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Ch 01 intro

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Raghav Gadgil
Raghav Gadgil

This document summarizes igneous petrology and the structure and composition of the Earth's interior. It discusses how the Earth is composed of layers including the crust, mantle, outer core, and inner core. The crust is divided into oceanic and continental crust. The mantle makes up most of the Earth's volume and is composed of ultramafic rock. Heat transfer mechanisms like conduction, convection, and advection are described. The geothermal gradient and how temperature increases with depth is also summarized. Plate tectonics and mantle convection are driving the dynamic cooling of the Earth.

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IgneousIgneous PetrologyPetrology Thanks to John WinterThanks to John Winter 1
IntroductionIntroduction Igneous petrology is the study of melts (magma) and the rocks that crystallize from such melts, encompassing an understanding of the processes involved in melting and subsequent rise, evolution, crystallization, and eruption or emplacement of the eventual rocks. 2
All terrestrial rocks that we now find at earths surface were derived initially from the mantle, although some have gone cycles of sedimentation and metamorphism. If these rocks have ultimate origin at depth, it follows that we need to know what makes up the earth if we want to understand their origin more fully. Compositionally there is crust, mantle and core delineated by seismic discontinuities. The Earth’s InteriorThe Earth’s Interior 3
The Earth’s InteriorThe Earth’s Interior Crust: Oceanic crust: Basaltic Thin: 10 km Relatively uniform stratigraphy = ophiolite suite: sediments pillow basalt sheeted dikes more massive gabbro ultramafic (mantle) Plate tectonics is creating oceanic crust at mid-ocean ridges and consuming it at subduction zones, the oceanic crust is continually being renewed and recycled. 4
The Earth’s InteriorThe Earth’s Interior Continental Crust Thicker: 20-90 km average ~35 km Highly variable composition Average ~ granodiorite Covers ~40% of earths surface The amount of continental crust has been increasing over the past 4 Ga. The stable continents (Cratons) consist of more ancient shield and stable platforms. several marginal orogenic belts also flank many cratons. 5
The Earth’s InteriorThe Earth’s Interior Mantle: 83% of earth’s volume Peridotite (ultramafic) There are layers distinguished by more physical rather than chemical differences. 80-220km-LVZ-1-5% Partial melting. The melt probably forms thin discontinuous film along mineral grain boundaries. It makes mantle behave in a more ductile fashion. 6
The Earth’s InteriorThe Earth’s Interior Below the LVZ, 2 more seismic discontinuities are encountered. Upper to 410 km (olivine → spinel) Low Velocity Layer 60-220 km Transition Zone as velocity increases ~ rapidly 660 spinel → perovskite-type SiIV → SiVI Both these discontinuities result in abrupt increase in the density of the mantle, accompanied by a jump in seismic velocities. 7
The Earth’s InteriorThe Earth’s Interior Below 660-km discontinuity, the velocities of seismic waves increase fairly uniformly with depth. At the very base of mantle is a ~200km thick heterogenous layer of anomalously low seismic velocity called the D” layer. A thin ~40km, apparently discontinuous layer has been recently resolved at the mantle-core boundary, clearly beneath central Pacific. The nature of these 2 layers is not clear, but their properties are sufficiently anomalous to require more than a thermal boundary perturbation, and they probably represent layer of different composition, probably dense ‘dregs’ of subducted oceanic lithosphere that have settled at the base of lower mantle. 8
The Earth’s InteriorThe Earth’s Interior Core: Silicates of the mantle give way to a much denser Fe rich metalic alloy with minor amounts of Ni, S, Si, O etc. Outer Core is liquid No S-waves Inner Core is solid Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 9
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. 10
The PressureThe Pressure GradientGradient P increases = ρgh Hydrostatic pressure=lithostatic pressure Density increases with depth as the rocks are compressed Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science. 12
The PressureThe Pressure GradientGradient P increases = ρgh Nearly linear through mantle ~ 30 MPa/km ≈ 1 GPa at base of ave crust Core: ρ incr. more rapidly since alloy more dense Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science. 13
Heat SourcesHeat Sources in the Earthin the Earth 1. Heat from the early accretion and differentiation of the Earth (Secular cooling) Primordial heat developed early in the history of the Earth from the processes of accretion and gravitational differentiation Some continued gravitational partitioning of iron in the inner core may contribute some heat as well. Estimates are 10-25% of total heat flux 14
Heat SourcesHeat Sources in the Earthin the Earth 2. Heat released by the radioactive breakdown of unstable nuclides • Most of the radioactive elements are concentrated in the continental crust. • 75-90% heat is produced that reaches the surface 15
Heat TransferHeat Transfer 1. Radiation: Sufficiently transparent or translucent material can transfer heat by radiation. It is the movement of the particles/waves through another medium. Eg. Lamp losing heat or surface of the earth losing heat. 2. Conduction: Opaque and rigid substances transfer heat by conduction. It involves transfer of kinetic energy from hotter atoms to adjacent cooler ones. It is efficient for metals. Conduction is poor for silicate minerals but relatively efficient in the core. 16
