Appendix B—
Volcanic Rock Classifications and Data

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Classification Methods

Classification methods have been developed for pyroclastic materials, lavas, and in some cases, their intrusive equivalents. The following classification schemes are taken from Williams et al . (1982) and Fisher and Schmincke (1984). In general, classification systems include a rock chemistry designation, which may be derived from either a major-element chemical analysis or color and phenocryst content. Some textural classifications are based on hand sample inspection, but in the case of fine-grained rocks and tuffs or rocks that have been altered during diagenesis or metamorphism, it is necessary to use in addition a textural analysis by petrographic or scanning electron microscope.

Chemical Classification

Most volcanic rocks are composed of silicate minerals and glass; notable exceptions are carbonatites, which are composed of carbonate minerals, and rare lavas that are dominated by magnetite or sulfur. The SiO₂ content is the most general basis for classification: Silicic (acid) rocks have >66 wt%, intermediate rocks range from 52 to 66 wt%, mafic (basic) rocks have between 45 to 52 wt%, and ultramafic (ultrabasic) rocks <45 wt%. Alkali-silica variation diagrams (see Fig. 1.3) are widely used to classify volcanic rocks. Table B.1 shows average major-element chemical compositions for common volcanic rock types in order of increasing silica content.

A simple petrographic technique can also be used to estimate the SiO₂ content of volcanic rocks that contain glass. This technique is based on the decreasing refractive index of nonhydrated glass with increasing SiO₂ content; such a relationship is shown in Fig. B.1.

Williams et al . (1982) demonstrated that a reasonable chemical classification can be assigned to rocks and tephra containing phenocrysts because these minerals have characteristic SiO₂ contents that are a key to the bulk composition. Williams et al . (1982) listed the SiO₂ contents of felsic and mafic minerals as a useful guide.

Identification of the phenocryst content also makes it possible to use the international classification scheme of Streckheisen (1967), which was discussed in Chapter 1 (Fig. 1.3).

Where phenocryst abundances are significant (>4%), the rock name can be prefixed by the names of the significant phenocrysts in order of increasing abundance (for example, hornblende-biotite rhyodacite, pyroxene andesite, and olivine basalt).

Textural Classification

Textural classification can be very detailed, especially if it is determined by petrographic microscopic observation. Williams et al .

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Fig. B.1
The range of glass refractive index as a function
of silica content is shown by the shaded
band; the average values for several volcanic
rock types are indicated by dots.
(Adapted from Best, 1982.)

(1982) described and illustrated many textural features of volcanic rocks, but for the sake of simplicity here, we limit lava textural terminology to some hand-sample features (Table B.2).

Pyroclastic rocks in general are called tephra where they are unconsolidated and pyroclastic rock where they are consolidated. In the case of ash-size pyroclasts (see Table B.3), the unconsolidated deposit is simply termed ash , whereas the consolidated deposit is denoted tuff . Fisher and Schmincke (1984) based the textural classification of well-sorted pyroclasts on their granulometric character. For volcanic rocks composed of poorly sorted pyroclasts, Fisher and Schmincke advocated the system shown in Table B.4; however, some samples may contain a mixture of pyroclasts that spans the

size categories (Table B.3), and in that case, the ternary classification system shown in Fig. B.2 is prescribed.

Because pyroclastic rocks are composed of various proportions of vitric, crystal, and lithic constituents of juvenile, cognate, or accidental origin, the classification should also be made according to the proportion of these constituents in a sample, as is illustrated in Fig. B.3.

Finally, where the environment of deposition or mode of emplacement can be determined (as discussed in Chapter 1), classification may include such a designation. For example, tuff deposited in a marine environment is called submarine tuff , which distinguishes it from subaerial or lacustrine tuff. Tuff deposited by fallout is denoted fallout tuff, but tuff emplaced by pyroclastic flow is generally termed ash-flow tuff. Reworked tuff may be aeolian tuff where wind-reworked or fluvial tuff where deposited by a river or a stream. Combining these classification schemes produces terms such as crystal-lithic lapilli tuff, lithic tuffaceous breccia , or lithic-vitric fallout agglomerate .

