Geology Reference
In-Depth Information
12.1. Magnetic Signatures
Robert S. Sternberg
Scoria Point Overlook, South Unit, Theo-
dore Roosevelt National Park, North
Dakota. Scoria is the local name of the red
clinker that formed when Paleocene
lithostratigraphic units were exposed and
burned during the Quaternary.
Photo by Robert S. Sternberg, 2003.
Introduction
F
or coal fires that burn near the Earth
s surface, geophysics can be used to make inferences about the extent of the
burn. For smaller active burns, the conversion of magnetic minerals in the overlying soil can enhance magnetic
susceptibility (the ability of a material to become temporarily magnetized in the presence of an applied magnetic
field).
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measurements of the susceptibility or detection of the resulting magnetic anomalies (magnetic field
readings that deviate from the generally smooth magnetic field on the Earth
In situ
'
s surface) will both reflect this
mineralogical conversion.
The formation of pyrometamorphic rocks, i.e., clinkers, by larger fires is accompanied by the acquisition of a
significant thermoremanent magnetization (TRM), a permanent magnetization acquired when ferromagnetic
minerals are heated to several hundred degrees centigrade, which will generate larger magnetic anomalies.
These clinker anomalies allow the geophysical delineation of older coal burns and can locate the transition
from coal seams, used as an energy resource, to the less valuable clinker, used as an aggregate. Because the
magnetic signatures are generated under unusual geologic conditions of low pressure, high temperature, and
high oxygen fugacity, these soils and rocks might also be used to understand magnetization acquired under
these conditions.
This chapter will provide a brief overview on the use of magnetometry, although different techniques of geophysics
and remote sensing have been applied to the study of coal fires. Zhang et al. (2004) present an excellent discussion
of the general nature of coal fires and the use of remote sensing methods to detect them. Other ground-based
geophysical techniques include seismic refraction (Sontag, 1984), ground penetrating radar (Lindner and Rüter,
2007), and electromagnetics (Schaumann et al., 2008).
Creation of clinker by burning coal seams (Heffern and Coates, 2004) is a special case of combustion
metamorphism (CM), when organic-rich sediments burn. Combustion metamorphism is also found in the
bitumen-rich Upper Cretaceous
Lower Tertiary Hatrurim Formation in Israel (Matthews and Gross, 1980).
Cisowski and Fuller (1987) looked at combustion metamorphism due to oxidation of fine pyrite in oil-
bearing marine carbonaceous rocks in the Monterey and Sisquoc Formations in southern California and the
Marcelina Formation in Venezuela. These diagenetic sources of magnetic anomalies over sedimentary rocks
contrast with syngenetic magnetic anomaly sources as discussed by Gay and Hawley (1991). According to
these authors, both anomaly types were found to coexist in the Kaiparowits Plateau, Utah, in the south-
westernUnitedStates.
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