Heat TransferHeat Transfer 3. Convection: If the material is more ductile and if it can be moved, this is the process. It is the movement of material due to density differences caused by thermal or compositional variations. 17
Heat TransferHeat Transfer 3. Convection: It affects ductile rocks and liquids. • It may involve flow in a single direction, in which case the moved hot material will accumulate at the top of the ductile portion of the system. • Convection may also occur in a cyclic motion, typically in a closed cell above a localized heat source. • Here, the heated material rises and moves laterally as it cools and is pushed aside by later convective matter. • Material pushed to the margin cools, contracts, and sinks towards the heat source, where it becomes heated and cycle continues. 18
Heat TransferHeat Transfer 3. Advection: • It involves the transfer of heat with rocks that are otherwise in motion. • For eg, if a hot region at depth is uplifted by either tectonism, induced flow or erosion and isostatic rebound, heat rises physically with the rocks. • All rocks, whether igneous, sedimentary or metamorphic, owe their origins directly or indirectly to the advective cooling of the earth. • The rise of magma into the crust or its extrusion onto the surface is an obvious example of advection. 19
The Geothermal GradientThe Geothermal Gradient • Convection works well in the liquid core and in the somewhat fluid mantle. • It is also a primary method of heat transfer in hydrothermal systems above magma bodies. • Beyond these areas, conduction and advection are the only available methods of heat transfer. • Transfer of heat controls metamorphism, melting, crystallization, as well as the motion, mixing and mechanical properties of Earth materials. • We will consider heat transfer later. 20
The Geothermal GradientThe Geothermal Gradient Diagrammatic cross-section through the upper 200-300 km of the Earth showing geothermal gradients reflecting more efficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere (steeper gradient in blue) ) and less efficient conductive heat transfer through the more rigid lithosphere (shallower gradient in red). The boundary layer is a zone across which the transition in rheology and heat transfer mechanism occurs (in green). The thickness of the boundary layer is exaggerated here for clarity: it is probably less than half the thickness of the lithosphere. Convection in the mantle redistributes heat more faster than conduction 21
The Geothermal GradientThe Geothermal Gradient A similar example for thick (continental) lithosphere. The curved part of the geotherm represents a zone of gradual transition – boundary layer – where, as temperature rises, rocks become progressively more ductile and begin to participate in convection. 22
The Geothermal GradientThe Geothermal Gradient Notice that thinner lithosphere allows convective heat transfer to shallower depths, resulting in a higher geothermal gradient across the boundary layer and lithosphere. TP is the potential temperature. It permits comparison of (estimated) temperatures at depth from one locality to another. Because temperature varies with depth, one must select some reference depth. In this case the surface was chosen. One simply extrapolates adiabatically from the T and P in question to the surface. Potential temperature, is the hypothetical surface temperature a deeply buried rock would end up at if it were transported to the surface along the adiabat passing through PT , without melting. 23
The Geothermal GradientThe Geothermal Gradient Figure 1.10 Temperature contours calculated for an oceanic plate generated at a mid-ocean ridge (age 0) and thickening as it cools. The 1300o C isotherm is a reasonable approximation for the base of the oceanic lithosphere. The plate thickens rapidly from zero to 50 Ma and is essentially constant beyond 100 Ma. From McKenzie et al. (2005). 26
The Geothermal GradientThe Geothermal Gradient Figure 1.11 Estimates of oceanic (blue curves) and continental shield (red curves) geotherms to a depth of 300 km. The thickness of mature (> 100Ma) oceanic lithosphere is hatched and that of continental shield lithosphere is yellow. • Because the thermal conductivity of shallow crustal rocks is low, heat transfer is slow. Extrapolating this leads to very high geothermal gradient. 27
The Geothermal GradientThe Geothermal Gradient • Shallow gradients in old oceanic lithosphere range from about 17- 200 C/Km, gradually steepening to about 70 C/Km with depth. • Shallow shield gradients range from about 12-180 C/Km, transitioning to about 4-60 C/Km below about 30Km. • The oceanic v/s shield temperature differences are essentially restricted to the lithosphere and boundary layer, so the oceanic and continental curves converge by about 250-300Km where convection has homogenizing influence. 28
The Geothermal GradientThe Geothermal Gradient Figure 1.12 Estimate of the geothermal gradient to the center of the Earth (after Stacey, 1992). The shallow solid portion is very close to the Green & Ringwood (1963) oceanic geotherm in Fig. 1–11 and the dashed geotherm is the Jaupart & Mareschal (1999) continental geotherm. • Nearly adiabatic convective heat flow deeper than about 300Km results in a linear geothermal gradient of approx. 0.30 C/Km. • The gradient shallows across the D” layer and then becomes steep in the metallic core, where the thermal conductivity and convection is very high. 29
