UKC

Just spotted this on BBC news website

http://news.bbc.co.uk/1/hi/scotland/highlands_and_islands/8042399.stm

Bit that really caught my attention was

"Rescue experts said compasses can lose their bearings in some parts of the Cairngorms because of a magnetic anomaly."

I've been walking in the Cairngorms for a good number of years and this is a new one on me.

Anyone able to provide more info.

In reply to Jim Braid:

Huge lumps of granite can affect compasses I believe, however this has never happened to me in the cairngorms as far as I can tell.

Maybe the compass was faulty?

In reply to link:

Granite tends not to be that magnetic (gabbro, now, that is a different matter), so this surprises me.

Y.

In reply to Jim Braid:

The report is contradictory. In one line it says his compass was "broken" and later it talks about "magnetic anomality". So which was it?

In reply to Yrmenlaf: Agreed, gabbro contains a lot of iron and hence locks in prehistoric magnetic fields when it cools below the curie temperature, apparently. Granite is alumino silicate type stuff and obviously much less ferrous. I've never experienced problems with my compass in the cairngorms, other than not totally believing it, then following it and finding it was of course correct.

In reply to errrrm?: I've broken a compass on the Cairngorms, slipped on ice and put a rock through the housing, very easy to do because if right handed and you walk with the compass in your right hand that is the one people will instinctively stick out to break the fall.

In reply to Jim Braid:

If you can read it, the 2nd paragraph...

http://books.google.co.uk/books?id=ObdepEp9r7kC&pg=PA20&lpg=PA20&am...

"Thin reversely magnestised Tertiary Dykes, associated with the igneous centres, large polyphase granitic extrusions such as Cairngorm" etc etc

In reply to Dominion:

And

http://www3.interscience.wiley.com/journal/119579183/abstract?CRETRY=1&...

ABSTRACT

Summary. The published results of the LISPB experiment in northern Britain indicate that the upper crustal structure beneath the Southern Uplands and Grampian Highlands areas cannot be clearly defined. This paper examines the relationship of this uncertainty with other geophysical and geological data for the latter area. A combined gravity and magnetic model along the LISPB line indicates the existence of a highly granitized belt as the cause of the uncertainty in the upper crustal structure of the Grampian Highlands area. The model constructed is in agreement with the LISPB data, magnetotelluric information and the geology of the area.

In reply to Jim Braid:

Well, you live and learn

But unless the dykes were very dense (are they? I don't know), or you are walking along them, you'd expect that the anomaly would be very local. The paper you linked to is a tad techie for me, I'm afriad.

Y.

In reply to IainRUK:

Agree that a broken compass could happen to anyone - including me. Same circumstances as you except I was wandering about Creag Meaghaidh on my own in winter. Not something you easily forget!

In reply to Dominion:

Fascinating. Is there a geologist out there who could comment on the practical implications? Anomaly exists but would it have an effect on a hillwalker's compass enough that he ends up lost? If so I'm surprised that given the number of folk wandering around the Cairngorms I haven't seen this reported as a reason for some ending up lost.

In reply to Yrmenlaf:

(If it exists) it's a magnetic anomaly, not gravitational.

In reply to Snorkers: There used to be a warning on SMC maps to Skye that this phenomena exists.
In mist on Bleaklow compasses can be pulled out by unseen aircraft wreckage.

In reply to Jim Braid:

A mobile phone emits a large magnetic field and makes the compass go haywire - try it.

In reply to Jim Braid: Can I be the first to say this wouldn't have happened if he'd been using a nice, traditional GPS device?

In reply to Jim Braid:

Magnetic anomaly my arse.
Rocks which would create an anomaly contain magnetite and include gabbro, diorite, basalt and other “mafic” igneous rocks. Felsic igneous rocks, like granite or rhyolite, and most sedimentary rocks are notably non-magnetic except in rare cases.

Slugain

In reply to Trangia:
How large an effect can you get off a mobile phone mast? I took a bearing from near near the shed on top of Aonach Mor and it was all over the place. I somehow missed Easy gulley by around 30yards in dense cloud and had great fun trying to retrace my steps as reversing my 'original bearing' didn't take me back to the shed. Personally I'm blaming Vodaphone rather than my own lack of micronavigation ability.

In reply to Al Evans:

Point still stands: you won't find anything on those maps saying that the density of rocks is a factor, because anomalies in density and magnetic susceptibility have completely different causes.

Geophysics 101, Al!

In reply to Al Evans:


Also on Black Hill. There is a large vein of Iron ore which pulls compasess off

Land Rover bonnets can be a bit distracting too

In reply to Reach>Talent:


It's funny... I found that shed had exactly the same effect on my unbranded oh-bugger-I've-forgotten-the-compass-look-at-the-sun-and-guestimate. Threw it off by a good 20 yards

jk

In reply to jkarran:
I'd have used the sun if some thieving bugger hadn't pinched it, I went up again the next day and found it fine when I could actually see.