Hot Volcanic Rock?

[Calvin.Coin]

Why do some pieces of volcanic rock make my detector beep like mad and others don't? They are indistinguishable by look or feel...and only a few here and there will cause a beep, not very many.

cheers,
cc

 

[Gold_Fever]

Or are the hot ones meteorites?

 

[Calvin.Coin]

I don't think so...when busted up they appear no different from the regular ones. There is no metal evident and a magnet doesn't stick.

cheers,
cc

 

[woof!]

As igneous rocks are subject to weathering their iron mineral crystal structure changes, resulting in different magnetic properties. With weathering the color usually shifts from black/gray toward reds and browns, but sometimes the color shift isn't obvious if you don't look at it carefully. Also, in rare situations, there can be igneous rocks which due to their particular cooling history have magnetite crystals in a narrow size range which gives them peculiar magnetic properties (as seen by a metal detector, not by a magnet) but which are no different in naked eye appearance from rocks which cooled from the same melt material but under a slightly different temperature history.

--Dave J.

 

[Sandman]

Dave nailed it. Remember not all rocks are created equal.

 

[Calvin.Coin]

Cool guys, thanks! That's what I like about this hobby, so much to learn...geology, archaeology, history etc.

It is an odd situation. The softball size chunks of volcanic rock are laid like cobblestones. When one would beep I would move it aside and look underneath...signal gone! Took a minute to figure out it was the rock. LoL. The hot ones register anywhere from iron->nickle->pull-tab with rare sporadic jumps into the coin range if really hot. Don't know how I'm gonna be able to search under that 'patio' yet, since discrim seems like it would eliminate what might be there (cache, jewelery, etc.)

cheers,
cc