Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Alandriel on 19/12/2008 09:08:39

Title: Rocks and electro-magnetic fields - a guide for the uninitiated - please!
Post by: Alandriel on 19/12/2008 09:08:39
I’ve  been following the ITV series ‘CATASTROPHE’ with interest and one thing really stuck out for me:

- dating / tracing of rocks using measurements of their electro-magnetic field *

The guy, a palaeonto-geologist – I think, or did he call himself a geo-biologist  (if any of that actually exists or  was just created by my overactive imagination…) – mentioned that each type of rock holds a unique electro-magnetic signature and through that, yes it get’s better still,  can not only be dated but traced back to its origin

[they were talking about rocks having been displaced for many miles by glaciers during snowball earth]

This is all probably an old hat for you smart and learned people but it sure is totally news to me…. and of course, being a read head (no sorry, not blonde)  I can’t make heads and tails of it.

I’ve tried reading up bits here and there on background radiation and geomagnetic fields, looked at many dazzling graphs and gorgeous mathematical equations but if anything I’m more confuzzled than ever.

HELP! Could some kind soul explain this all in plain English?

Oh – PS  - and while I’m here… and still have your attention.......I found this also on wiki

<<The level of natural background radiation varies depending on location, and in some areas the level is significantly higher than average.[3] Such areas include Ramsar in Iran,[4] Guarapari in Brazil, Kerala in India,[5] and Yangjiang in China.[6] In Ramsar a peak yearly dose of 260 mSv has been reported>>

Something tells me that this is related to my question above…. Why those places? What happened in the past that made it so (and continues to keep it so today…)

                               Questions – questions that need answeres

Thanks ! Much obliged  [:)]

* disclaimer: it was late, I was tired, I might have all gotten it completely wrong and of course I’ve NOT (DOH!) recorded it so I could now go back and rewatch. So, in case I’m totally off the rocker (is that the saying you use in English?) be kind….. And sorry, not nice piccie  [>:(]Couldn't find anything pretty and relevant that would not immediately trigger a headache
Title: Rocks and electro-magnetic fields - a guide for the uninitiated - please!
Post by: Bass on 20/12/2008 20:31:10
Don't have much time to respond to this- so will do my best to add to your confusion:

Paleomagnetism is the study of remnant magnetism in rocks- or - how magnetic minerals are oriented in rocks over time.  The magnetic poles reverse- or switch on a regular basis (in recent times average is about 200,000 years).  Geologists have been able to figure out the age of reversals at least back to the Cambrian (around 500 million years ago).  By knowing the polarity (whether the poles were normal or reversed), scientists can narrow down the age ranges of rocks- however, knowing the polarity alone will not age date rocks.  Paleomagnetic dating is probably best only back about 80 million years- and becomes more unreliable as rocks get older.

I borrowed this illustration from the USGS:
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It shows a mid-oceanic ridge, where new crust is created as the plates on each side split apart.  As the new lava cools, the magnetic minerals align themselves with earth's magnetic field.  The polarity changes each time the earth's magnetic field flips- creating the "zebra" striped pattern during spreading of the ocean floor. 

Continental rocks also show remnant magnetism- as molten rocks cool below the Curie point, the magnetic minerals align themselves with the poles.  Sedimentary rocks, especially clays, also align with the poles- as do hydrothermal deposits- even limonites created during weathering.

Tectonic plates and the poles also wander through time.  So not only can the polarity be determined, but where the pole existed relative to the rocks when they were created can be used to help establish ages.

However, all this depends on the rock being in place so it can be measured accurately.  It is almost impossible to determine the age and/or source of glacial erratics using only paleomagnetism.  Archeologist also use magnetic polarity to help determine the age of pots and shards- these also would acquire a remnant magnetism when they were fired.

As to background levels of radiation- uranium (and thorium and potassium) are commonly concentrated in certain types of rocks (uranium in granite, for example).  Where you get higher concentrations of radiogenic elements, background radiation will increase in those areas.  Radon, a daughter product of uranium decay, is one of the main culprits- as a gas, it can move freely in rocks, water or air.  In fact, the US CDC says radon is second leading cause of lung cancer- though it is far outweighed by tobacco use.

Hope this helps.
Title: Rocks and electro-magnetic fields - a guide for the uninitiated - please!
Post by: Alandriel on 23/12/2008 22:03:57
Thanks Bass  [:)] for a handle on this. I read it several times with interest and will probably a few times more.

Just goes to show how much highly fascinating stuff I still have to discover... and so little time.

But let me ask you another thing: you mention  <<> uranium - commonly concentrated in certain types of
rock (uranium in granite for example)
>>> my native Switzerland is full of granite. Does that mean we have
a lot of background radiation?

Or am I spinning this too far?

Merry Christmas btw to any and all reading along  [:)]
Title: Rocks and electro-magnetic fields - a guide for the uninitiated - please!
Post by: Bass on 24/12/2008 01:40:58
Link to Switzerland radon map (

I'm not familiar enough with Swiss geology to answer your question- and too lazy to look it up tonight- but hopefully this map will help you evaluate radon (natural radioactivity) risk.