SPECTROSCOPIC ANALYSIS OF TEPHRA AS A SITE 

CHARACTERIZATION TOOL 

Prepared for 

U.S. Nuclear Regulatory Commission 

Contract NRC–02–07–006 

Prepared by 

Donald M. Hooper 

D. Marius Necsoiu 

Center for Nuclear Waste Regulatory Analyses 

San Antonio, Texas 

September 2011 
ii 

ABSTRACT 

Tephra (ash) deposits from Sunset Crater volcano, Arizona, were investigated to understand the 

relationship between visible near-infrared reflectance and physical properties of volcanic 

material as related to the erosion and weathering of a 900-year-old deposit in a semiarid 

climate. Quantitative and qualitative analyses were performed on tephra modified by aeolian 

(i.e., wind) processes to investigate the effects of grain size, shape, texture, and weathering on 

spectral response. Reflectance spectra were collected from homogeneous sample splits 

separated by sieve fraction. Principal component analysis was applied to decompose data by 

finding maximum variances, so the complexity of the spectral response of tephra samples could 

be more easily interpreted. Partial least squares analysis was used for developing a linear 

calibration model between grain size and spectral reflectance of sieve fractions. The model was 

used to estimate grain-size distributions of other tephra samples collected from the study area. 

The trends observed in the spectral reflectance of these samples showed that a complex 

relationship exists between reflectance and geometry/grain size of the analyzed fractions. The 

first and second principal components were useful to separate the samples based on shape, 

texture, and the amount of weathering. As expected, the grain size of a homogeneous sample 

affects the reflectance properties such that an increase in grain size produces a decrease in 

reflectance. This trend is noticeable for grain-size sieve fractions less than 0.6 mm [0.02 in]. 

Through this research, we have improved understanding of the relationships between physical 

properties and spectral response of volcanic material in the visible and near-infrared regions. 

These relationships may be used to support site characterization and the investigation of the 

extent of tephra deposit remobilization and redistribution, resuspension, rates of weathering and 

erosion, and grain-size characteristics. 
iii 

TABLE OF CONTENTS 

ABSTRACT ................................................................................................................................... ii 

FIGURES ..................................................................................................................................... iv 

TABLE .......................................................................................................................................... v 

ACKNOWLEDGMENTS .............................................................................................................. vi 

1 INTRODUCTION ................................................................................................................ 1-1

2 BACKGROUND .................................................................................................................. 2-1

2.1 Sunset Crater Volcano .............................................................................................. 2-1

2.2 Tephra Deposit .......................................................................................................... 2-1

2.3 Tephra Redistribution ................................................................................................ 2-2

3 SAMPLE DATA COLLECTION AND PREPARATION ....................................................... 3-1

3.1 Granulometric Analyses ............................................................................................ 3-1

3.2 Spectroscopic Analyses ............................................................................................ 3-1

3.3 Classification Scheme ............................................................................................... 3-2

4 DATA RESULTS AND ANALYSIS ...................................................................................... 4-1

5 CONCLUSIONS .................................................................................................................. 5-1

6 REFERENCES ................................................................................................................... 6-1
iv 

FIGURES 

Figure Page 

2-1 Sunset Crater Scoria Cone Viewed From the South ...................................................... 2-2 

2-2 Map of Tephra Thickness Reproduced From Hooten, et al. (2001) and 

Superimposed Upon Landsat ETM+ Satellite Data of the Study Area ........................... 2-3 

2-3 Aeolian-Reworked Sunset Crater Tephra in the Distal Area .......................................... 2-5 

3-1 Sieved Sample MN62906-32. ........................................................................................ 3-2 

3-2 Grain Shape Visual Chart .............................................................................................. 3-3

4-1 Residual Validation Variance for the Model and Predicted Versus Measured Y Plot .... 4-2 

4-2 Plots Demonstrating the Results From Principal Component Analysis .......................... 4-2 

4-3 Spectral Response of a Tephra-Sieved Sample (MN62906-32) .................................... 4-3 
v 

TABLE 

Table Page 

3-1 Sieve Characteristics for Sampling Protocol .................................................................. 3-2 
vi 

ACKNOWLEDGMENTS 

This report was prepared to document work performed by the Center for Nuclear Waste 

Regulatory Analyses (CNWRA®) for the U.S. Nuclear Regulatory Commission (USNRC) under 

Contract No. NRC–02–07–006. The activities reported here were performed on behalf of the 

NRC Office of Nuclear Material Safety and Safeguards, Division of High-Level Waste 

Repository Safety. The report is an independent product of CNWRA and does not necessarily 

reflect the views or regulatory position of USNRC. 

