Composition, Temperature, and Pressure in the Earth 


I. Composition
A. Compositional structure
1.Oceanic (basalt): created at MOR (Mid Oceanic Ridge) and destroyed at SZ (subduction zone), oldest = 160 Ma
2. Continental (granodiorite): created over time, doesn''t subduct, oldest = 4 Ga. Base = Moho
Mantle : composed of peridotite (dense: Fe-, Mg-silicates), extends to 2900 km depth , contains several smaller discontinuities (Low-velocity layer and Related to structural changes in mantle minerals), base marked by decrease in P velocity, disappearance of S-waves
Core: outer core is liquid, inner is solid, composed of iron-nickel alloy
B. Rheological structure
C. Chemical elements
Abundance - distance from center of solar system: refractory elements in inner planets, volatiles in outer planets , imperfect differentiation
Major elements: 97% of Earth''s mass O, Mg, Fe, Si, S, Al, Ca; most of Fe, S in core; all found in mantle and crust
Crust enriched in minor and trace elements
Composition at depth is of interest because that''s where melts form and metamorphism takes place
II. Pressure in the Lithosphere
A. Lithostatic pressure = pressure resulting from load of overlying rocks.
Applies at depths where rocks behave plastically
At shallow depths, rocks are not plastic

B. Pressure-depth relation
-P at depth z = (density) x (gravity) x (z)
-This formula applies for density and gravity = constants . Density increases downward => modify formula . Gravity depends on: mass, which decreases with increasing z , density, which increases with increasing z , these cancel out in crust through upper mantle
III. Temperature Gradients and Heat Flow in the Lithosphere
A. Direct evidence: measurements in deep mines and drill holes
B. Indirect evidence
Laboratory experiments, Limits on the geotherm from seismic waves: Top of LVL: T ~ 1000 deg C ; Base of LVL: 1500 deg C; Core/mantle boundary: T = 5000 deg C; Inner core/outer core boundary: T = 6500 deg C
C. Heat flow
-Heat energy of core and mantle results from: impacts during planetary accretion; energy released during separation of core; decay of short-lived radiogenic isotopes
-Surface heat flow = influx from below + radioactive decay in crust
-Expressed as J = quantity of heat passing through unit area in unit time. Depends on temperature gradient and K = coefficient of thermal conductivity
-Examples : young oceanic crust -250 mW/sq m; old oceanic crust -38 mW/sq m; young orogenic belts -150 mW/sq m; old continental crust -40 mW/sq m
-Formula for heat flow
J1 = J2 + Ad. where J1 = J at top of layer , J2 = J from beneath layer , A = radiogenic heat production of layer , d = thickness of layer
-Formula for temperature
T2 = T1 + (J1 / K) * (z2 - z1) -(A/2K) * (z2 - z1)^2, where T1, z1 refer to top of a layer, T2, z2 refer to bottom of the layer , A, K apply within the layer

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