Hi All. Whenever Petra and I take a field trip I always manage to find little pieces of a large puzzle that looks for a place to fit. Our recent trip to the geothermal fields near Middletown was just such a trip.

Our tour guide Lyle La Faver was informative, but no more so then any others. He did however say a couple of things that got me to thinking. One of those was the connection between the Mendocino Triple Junction and the Geyser-Clear Lake volcanic fields. He explained to us the various different elements and gases found in the steam and water that is pumped up from below. Just out of curiosity I asked him if any Helium 3 had ever been detected. He said he didn’t know, but we could check on that when I got back to the tour center. When we got back to the tour center he got a book in which there was a chapter on the various gases and other elements found. Helium 3 was not listed.

He then took me the room where there was map showing the various different areas of the geothermal field. This was not a part of the tour. He pointed out an area in which water was known to be rising to the surface. This is in an area that isn’t being worked. Not having the Lat/Long of the area I’m unable to locate it on a map, or check for quakes specific to that area.

How ever in searching for the connection between the Mendocino Triple Junction and geothermal fields I did find the following.

“Over the past 20 years, geological and geophysical studies of the geometry and evolution of the Gorda, North American and Pacific plate boundaries have led to models of asthenospheric upwelling into a "slab gap" under the California Coast Ranges south of the Mendocino Triple Junction (MTJ). Although the existence of a "slab gap" is widely accepted, several aspects of the phenomenon remain poorly constrained. The predicted warming of the North American plate as it slides off the Gorda plate is confirmed by the general increase in heat flow with distance south of the MTJ, but the vertical, lateral and temporal extent of the gap are open to question, as is the possible role of the asthenospheric upwelling in generating magma beneath the Coast Ranges in general and the Geysers-Clear Lake volcanic field in particular.”

“We address some of these uncertainties with an integrated comparison of models for the thermal state of the crust south of the MTJ with published heat-flow data, recently released geothermal industry heat-flow data, and the thermal conditions implied by the shallowing base of the seismogenic zone southward along the Maacama, Healdsburg, Rodgers Creek, and Hayward faults (MF-HBF-RCF-HF). Both the heat flow and base of the seismogenic zone can be matched to the thermal field produced by an asthenosphere rising to the base of a 20-km thick North American plate. Significantly shallower or deeper heat sources fail to match the rate and magnitude of warming indicated by the available data.”

As one can see there isn’t a clear connection between the MTJ and the geothermal fields. It is known however that the Maacama fault does cut all the way down to the mantel so this could indeed be the heat source for the geothermal fields. Take Care…Don in creepy town

• Geothermal Fields