**APPENDIX D**

**VOLCANIC Materials: SCIENCE LANGUAGE for their NAMING and
CHARACTERIZATION IN DIGITAL GEOLOGIC-MAP DATABASES**

Version 1.0

North American Geologic-map Data Model Steering Committee

**Science Language Technical Team (SLTT)**

Volcanic Subgroup

This document should be cited as:

North American Geologic-Map Data Model Science Language Technical Team,
2004, Report on progress to develop a North American science-language
standard for digital geologic-map databases; Appendix D -- Volcanic
materials: Science language for their naming and characterization in
digital geologic-map databases, Version 1.0 (12/18/2004), *in* Soller,
D.R., ed., Digital Mapping Techniques '04---Workshop Proceedings: U.S.
Geological Survey Open-File Report 2004-1451, 20 p. Appendix D accessed
at http://pubs.usgs.gov/of/2004/1451/sltt/appendixD/.

\[This report was formerly available at the North American Geologic Map
Data Model Steering Committee website as
http://nadm-geo.org/sltt/products/sltt_volcanic_12_18_04.doc.\]

TABLE OF CONTENTS

[1 EXECUTIVE SUMMARY 3](#executive-summary)

[2 INTRODUCTION 4](#introduction)

[2.1 Purpose 4](#purpose)

[2.2 Intended Use 4](#intended-use)

[2.3 Who developed this document, and how?
4](#who-developed-this-document-and-how)

[2.4 Acknowledgments 5](#acknowledgments)

[3 BASIS OF SLTT VOLCANIC 1.0 6](#basis-of-sltt-volcanic-1.0)

[3.1 Philosophical approach 6](#philosophical-approach)

[3.2 Special problems 6](#special-problems)

[4 ATTRIBUTION BASED ON COMPOSITION
7](#attribution-based-on-composition)

[5 ATTRIBUTION BASED ON TEXTURE 8](#attribution-based-on-texture)

[6 ATTRIBUTION BASED ON EMPLACEMENT CHARACTERISTICS
8](#attribution-based-on-emplacement-characteristics)

[7 REFERENCES CITED 9](#references-cited)

[8 APPENDIX A -- TERMS BASED ON COMPOSITION
10](#appendix-a-terms-based-on-composition)

[9 APPENDIX B -- TERMS BASED ON COMPOSITION
13](#appendix-b-terms-based-on-composition)

[10 APPENDIX C -- TERMS BASED ON EMPLACEMENT CHARACTERISTICS
15](#appendix-c-terms-based-on-emplacement-characteristics)

[11 APPENDIX D -- PICKLISTS OF VOLCANIC TERMS
17](#appendix-d-picklists-of-volcanic-terms)

# EXECUTIVE SUMMARY

This document presents a classification of volcanic lithologies for use
in digital geologic-map databases produced and distributed by
public-sector geoscience agencies. The classification was developed by a
multi-constituency panel of geoscientists representing the Geological
Survey of Canada, the U.S. Geological Survey, State geological surveys,
and the Bureau of Land Management.

The classification of volcanic lithologies and their attributes is part
of a larger effort to develop a standardized data model for the storage,
manipulation, analysis, management, and distribution of digital
geologic-map information. This continent-wide effort is taking place
under the auspices of a North American Geologic-map Data Model Steering
Committee[^1] composed of representatives from Canadian and American
geoscience agencies. The data-model effort has several components:

(1) A standard conceptual model for storing digital data, and for
    manipulating these data in a relational and (or) object-oriented
    database environment;

(2) Standardized science language that allows geologic materials and
    geologic structures to be described, classified, and interpreted;

(3) Software tools for entering data into the standardized model at the
    front end (data-producer) and for extracting the data at the back
    end (data-user);

(4) Methodologies and techniques for exchanging data sets having
    different structures and formats.

This document addresses the science language of volcanic materials in
terms of their composition, their texture, and their emplacement style.