The Geothermal GradientThe Geothermal Gradient • The density contrast across the core-mantle boundary prohibits the core material from rising into the mantle, so heat can only be transferred upward by conduction, resulting in a thermal boundary layer across which the thermal gradient is estimated to be 300 – 10000 C. 30
The geothermal gradientThe geothermal gradient The rate of convective flow in the mantle is measured in terms of cms/year. Slabs of cooler oceanic lithosphere, mostly mantle rock, too viscous to convect within themselves, sink into the underlying hotter and relatively less dense underlying mantle. Computed tomography of the Earth using seismic waves has shown that at least some subducting lithospheric slabs sink all the way through the mantle and come to rest on top of the dense metallic core 31
Even though the rate at which the mantle convects, on the order of a few centimeters/year, seems minuscule, it is far greater than the rate of heat transfer by conduction. The fact that lithospheric slabs can sink convectively over tens of millions of years about 2800 km through the hotter mantle but still maintain a recognizable cooler T throughout their approximately 100-km thickness demonstrates how slowly they are conductively heated. Conversely, if heat conduction were more rapid than convection, the subducting slabs would absorb heat and lose their density contrast and identity before sinking very far. 32
33
Dynamic cooling of the earthDynamic cooling of the earth Previous figure was intended to illustrate the thermal structure at shallow levels of Earth’s interior in stabilized areas, similar to a static, chemically and physically stratified earth with continents and oceans. The earth is far more dynamic. Temperature variations that drive convective flow are concentrated in thin boundary layers that are much smaller than the overall circulation pattern. The upper boundary of this system is a cool thermal boundary layer with low viscosity: the lithosphere. 34
Dynamic cooling of the earthDynamic cooling of the earth In laboratory models of convecting fluids, cool dense upper thermal boundary layer descend as either cylindrical downwellings or as larger networks of partially connected tabular-shaped downwelling slabs, depending on the rheology of the material. It generally leads to slab-like downwelling. Mantle convection and plate tectonics are inseparable manifestations of the heat-driven dynamic cooling process of the Earth, given its present thermal state and rheological properties. For the Earth, plate tectonics is mantle convection. 35
Plate tectonicsPlate tectonics The negative buoyancy of the descending plates has been called slab pull, dense subducting slab pulls the rest of the plate after it. The motion of the lithospheric plates down and away from the elevated mid- oceanic ridges is called as ridge-push. Both are body forces affecting whole plate. As the subducting slab descends, the slab cant pull significantly on the rest of the plate, which would simply fault and break. 36
Plate tectonicsPlate tectonics Instead, as it sinks, the slab sets up circulation patterns in the mantle that exert a sort of suction force. As the slab descends from the surface, the pressure behind it is lowered, which is immediately compensated by feeding more plate into the subduction zone. 37
Dynamic cooling of the earthDynamic cooling of the earth Ridge push is gravitational sliding of a plate off the elevated ridge, not an active mantle push by ascending mantle flow, which would buckle the plate. Plate motion velocities are roughly proportional to the percentage of plate’s perimeter that corresponds to subduction zones and not to ridges (Forsyth & Uyeda, 1975) The predominance of slab pull suggest that mantle upwelling at divergent plate boundaries, which moves heat upward, is essentially a passive response to plate separation and descent. 38
TheThe GeothermalGeothermal GradientGradient Fig 1.13. Pattern of global heat flux variations compiled from observations at over 20,000 sites and modeled on a spherical harmonic expansion to degree 12. From Pollack, Hurter and Johnson. (1993) Rev. Geophys. 31, 267-280. Cross-section of the mantle based on a seismic tomography model. Arrows represent plate motions and large-scale mantle flow and subduction zones represented by dipping line segments. EPR =- East pacific Rise, MAR = Mid-Atlantic Ridge, CBR = Carlsberg Ridge. Plates: EA = Eurasian, IN = Indian, PA = Pacific, NA = North American, SA = South American, AF = African, CO = Cocos. From Li and Romanowicz (1996). J. Geophys. Research, 101, 22,245-72. 39
TheThe GeothermalGeothermal GradientGradient Thermal structure in a 3D spherical mantle convection model (red is hot, blue is cold). J. H. Davies and H.-Peter Bunge http://www.ocean.cf.ac.uk/people/huw/AG U99/mantlecirc.html 40
Dynamic cooling of the earthDynamic cooling of the earth What happens at other deeper boundary? The density more than doubles from lower mantle to the core, thus preventing convection across the boundary, regardless of thermal differences. The liquid outer core must be internally convecting to produce Earth’s magnetic field. The D” layer may be the resulting mantle side thermal boundary layer, across which the temperature gradient is estimated to be 300 to 10000 C. Convection within the mantle results from thermal instabilities as the heated by D” layer becomes positively buoyant. 43