The author gratefully acknowledges M. Roberts for her technical review, E. Pearcy for his 

programmatic review and editorial review, and A. Ramos for his secretarial support. Other 

CNWRA staff members contributing to this work were acknowledged in previous reports. 

QUALITY OF DATA, ANALYSES, AND CODE DEVELOPMENT

DATA: All CNWRA-generated original data contained in this report meet the quality assurance 

requirements described in the Geosciences and Engineering Division, Quality Assurance 

Manual. Sources for other data should be consulted for determining the level of quality for 

those data. Field measurement data and laboratory analyses have been recorded in CNWRA 

Scientific Notebooks 747E and 748E (Necsoiu, 2005; Hooper, 2005). 

ANALYSES AND CODES: No scientific and engineering software was used in the analyses 

contained in this report.

REFERENCES 

Hooper, D. “Sunset Crater Field Studies.” Scientific Notebook No. 748E. San Antonio, Texas: 

CNWRA. pp. 1–47. 2005. 

Necsoiu, M. “Spectral Field Measurements, Post-Fieldwork Analysis, Lab Processing of 

Geologic Samples and Remote Sensing Data Analysis in Support of the Airborne Transport of 

Radionuclides Project (DIRECT2).” Scientific Notebook No. 747E. San Antonio, Texas: 

CNWRA. 122 pp. 2005. 
1-1 

1 INTRODUCTION 

Remote sensing (e.g., satellite and airborne image data) is a potentially valuable tool for site 

characterization because it affords an opportunity for a reconnaissance-scale analysis of the 

surface geology of large geographic areas in a short timeframe. However, spectroscopic data 

that characterize a sample of a mineral or rock type in a laboratory setting may differ from 

remotely sensed information from the same material in a natural setting. For example, 

differences in grain size, variations in mineral distribution in rocks, and alteration of rocks due to 

weathering will affect the spectroscopic data. As part of U.S. Nuclear Regulatory Commission 

prelicensing activities related to the proposed high-level waste repository at Yucca Mountain, 

Nevada, to enhance understanding of the factors that influence the above variations, Center for 

Nuclear Waste Regulatory Analyses staff conducted field and laboratory investigations of 

heterogeneities in physical properties such as grain size, shape, texture, and weathering of 

tephra from Sunset Crater volcano in north-central Arizona. Sunset Crater is a suitable analog 

for volcanic processes and post-eruption surface processes at Yucca Mountain, Nevada. The 

focus of the investigation was to contrast the physical properties of the tephra with measured 

variations in the visible and near-infrared reflectance of the samples. Volcanic ejecta includes 

scoriaceous ash {<2 mm [0.08 in]}, lapilli (or cinders) {2–64 mm [0.08–2.5 in]}, and blocks and 

bombs {>64 mm [2.5 in]}. When transported through the air, these ejecta are collectively 

termed tephra. Aeolian tephra deposits were sampled because they offer the opportunity to 

examine basaltic volcanic material that has been reworked by surface processes and 

redistributed. The techniques used in this investigation and the improved understanding of 

factors that affect such measurements may be useful for future site characterization

applications. This approach is not restricted to volaniclastic deposits or volcanic (igneous) rock 

types, but can be expanded to other sediment and rock types and/or other locations as part of a 

comprehensive site characterization.
2-1 

2 BACKGROUND

2.1 Sunset Crater Volcano

Sunset Crater is a 900-year-old (Smiley, 1958; Champion, 1980; Ort, et al., 2001) scoria-cone 

volcano composed of alkali olivine basalt (Holm and Moore, 1987). The volcanic cone stands 

314 m [1,030 ft] in height and is interpreted to have been created by a Strombolian-type 

eruption (Figure 2-1). These volcanoes are also known as cinder cones because of the 

abundance of vesicular cinder or scoria. 

Sunset Crater rests on the edge of the Colorado Plateau at an elevation of 2,135 m [7,000 ft]. 

