Magnetic weirdness at Hartz Peak

[alliecat]

I was up at Hartz Peak on Saturday. Visibility was lousy - between 5m and 50m most of the time. So before I headed off to try and make it to Mt Snowy, I thought I'd check the trusty compass to make sure I was heading in the right general direction, as I hadn't seen Snowy all day. I was pretty sure which way North was, I just wanted to be doubly sure before I headed off along the invisible (in the clouds) ridge line. So I pull the compass out, face what I'm sure is SE towards Snowy and look down. Only to find that the compass is telling me I'm facing NW rather than SE!

I give the compass a gentle tap and shake, and watch it settle down. Nope, still confused. By this time, given the poor visibility, and the fact that I'd been rained on for hours, this shook my confidence a bit. Could I have got turned around completely? Could the tack not match the map? had the track been rerouted? So I decide rather than try for Snowy, I'll retreat back down the track and see how I feel after a bit. As I walk down the track, the compass needle slowly swings around until, maybe 300m from the last little scramble to the summit, it's now pointing to where I was sure North lay. Back up the track - needle swings around. Hmm. Very suspicious.

Has anybody else experienced compass weirdness at Hartz Peak or anywhere else? On a clear day, it'd be just something to laugh at. If I was off track and in cloud, it'd be a bit unnerving. Maybe a GPS is a worthwhile investment after all.

Just thought I'd share that little experience.

Cheers,
Alliecat

[BarryJ]

There are a number of magnetic anomalies around Tasmania where the underlying rock types interfere with compasses. The Hartz Peak area is probably one of them.

[Son of a Beach]

Most of the area behind the Great Western Tiers up on the central plateau causes problems with compasses due to the iron in the rock (eg, "Ironstone Mountain"). This is nowhere near Hartz Peak, but it could be a similar issue. I find that holding the compass up higher (away from the rock) helps a little, but not enough so that I'd rely on it completely in those areas, if visibility was poor.

[johnw]

To offer another possibility. On my only trip up Hartz Peak, I remember some sort of broken concrete (and metal?) fitting at the summit. Maybe something that used to be there with a lot of metal in its base? Otherwise I'm inclined to agree with iron content in the rock. We have locations in NSW with the same issue.

[stu]

I have also heard of this anomaly, the King William range (especially KW II) apparently suffers the same compass issues & probably a lot of other areas in the dolerite ranges.

I would guess, as SOB has put forward, that it is to do with the iron content (& magnetic intensity) in the dolerite, some areas being richer in the mineral than others?

This link has mapping of the magnetic intensity of Australia, showing the various anomalies (couldn't find a specific detailed Tassie version).

http://www.ga.gov.au/image_cache/GA1689.pdf

My father is a Geo-technical engineer, I'll quiz him next time I see him.

A map of 'danger' areas for compass users would be very handy, I'll see what I can come up with.

Stu.

[tasadam]

Digging around in these pages, someone might know something.
You could try emailing perhaps?
See the bit at the bottom - Contact: Geomatic Specialist

[Mickeymoo]

I'm pretty sure that areas (especially dolerite) that are subject to lightning strikes i.e. the tops of peaks and high rocks sticking out all by themselves can be affected by the lightning strikes and give weird compass readings.

Michael.

[Swifty]

Mickeymoo is on the right track. The igneous rock dolerite which makes up the top of many of Tasmania's peaks - including the Hartz Mountains - contains the iron oxide mineral magnetite. When the magmas crystallized during the Jurassic - about 170 million years ago as Gondwana started breaking up - the magnetite grains were distributed more or less equally throughout the rock. In most cases, this magnetite is randomly oriented so the rock does not produce an overall dipole effect, but the rocks are magnetic in the sense that a swing magnet will be attracted to the rocks if placed next to them (try it with your compass) but at any reasonable distance (say waist-height) the effect won't normally be a problem. Running a huge current through the rock (that's the lightning strikes' accumulated effect over millenia) can and does realign the magnetite crystals so that the whole rock does act like a big magnet (as with naturally occuring magnets called "lodestones", which are just lumps of magnetite-rich rock). Then this will have a big effect on your compass, until you walk off the strike-affected area's zone of magnetic influence. How big can this area be? Several hundred meters, perhaps, more common towards the summits of mountains where the lightning tends to strike more often. Just imagine, these lightning hits have been happening since these rocks were uplifted and exposed - hundreds of thousands of years at least!
This should never occur over the non-magnetic sediments that make up a lot of the south west ranges - e.g. the Eastern and Western Arthurs - there is not enough magnetite in these rocks. The effect of man-made structures containing iron will also only be really local - no effect 10 meters away from them as far as your compass is concerned.
The worst experience I had with compass navigation when the compass was "wrong" happened in the fog on top of the Denison Range. Of course, these are not magnetic rocks and clearly the problem was with the navigator, not the compass

[geoskid]

A map of 'danger' areas for compass users would be very handy, I'll see what I can come up with.

Stu.

Have you had any further thoughts on this Stu.

I was up on Ben Lomond for a night in the school holidays (not bushwalking - stayed in a lodge so the kids could tobbogan and generally muck about in the snow). There was'nt a lot of snow around, and none down Meadow Vale so went for a quick 'look' (with my feet ). I've now got the bug to return and walk the plateau and camp at Lake Youl. Anyone know
whether the rocks up there are of the type that affect a compass. The rocks looked rounded (Granite?). The weather closed in a couple of times while we were up there - enough that if the compass did wierd things it would be unsettling.

[Son of a Beach]

It's dolerite up there which is what causes problems in some areas bit it all depends on the iron content. I don't know about the iron content on Ben Lomond and haven't tried a compass up there.