Dynamic cooling of the earthDynamic cooling of the earth The viscosity is much lower than that of the plates at such elevated temperatures, and the instabilities take the alternative form of rising cylindrical plumes rather than slabs. These plumes are more of an active upwelling, in comparison to the more passive shallow mantle rise in response to plate separation. The plumes appear to be largely independent of plate tectonics, but in many cases are sufficiently vigorous to penetrate the lithosphere and reach the Earth’s surface, where they result in hotspot activity. Plate tectonics thus appears to be the method by which a largely internally heated and viscous mantle cools by convection, whereas plumes essentially cool the core and have an increasing effect on lower mantle. 44
Dynamic cooling of the earthDynamic cooling of the earth Even general model of mantle convection depends on the influence of 660-km phase transition. Some investigators consider the density contrast across the boundary insufficient to impede convection, so there is whole mantle convection. Others believe the 660-km transition can impede convection, so that warm, buoyant mantle material in rising portions of deep convection cells would be less buoyant than the material above transition, causing the rising material to spread laterally instead. If so, heat can transfer across the boundary only by conduction, inducing convection in the upper mantle. 45
Figure 1.14. Schematic diagram of a 2- layer dynamic mantle model in which the 660 km transition is a sufficient density barrier to separate lower mantle convection (arrows represent flow patterns) from upper mantle flow, largely a response to plate separation. The only significant things that can penetrate this barrier are vigorous rising hotspot plumes and subducted lithosphere (which sink to become incorporated in the D" layer where they may be heated by the core and return as plumes). Plumes in core represent relatively vigorous convection (see Chapter 14). After Silver et al. (1988). Mantle dynamicsMantle dynamics Is the 660 km transition a barrier to whole-mantle convection? Maybe? Partly? No? 46
Mantle dynamicsMantle dynamics The 660-km boundary layer in such models allows some material transfer but impedes wholesale mantle convection from homogenizing the full vertical extent of the mantle and thus permits the composition of the shallow and deep mantle to evolve independently. 47
Plate Tectonic - Igneous GenesisPlate Tectonic - Igneous Genesis 1. Mid-Ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-Arc Basins 6. Ocean Island Basalts 7. Miscellaneous Intra- Continental Activity kimberlites, carbonatites, anorthosites... 49

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Ch 01 intro

  • 1. IgneousIgneous PetrologyPetrology Thanks to John WinterThanks to John Winter 1
  • 2. IntroductionIntroduction Igneous petrology is the study of melts (magma) and the rocks that crystallize from such melts, encompassing an understanding of the processes involved in melting and subsequent rise, evolution, crystallization, and eruption or emplacement of the eventual rocks. 2
  • 3. All terrestrial rocks that we now find at earths surface were derived initially from the mantle, although some have gone cycles of sedimentation and metamorphism. If these rocks have ultimate origin at depth, it follows that we need to know what makes up the earth if we want to understand their origin more fully. Compositionally there is crust, mantle and core delineated by seismic discontinuities. The Earth’s InteriorThe Earth’s Interior 3
  • 4. The Earth’s InteriorThe Earth’s Interior Crust: Oceanic crust: Basaltic Thin: 10 km Relatively uniform stratigraphy = ophiolite suite: sediments pillow basalt sheeted dikes more massive gabbro ultramafic (mantle) Plate tectonics is creating oceanic crust at mid-ocean ridges and consuming it at subduction zones, the oceanic crust is continually being renewed and recycled. 4
  • 5. The Earth’s InteriorThe Earth’s Interior Continental Crust Thicker: 20-90 km average ~35 km Highly variable composition Average ~ granodiorite Covers ~40% of earths surface The amount of continental crust has been increasing over the past 4 Ga. The stable continents (Cratons) consist of more ancient shield and stable platforms. several marginal orogenic belts also flank many cratons. 5
  • 6. The Earth’s InteriorThe Earth’s Interior Mantle: 83% of earth’s volume Peridotite (ultramafic) There are layers distinguished by more physical rather than chemical differences. 80-220km-LVZ-1-5% Partial melting. The melt probably forms thin discontinuous film along mineral grain boundaries. It makes mantle behave in a more ductile fashion. 6
  • 7. The Earth’s InteriorThe Earth’s Interior Below the LVZ, 2 more seismic discontinuities are encountered. Upper to 410 km (olivine → spinel) Low Velocity Layer 60-220 km Transition Zone as velocity increases ~ rapidly 660 spinel → perovskite-type SiIV → SiVI Both these discontinuities result in abrupt increase in the density of the mantle, accompanied by a jump in seismic velocities. 7
  • 8. The Earth’s InteriorThe Earth’s Interior Below 660-km discontinuity, the velocities of seismic waves increase fairly uniformly with depth. At the very base of mantle is a ~200km thick heterogenous layer of anomalously low seismic velocity called the D” layer. A thin ~40km, apparently discontinuous layer has been recently resolved at the mantle-core boundary, clearly beneath central Pacific. The nature of these 2 layers is not clear, but their properties are sufficiently anomalous to require more than a thermal boundary perturbation, and they probably represent layer of different composition, probably dense ‘dregs’ of subducted oceanic lithosphere that have settled at the base of lower mantle. 8