# INTRODUCTION

The Science Language Technical Team (SLTT) of the North American Data
Model Steering Committee (NADMSC) is a multi-constituency group of
geologic map producers and users that is developing prototype lists of
descriptive lithologic terms for use in digital geologic map databases.
The increasing use of digital geologic map databases highlights the need
for standardized terminology to facilitate their widespread interchange
and use.

The NADMSC was formed in 1999 as a partnership between the U.S.
Geological Survey (USGS), the Association of American State Geologists
(AASG), and the Geological Survey of Canada (GSC). This committee
identified the need for standardized science language for use in North
America and chartered the SLTT, first convened in early 2000.

## Purpose

The purpose of this report is to develop standardized nomenclature for
use in digital geologic map databases, specifically to describe
lithologies in volcanic rock units. Although this nomenclature takes the
form of a hierarchy of terms, it is important to note that this is not
the same as a formal rock-naming system. Similarly, it is not a system
for naming geologic map units and has nothing directly to do with
stratigraphic nomenclature.

## Intended Use

The intended use of this report is to provide standardized lists of
attributes for use in databases that describe geologic maps.

## Who developed this document, and how?

This document was developed by geoscientists from American and Canadian
geoscience agencies (Table 2.3.1). The group was assembled in early 2000
as the Volcanic Science Language Technical Team (SLTT) of the North
American Geologic-map Data Model Steering Committee. Members were
appointed in the following ways:

(1) Most participants from the U.S. Geological Survey (USGS) were
    identified by Regional Geologic Executives from the USGS Western,
    Central, and Eastern Regions. Some USGS scientists were appointed by
    Coordinators of USGS line-item science programs;

(2) Scientists from the Geological Survey of Canada (GSC) were
    identified by Canadian members of the North American Geologic-map
    Data Model Steering Committee;

(3) Scientists from State geological surveys were identified by the
    Digital Geologic-Mapping Committee of the Association of American
    State Geologists (AASG);

(4) Scientists from the U.S. Bureau of Reclamation were selected by the
    overall chair of the NADMSC Science Language Technical Team.

+------------------------------+---------------------------------------+
| **Committee Members**        |                                       |
|                              |                                       |
| **Volcanic Science Language  |                                       |
| Technical Team**             |                                       |
|                              |                                       |
| **Steve D. Ludington and     |                                       |
| Robert L. Christiansen,      |                                       |
| co-Chairs**                  |                                       |
+------------------------------+---------------------------------------+
| R. Ernest Anderson           | U.S. Geological Survey                |
+------------------------------+---------------------------------------+
| Robert L. Christiansen       | U.S. Geological Survey                |
| (Co-Chair)                   |                                       |
+------------------------------+---------------------------------------+
| Claudia C. Faunt             | U.S. Geological Survey                |
+------------------------------+---------------------------------------+
| Bruce R. Johnson             | U.S. Geological Survey                |
+------------------------------+---------------------------------------+
| Alison Klingbyle             | Natural Resources Canada              |
+------------------------------+---------------------------------------+
| Reed S. Lewis                | Idaho Geological Survey               |
+------------------------------+---------------------------------------+
| Stephen D. Ludington         | U.S. Geological Survey                |
| (Co-Chair)                   |                                       |
+------------------------------+---------------------------------------+
| Jonathan C. Matti (ex        | U.S. Geological Survey                |
| officio)                     |                                       |
+------------------------------+---------------------------------------+
| William C. Steinkampf        | U.S. Geological Survey                |
+------------------------------+---------------------------------------+
| Lambertus C. Struik          | Geological Survey of Canada           |
+------------------------------+---------------------------------------+
| David L. Wagner              | California Geological Survey          |
+------------------------------+---------------------------------------+
| Richard B. Waitt             | U.S. Geological Survey                |
+------------------------------+---------------------------------------+
| Richard Watson               | U.S. Bureau of Land Management        |
+------------------------------+---------------------------------------+
| Frederic H. Wilson           | U.S. Geological Survey                |
+------------------------------+---------------------------------------+

Table 2.3.1

The subgroup on volcanic rocks met face-to-face on May 1, 2001[^2], to
consider the best way to refer to the lithologies of volcanic rock units
in a digital database for geologic maps. An initial schema was developed
at that meeting, and subsequent refinements of the subgroup's
recommendations have taken place among all the members by e-mail and
telephone. Coordinators Steve Ludington and Bob Christiansen wrote this
report.