Mean annual precipitation in the region is approximately 500 mm [20 in] (Sellers and Hill, 1974; 

Staudenmaier, et al., 2007). The climate is semiarid and supports pine forests at higher 

elevations and pinyon-sagebrush communities at lower elevations where precipitation amounts 

are lower. The prevailing wind direction is from the southwest, but it becomes more northerly 

during the winter months (Sellers and Hill, 1974; Staudenmaier, et al., 2007). 

2.2 Tephra Deposit

The fall deposit from the Sunset Crater eruption is a widespread blanket of fresh black and 

subordinate red tephra. Pyroclasts are typically vitreous, highly vesicular, and acnelithic in 

morphology. Individual fall units are unconsolidated, well sorted, and demonstrate both reverse 

and normal grading. Grain size range is typical of Strombolian style eruptions (i.e., coarse 

scoriaceous ash to lapilli). Granulometric analyses are described in a previous report 

(Hooper, et al., 2008). Secondary mineralization of Sunset Crater pyroclast surfaces by 

carbonate and/or silica is rare. Lithic fragments within the tephra are uncommon. When lithics 

are present, they make up an extremely low percentage (~ <0.5 percent) of their associated 

tephra package (Amos, 1986; Hooten, et al., 2001). Amos reports occasional carbonate lithic 

fragments from the Kaibab Formation. Amos (1986) identified eight discrete tephra-fall units 

associated with the Sunset Crater eruption with an estimated total volume of 0.75 km3

 [0.18 mi3] 

{dense rock equivalent volume of 0.41 km3

 [0.10 mi3]}. Each unit ranges in thickness from 

10 mm to over 1 m [0.4 in to 3.3 ft] and represents a separate phase of the eruption. 

Sunset Crater tephra drapes preexisting scoria cones, and the older pyroclasts are noticeably 

more weathered and oxidized. When pits were dug to the base of the tephra-fall deposit, a 

substrate of oxidized, cohesive material usually was encountered (Hooper, et al., 2008). This is 

the preeruption surface (or paleosol) comprising a red-colored, clay-rich layer that sharply 

contrasts with the black, fine-grained and coarse-grained tephra layers. 

Tephra dispersal from the Sunset Crater eruption can be divided into a continuous and 

discontinuous deposit. The continuous tephra-fall deposit mantles the preexisting landscape in 

the proximal and medial regions and covers an area of approximately 260 km2

 [100.4 mi2] 

(Amos, 1986; Colton, 1967, 1932). In the distal region, tephra grades into a discontinuous 

deposit contingent upon local topography, erosion, and initial deposition (dependent upon wind 

direction). Hooten, et al. (2001) supplemented the study by Amos (1986) with additional data 

points and generated a new isopach map (Figure 2-2). Hooten, et al. (2001) is the full technical 

report, but a general discussion of tephra dispersal, complete with an isopach map, can be 

found in Elson and Ort (2003). Hooten, et al. (2001) proposed that preexisting local topographic 

features played a significant role in the distribution and thickness in the proximal zone. 
2-2 

Figure 2-1. Sunset Crater Scoria Cone Viewed From the South. A Deposit of Black 

Tephra Covers the Surrounding Landscape. 

Rapid decreases in thickness likely correspond to older volcanic edifices acting as a 

topographic barrier to initial tephra deposition. 

2.3 Tephra Redistribution

Primary tephra is recognizable in the field because it is glassy, vesicular, and unconsolidated. 

In contrast, fluvially reworked tephra is abraded, rounded, and often mixed with older sediment. 

Tephra that has been reworked by aeolian processes is usually finer grained, well sorted by 

wind transport, and often remobilized into dune forms. 

The top of the Sunset tephra sequence, especially in the proximal region, is exposed directly on 

the surface without an overlying distinguishable soil horizon and is subjected to active surface 

processes. Because of the potential for erosion after 900 years since the time of emplacement, 

the initial thickness of tephra is unknown and the amount present today is only a measure of the 

minimum thickness immediately after the eruption. Careful inspection of the sequence reveals a 

topmost layer of bioturbated tephra mixed with aeolian fines (or potentially colluvium). This 

horizon is a surficial layer of scoria (cinder) with mixed quartz-rich sediment ranging from 8 to 

15 cm [3.2 to 5.9 in] in thickness as measured in numerous pits (Hooper, et al., 2008). In other 

locations, the primary tephra is covered by a protective layer of alluvium—usually mixed with 

Sunset scoria—of the same thickness. 
2-3 

Figure 2-2. Map of Tephra Thickness Reproduced From Hooten, et al. (2001) and 

Superimposed Upon Landsat ETM+ Satellite Data of the Study Area 

 
2-4 

The U.S. Geological Survey has mapped areal distribution of alluvial and aeolian deposits as 

part of the geologic map of the eastern San Francisco Volcanic Field (Moore and Wolfe, 1976). 

