STM imaging of MBE-grown pseudomorphic (100) Fe3O4 surfaces reveals terrace widths that are typically a few hundred ångströms broad, and can be as broad as 1000 Å. These terraces are separated by steps that are 1/4 of the spinel lattice constant high, corresponding to the distance (2.1 Å) between planes of oxygen (or equivalent iron) atoms. The images show that the p(1 × 1) surface reconstruction is caused by a clustering of atoms in the unit cell. These clusters are aligned along a  direction, and change direction on alternate terraces. The reconstruction is driven by the tetrahedral iron atoms, which have dangling bonds that rotate by 90° from one atomic plane to the next. Some regions of the surface also show a high-symmetry close-packed structure with 3 Å spacing between atoms. The presence of stacking faults is revealed by the orientation of the unit cells. In one image, the two possible orientations of the unit cells are present on the same terrace, separated by a disordered band, which must contain a stacking fault. In another case, the unit cells are oriented in the same direction on two terraces separated by a 2.1 Å step. Again a disordered region appears at the boundary between the two terraces. Single-domain regions are as large as a few hundred ångströms wide, which indicates that the surface diffusion length of the iron atoms during the initiation of growth on the higher symmetry MgO substrate is of this same order.