## Acknowledgments

The following persons provided comments, evaluations, and constructive
criticism of various drafts of this report: Fred Fisher (U.S. Geological
Survey, emeritus), J. Wright Horton, Jr. (U.S. Geological Survey),
Richard Moore (U.S. Geological Survey), and Steve M. Richard (Arizona
Geological Survey). We gratefully acknowledge their thoughtful feedback
and contributions, even if we didn't always adopt their suggestions.

# BASIS OF SLTT VOLCANIC 1.0

## Philosophical approach

The fundamental spatial characteristic of a geologic map is the map
unit, which is defined and characterized at the discretion of the author
of the map. Sometimes lithology is an important aspect of a unit
definition, sometimes not. A geologic map unit may comprise only a
single lithology, but more commonly it includes several different
lithologies.

We consider it critical to remember that the purpose of our hierarchical
subdivision of terms is to describe the *lithologic characteristics* of
*geologic map units*. It is to be used to logically retrieve or select
those map units that contain a specified set of lithologic
characteristics. Thus, it must be flexible enough to accommodate the
extremely varied and unsystematic way in which map units are described
and defined by various authors. This report groups lithologic features
necessary to adequately characterize **volcanic materials** in the map
units of a geologic map database into three fundamental classes based on
**composition, texture**, and **emplacement characteristics**.

No one of these classes is primary, and any or all may be used to select
the lithologies of map units. The subdivision of any one of the
fundamental classes consists of a list of words, arranged in a hierarchy
that can be used to select lithologies. The words that describe these
subdivisions are not given formal definitions here, but brief
descriptions are given in the appendices. Many of the words have
multiple, sometimes conflicting definitions and have been used
differently over the years by different map authors. We have attempted
to make the hierarchy sufficiently comprehensive, especially at the
higher levels, to allow adequate lithologic characterization and to
accommodate the vast majority of lithologic descriptions on existing
geologic map legends.

For any descriptive parameter there may be many words that have been
used to describe lithology. Some of them have been used widely, others
only rarely. Some of them are commonly misused or are used
inconsistently. We have attempted to place most of those words into a
hierarchy that fulfills two important requirements:

(1) the hierarchy should not have 'holes', i.e., all commonly described
    lithologies should have a place in it;

(2) the parent-child relationships in the hierarchy can be generally
    agreed upon.

It is assumed that any particular map-unit lithology can be described in
terms of any one, two, or all three of the first-level classes in the
hierarchy of terms. Within any one of those classes, a lithology will be
uniquely described by one control term, and any of those control terms
will be defined by a unique logical path starting at the first level.
Depending on the amount of information available, the path can end at
any level. Thus, within one class, a particular lithology might be
described by only a single very general term (*e.g.,* "volcanic rock"),
or by a highly specific term at a different level in the hierarchy
(*e.g.,* "shoshonite"). For any lithology, a complete geologic map
database would include a term from each of the first-level classes
(*e.g.*, rhyolite; obsidian; lava flow).

## Special problems

Two special problems pertaining to volcanic lithologies in geologic map
units require comment.

(1) Volcanic rock sequences commonly contain sedimentary materials. An
    example might be sandstone and conglomerate beds consisting largely
    or entirely of volcanic materials and intercalated with lava flows
    in a thick volcanic sequence. Such rocks are commonly termed
    volcanic sedimentary rocks, but we do not provide a place for them
    in our hierarchy. They are described in the SLTT report on
    sedimentary terminology.

(2) A similar situation pertains to subvolcanic intrusive rocks. Most
    stratovolcanoes, for example, contain dikes and sills of intrusive
    rock similar in composition to the lavas and pyroclastic rocks that
    make up most of the edifice and are commonly intimately associated
    with the predominantly eruptive volcanic lithologies. They are to be
    described in the SLTT report on plutonic terminology.

