ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
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Heat Flow-2
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ÖNCEL AKADEMİ: SOLID EARTH GEOPHYSICS
- 1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Heat Flow Solid Earth Geophysics
- 2. What could you say about the qualitative behavior of the Heat Conduction equation after so many things? 1D Heat Conduction Equation 3D Heat Conduction Equation Specific heat Specific heat is defined as the amount of heat required to raise 1 kg of material by 1 C. Thus, Wkg -1 o C -1 is the unit for specific heat.
- 3. Calculation of Simple Geotherms – Equilibrium Geotherms pp. 279 of Fowler, 2005
- 8. Oceanic Heat Flow Heat flow is higher over and more scattered over young oceanic crust, which is formed by intrusion of basaltic magma from below. The heat drives water convection due to very permeable of the fresh basalt despite the fact that ocean crust is gradually covered by impermeable sediment and water convection ceases. Ocean crust ages as it moves away from the spreading center. It cools and it contracts. pp. 289 Fowler, 2005
- 9. d = 5.65 – 2.47e -t/36 d (km) = bathymetric depth t (Ma) = Lithosphere age Q=Heat Flow pp. 289-290 Fowler, 2005 These data have been empirically modeled in two ways: d = 2.6 + 0.365t 1/2 For ages <20 my: For ages >20 my: For ages >55 my: For ages <55my: Q = 510 t- 1/2 Q = 48+ 96e -t/36
- 10. Oceanic Heat Flow, Mean Depth, t 1/2 , t -1/2 Age Law For ages <70 my: For ages <120 my: See for detail on Table 7.5, pp. 296, Fowler, 2005
- 11. Half Space Model: Specified temperature at top boundary. No bottom boundary condition. Cooling and subsidence are predicted to follow square root of time as discussed by: Plate Model: Specified temperature at top and bottom boundaries. Cooling and subsidence are predicted to follow an exponential function of time. Roughly matches Half Space Model for first 70 my. pp. 294 Fowler, 2005 Lithosphere
- 12. pp. 298 Fowler, 2005 Plate Motion The base of the mechanical boundary layer is the isotherm chosen to represent the transition between rigid and viscous behavior . The base of the thermal boundary layer is another isotherm, chosen to represent correctly the temperature gradient immediately beneath the base of the rigid plate. In the upper mantle beneath these boundary layers, the temperature gradient is approximately adiabatic. At about 60-70 Ma , the thermal boundary layer becomes unstable, and small-scale convection starts to occur. With a mantle heat flow of about 38x10-3 Wm-2 the equilibrium thickness of the mechanical boundary layer is approximately 90 km. Thermal Structure of oceanic lithospheric plate.
- 13. Radioactive Heat Generation Radioactive elements: Uranium ( 238 U, 235 U), Thorium ( 232 Th) and Potassium ( 40 K)
- 14. Continental Heat Flow Heat flow versus crustal age for the continents. The heights of the boxes indicate the standard deviation about the mean heat flow, and the widths indicate the age ranges (After Sclater et al., 1980) pp. 299 Fowler, 2005
- 15. Heat Flow Provinces from Eastern USA pp. 299 Fowler, 2005 Internal heat generation Measured Heat Flow
- 17. The model of plate cooling with age generally works for continental lithosphere, but is not very useful. Variations in heat flow in continents is controlled largely by changes in the distribution of heat generating elements and recent tectonic activity. Continental Heat Flow pp. 302 Fowler, 2005
- 18. Range of Continental and Oceanic Geotherms in the crust and upper mantle pp. 303 Fowler, 2005
- 19. Oceanic Lithosphere Thermal models of the lithospheric plates beneath oceans and continents. The dashed line is the plate thickness predicted by the PSM plate model; k (values of 2.5 and 3.3) is the conductivity in Wm -1 C -1 .
Editor's Notes
- Gary A. Glatzmaier of the Institute of Geophysics & Planetary Physics at Los Alamos National Laboratory explains the computer modeling of field reversals. The first dynamically-consistent, three-dimensional computer simulation of the geodynamo (the mechanism in the Earth's fluid outer core that generates and maintains the geomagnetic field) was accomplished and published by Paul H. Roberts of the University of California at Los Angeles and myself in 1995. We programmed supercomputers to solve the large set of nonlinear equations that describe the physics of the fluid motions and magnetic field generation in the Earth's core. Image: Gary A. Glatzmaier, Paul H. Roberts COMPUTER SIMULATION shows a magnetic pole reversal taking place over a period of about 1,000 years. Magnetic field lines are blue where the field is directed inward and yellow where it is directed outward. The simulated geomagnetic field, which now spans the equivalent of over 300,000 years, has an intensity, a dipole-dominated structure and a westward drift at the surface that are all similar to the Earth's real field. Our model predicted that the solid inner core, being magnetically coupled to the eastward fluid flow above it, should rotate slightly faster than the surface of the Earth. This prediction was recently supported by studies of seismic waves passing through the core. In addition, the computer model has produced three spontaneous reversals of the geomagnetic field during the 300,000-year simulation. So now, for the first time, we have three-dimensional, time-dependent simulated information about how magnetic reversals can occur. The process is not simple, even in our computer model. Fluid motions try to reverse the field on a few thousand-year timescale, but the solid, inner core tries to prevent reversals because the field cannot change (diffuse) within the inner core nearly as quickly as in the fluid, outer core. Only on rare occasions do the thermodynamics, the fluid motions and the magnetic field all evolve in a compatible manner that allows for the original field to diffuse completely out of the inner core so the new dipole polarity can diffuse in and establish a reversed magnetic field. The stochastic (random) nature of the process probably explains why the time between reversals on the Earth varies so much. Answer originally posted on April 6,1998. « previous