Volcanology and Geothermal Energy "d0e18336"

Geophysical Properties

A set of geophysical properties for a volcanic rock includes its elastic constants, strength, seismic velocity, heat capacity and thermal conductivity, radioactivity, electrical resistivity, and well-log parameters. Although data for many volcanic rock types are sparse, in Tables B.7 through B.13 we list some typical values [chiefly from Clark (1966) and Carmichael (1984)]. However, volcanic rocks show considerable variability in their geophysical properties and these values listed below are provided as examples—useful for rough calculation but not for strict application in geothermal exploration. Needless to say, specific data should be obtained for volcanic rocks in the field of interest.

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Table B.6. Range of Permeabilities for Common Rock Types^a

Table B.7. Elastic Constants of Selected Volcanic Rocks^a
*Rock Type*	Density (Mg/m³ )	*Young's Modulus [E] (Mb)*	*Shear Modulus [G] (Mb)*	*Poisson's Ratio [v]*
Obsidian	2.446	0.656	0.303	0.08
		0.652	0.278	0.17
		0.718	0.303	0.18
Silicic Tuff
Lithic	1.45	0.14		0.11
Bedded	1.6	0.042	0.021
Welded	2.2	0.116	0.054	0.12
Andesite	2.57	0.54		0.18
Basalt	2.85	0.61	0.27
	2.97	0.85	0.34
	2.74	0.63		0.25
	2.82	0.485		0.384
^a From Birch (1966).

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Table B.8. Strength of Selected Volcanic Rocks^a
*Rock Type*	Ultimate Strength (kb)	Crushing Strength (kb)	Cohesive Strength (kb)
Rhyolite^b	8.00
Rhyolite^c	10.47
Rhyolite Tuff^d		0.067-0.482
Lithic Tuff		0.250	0.050
Andesite		1.320	0.290
		1.290	0.280
Basalt^e
T=297 K	15.40
T=673 K	13.80
T=873 K	10.30
T=973 K	5.31
T=1073	2.63
Basalt^f	2.62
^a From Handin (1966).
^b Confining pressure = 1.01 kb; temperature = 423 K.
^c Confining pressure = 5.05 kb; temperature = 773 K.
^d Information from Zalessky (1961).
^e Confining pressure = 5.00 to 5.07 kb.
^f Confining pressure = 0.00 kb; Temperature = 297 K.

Table B.9. Seismic Velocities of Selected Volcanic Rocks^a
*Rock Type*	v_p (km/s)	v_s (km/s)
Tuff	1.43	0.87
	0.76-4.57^b
Silicic Tuff	2.16	0.83
Rhyolite	3.27	1.98
Latite	3.77	2.21
Volcanic Breccia	4.22	2.49
Trachyte	5.41	3.05
Andesite	5.23	3.06
Basalt	3.35	1.64
	4.76	2.19
	5.06	2.72
	5.41	3.21
	6.4	3.2
Basaltic Scoria	4.33	2.51
^a Adapted from Press (1966) and Christensen (1984); v_p = compressional velocity and v_s = shear velocity.
^b Gardner and House (1987.)

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Table B.10. Heat Capacities and Thermal Conductivities of Selected Volcanic Rocks^a
*Rock Type*	Heat Capacity (kJ/kg-K)	*Conductivity Range (W/m-K)*	*Conductivity Mean (W/m-K)*
Rhyolitic Tuff^b		0.20–0.40	0.3
Rhyolite	1.06^c	1.58–4.33	3.0
Obsidian
T = 273 K			1.34
T = 373 K			1.46
T = 473 K			1.56
T = 573 K			1.67
T = 673 K			1.78
T = 773 K			1.89
Altered Rhyolite		3.1–3.7	3.44
Dacite^d	1.17	0.54–0.97	0.69
Andesite	1.04^c	1.35–4.86	3.7
Lava^e		2.6–3.6	3.10
Lava^e		2.7–3.3	3.01
Lava^f		1.7–2.8	2.10
Basalt	1.05^c	1.12–2.38	1.8
Diabasic Basalt
T = 303 K			1.69
T = 348 K			1.73
^a From Clark (1966) and Nathenson et al . (1982).
^b From W. Sibbett, personal communication (1978).
^c Heat capacity at 1000 K; from Bacon (1975).
^d From Friedman et al . (1981) for Mount St. Helens dacite.
^e Ventersdorp Lava, Orange Free State.
^f Portage Lake Lava, Calumet, Michigan.

Table B.11. Radioactivity in Selected Volcanic Rocks as Noted by Potassium, Uranium, and Thorium Abundances^a
*Rock Type*	Potassium (%)	Uranium (ppm)	Thorium (ppm)
Rhyolite	4.2	5
Feldspathic Tuff	2.04	5.96	1.56
Andesite	1.7	0.8	1.9
Basalt
Alkali	0.61	0.99	4.6
Alkali-Olivine	<1.4	<1.4	3.9
Tholeiite
Orogenic	<0.6	<0.25	<0.05
Nonorogenic	<1.3	<0.50	<2.0
^a From Fertl and Overton (1982).

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Table B.12. Electrical Resistivity Ranges of Selected Water-Bearing Volcanic Rocks^a
*Rock Type*	Lower (W -m)	Upper (W -m)
*Volcanic Rocks*
Quaternary-Tertiary	10	200
Mesozoic	20	500
Carboniferous	50	1000
Paleozoic	100	2000
Precambrian	200	5000
*Tertiary Tuff*
Granular	17.2	59.1
Welded	217	1410
^a From Keller (1966).

Table B.13. Simple Classification of Volcanic Rocks by Well Log Response Parameters^a

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