As is typical for this semiarid region, the drainage system is ephemeral or intermittent and is 

characterized by dry, flat-floored channels (i.e., washes or arroyos). The regional slope for the 

study area is to the east-northeast toward the Little Colorado River. However, the coarse and 

porous nature of the Sunset Crater tephra deposit prohibits significant fluvial activity such as 

overland flow and runoff. Overland flow is more significant in the distal portion of the 

Sunset Crater fall deposit where the tephra covering is thin or intermittent and a weathered, 

preeruption surface dominates. 

Wind action has subsequently redistributed and reworked the tephra into a variety of 

dark-colored, aeolian deposits and dune forms (Figure 2-3). Aeolian dunes accumulate when a 

supply of sand-sized granular material is carried downwind by bouncing or jumping motions 

(saltation) and is deposited when these winds weaken below the threshold for transport. 

Aeolian deposits composed of basaltic sediment from the Sunset Crater eruption include 

coppice dunes, falling dunes, wind ripples, and sand streaks (Hooper and Necsoiu, 2008; 

Hooper, et al., 2008). Coppice (nebkha) dunes form where clumps of vegetation trap saltating 

particles and create small mounds or hummocks. The spatial distribution of aeolian deposits 

relates to patterns of sedimentary deposition and erosion of source materials. In the vicinity of 

Sunset Crater, topographic barriers, such as mountains, escarpments, mesas, boulders, and 

the margins of lava flows generate zones of airflow turbulence, acceleration, and deceleration. 

This in turn strongly influences the accumulation or transport of the sand-size particles. 

Identifying the type and pattern of aeolian deposition can provide insight into sedimentary 

processes and paleotopography. 

Wind and human activity can disturb fallen fine-grained volcanic deposits for months or years 

after an eruption, presenting a long-term health hazard. Airborne particle mass concentrations 

were measured using a personal sampler under a variety of surface-disturbing activities within 

different depositional environments at both volcanic and nonvolcanic sites surrounding 

Sunset Crater (Hooper, et al., 2008; Benke, et al., 2009). Light surface-disturbing activities 

include walking or setting up equipment, while heavy surface-disturbing activities include 

digging and hoeing. The level of surface-disturbing activity was found to be the most influential 

factor affecting the measured airborne particle concentrations, which increased over three 

orders of magnitude relative to ambient (i.e., without human activity) conditions. Under 

ambient conditions, the total airborne particle concentration in the Sunset Crater region is 0.1 

to 0.4 mg/m3

 [6.2 × 10−9 to 2.5 × 10−8 lb/ft3] (Benke, et al., 2009). 

 
2-5 

Figure 2-3. Aeolian-Reworked Sunset Crater Tephra in the Distal Area. Black-Colored 

Tephra, Which Has Been Remobilized by the Wind, Drapes the Non-Volcanic Red-Colored 

Moenkopi Formation. The Sedimentary Rock is Capped by an Old Basaltic Lava Flow on 

the Skyline. Kana-a Wash Is in the Foreground. 
3-1 

3 SAMPLE DATA COLLECTION AND PREPARATION

The erosional change over time (i.e., geomorphologic evolution) of a basaltic tephra deposit in a 

semiarid climate can be evaluated by studying the relationship between visible near-infrared 

reflectance and physical properties of aeolian (volcaniclastic) material. Several previous studies 

(e.g., Leu, 1977; Johnson, et al., 1992; and Okin and Painter, 2003) have investigated the 

relationship between reflectance spectroscopy and the grain size of unconsolidated or 

powdered rocks and minerals. The goal for spectroscopic analysis is to understand the 

relationship between physical properties of tephra and near-infrared reflectance and compare 

these results to the expected relationship noted by previous studies. This analysis provides a 

basis for future site characterization applications. 

