The Black Cuillin are regarded as the most amazing hills in the UK. They are quite young and were the magma chamber of a super-volcano. The magma was the mafic type that makes the oceanic crust. The magma cooled slowly and formed big dark crystals on a pale rock. The rock is called gabbro and is very abrasive so it has excellent grip for climbing. It also has a reputation for distorting compass bearings because one of the minerals in it is a compound of iron called magnetite. I was too blown away by the view to remember to check my compass when we went but I did bring back some samples. I have tested them with the Hall probe. The top picture shows the magnetic flux density in my front room. The second reading is when the probe is placed into the box of gabbro. There is quite a difference. The only thing I should warn you is that the readings are very variable and do depend a lot on the direction of the probe. I've tried to be careful but will need to do some further measurements. It seems that there is a measurable effect though.
