EES215
Lecture 8
Example of heat flow calculation:
q = - K DT/Dh
What is the temperature increase in Basalt at 100 m with surface heatflow of 60 x 10-3 W/m2 and thermal conductivity of 2 W/m-oC:
DT = q Dh/K
DT = 60x10-3 W/m2
100m / 2 W/m-oC = 3 oC
For center of earth: Dh
= 6,300 km à
DT - 189,000 oC unrealistic:
real temperature range at center of Earth between 4000oC and 6000oC. Reason for discrepancy: distribution of heat
generating elements not uniform; more efficient heat transfer mode –
convection.
Conduction is important, but slow
Convective heat transfer: Fluids (as do most substances) expand with increasing temperature - density decreases. If a fluid medium is heated from below (or within) warm fluid raises and cold, denser fluid sinks – convection. Heat is transported by material at increased temperatures; important process in mantle and core; in geothermal systems, but not of great importance in crust. Convection is much faster and more efficient than conduction in transferring heat.
Important parameters: convection increases with temperature gradient; coefficient of thermal expansion; decreases with viscosity, thermal conductivity.
Rayleigh number, measure for velocity of convection: Ra = (a DT H)/(n K), with a - expansion coefficient; DT
- T difference; H - vertical dimension; n - viscosity (resistance to flow); K thermal
conductivity of fluid
critical Ra # has to be exceeded before convection can start
Important process in mantle, core, geothermal systems, oceans, atmosphere
Radiative transfer: hot substance emits radiation, most in
infrared and visible region of spectrum, efficiency depends on transparency of
medium, probably contributes in inner of earth, important for energy status at
surface (solar radiation)
Continental heat flow: Average 0.06 W/m2
often linear relation between heat flow and heat generation due to radioactive
decay in granites - two components in continents: deep heat flow (from mantle)
and heat production in crust: Q0 = Qd + Ad
with Qd called the reduced heat flow (coming from the mantle
or so) and d the thickness of the layer with heat generation A
A general decrease in heat flow from young to old crust is observed in continental crust
Oceanic heat flow: Heat generation lower in basalt, but heat flow similar to continental average è different components contribute to marine and continental heat flow
Observed heat flow discrepancy at mid-ocean ridges (Fig.
1) è hydrothermal convection;
Volcanism: Distribution of volcanoes è plate boundaries and hot spots (Fig. 3)
Type of volcanism: basaltic, andesitic (Fig. 4)
Volcanic deposits:
Lava flow, pahoehoe; aa; pillow lavas
Pyroclastic deposit
Eruptive styles depend on composition of magma, presence of
volatiles and viscosity of magma: basaltic has low viscosity; andesitic high
viscosity
Flood basalts: low viscosity allows to form large sheets of basalts (e.g.
Phreatic eruption – high content of volatiles in high viscosity magma:
explosive eruption;
Shield volcanoes (Fig. 5)
Volcanic domes; Cinder cones (Fig. 6)
Composite cone (strato volcano) (Fig. 7)
o Crater:
typical opening at top of volcanoes, access of active magma/lava to the
surface.
Caldera: Remnant of explosive eruption, can be tens of km across
(largest caldera is