# ATTRIBUTION BASED ON COMPOSITION

Volcanic materials form a compositional continuum that makes any
compositional classification arbitrary. There are numerous
classification schemes in the literature for naming volcanic rocks.
Probably the most important is that of Le Maitre and others (2002),
which contains the formal recommendations of the IUGS Subcommission on
the Systematics of Igneous Rocks. We have tried to construct a scheme
that (1) includes most commonly used names, (2) is hierarchical, and (3)
does no violence to other commonly used classification schemes. To the
extent practical, we utilize the recommended compositional terms of Le
Maitre and others, but we have included a few others that are in common
use and fill descriptive needs. Note that we do not provide strict
definitions of the boundaries between these classes based on either
chemical or mineralogical criteria. Each method has its place, and both
have been extensively used for existing geologic maps. For selection and
retrieval of lithologies among the units of a geologic map database, the
most important requirement is that the hierarchy be valid (Fig. 1).

Many phaneritic volcanic rocks are described using compositional terms
based upon plutonic-rock nomenclature (perhaps with the prefix "micro-"
or with the additional appellation "porphyry"). For such materials,
plutonic compositional nomenclature will probably suffice and is not
further developed here. Other volcanic lithologies can be divided
compositionally into **felsic volcanic material, mafic volcanic
material, ultramafic volcanic material, high-alkali volcanic material,
volcanic carbonatite**, and **lamprophyre**.

The felsic volcanic materials depicted in most geologic maps can be
divided into **rhyolite, rhyodacite, dacite, trachydacite**, and
**trachyte**.

Most mafic volcanic materials are divided into **andesite, basaltic
andesite (basoandesite), basalt, trachyandesite,** and **trachybasalt**.
A subtype of basoandesite is **icelandite**. Basalt subtypes may include
**tholeiitic basalt, calc-alkali basalt**, and **alkali basalt**.
Trachyandesite subtypes may include **benmoreite, latite, mugearite**,
and **shoshonite**. Trachybasalt subtypes may include **hawaiite** and
**absarokite**.

Ultramafic volcanic materials include **picrobasalt, picrite**, and
**komatiite**.

High-alkali volcanic materials can be divided into **alkali rhyolite,
alkali trachyte, phonolite, tephriphonolite, phonotephrite,**
**tephrite, basanite**, and **foidite**. High-alkali rhyolite subtypes
include **comenditic rhyolite** and **pantelleritic rhyolite,** and the
high-alkali trachyte subtypes include **comenditic trachyte** and
**pantelleritic trachyte**. Typical names for foidites are
**nephelinite, leucitite**, and **melilitite**.

Two additional types of volcanic materials not included among the
preceding groups are **volcanic carbonatite** and **lamprophyre**.
Although varied mafic mineral assemblages are used to classify
lamprophyre types, they are not reviewed or listed here.

Appropriate mineral-name modifiers can be appended to most of the names
of volcanic materials on the above lists.

The hierarchy of terms for composition is shown schematically in the
accompanying figure 1, and brief descriptions of the terms used are in
appendix A.

# ATTRIBUTION BASED ON TEXTURE

The highest-level textural division for volcanic lithologies (Fig. 2) is
between **unconsolidated volcanic deposit** and **volcanic rock**.
Unconsolidated volcanic deposits are subdivided with the following
grain-size terms: **ash, lapilli-ash, lapilli, block-ash, blocks, bombs,
scoria, pumice**.

Volcanic rocks are first divided into **fragmental volcanic rock** and
**lava rock**. Fragmental volcanic rocks can be further subdivided using
the terms **tuff, lapilli tuff, lapillistone, tuff breccia, pyroclastic
breccia, agglomerate.**

A subtype of lava rock is **vitric lava rock,** which can then be
further subdivided as **obsidian, vitrophyre, pitchstone,** or
**perlite**.

The hierarchy of terms for texture is shown schematically in the
accompanying figure 2, and brief descriptions of the terms used are in
appendix B.