Density

Volcanic rocks show a range of densities—from <1.0 Mg/m³ for silicic pumice to ~2.9 Mg/m³ for basalt. Because of the degree of vesiculation, crystallization, fragmentation, and postemplacement compaction, it is clear that after eruption, volcanic rock densities change from those of their parental magma. Bottinga and Weill

(1970) demonstrated the relationship of magma density to composition as a function of temperature. With increasing water content, magma densities generally decrease—as do their vesiculated volcanic equivalents. Table B.5 lists some average densities for common volcanic and intrusive rock types. The densities for intrusive equivalents exhibit maximum ranges for a given composition, whereas those for volcanic glasses fall in the minimum ranges. Because tuffs have pore space as a result of vesicles and intergranular voids, their densities (shown in Table B.5 for silicic rocks) are commonly 40 to 60% of those for their glassy lava equivalents. Vesicles may make up as much as 80% of the volume of pumices, for example.

Porosity and Permeability

Although there is no direct relationship between porosity and permeability, both of these rock properties are extremely important when assessing the reservoir potential of a given rock type. Porosity in volcanic rocks is mainly defined by the abundance of vesicles. In the case of pyroclastic rocks, grain size distribution and sorting determine the packing density of clasts. The porosity of a pyroclastic rock generally imparts a primary permeability; if subjected to hydrothermal fluid circulation, this permeability may change as a result of the dissolution of glass and the growth of secondary minerals. As we said earlier in the section on density, the porosity of pyroclastic rocks may reach 80%, but for fresh, nonaltered pyroclastic rock, porosity is generally in the range of 40 to 60%. Lavas, on the other hand, exhibit porosity only if they are brecciated during emplacement or contain vesicles and other gas cavities such as lithophysae; in these cases, lava porosity is generally <20%.

The bulk permeability of volcanic rocks is a function of primary and secondary permeability. Primary permeability (sometimes called formation permeability ), as discussed above, develops from the original texture of the rock (for example, interconnected pores and vesicles and grain boundaries). In contrast, secondary permeability (sometimes called fracture permeability ) is promoted by rock fracture and foliation, and where it occurs, it is generally the dominant type of permeability. Volcanic rock fracture has numerous origins, such as tectonic movement and proximity to faults, differential compaction that causes stress fractures, cooling contractions, thermal spallation, and eruptive/emplacement brecciation. Typical permeabilities for all rock types range from 10^-20 m² (0.01 µDarcy) to 10^-7 m²

Table B.4. Terms for Mixed Pyroclastic-Epiclastic Rocks Pyroclasticb Tuffites (Volcanic and/or Nonvolcanic) Epiclastic (Mixed Pyroclastic-Epiclastic) Average Clast Size (mm) Agglomerate, agglutinate, pyroclastic breccia Tuffaceous conglomerate, tuffaceous breccia Conglomerate, breccia 64 Lapillistone       (Ash) tuff coarse Tuffaceous sandstone Sandstone 2   fine Tuffaceous siltstone Siltstone 1/16     Tuffaceous mudstone shale Mudstone, shale 1/256 [Full Size]   a From Schmid (1981). b Terms are those used in Table B.3.

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Fig. B.2
Classification scheme for pyroclastic samples
composed of a mixture of fragment sizes;
the term lapilli-tuff is
synonymous with lapillistone.
(Adapted from Schmid, 1981.)

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Fig. B.3
Classification scheme for pyroclastic samples
composed of a mixture of constituents.
(Adapted from Cook, 1965.)

(0.1 MDarcy), as shown in Table B.6. The permeabilities of unaltered pyroclastic rocks should be similar to those of silty and clean sand—in the range of 10^-14 to 10^-10 m² (0.01 to 100.0 Darcy).

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.

Table B.6. Range of Permeabilities for Common Rock Typesa [Full Size]

Table B.13. Simple Classification of Volcanic Rocks by Well Log Response Parametersa [Full Size]