  • 9. The Earth’s InteriorThe Earth’s Interior Core: Silicates of the mantle give way to a much denser Fe rich metalic alloy with minor amounts of Ni, S, Si, O etc. Outer Core is liquid No S-waves Inner Core is solid Figure 1.2 Major subdivisions of the Earth. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 9
  • 10. 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. 10
  • 11. The PressureThe Pressure GradientGradient P increases = ρgh Hydrostatic pressure=lithostatic pressure Density increases with depth as the rocks are compressed Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science. 12
  • 12. The PressureThe Pressure GradientGradient P increases = ρgh Nearly linear through mantle ~ 30 MPa/km ≈ 1 GPa at base of ave crust Core: ρ incr. more rapidly since alloy more dense Figure 1.8 Pressure variation with depth. From Dziewonski and Anderson (1981). Phys. Earth Planet. Int., 25, 297-356. © Elsevier Science. 13
  • 13. Heat SourcesHeat Sources in the Earthin the Earth 1. Heat from the early accretion and differentiation of the Earth (Secular cooling) Primordial heat developed early in the history of the Earth from the processes of accretion and gravitational differentiation Some continued gravitational partitioning of iron in the inner core may contribute some heat as well. Estimates are 10-25% of total heat flux 14
  • 14. Heat SourcesHeat Sources in the Earthin the Earth 2. Heat released by the radioactive breakdown of unstable nuclides • Most of the radioactive elements are concentrated in the continental crust. • 75-90% heat is produced that reaches the surface 15
  • 15. Heat TransferHeat Transfer 1. Radiation: Sufficiently transparent or translucent material can transfer heat by radiation. It is the movement of the particles/waves through another medium. Eg. Lamp losing heat or surface of the earth losing heat. 2. Conduction: Opaque and rigid substances transfer heat by conduction. It involves transfer of kinetic energy from hotter atoms to adjacent cooler ones. It is efficient for metals. Conduction is poor for silicate minerals but relatively efficient in the core. 16
  • 16. Heat TransferHeat Transfer 3. Convection: If the material is more ductile and if it can be moved, this is the process. It is the movement of material due to density differences caused by thermal or compositional variations. 17
  • 17. Heat TransferHeat Transfer 3. Convection: It affects ductile rocks and liquids. • It may involve flow in a single direction, in which case the moved hot material will accumulate at the top of the ductile portion of the system. • Convection may also occur in a cyclic motion, typically in a closed cell above a localized heat source. • Here, the heated material rises and moves laterally as it cools and is pushed aside by later convective matter. • Material pushed to the margin cools, contracts, and sinks towards the heat source, where it becomes heated and cycle continues. 18
  • 18. Heat TransferHeat Transfer 3. Advection: • It involves the transfer of heat with rocks that are otherwise in motion. • For eg, if a hot region at depth is uplifted by either tectonism, induced flow or erosion and isostatic rebound, heat rises physically with the rocks. • All rocks, whether igneous, sedimentary or metamorphic, owe their origins directly or indirectly to the advective cooling of the earth. • The rise of magma into the crust or its extrusion onto the surface is an obvious example of advection. 19
  • 19. The Geothermal GradientThe Geothermal Gradient • Convection works well in the liquid core and in the somewhat fluid mantle. • It is also a primary method of heat transfer in hydrothermal systems above magma bodies. • Beyond these areas, conduction and advection are the only available methods of heat transfer. • Transfer of heat controls metamorphism, melting, crystallization, as well as the motion, mixing and mechanical properties of Earth materials. • We will consider heat transfer later. 20
  • 20. The Geothermal GradientThe Geothermal Gradient Diagrammatic cross-section through the upper 200-300 km of the Earth showing geothermal gradients reflecting more efficient adiabatic (constant heat content) convection of heat in the mobile asthenosphere (steeper gradient in blue) ) and less efficient conductive heat transfer through the more rigid lithosphere (shallower gradient in red). The boundary layer is a zone across which the transition in rheology and heat transfer mechanism occurs (in green). The thickness of the boundary layer is exaggerated here for clarity: it is probably less than half the thickness of the lithosphere. Convection in the mantle redistributes heat more faster than conduction 21
  • 21. The Geothermal GradientThe Geothermal Gradient A similar example for thick (continental) lithosphere. The curved part of the geotherm represents a zone of gradual transition – boundary layer – where, as temperature rises, rocks become progressively more ductile and begin to participate in convection. 22
  • 22. The Geothermal GradientThe Geothermal Gradient Notice that thinner lithosphere allows convective heat transfer to shallower depths, resulting in a higher geothermal gradient across the boundary layer and lithosphere. TP is the potential temperature. It permits comparison of (estimated) temperatures at depth from one locality to another. Because temperature varies with depth, one must select some reference depth. In this case the surface was chosen. One simply extrapolates adiabatically from the T and P in question to the surface. Potential temperature, is the hypothetical surface temperature a deeply buried rock would end up at if it were transported to the surface along the adiabat passing through PT , without melting. 23