3.1 Granulometric Analyses

Granulometric analyses are the main source of data when examining the grain-size variations in 

unconsolidated pyroclastic (tephra) deposits. Systematic measurements of maximum particle 

size are also used to analyze the energetics of pyroclastic fall eruptions. Samples were first 

dried and then placed in a set of stacked sieves with standard mesh sizes. The stack of sieves 

was placed on a shaking machine (Ro-Tap) for 10 minutes. For aeolian deposits, analyses 

focused on a grain-size range from 0.106 to 2 mm [0.004 to 0.078 in]—the desired range for 

spectroscopic analyses (Table 3-1). The sample volume for fractions less than 0.106 mm 

[0.004 in] were too small for the collection of spectral data. 

Because of the broad distribution of grain sizes found in many geologic materials, it is common 

to use a logarithmic transformation of grain diameters called the phi (Φ) scale 

(Wentworth, 1922) 

Φ = −log2(dmm) (3-1) 

for which dmm is the grain diameter in millimeters [1 mm = 0.04 in]. For example, −4 Φ is equal 

to 16 mm [0.63 in], −2 Φ is equal to 4 mm [0.16 in], 0 Φ is equal to 1 mm [0.04 in], 2 Φ is equal 

to 0.25 mm [0.01 in], and 4 Φ is equal to 0.062 mm [0.0024 in]. 

3.2 Spectroscopic Analyses

The geologic samples collected in the field were analyzed in the laboratory with an Analytical 

Spectral Devices FieldSpec® 3 portable spectroradiometer which is designed to collect 

reflectance (solar or built-in light source), radiance, and irradiance measurements. This 

instrument collects a high volume of quality data in a short time interval and performs contact 

and noncontact National Institute of Standards and Technology-calibrated reflectance 

measurements of soil, vegetation, and rock samples over a wide spectral range. For 

these analyses, diffuse-reflectance spectra were collected from five samples (MN62406-5, 

MN62406-6, MN62606-13, MN62706-27, MN62906-32) with each of the samples being sieved 

into eight different sieve fractions or splits (Figure 3-1). Each spectral measurement was 

performed without interference from noise (in this case specular reflectance). The setup 

configuration, such as the angle of incident light and the distance of light illumination and 

sample surface, were consistent throughout the measurement process. A large number of 

samples (e.g., 60) were averaged per measurement because random noise is reduced by 

spectral averaging (as the square root of the number of averaged spectra). In addition, several 

measurements were performed on different locations of the sample to obtain measurement 

results that are representative of the entire sample. 
3-2 

Table 3-1. Sieve Characteristics for Sampling Protocol 

Sieve No. 

Mesh Size 

(Diameter, inches) 

Mesh Size 

(Diameter, mm) 

Phi Units 

(−log2 Diameter) 

10 0.078 2 −1.0 

18 0.039 1 0.0 

30 0.024 0.6 0.75 

40 0.017 0.425 1.25 

50 0.012 0.300 1.75 

70 0.008 0.212 2.25 

100 0.006 0.150 2.75 

140 0.004 0.106 3.25 

Figure 3-1 Sieved Sample MN62906-32. Grain-Size Range From 0.106 mm to 2 mm 

[0.004 to 0.089 in] Scale Bar Represents 2 mm [0.089 in]. 

3.3 Classification Scheme

In addition to grain size, grain shape, texture (vesicularity), and degree of weathering were 

measured for the sieve fractions. Grain shape was based on standard visual charts 

(e.g., Lindholm, 1987) and consisted of subdivisions representing angular, subangular, 

subrounded, and rounded (Figure 3-2). Grain texture was defined by degree of vesicularity. 

Vesicularity was classified as very high when the majority of grains are significantly covered in 

vesicles (bubble voids). Vesicularity grades through high, medium, low, and very low. For very 

low vesicularity, most grains have no or very few vesicles. For these cases, vesicles are 

smaller and the surface area is smooth and usually lustrous or shiny. The degree of weathering 

or amount of weathered material in vesicles or on the grain surface 
3-3 

Figure 3-2. Grain Shape Visual Chart 

was classified as very high when most grains were predominantly or totally coated in weathered 

material, grading through high, medium, low, and very low when most grains had little or 

no weathering. Typical weathering products include precipitation of calcium carbonate and clay 

mineralization and alteration. 
4-1 

4 DATA RESULTS AND ANALYSIS

Quantitative and qualitative analyses were performed on tephra modified by aeolian processes 

to investigate the effects of grain size, shape, texture, and weathering on spectral response 

