Studies of the magnetic properties of hundreds of surface samples and more than 500 underground samples of nonwelded and welded rhyolitic tuff, rhyolite lavas, lavas of intermediate composition, and trachybasalt lavas show that remanent magnetization is responsible for almost all of the prominent aeromagnetic anomalies associated with Tertiary volcanic rocks in the Nevada Test Site and vicinity. Although average induced magnetization ranges from about 1.5 × 10−4 emu/cc for rhyolitic lava and tuff to 3.6 × 10−4 for trachybasalt, this effect is not sufficient to explain measured anomalies. Remanent intensities vary from about 1.5 × 10−4 emu/cc for nonwelded tuff to about 30 × 10−4 for strongly magnetized welded tuff, to about 85 × 10−4 for a very magnetic facies of rhyolite, and to about 70 × 10−4 for trachybasalt.
Of the more than 50 volcanic units investigated to date, only 14 have the remanent intensity and thickness required to produce aeromagnetic anomalies. Of the 14 units, 7 are normally magnetized and give positive anomalies, 6 are reversely magnetized and give negative anomalies, and 1 has normal and reverse magnetization, giving both positive and negative anomalies. The remanent directions in rock which is free from near-surface lightning effects are consistent within individual stratigraphie units, and are either approximately along (normal polarity) or opposite to (reverse polarity) the present geomagnetic field. Application of rock polarity measurements assisted the geologist in making correlations of exposed volcanic units.