  • 23. The Geothermal GradientThe Geothermal Gradient Figure 1.10 Temperature contours calculated for an oceanic plate generated at a mid-ocean ridge (age 0) and thickening as it cools. The 1300o C isotherm is a reasonable approximation for the base of the oceanic lithosphere. The plate thickens rapidly from zero to 50 Ma and is essentially constant beyond 100 Ma. From McKenzie et al. (2005). 26
  • 24. The Geothermal GradientThe Geothermal Gradient Figure 1.11 Estimates of oceanic (blue curves) and continental shield (red curves) geotherms to a depth of 300 km. The thickness of mature (> 100Ma) oceanic lithosphere is hatched and that of continental shield lithosphere is yellow. • Because the thermal conductivity of shallow crustal rocks is low, heat transfer is slow. Extrapolating this leads to very high geothermal gradient. 27
  • 25. The Geothermal GradientThe Geothermal Gradient • Shallow gradients in old oceanic lithosphere range from about 17- 200 C/Km, gradually steepening to about 70 C/Km with depth. • Shallow shield gradients range from about 12-180 C/Km, transitioning to about 4-60 C/Km below about 30Km. • The oceanic v/s shield temperature differences are essentially restricted to the lithosphere and boundary layer, so the oceanic and continental curves converge by about 250-300Km where convection has homogenizing influence. 28
  • 26. The Geothermal GradientThe Geothermal Gradient Figure 1.12 Estimate of the geothermal gradient to the center of the Earth (after Stacey, 1992). The shallow solid portion is very close to the Green & Ringwood (1963) oceanic geotherm in Fig. 1–11 and the dashed geotherm is the Jaupart & Mareschal (1999) continental geotherm. • Nearly adiabatic convective heat flow deeper than about 300Km results in a linear geothermal gradient of approx. 0.30 C/Km. • The gradient shallows across the D” layer and then becomes steep in the metallic core, where the thermal conductivity and convection is very high. 29
  • 27. The Geothermal GradientThe Geothermal Gradient • The density contrast across the core-mantle boundary prohibits the core material from rising into the mantle, so heat can only be transferred upward by conduction, resulting in a thermal boundary layer across which the thermal gradient is estimated to be 300 – 10000 C. 30
  • 28. The geothermal gradientThe geothermal gradient The rate of convective flow in the mantle is measured in terms of cms/year. Slabs of cooler oceanic lithosphere, mostly mantle rock, too viscous to convect within themselves, sink into the underlying hotter and relatively less dense underlying mantle. Computed tomography of the Earth using seismic waves has shown that at least some subducting lithospheric slabs sink all the way through the mantle and come to rest on top of the dense metallic core 31
  • 29. Even though the rate at which the mantle convects, on the order of a few centimeters/year, seems minuscule, it is far greater than the rate of heat transfer by conduction. The fact that lithospheric slabs can sink convectively over tens of millions of years about 2800 km through the hotter mantle but still maintain a recognizable cooler T throughout their approximately 100-km thickness demonstrates how slowly they are conductively heated. Conversely, if heat conduction were more rapid than convection, the subducting slabs would absorb heat and lose their density contrast and identity before sinking very far. 32
  • 30. 33
  • 31. Dynamic cooling of the earthDynamic cooling of the earth Previous figure was intended to illustrate the thermal structure at shallow levels of Earth’s interior in stabilized areas, similar to a static, chemically and physically stratified earth with continents and oceans. The earth is far more dynamic. Temperature variations that drive convective flow are concentrated in thin boundary layers that are much smaller than the overall circulation pattern. The upper boundary of this system is a cool thermal boundary layer with low viscosity: the lithosphere. 34
  • 32. Dynamic cooling of the earthDynamic cooling of the earth In laboratory models of convecting fluids, cool dense upper thermal boundary layer descend as either cylindrical downwellings or as larger networks of partially connected tabular-shaped downwelling slabs, depending on the rheology of the material. It generally leads to slab-like downwelling. Mantle convection and plate tectonics are inseparable manifestations of the heat-driven dynamic cooling process of the Earth, given its present thermal state and rheological properties. For the Earth, plate tectonics is mantle convection. 35
  • 33. Plate tectonicsPlate tectonics The negative buoyancy of the descending plates has been called slab pull, dense subducting slab pulls the rest of the plate after it. The motion of the lithospheric plates down and away from the elevated mid- oceanic ridges is called as ridge-push. Both are body forces affecting whole plate. As the subducting slab descends, the slab cant pull significantly on the rest of the plate, which would simply fault and break. 36
  • 34. Plate tectonicsPlate tectonics Instead, as it sinks, the slab sets up circulation patterns in the mantle that exert a sort of suction force. As the slab descends from the surface, the pressure behind it is lowered, which is immediately compensated by feeding more plate into the subduction zone. 37