# ATTRIBUTION BASED ON EMPLACEMENT CHARACTERISTICS

The most fundamental distinction in the mode of emplacement of volcanic
(or predominantly volcanic) materials (Fig. 3) is among **intrusive
volcanic rock, extrusive rock** (both of them formed more-or-less
directly from erupted magma), and **volcaniclastic material**, composed
of aggregated volcanic materials that were previously fragmented. Terms
that characterize intrusive volcanic rocks would include terms like
**volcanic dike, volcanic sill, volcanic laccolith, volcanic stock,
volcanic plug, intrusive volcanic breccia**, etc. Most extrusive
volcanic rocks can be described with the following subdivisions: **lava
flow, lava dome, stratocone, shield**, etc. Lava flows can be further
subdivided into **pahoehoe, aa, block lava, massive lava**, and **pillow
lava**. Volcaniclastic materials can be further subdivided with the
terms **pyroclastic material** and **volcanic epiclastic material**.
Pyroclastic materials can be further described with the terms
**pyroclastic flow, pyroclastic surge**, and **pyroclastic fall**.
Pyroclastic fall can include **agglutinate (spatter), ejecta blanket,
cinder cone, tuff cone**, and **tuff ring**.

The hierarchy of terms for emplacement characteristics is shown
schematically in the accompanying figure 3, and brief descriptions of
the terms used are in appendix C.

# REFERENCES CITED

Le Maitre, R.W. (editor), Streckeisen, A., Zanettin, B., Le Bas, M.J.,
Bonin, B., Bateman, P., Bellieni, G., Dudek, A., Efremova, S., Keller,
J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H., and Woolley,
A.R., 2002, Igneous rocks: A classification and glossary of terms:
Recommendations of the International Union of Geological Sciences
Subcommission on the Systematics of Igneous Rocks: Cambridge, Cambridge
University Press, 236 p.

# APPENDIX A -- TERMS BASED ON COMPOSITION[^3]

*Felsic (high-silica) volcanic material* -- volcanic material
characterized by high SiO~2~ or by a high relative abundance of
feldspar, feldspathoids, and/or quartz.

*Rhyolite* -- felsic volcanic material characterized by abundant modal
or normative quartz and alkali feldspar and that generally contains more
than about 69 weight percent SiO~2~.

*Rhyodacite* -- felsic volcanic material, sometimes distinguished from
rhyolite or dacite, that generally contains from about 69 to about 73
weight percent SiO~2~.

*Dacite* -- felsic volcanic material characterized by some modal or
normative quartz, alkali feldspar about equal to plagioclase, and that
generally contains from 63 to about 69 weight percent SiO~2~.

*Trachydacite* -- felsic volcanic material, sometimes distinguished from
trachyte or dacite, that has SiO~2~ similar to trachyte but higher modal
or normative quartz.

*Trachyte* -- felsic volcanic material characterized by a low abundance
of modal or normative quartz and that generally contains from about 60
to 69 percent SiO~2~ and high Na~2~O and K~2~O.

*Mafic (low-silica) volcanic material* -- volcanic material
characterized by low SiO~2~ or by a high relative abundance of Mg- and
Fe-rich minerals and calcic plagioclase.

*Andesite* -- mafic volcanic material of intermediate composition,
generally with 57 to 62 weight percent SiO~2~.

*Basaltic andesite (basoandesite)* -- mafic volcanic material of
composition intermediate between andesite and basalt, generally between
52 and 57 weight percent SiO~2~.

*Icelandite* -- basoandesite characterized by a relatively high ratio of
iron to aluminum.

*Basalt* -- mafic volcanic material that contains essential plagioclase
and generally has SiO~2~ between 45 and 52 weight percent; typical
minerals include plagioclase of An content ≥50, pyroxene, and olivine.

*Tholeiitic basalt* -- basalt of an iron-rich compositional series,
generally characterized by disequilibrium between olivine and the
groundmass.

*Calc-alkali basalt* -- basalt that is neither particularly iron-rich
nor nepheline-normative.

*Alkali basalt* -- basalt of a low-iron compositional series
characterized by olivine in equilibrium with groundmass, relatively high
alkali metals, and normative nepheline.