(Necsoiu, et al., 2007a,b). Each reflectance spectra was jump-corrected at 1,000 and 1,800 nm 

[4.0 × 10−5

 and 7.0 × 10−5 in] (i.e., spectral discontinuities due to the spectrophotometer), and 

multiple measurements were averaged per sieving. Principal component analysis was applied 

to decompose data by finding maximum variances so the complexity of tephra samples could be 

interpreted. A partial least squares method was used for developing a linear calibration model 

between grain size {0.106 to 0.6 mm [0.004 to 0.024 in]} and spectral reflectance of sieve 

fractions (Figures 4-1 and 4-2). Chemometric techniques were used for quantitative analysis. 

These techniques were utilized to extract the information hidden in a complex data set and 

involved collecting a sufficient number of representative and diverse samples and then 

performing reference analyses for the parameters of choice. The resulting scores were used to 

detect sample patterns, groupings, similarities, or differences. Over 98 percent of the variance 

in the data was explained by the first two components of the principal component analysis. 

Reflectance spectra, which were collected from homogeneous sample splits separated by sieve 

fraction, displayed a complex relationship between spectral responses and the geometry and 

grain size of the analyzed fractions (Figure 4-3). 

The trends observed in the spectral reflectance of the analyzed fractions of tephra samples 

showed that an orderly relationship exists between reflectance and geometry, grain size, and 

mixing with nontephra grains. In the near-infrared wavelength range, the grain size of a 

homogenous sample generally affects the reflectance properties such that an increase in grain 

size produces a decrease in reflectance (Clark, 1999). This observation seems to generally 

agree with our aeolian-modified samples such that an increase in grain size produces a 

decrease in reflectance. This trend is particularly true for grain-size sieve fractions less than 

0.6 mm [0.024 in]. For grain-size sieves greater than 0.6 mm [0.024 in], the effect is reversed, 

probably due to the presence of weathering material on the surface of individual grains. 

The analysis reveals a subtle grain shape change from angular to subangular as grain size 

decreases from 2 to 0.106 mm [0.078 to 0.004 in], which may be related to the change in 

vesicularity and its overall effect on grain texture and morphology (coarser particles being 

more vesicular than finer particles). This trend agrees with the observation by Sandia National 

Laboratory (2007) that larger particles tend to have a higher fraction of vesicles than 

small particles. 

High spectral response of the fine grain sizes is also related to the degree of mixing. The 

percentage of nontephra grains increases in the finer grain-size splits, where the finest 

grain-size split is the most mixed. This is due to fine-grained aeolian sand and dust 

(mostly quartz), which is not related to the basaltic tephra from Sunset Crater. The coarser 

grain-size splits display slightly more weathering. Broadly, this includes oxidation, possible 

development of a weathering rind (usually best observed on larger particles), mineralization and 

cementation (may include clay minerals or a calcium carbonate coating), and similar processes 

that coat individual grains and fill vesicles or other pores. The observation that coarser grains 

are more weathered may be connected to another observed relationship, that coarser grains are 

more vesicular than fine-grained particles. These bubble voids appear to be the focus of most 

postdepositional mineralization. 

 
4-2 

Figure 4-1. Residual Validation Variance for the Model and Predicted Versus Measured 

Y Plots 

Figure 4-2. Plots Demonstrating the Results From Principal Component Analysis. After 

Quantitative Analysis, Reflectance Spectra of Sieved Samples Were Grouped by 

(a) Sample Number, (b) Shape, (c) Texture, and (d) Degree of Weathering. 
4-3 

Figure 4-3. Spectral Response of Sieved Tephra Sample (MN62906-32) Over Spectral 

Range (wavelength) From 350–2,500 nm [1.4 × 10−5

–9.8 × 10−5 in]. 
5-1 

5 CONCLUSIONS

Given the considerable areal extent of the fall deposits at Sunset Crater and the vegetation 

patterns within the depositional area, the use of satellite and airborne image data for 

reconnaissance mapping is time-efficient and cost-effective. Geomorphologic and structural 

relationships can be discerned over large areas with minimal ground control, and the 

identification of lithologic and vegetation units can be extrapolated on the basis of spectral 

characteristics. Furthermore, satellite and airborne image data can be used to evaluate 

uncertainties in published tephra deposit patterns and distribution characteristics. 