  • 35. Dynamic cooling of the earthDynamic cooling of the earth Ridge push is gravitational sliding of a plate off the elevated ridge, not an active mantle push by ascending mantle flow, which would buckle the plate. Plate motion velocities are roughly proportional to the percentage of plate’s perimeter that corresponds to subduction zones and not to ridges (Forsyth & Uyeda, 1975) The predominance of slab pull suggest that mantle upwelling at divergent plate boundaries, which moves heat upward, is essentially a passive response to plate separation and descent. 38
  • 36. TheThe GeothermalGeothermal GradientGradient Fig 1.13. Pattern of global heat flux variations compiled from observations at over 20,000 sites and modeled on a spherical harmonic expansion to degree 12. From Pollack, Hurter and Johnson. (1993) Rev. Geophys. 31, 267-280. Cross-section of the mantle based on a seismic tomography model. Arrows represent plate motions and large-scale mantle flow and subduction zones represented by dipping line segments. EPR =- East pacific Rise, MAR = Mid-Atlantic Ridge, CBR = Carlsberg Ridge. Plates: EA = Eurasian, IN = Indian, PA = Pacific, NA = North American, SA = South American, AF = African, CO = Cocos. From Li and Romanowicz (1996). J. Geophys. Research, 101, 22,245-72. 39
  • 37. TheThe GeothermalGeothermal GradientGradient Thermal structure in a 3D spherical mantle convection model (red is hot, blue is cold). J. H. Davies and H.-Peter Bunge http://www.ocean.cf.ac.uk/people/huw/AG U99/mantlecirc.html 40
  • 38. Dynamic cooling of the earthDynamic cooling of the earth What happens at other deeper boundary? The density more than doubles from lower mantle to the core, thus preventing convection across the boundary, regardless of thermal differences. The liquid outer core must be internally convecting to produce Earth’s magnetic field. The D” layer may be the resulting mantle side thermal boundary layer, across which the temperature gradient is estimated to be 300 to 10000 C. Convection within the mantle results from thermal instabilities as the heated by D” layer becomes positively buoyant. 43
  • 39. Dynamic cooling of the earthDynamic cooling of the earth The viscosity is much lower than that of the plates at such elevated temperatures, and the instabilities take the alternative form of rising cylindrical plumes rather than slabs. These plumes are more of an active upwelling, in comparison to the more passive shallow mantle rise in response to plate separation. The plumes appear to be largely independent of plate tectonics, but in many cases are sufficiently vigorous to penetrate the lithosphere and reach the Earth’s surface, where they result in hotspot activity. Plate tectonics thus appears to be the method by which a largely internally heated and viscous mantle cools by convection, whereas plumes essentially cool the core and have an increasing effect on lower mantle. 44
  • 40. Dynamic cooling of the earthDynamic cooling of the earth Even general model of mantle convection depends on the influence of 660-km phase transition. Some investigators consider the density contrast across the boundary insufficient to impede convection, so there is whole mantle convection. Others believe the 660-km transition can impede convection, so that warm, buoyant mantle material in rising portions of deep convection cells would be less buoyant than the material above transition, causing the rising material to spread laterally instead. If so, heat can transfer across the boundary only by conduction, inducing convection in the upper mantle. 45
  • 41. Figure 1.14. Schematic diagram of a 2- layer dynamic mantle model in which the 660 km transition is a sufficient density barrier to separate lower mantle convection (arrows represent flow patterns) from upper mantle flow, largely a response to plate separation. The only significant things that can penetrate this barrier are vigorous rising hotspot plumes and subducted lithosphere (which sink to become incorporated in the D" layer where they may be heated by the core and return as plumes). Plumes in core represent relatively vigorous convection (see Chapter 14). After Silver et al. (1988). Mantle dynamicsMantle dynamics Is the 660 km transition a barrier to whole-mantle convection? Maybe? Partly? No? 46
  • 42. Mantle dynamicsMantle dynamics The 660-km boundary layer in such models allows some material transfer but impedes wholesale mantle convection from homogenizing the full vertical extent of the mantle and thus permits the composition of the shallow and deep mantle to evolve independently. 47
  • 43. Plate Tectonic - Igneous GenesisPlate Tectonic - Igneous Genesis 1. Mid-Ocean Ridges 2. Intracontinental Rifts 3. Island Arcs 4. Active Continental Margins 5. Back-Arc Basins 6. Ocean Island Basalts 7. Miscellaneous Intra- Continental Activity kimberlites, carbonatites, anorthosites... 49

Editor's Notes

  1. Myron Best: The difference in T between adjacent hotter and cooler masses, called the thermal gradient, is reduced and may eventually be eliminated over sufficient time, provided heat is not restored to the hotter mass. The rate at which heat is conducted over time from a unit surface area, called the heat flux or heat flow, is the product of the thermal gradient and the thermal conductivity, or heat flow = thermal conductivity x thermal gradient. Because of their extremely low thermal conductivity, compared with that of familiar metals, rocks are considered to be thermal insulators. All other factors being equal, copper conducts heat nearly 200 times faster than rock. Because of the low thermal conductivity of rock and the large dimension of the Earth (radius about 6370 km), little heat has been conducted from the deep interior over the lifetime (4.5 Ga) of the Earth (Verhoogen, 1980).
