1. Modeling Gravity Anomalies for the Medicine Lake
Highland Volcano, California
D. D. Torres
Sponsor: J. S. McClain
Department of Earth and Planetary Sciences | University of California, Davis
Introduction
The Medicine Lake Highland is a shield volcano of varying
compositions in northeastern California - active as recently as
950 years ago[1].
The highland is situated between two geologic provinces: The
extensional province of the Basin and Range, and the volcanic
arc of the Cascade range[2].
Figure 2: Modified from Donnelly-Nolan et al, 2008
Figure 1: Modified from Donnelly-Nolan et al, 1996
Figure 9: Array 2 – S-N profile across Medicine Lake Highland. Basalt magma
chamber to the south not optimal. RMS Misfit = 0.9382929 mgal
Figure 8: Array 1 - E-W profile across Medicine Lake Highland. Finished
product after utilizing the previous models derived from seismic velocities,
magnetotellurics data and surficial geology. Resistivity soundings
(magnetotellurics) report presence of smectite clay cap at 1700 masl.
RMS misfit 0.9074531 mgal.
Figure 6: Adaptation of East-West (array 1) profile of MLV, based on rock densities
derived from seismic velocities (Figure 5). Dark blue line represents calculated
gravity. Light blue line represents observed gravity. Different body colors indicate
bodies of different densities. Calculated gravity profile does not fit the observed line;
therefore, model does not work. RMS Misfit=7.89107 mgal
The Purpose of…
This study is to use regional gravity anomalies to identify
structures that influence the delivery of magma and thermal
waters to the Earth’s surface during volcanic eruptions.
The mechanics of this system could explain why the Medicine
Lake Highland exhibits anomalous distributions of gravitational
highs and lows. Regional anomalies could be attributed to the
location of magmatic intrusions beneath the Medicine Lake
volcano.
Results
Conclusions
1. Positive gravity anomalies correspond to rocks that are
denser than the assumed background density. While
negative gravity anomalies correspond to rocks that are
less dense than the assumed background density.
2. Presence of a subsolidus rhyolite chamber is supported
by the gravity models, which is consistent with the
seismic velocity model (Fig. 5).
3. The model based on surficial geology, magnetotellurics
and seismic velocity data fits the observed gravity best.
4. Eruptions over the past ~500 kya show lava beds of
varying compositions[2], which supports the conclusion
that there are several magma chambers of differing
compositions (Fig. 8, 9).
5. Presence of a dome-shaped smectite clay cap is
confirmed (Fig. 8, 9). Inclusion of a “clay cap” is
necessary to provide the best fit for the calculated CBA
to the observed values. This smectite cap is important
because the impermeable layer prevents hydrothermal
fluids from rising to the surface.
6. There is an unusually high anomaly to the south of
Medicine Lake, which could be representative of a large
basaltic magma chamber; however, the authors think
this unlikely.
– basalt, andesite, and rhyolite.
Figure 4: Oblique view of the Complete Bouguer Anomaly (CBA)
gridded and interpolated using MatlabTM. The CBA is the residual
gravity anomaly after the correction of gravity measurements for
instrument drift, latitude of measurement, elevation, and surrounding
topography. Black crosses mark the location of gravity stations.
High red areas correspond to regions of positive anomaly values. Low
blue areas correspond to regions of negative anomaly values.
CorrectedBouguerAnomaly(mgal)
Figure 3: Graph of Complete Bouguer Anomaly (CBA) versus position.
Red dashed circle highlights approximate location of volcanic basin.
Black lines indicate locations of modelled gravity profiles.
Easting (m)
Northing(m)
CompleteBouguerAnomaly(mgal)
Complete Bouguer Anomaly of Medicine Lake Highland
Array 1
Array 2
References Cited
1. Finn, C., & Williams, D. L. (1982). Gravity evidence for a shallow intrusion under
Medicine Lake volcano, California. Geology, 10(10), 503-507
2. Donnelly-Nolan, J. M., Grove, T. L., Lanphere, M. A., Champion, D. E., & Ramsey, D. W.
(2008). Eruptive history and tectonic setting of Medicine Lake Volcano, a large rear-arc
volcano in the southern Cascades. Journal of Volcanology and Geothermal Research,
177(2), 313-328.
3. Cumming, W., & Mackie, R. (2010, April). Resistivity imaging of geothermal resources
using 1D, 2D and 3D MT inversion and TDEM static shift correction illustrated by a glass
mountain case history. In Proceedings World Geothermal Congress 2010 (pp. 1-10).
4. Evans, J. R., & Zucca, J. J. (1988). Active high‐resolution seismic tomography of
compressional wave velocity and attenuation structure at Medicine Lake Volcano,
Northern California Cascade Range. Journal of Geophysical Research: Solid Earth (1978–
2012), 93(B12), 15016-15036.
5. Gardner, G. H. F., Gardner, L. W., & Gregory, A. R. (1974). Formation velocity and
density-the diagnostic basics for stratigraphic traps. Geophysics, 39(6), 770-780.
6. Lowenstern, J. B., Killgore, M., Mariner, R., Blakely, R. J., Smith, J. G., & Donnelly-
Nolan, J. M. (1998). 3-D dimensional visualization of the Medicine Lake Highland, CA;
topography, geology, geophysics, and hydrology (No. 98-777).
Acknowledgements
We would like to extend our gratitude to the Department of
Earth and Planetary Sciences, Colin Ferguson, and Samuel
Hawkes.
Figure 7: After addition of two low density alluvial layers the calculated gravity profile
better reflects the observed gravity. RMS misfit=2.567582 mgal
Easting (m)
Northing (m)
Position v. CBA
CompleteBouguerAnomaly(mgal)
Figure 5: Seismic velocity model modified from Evans and Zucca (1988). For this study the seismic velocities
were converted to density that was used to model the gravity anomalies. A key result of Evans and Zucca (1988)
was the presence of a magma chamber below the volcano at a depth of ~500 meters above sea level (masl).