*Trachyandesite* -- mafic volcanic material more alkali-rich than
andesite and that generally contains from about 52 to about 60 weight
percent SiO~2~.

*Benmoreite* -- trachyandesite that generally contains about 55 to about
60 weight percent SiO~2~ and a relatively low ratio of K~2~O to Na~2~O.

*Latite* -- trachyandesite that generally contains about 55 to about 60
weight percent SiO~2~ and a relatively high ratio of K~2~O to Na~2~O.

*Mugearite* -- trachyandesite that generally contains about 51 to about
55 weight percent SiO~2~ and a relatively low ratio of K~2~O to Na~2~O.

*Shoshonite* -- trachyandesite that generally contains about 51 to about
55 weight percent SiO~2~ and a relatively high ratio of K~2~O to Na~2~O.

*Trachybasalt* -- mafic volcanic material more alkali-rich than basalt
and that generally contains from 45 to about 52 weight percent SiO~2~.

*Hawaiite* -- trachybasalt characterized by a relatively low ratio of
K~2~O to Na~2~O.

*Absarokite* -- trachybasalt characterized by a relatively high ratio of
K~2~O to Na~2~O

*Ultramafic volcanic material* -- volcanic material characterized by
very low SiO~2~ or by a very high relative abundance of Mg- and Fe-rich
minerals, no or very little plagioclase, and no alkali feldspar or
quartz.

*Picrobasalt* -- ultramafic volcanic material intermediate between
basalt and foidite containing 41 to 45 weight percent SiO~2~ and less
than 12 percent MgO.

*Picrite* -- ultramafic volcanic material that is especially rich in
olivine, containing less than 42 percent SiO~2~ and 12 to 18 percent
MgO.

*Komatiite* -- ultramafic volcanic material containing more than 18
weight percent MgO and typically characterized by spinifex texture (a
needle-like network of olivine grains that encloses intergranular
pyroxenes).

*High-alkali volcanic material* -- volcanic material characterized by a
high percentage of Na~2~O and K~2~O or by essential alkali-rich
pyroxenes or amphiboles.

*Alkali rhyolite* -- rhyolite that contains alkali pyroxene or amphibole
or a very high chemical abundance of Na~2~O and K~2~O.

*Comenditic rhyolite* -- alkali rhyolite of peralkaline composition that
has a relatively high ratio of aluminum to iron.

*Pantelleritic rhyolite* -- alkali rhyolite of peralkaline composition
that has a relatively low ratio of aluminum to iron.

*Alkali trachyte* -- trachyte that contains alkali pyroxene or amphibole
or a very high chemical abundance of Na~2~O and K~2~O.

*Comenditic trachyte* -- alkali trachyte of peralkaline composition and
that has a relatively low ratio of iron to aluminum.

*Pantelleritic trachyte* -- alkali trachyte of peralkaline composition
and that has a relatively high ratio of iron to aluminum.

*Phonolite* -- high-alkali volcanic material that generally contains
from about 55 to about 64 weight percent silica, has higher Na~2~O and
K~2~O than trachyte, and contains feldspathoid minerals.

*Tephriphonolite* -- high-alkali volcanic material that corresponds to
high-SiO~2~ trachyandesite, generally with about 50 to about 55
weight-percent SiO~2~.

*Phonotephrite* -- high-alkali volcanic material that corresponds to
low-SiO~2~ trachyandesite, generally with about 46 to about 50
weight-percent SiO~2~.

*Tephrite* -- high-alkali volcanic material that corresponds to basanite
but is olivine-free.

*Basanite* -- high-alkali volcanic material that is olivine-bearing and
corresponds to basalt or trachybasalt but has higher Na~2~O and K~2~O
and generally contains from 41 to about 49 weight percent SiO~2~.

*Foidite* -- high-alkali volcanic material characterized by the presence
of significant amounts of feldspathoid minerals and the absence or very
low abundance of feldspar.

*Nephelinite* -- foidite in which the predominant feldspathoid mineral
is nepheline.