  2. 1. Radiation Requires transparent medium Rocks aren’t (except perhaps at great depth) 2. Conduction Rocks are poor conductors Very slow 3. Convection Material movement (requires ductility) Heat-induced expansion and buoyancy Much more efficient than conduction
  3. Convection from Myron Best: Movement of material having contrasting temperatures from one place to another is convection. Movement is caused by significant differences in density of different parts of the material so that, under the influence of gravity, less dense expanded material rises and more dense sinks. Density contrasts driving convection can also be related to contrasts in composition. For example, surface evaporation of water in saline lakes in hot arid regions increases the surface salt concentration, making the water more dense and causing it to sink, even though it may be warmer than underlying less saline water. It should be emphasized that, unlike heat transfer by radiation and conduction, convection depends upon gravity. Without gravity, there is no buoyancy force to act on density contrasts that move matter.
  4. But so is the development of contact metamorphic minerals through endothermic reactions. Similarly cold, crustal rocks that are buried and undergo regional metamorphism, which is largely endothermic, and then return to the surface of the earth provide another means of advectively removing heat from planet’s interior. Most metamorphic reactions involve the liberation of fluids. The upward flux of these hot fluids from metamorphic terranes is still another means of advectively cooling of earth. (Chapter 23 of Anthony R Philpotts) Advection: (Myron Best): Advection involves flow of a liquid through openings in a rock whose T is different from that of the liquid. Because all rocks near the surface of the Earth are fractured on some scale and because these fractures are, at least partly, filled with water, advection is a significant heat transfer process. For eg, hot water heated by nearby magmatic intrusion advects through cracks in cooler rocks, heating it while moderating the T of water. The greater heat capacity of water than of rock makes advective heat transport more effective. Advective heat transfer is also important where magma invades a cooler rock.
  5. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  6. The passage of seismic waves through the mantle demonstrates that it reacts as a solid to abrupt stress changes. The mantle is therefore solid, but the sub-lithospheric mantle is capable of flow when subject to slower stress changes, such as those associated with heating. Heat is thus transported more effectively by convection at depth but only by conduction across the stagnant rigid lithosphere. Both the conductive and convective geothermal gradients are nearly constant (linear) but of different magnitudes. Deep convection is sufficiently rapid that no heat is lost by conduction to the surroundings, so heat is retained within the rising material. Processes in which heat is neither lost nor gained are called adiabatic processes. Rheology (/riːˈɒlədʒi/; from Greek ῥέω rhéō, "flow" and -λoγία, -logia, "study of") is the study of the flow of matter, primarily in a liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force.
  7. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  8. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  9. The transition from rigid (lithospheric) to ductile (aesthenospheric) behavior is in the 1300 to 14000C range so the 13000C isotherm can be used as an approximation for the base of the oceanic lithospheric plate. The plate thus thickens from about 50 km at the ridge to about ~110 km at “maturity” of ~100 Ma, beyond which it essentially remains constant in thickness and thermal structure.
  10. As expected, the geothermal gradient is higher in the ocean setting than in the continental cratons because the oceanic lithosphere is thinner.
  11. As expected, the geothermal gradient is higher in the ocean setting than in the continental cratons because the oceanic lithosphere is thinner.
  12. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  13. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  14. Top text from Myron Best pg. 6-7
  15. Myron Best
  16. Plate tectonics by Wolfgang, Meschede and Blakey pg26
  17. Calculations of the excess heat required to create such swells have yeilded estimates of the total heat transported by plumes, which agree with the estimates of the heat flux from the core to the mantle (10%) plate tectonics thus appears to be the method by which a largely internally heated and viscous mantle cools by convection, whereas plumes essentially cool the core and have an increasing effect on lower mantle.
  18. From Alexander McBirney: The mantle is thought to turn over slowly, transferring heat generated in the core and in the mantle itself to the asthenosphere and to the lithosphere. The temperature within any convecting horizon is probably close to an adiabatic gradient. That is to say, the heat content of the rocks remains nearly constant while temperature varies with pressure, much as it does in Earth’s atmosphere, where the temperature descreases as air rises and expands but it increases as it descends and is compressed. And just as the temperature of air is buffered by the heat absorbed or released when water vaporizes or condenses, the temperature of the mantle must be moderated by the heat absorbed or released when minerals melt, crystallize or change their crystalline structure in response to changes in pressure.
  19. Continental Gradient higher than Oceanic Gradient Range for both Highest at Surface water and cold surface In the future we will often use average values rather than the ranges
  20. Myron Best: Associated with focused magmatic activity is the concept of petrotectonic associations: that specific types of rocks are found together in specific tectonic regimes. Although basaltic rocks composed mostly of plagioclase and pyroxene are created in most of the tectonic settings, there are significant differences from one tectonic regime to another in their chemical compositions, particularly in so-called trace elements such as Sr, Ba, Ta, and Nb. Also, the types of associated rocks are different. For example, andesites are common in convergent plate subduction zones but are essentially absent at oceanic spreading ridges. Rhyolites and their plutonic granitic rock counterparts are widespread along continental margin subduction zones but rare where two oceanic lithospheric plates converge, as in the island arcs of the western Pacific.