Paleomagnetic Measurements

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Exploration Technique: Paleomagnetic Measurements

Exploration Technique Information
Exploration Group: Lab Analysis Techniques
Exploration Sub Group: Rock Lab Analysis
Parent Exploration Technique: Rock Lab Analysis
Information Provided by Technique
Lithology: Can determine detailed information about rock composition and morphology
Stratigraphic/Structural: Historic structure and deformation of land
Hydrological:
Thermal:
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Paleomagnetic Measurements:
Paleomagnetism is the study of remnant magnetization in rocks. Paleomagnetic measurements are measurements of the magnetic properties in rocks; these properties are locked in during the formation of the rock. A rock samples magnetic direction and intensity is a record of the earth’s magnetic field at the time the rock formed.
Other definitions:Wikipedia Reegle


 
Introduction
Paleomagnetic measurements are magnetic measurements of rocks. By determining the magnetic intensity and orientation of multiple rock outcrops in an area much can be learned about the formation history, land movement, and geologic structure of the area. During the formation of rocks when its temperature drops below the Curie point (around 570°C for igneous rocks)[1] magnetic particles in the rock which are oriented with the earth’s magnetic field are locked into place. These orientations are later measured by geologists and sometime historical land movement and deformation can be deduced. Paleomagnetism has played a significant role in understanding and confirming the theories of plate tectonics, magnetic pole reversals, and mid oceanic ridge seafloor spreading.
 
Use in Geothermal Exploration
Paleomagnetic measurements can be useful in geothermal exploration to learn about the structure and historical deformation that has taken place in a geothermal area, which could potentially give clues as to where and how intense the geothermal heat source might be.[2]

 
Field Procedures
One field method is to take small core samples from outcrops in the field. The orientation of the core samples are recorded and the samples are taken to the lab where their magnetic orientation and age can be determined, and compared with their present day orientations.

(Left) A geologist uses a hand held coring device to drill a core sample in the field. (Right) a compass inclinometer used to measure the orientation of rocks prior to the core being collected.[1]



 
Best Practices
It is best to collect core samples from outcrops and exposed layers of rocks such as road cuts, where multiple layers can be easily accessed.
 
Potential Pitfalls
Paleomagnetic measurements can be useful for piecing together land movement and deformation but only the original orientation of the rock and its final orientation are known, so what happened in-between formation and present time is not always completely understood. Another problem is that sometime a rock may form then later become reheated above its Curie temperature, thus resetting the magnetic signature.








 
References
  1. 1.0 1.1 Colorado School of Mines and Imperial College London (Colorado School of Mines and Imperial College London). 2011. Geophysical Characterization of a Geothermal System Neal Hot Springs, Oregon, USA. Golden, Colorado: Colorado School of Mines and Imperial College London. Report No.: N/A.
  2. Christopher J. Pluhar,Robert S. Coe,Jonathan C. Lewis,Francis C. Monastero,Jonathan M.G. Glen. 2006. Fault Block Kinematics at a Releasing Stepover of the Eastern California Shear Zone: Partitioning of Rotation Style in and Around the Coso Geothermal Area and Nascent Metamorphic Core Complex. Earth and Planetary Science Letters. 250(1-2):134-163.




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