*Leucitite* -- foidite in which the predominant feldspathoid mineral is
leucite.

*Melilitite* -- foidite in which the predominant feldspathoid mineral is
melilite.

*Volcanic carbonatite* -- volcanic material composed primarily of
carbonate minerals.

*Lamprophyre* -- porphyritic volcanic material that contains abundant
panidiomorphic phenocrysts of biotite, amphibole, and pyroxene and some
feldspar or feldspathoid in a groundmass of the same minerals; K~2~O is
generally much more abundant than Na~2~O.

# APPENDIX B -- TERMS BASED ON COMPOSITION

*Unconsolidated volcanic deposit --* volcanic material that is not
lithified or indurated.

*Ash* -- unconsolidated volcanic deposit and ejected from a volcano and
characterized by grain size \<2 mm.

*Lapilli-ash* -- unconsolidated volcanic deposit ejected from a volcano
and characterized by a mixture of ash and lapilli.

*Lapilli* -- unconsolidated volcanic deposit ejected from a volcano and
characterized by grain size between 2 and 64 mm.

*Block-ash* -- unconsolidated volcanic deposit ejected from a volcano
and characterized by a mixture of ash, lapilli, and blocks.

*Blocks --* unconsolidated volcanic deposit ejected from a volcano and
characterized by angular fragments \>64 mm.

*Bombs --* unconsolidated volcanic deposit ejected from a volcano and
characterized by flight shaped fragments \>64 mm.

*Scoria (cinders)* -- unconsolidated volcanic deposit of highly
vesiculated material having thick aphanitic or dull glassy vesicle
walls.

*Pumice* -- unconsolidated volcanic deposit of highly vesiculated
material having thin glassy vesicle walls.

*Volcanic rock --* volcanic material that is lithified or indurated.

*Fragmental volcanic rock* -- volcanic rock formed by lithification of
an unconsolidated volcanic deposit.

*Tuff* -- volcanic rock formed by lithification of ash.

*Lapilli tuff --* volcanic rock formed by lithification of lapilli-ash.

*Lapillistone* -- volcanic rock formed by lithification of lapilli.

*Tuff breccia* -- volcanic rock formed by lithification of block-ash.

*Pyroclastic breccia* -- volcanic rock formed by lithification of
blocks.

*Agglomerate* -- volcanic rock formed by lithification of bombs.

*Lava rock* -- volcanic rock that cooled directly from lava or magma and
not as an aggregate of initially loose particles.

*Vitric lava --* lava rock composed primarily of glassy volcanic
material.

*Obsidian* -- vitric lava that is nonhydrated, is generally
dark-colored, and has few or no phenocrysts.

*Vitrophyre* -- vitric lava that has conspicuous phenocrysts in a matrix
of glass.

*Pitchstone* -- vitric lava characterized by hydrated glass with a dull
luster.

*Perlite* -- vitric lava characterized by hydrated glass with a
distinctive spherical fracture texture.

# APPENDIX C -- TERMS BASED ON EMPLACEMENT CHARACTERISTICS

*Intrusive volcanic rock* -- volcanic material emplaced beneath the
ground surface.

*Volcanic dike* -- a tabular body of intrusive volcanic rock emplaced
into a fracture, typically across preexisting volcanic materials.

*Volcanic sill* -- a tabular body of intrusive volcanic rock emplaced
between preexisting volcanic strata.

*Volcanic laccolith* -- a body of intrusive volcanic rock emplaced
between preexisting volcanic strata and that has bulged the overlying
strata upward.

*Volcanic stock* -- an equant body of intrusive volcanic rock.

*Volcanic plug* -- a body of intrusive volcanic rock emplaced into the
upper part of an eruptive volcanic conduit.

*Intrusive volcanic breccia* -- a body of intrusive volcanic rock
emplaced as fragmental material, typically not sorted.

*Extrusive rock --* volcanic material erupted as lava and emplaced on
the ground surface.

*Lava flow --* extrusive rock resulting from outpouring of lava and
having a tongue-like or sheet-like form.

