A possible cause of high magnetic stability in volcanic rocks

A model based on ellipsoidal particles is presented to account for many of the magnetic characteristics of basic volcanic rocks. Elongated grains are common in those basic rocks in which high-temperature oxidation has produced ilmenite lamellas in the titanomagnetite crystals. This shows that many of these grains are small enough and elongated enough to behave as single domains. The model explains (1) why only volcanic rocks that have extremely small grains from initial crystallization or that show extensive lamella development in large grains are magnetically stable; (2) the observation that warming a specimen from −143°C in zero field does not destroy the stable TRM; and (3) the extremely stable fraction of TRM found in volcanic rocks, even when they are demagnetized in fields of 1000 oe or more.