Location of ground magnetic surveys near Yucca Mountain, Nevada. Plio-Quaternary volcanoes (shown in yellow) and aeromagnetic anomalies (solid green circles) are approximately 10-20 km from the proposed repository (marked). Faults are indicated by the red lines and roads by black lines.
Ground magnetic surveys were needed to understand patterns in volcanic activity during Miocene to Quaternary times and to identify additional buried volcanic centers. The surveys would also detect possible relationships between volcanism and the faults that bound and penetrate the Yucca Mountain block.
Magnetic Surveys "See" Beneath the Ground
We obtained magnetic readings using a cesium- vapor magnetometer interfaced to a differential GPS. Ground resolution of this particular differential GPS is better than 5 m and is typically 1–2 m. The data were downloaded into a Geographic Information System (GIS) for postprocessing. Although the error associated with the cesium-vapor magnetometer is on the order of 0.1 nanotesla (nT), total error in the survey is typically 1 nT due to errors in position, and it reaches 10 nT in areas of high magnetic gradients associated with basalt cover.
Using this instrumentation, we could traverse the area, quickly locating anomalies while continuously recording magnetic field and position data. Because the instrumentation provides real-time feedback, the survey team can make rapid and informed decisions to make more observations where anomaly wavelengths are short enough to warrant it and to sample less interesting areas more sparsely. A short-wavelength anomaly is one produced by shallow magnetized rock, like basaltic lavas around buried volcanoes. For surveys of three separate areas, more than 25,000 magnetic measurements distributed along approximately 60 km of traverse lines were taken. Line spacing varied between 25 and 500 m, depending on wavelength and complexity of observed anomalies. Our surveys represent the first detailed ground magnetic maps made of the region. Prior to the survey, researchers have have relied on aeromagnetic surveys supplemented by ground magnetic profiles to map volcanic hazards at the site.
Volcanoes Line Up in Amargosa `Anomaly A'
In previous studies, geologists identified five anomalies in Amargosa Valley as potential buried volcanoes. Amargosa Anomaly A (Figure 1) is the most complex and difficult of these anomalies to interpret from aeromagnetic maps. Yet, anomaly A is of great interest because it is near the Lathrop Wells cinder cone and the proposed repository site (Figure 1). With these factors in mind, we completed a ground magnetic survey of anomaly A to determine its origin and map its distribution in the subsurface.
The ground magnetic map of data collected over Amargosa Anomaly A delineates three separate anomalies associated with shallowly buried, reversely magnetized rock (Figure 2).
These anomalies are distributed over 4.5 km on a northeast trend, and the magnetization of each differs 70–150 nT from the surrounding area.
Ground magnetic map of the Amargosa Anomaly A showing details of three anomalies, most likely produced the three aligned and buried volcanoes. The high resolution magnetic survey shows that each anomaly is elongated and characterided by reversed
magnetization, indicating that the anomalies are caused by basaltic rock that was formed when the Earth's magnetic field was reversed. Contour interval is 10 nT.
Although these features can be partially resolved with aeromagnetic data, well-defined details emerge from the ground magnetic survey.
Examples of these details are the distinctive southernmost anomaly and the northeast-trending structure within the negative portion of the central anomaly that runs parallel to the overall alignment.
The ground magnetic data also enhance the small positive anomalies north of each of the three larger amplitude negative anomalies. These pairs of positive and negative magnetic anomalies, called dipolar anomalies, reinforce our interpretation that anomaly A is produced by coherent basalt edifices with strongly reversed magnetizations.
A key result of this study is finding the northeast trend of the anomaly A, which is similar to the orientation of the alignment of five Quaternary cinder cones in Crater Flat (Figure 1) and to the Sleeping Butte cinder cones, a Quaternary vent alignment 40 km to the northeast.
These findings suggest that development of northeast-trending cone alignments is a pattern of volcanism that has persisted through time in the Yucca Mountain region.
This alignment might occur because ascending dikes intrude along northeast trending faults, in a fashion similar to those mapped at Yucca Mountain (see figure), or because dikes rotated to the northeast because of stress conditions in the crust.
Small-volume basaltic volcanism has occurred at southern Crater Flat since large-volume eruptions ended during the mid-Miocene, but young alluvial sediments (deposited by streams or rivers) have partially or completely obscured these volcanoes in the southern part of the basin.
We used magnetic surveys near the Little Cones (two Quaternary cinder cones in the southern part of the basin) to map the buried volcanoes.
A clustered distribution of short-wavelength, large-amplitude anomalies showed the extent of Quaternary lava flows from the Little Cones.
The flows, which are buried 15–30 m below a nearly featureless alluvial plain, crop out at the surface about 400 m south of the cones. Ground magnetic surveys indicate that the Little Cones lavas extend approximately 1.5 km south of the Little Cones and that their volume is about 10 times greater than indicated by the area exposed at the outcrops.
Rather than being the exceptionally small volcanoes that they appear to be at the surface, the Little Cones and their lava flows represent the final burial stages of two young cinder cones in an actively subsiding basin.
This subsidence continues today.
The magnetic data also detected an older, completely buried volcano 2 km south of the Little Cones. First observed on aeromagnetic maps, this large-amplitude positive anomaly is beautifully symmetric, with a width of 750 m and a peak-to-peak amplitude of 1100 nT.
We also discovered a linear magnetic anomaly, probably caused by a shallow dike associated with the Miocene basalts, that extends approximately 500 m from the southern end of the survey area to the north-northwest.
After identifying the feature in the field, we immediately modified our survey to better define the shorter wavelengths and smaller amplitudes observed in this area.
Located in Crater Flat approximately 8 km from the repository site, Northern Cone is the closest Quaternary volcano to Yucca Mountain, which makes its structure of particular interest to volcanic hazard assessment.
Large-amplitude, short-wavelength anomalies were observed over the cone. Rather than finding northeast-trending structures that could relate Northern Cone to the rest of the Quaternary Crater Flat cinder cone alignment, we found prominent north-south trending anomalies surrounding Northern Cone with amplitudes of up to 400 nT.
These anomalies are produced by shallow fault rock buried beneath sediment.
When the ground magnetic map is compared with topographic and fault maps, the north-trending anomalies at Northern Cone roughly coincide with mapped faults just north of the survey area.
The mapped faults and the faults inferred from the magnetic map are all oriented north to north-northeast. Faults of this orientation are favorable for dike intrusion because the Earth's crust is extending in a west to northwest direction in this area.
Faults trending perpendicular to this direction of extension are easier to dilate as dikes rise through the crust. Thus the Northern Cone magnetic survey further supports the idea that volcanism on the eastern margin of Crater Flat was localized along faults.
Source: Eos, February 18, 1997, p. 73.
I have studied active volcanoes in Colombia, Nicaragua, Mexico, Kamchatka (Russia), Indonesia, and elsewhere. One of the lessons I have learned during this research is that individuals and societies often underestimate the risks of volcanic activity with terrible consequences.
Our society cannot afford to underestimate risks at Yucca Mountain—not only to our society, but also to people living in the area far into the future.
That is why my colleagues and I work hard to develop and use the best geophysical techniques possible to understand volcanic and tectonic hazards at Yucca Mountain.
Hidden Volcanoes of Southern Crater Flat
Northern Cone Volcanoes Line Up With Faults
GLOSSARY
A Few Words From Author Charles Connor ...
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