*Pahoehoe --* lava flow having a continuous, commonly folded or ropy
vesicular crust.

*Aa --* lava flow having a carapace of rough spiny-textured blocks.

*Block lava --* lava flow having a carapace of relatively smooth broken
blocks.

*Massive lava --* lava flow having no conspicuous crust or carapace.

*Pillow lava --* lava flow having prominent bulbous, flattened, or
elongate sack-like forms resulting from subaqueous emplacement.

*Lava dome --* extrusive rock resulting from outpouring of lava and
having a bulbous form.

*Stratocone --* a volcanic edifice consisting largely of layered
extrusive rocks, both lava flows and pyroclastic materials, erupted from
one or more central vents and displaying steeply sloping flanks.

*Shield volcano --* a volcanic edifice consisting largely of lava flows
erupted from one or more vents and typically displaying gently sloping
flanks.

*Volcaniclastic material --* volcanic material that was fragmented
before deposition of the unit in which it is found, including clastic
volcanic materials formed by various processes of fragmentation,
transporting agents, and depositional environments, and possibly mixed
with subordinate nonvolcanic fragments.

*Pyroclastic material --* an aggregate of volcaniclastic material
deposited directly after ejection from a volcanic vent.

*Pyroclastic flow --* pyroclastic material emplaced by lateral flowage
of an essentially unsorted mixture of hot gases and pyroclastic ejecta.

*Pyroclastic surge --* pyroclastic material sorted and emplaced by
impulsive density currents of hot gases and pyroclastic ejecta.

*Pyroclastic fall --* pyroclastic material ejected, sorted in the air,
and deposited in beds.

*Agglutinate* -- pyroclastic fall forming a coherent deposit solidified
from originally plastic ejecta.

*Ejecta blanket --* pyroclastic fall that covers a broad area and does
not exhibit a distinctive morphologic form.

*Cinder cone --* pyroclastic fall of loose vesicular fragments and
forming a conical volcanic edifice.

*Tuff cone* -- pyroclastic fall of coherent pyroclastic material that
was water saturated during emplacement and forming an edifice with a
high rim and steeply sloping flanks.

*Tuff ring --* pyroclastic fall of coherent pyroclastic material that
was water saturated during emplacement and forming an edifice with a low
rim and gently sloping flanks.

*Volcanic epiclastic material --* an aggregate of volcaniclastic
material that was re-deposited by sedimentary processes.

*Volcanic avalanche --* volcanic epiclastic material, possibly mixed
with other material, emplaced by gravitational collapse from a volcanic
edifice and subsequent rapid flowage.

*Volcanic debris flow --* volcanic epiclastic material consisting of
initially water-saturated rock debris that flowed rapidly under the
force of gravity; this term includes most deposits designated by authors
as lahars.

# APPENDIX D -- PICKLISTS OF VOLCANIC TERMS

The accompanying file "SLTT_volcanic_picklists_12_18_04.xls" provides
picklists of volcanogenic terminology developed from Figures 1, 2, and
3.

[^1]: Visit [http://nadm-geo.org](http://nadm-geo.org/) for information
    about the North American Geologic-map Data Model Steering Committee,
    and <http://nadm-geo.org/sltt> for information about the Steering
    Committee's Science Language Technical Team. See the Federal
    Geographic Data Committee (FGDC) website for the Subcommittee on
    Geologic Information at
    <http://ncgmp.usgs.gov/fgdc_gds/datamodelprop.html>. to learn about
    what the FGDC is doing about the science language of planar and
    linear geologic features.

[^2]: Members attending: Steve Ludington, Bob Christiansen, Bruce
    Johnson, Reed Lewis, Richard Watson, Bill Steinkampf, Dave Wagner

[^3]: It should be noted clearly that this is not a formal
    classification or naming system for volcanic rocks but, rather, a
    listing of commonly used terms that fit within a hierarchical
    nomenclature for describing geologic map units. The descriptions
    here are intended only to be helpful but qualitative guides for
    understanding the hierarchy. For formal definitions, the IUGS report
    by Le Maitre and others (2002) should be consulted.