The aureole of grooved terrain that surrounds the large shield volcano Olympus Mons consists of several overlapping roughly circular sheets, each 0.5 to 1.5 km thick. The surfaces of these materials bear abundant curvilinear ridges and troughs 10 to 100 km long and 1 to 5 km wide, which form anastomosing patterns that vary in length and width over the aureole. The aureole is asymmetric to Olympus Mons, extending almost 1000 km northwest from the center of the volcano but only 600 km southeast. The lobate form of the deposits, the pressure ridges on their surfaces, and the deflection of aureole material around barriers indicate that the deposits were emplaced as viscous flows. High-resolution Viking orbiter pictures show the aureole material to be unstratified, and it is inferred to contain randomly distributed large dark blocks whose weathering products form dark streaks in the talus on the flanks of ridges. A pyrocastic origin is proposed for the aureole. The following sequence is suggested. At least six great pyroclastic eruptions occurred prior to the construction Olympus Mons, they formed very fluid ash flows that became viscous during late stages of emplacement, and pressure ridges formed on the surfaces of the deposits before they finally came to rest. the oldest and most extensive aureole deposit (finely grooved terrain) was considerably modified by erosion prior to being partly covered by later eruptions. A positive gravity anomaly over the Olympus Mons area may define the site of a near-surface magma chamber that was the source of the pyroclastic eruptions.