The complex relationships among tephra thickness, vegetation, elevation, lithology, and 

weathering in the Sunset Crater area can also be assessed using field and laboratory 

spectroscopy. Using a portable instrument, field spectroscopy includes studies of reflectance or 

radiance properties of vegetation, soils, rocks, and water bodies under solar illumination. 

Near-infrared spectroscopy is a successful technique to characterize volcanic materials based 

on their physical properties. A model that relates spectral response to grain size was developed 

and successfully validated using a systematic cross-validation method; however, the test was 

performed on a small number of samples from the Sunset Crater tephra. Results indicate that 

physical characteristics of analyzed samples—shape, texture, and weathering—are 

intercorrelated. Samples with a high degree of weathering were identified as being angular with 

high vesicularity. These characteristics appear to be independent of the percent of black tephra 

mixing with oxidized tephra and nonvolcanic material. Tephra samples appear to be 

distinguishable based on the degree of oxidized tephra and nontephra material. An approach 

using a combination of near-infrared spectroscopy and airborne/satellite image data is a 

powerful tool for distinguishing lithological variations over a large areal extent. 

This research improves understanding of the relationship between physical properties and 

spectral response of basaltic tephra in the visible and near-infrared regions. These 

relationships can be used to support investigations of a broad range of volcanic deposits and 

eruption types, models of tephra dispersal, the extent of tephra deposit remobilization and 

redistribution, rates of weathering and erosion of tephra deposits, and grain-size characteristics. 

Finally, these relationships and analyses are not restricted to volaniclastic deposits or volcanic 

(igneous) rock types, but can be expanded to other sediment and rock types as part of a 

comprehensive site characterization.
6-1 

6 REFERENCES 

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Master Thesis. Arizona State University. Tempe, Arizona. 1986. 

Benke, R.R., D.M. Hooper, J.S. Durham, D.R. Bannon, K.L. Compton, M. Necsoiu, and 

R.N. McGinnis, Jr. “Measurement of Airborne Particle Concentrations Near the Sunset Crater 

Volcano, Arizona.” Health Physics. Vol. 96, No 2. pp. 97–117. 2009. 

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Implications for the Global Geomagnetic Field.” U.S. Geological Survey Open-File 

Report 80-824. p. 326. 1980. 

Clark, R.N. “Chapter 1: Spectroscopy of Rocks and Minerals, and Principles of Spectroscopy.” 

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Sciences. New York City, New York: John Wiley & Sons. pp. 3–58. 1999. 

Colton, H.S. “The Basaltic Cinder Cones and Lava Flows of the San Francisco Volcanic Field: 

Flagstaff, AZ.” Museum of Northern Arizona Bulletin. Revised 1967. 

–––––. “Sunset Crater: The Effects of a Volcanic Eruption on an Ancient Pueblo People.” 

Geographical Review. Vol. 22. pp. 582–590. 1932. 

Elson, M.D. and M.H. Ort. “Collaborative Research at Sunset Crater Volcano.” 

Archaeology Southwest. Vol. 17, No. 1. pp. 4–6. 2003. 

Holm, R.F. and R.B. Moore. “Holocene Scoria Cone and Lava Flows at Sunset Crater, Northern 

Arizona.” Geological Society of America Centennial Field Guide–Rocky Mountain Section. 

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(USA).” Reykjavik, Iceland: IAVCEI (International Association of Volcanology and Chemistry of 

the Earth's Interior) General Assembly. August 17–22, 2008. 

Hooper, D.M., M. Necsoiu, and R.R. Benke. “Status Report of Field Observations from Sunset 

Crater, Arizona.” San Antonio, Texas: Center for Nuclear Waste Regulatory Analyses. 2008. 

Hooten, J.A., M.H. Ort, and M.D. Elson. “Origin of Cinders in Wupatki National Monument.” 

Technical Report No. 2001-12. Tucson, Arizona: Desert Archaeology, Inc. 2001. 

Johnson, P.E., M.O. Smith, and J.B. Adams. “Simple Algorithms for Remote Determination of 

Mineral Abundance and Particle Sizes from Reflectance Spectra.” Journal of Geophysical 

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