GEO 5/6690 Geodynamics 27 Aug 2014

© A.R. Lowry 2014

Last Time: Concepts of Lithosphere *Generally* Includes Crust & Uppermost Mantle • Tectonic plate• Thermal boundary layer• Strong upper layer supporting flexural bending stress• Rheologically strong layer (“Mechanical Thickness”)• Seismogenic layer• Seismic “Lid”• “Tectosphere”: Basalt-depleted mantle (major elmt chem)• Isotopes/REE: Depleted or enriched (depending) in Large-Ion Lithophile elements; Light Rare Earth elements; other incompatible els. (U, Th, Nb, Ta, Pb).

These (observation-based) concepts derive from severaldifferent/related processes, so have different depth ranges!

Read for Fri 29 Aug: T&S 132-149 (§4.1-4.12)

How to use these powerpoints:

• Review them often: Before each class, and while doing homework exercises

• The most important points of each lecture (in my opinion) are summarized on the first slide of the next lecture

• Note the notation! Arial Black, italic means Important, pay attention… Arial Black, italic, red font means Critically important concept or terminology that I expect you to understand intimately for exercises Times New Roman, italic, black font means this is an equation or an algebraic variable

A red box with grey background means this is an especially important concept or equation

Today: The GeothermFour Types of Heat Transfer Relevant to the Earth:

• I. Conduction Molecular collisions Slow & inefficient: ~ 100 Myr “traveltime”

for a thermal pulse to travel 100 km Dominates in Thermal Boundary Layer

Four Types of Heat Transfer Relevant to the Earth:

• II. Convection Thermal buoyancy-driven, activetransport of heat in a fluid

Implies Rayleigh # exceeds a critical value;has a preferred length scale ~ verticalthickness of convecting layer for 2D

General circulation Everything is moving…

Image courtesy Harro Schmeling

Four Types of Heat Transfer Relevant to the Earth:

• III. Advection Heat carried by flux of material, other than by general circulation

Examples include magmatic intrusion, erosion/deposition, tectonic strain, groundwater

Brune et al., Nat. Comm., 2014

Four Types of Heat Transfer Relevant to the Earth:

• IV. Radiation Transfer by electromagneticradiation (light)

Not usually considered important in the solid Earth, but some controversial

measurementshave suggestedit couldcontribute tomantle transfer…

The Geotherm:

Dep

thTemperature

00 1200-1400 ºC

50-300 km

Conductive

Convective

Adiabat: TemperatureIncreases with PressureBut Energy Doesn’t Change

ThermalBoundary Layer

Conduction in one-dimension: Fourier’s Law

where q is heat flux (flow of heat energy acrossunit area per unit time = W/m2)

k is thermal conductivity (W/mºK)T is temperaturez is depth (positive downward).

Assumes:• Conduction only• No heat sources or sinks within the medium• One-dimensional. However 3D generalizes to

• Steady-state (no time dependence of T!)

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q = −kdT

dz

€

q = −kr

∇T