT SOURCES IN THE EARTH

HEAT SOURCES IN THE EARTH 1. Heat from the early accretion and differentiation of the Earth

•

“original heat” of early core separation (PV work of compression)

•

still slowly reaching surface as geotherm decays

HEAT SOURCES IN THE EARTH 1. Heat from the early accretion and differentiation of the Earth

•

“original heat” of early core separation (PV work of compression)

•

still slowly reaching surface as geotherm decays

2. Heat released by the radioactive breakdown of unstable nuclides

•

heat production (A) from decay of U, Th & K

•

mostly in crustal rocks

HEAT SOURCES IN THE EARTH 1. Heat from the early accretion and differentiation of the Earth

•

“original heat” of early core separation (PV work of compression)

•

still slowly reaching surface as geotherm decays

2. Heat released by the radioactive breakdown of unstable nuclides

•

heat production (A) from decay of U, Th & K

•

mostly in crustal rocks

3. Latent heat associated with outer core crystallization

•

continues today!

HEAT PRODUCTION Rock

Abundance of radioactive element

Heat produced

(joules/kg/yr)

U

Th

K

A

Granite

4

13

4

0.03

Basalt

0.5

2

1.5

0.005

Peridotite

0.02

0.06

0.02

0.0001

from Decker & Decker (1981)

HEAT PRODUCTION Rock

Abundance of radioactive element

Heat produced

(joules/kg/yr)

U

Th

K

A

Granite

4

13

4

0.03

Basalt

0.5

2

1.5

0.005

0.02

0.06

0.02

0.0001

If more heat-producing elements in continental crust, why are T’s lower??

Peridotite

from Decker & Decker (1981)

HEAT PRODUCTION Crustal T’s are lower because of: 1. less of it compared to mantle 2. continental crust is a good insulator 3. it’s thicker 4. it suffers from surface cooling

HEAT TRANSFER 1. radiation

•

conversion of IR energy from hot body; travels as a wave

•

efficient in a vacuum or transparent material; not efficient in rocks!

HEAT TRANSFER 2. conduction

•

transfer of EK by vibration & contact, one molecule to another

•

does not occur in a vacuum

•

greatest transfer with greatest ΔT (thermal gradient)

•

conduction increases with increasing surface area

•

depends on thermal conductivity (k) of material, given by: q = kΔT/Δz

HEAT TRANSFER 3. convection

•

movement of material with contrasting T, which changes density

•

gravity acts on Δρ, in which less dense material (ie, hotter) rises

•

movement of solid can occur in viscous mantle rocks, including rise of plumes

HEAT TRANSFER 4. advection

•

heat “carried” by flowing liquids or viscous bodies (e.g., water, magma) to cooler surroundings

LAVA FLOW

MAGMA

WHAT FORMS OF HEAT TRANSFER?

GEOTHERMAL GRADIENT

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT ΔT/Δz is the slope of T variation

with depth

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT ΔT/Δz is the slope of T variation

with depth

gradient drives conductive cooling toward the surface

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT ΔT/Δz is the slope of T variation

with depth

gradient drives conductive cooling toward the surface why steeper curve at depth?

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT ΔT/Δz is the slope of T variation

with depth

gradient drives conductive cooling toward the surface why steeper curve at depth? more heat production in crust

Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

GEOTHERMAL GRADIENT ΔT/Δz is the slope of T variation

with depth

gradient drives conductive cooling toward the surface why steeper curve at depth? more heat production in crust more efficient convective mixing of heat in the mantle Figure 1-9. Estimated ranges of oceanic and continental steadystate geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.

IGNEOUS TECTONIC SETTINGS 1. mid-ocean ridge

5. back-arc basin

2. continental rift

6. oceanic hotspot

3. oceanic island arc

7. continental hotspot

4. continental-margin arc

IGNEOUS TECTONIC SETTINGS 1. mid-ocean ridge

5. back-arc basin

2. continental rift

6. oceanic hotspot

3. oceanic island arc

7. continental hotspot

4. continental-margin arc

WHAT’S MELTING? HOW? WHERE DOES IT GO?

IGNEOUS ROCKS

next week we’ll begin igneous rocks!