Volcanic basalt samples originating from two historic eruptions, that is, the 1991 C.E. Hekla,Iceland, and 1944 C.E. Vesuvius, Italy, have been studied to determine the 3D tomographic and spatialdistributions of their constituent (titano)magnetite minerals using SEM‐FIB slice‐and‐view. Determining themorphology is key to quantifying the magnetic recording fidelity of a rock, as grain morphology is a primarycontrol of the magnetic (domain) state of a grain, which in turn determines magnetic recording fidelity. Smallergrains are magnetically uniform and are termed single domain (SD). A surface morphology resolution of ∼2 nmwas achieved and the smallest grains that were resolved with ∼21 nm in diameter; a total of 971 particles wereanalyzed. We determined a median equivalent‐volume spherical diameter of 70 nm for the Hekla sample, and135 nm for the Vesuvius sample. The particles had nearest‐neighbor distances of 184 and 355 nm, indicate themajority of grains were free from magnetostatic interactions. In both samples there was a roughly even splitbetween oblate and prolate grains. This number of oblate grains is much higher than traditionally assumed, andwill have implications for many paleomagnetic methods which assume prolate grains, for example, anisotropyof magnetic susceptibility analysis. Numerical micromagnetic analysis of the grain‐morphologies, predict that∼64% of the Hekla grains have SD ground‐states (∼6% by volume), but only ∼26% of the Vesuvius grains haveSD ground‐states (∼1% by volume). Both samples are predicted to be excellent paleomagnetic recorders, withmedian relaxation times far larger than the length of the Universe