20 north and south of the equator are subject to strong heat and rainfalls. This changes the soil's electrical parameters significantly increasing the natural content of Al2O3 or Fe2O3.
Stones become 'metalised'.
These areas can contain between four and seven per cent of metal oxide. The signature of a 0.3kg stone containing about 20g of metal oxide can generate much stronger signals in metal detectors than mines containing as little as 0.2g. Soil can show even larger masses and often is non-homogenous so that sophisticated locating technology is needed to achieve a good level of landmine clearance. Tropical zones have changed and laterite can now be found in milder climatic zones such as Europe. Conductive soil and soil with a high magnetic permeability have different effects on hand-held metal detectors.