on the upper flanks of the volcano but become much wider and lobate toward their downstream (distal) ends. Most appear to originate near the Alba and Tantalus Fossae fracture ring, but the actual vents for the sheet flows are not visible and may have been buried by their own products. Flow thicknesses have been measured for a number of sheet flows based on MOLA data. The flows range from to thick and are generally thickest at their distal margins. The second major type of lava flows on the flanks of Alba Mons are called tube- and channel-fed flows, or crested flows. They form long, sinuous ridges that radiate outward from the central region of the volcano. They are typically - wide. An individual ridge may have a discontinuous channel or line of pits that run along its crest. Tube- and channel-fed flows are particularly prominent on the western flank of the volcano where individual ridges can be traced for several hundred kilometers. The origin of the ridges is uncertain. They may form by successive buildup of solidified lava at the mouth of a channel or tube, with each pulse of flowing lava adding to the length of the ridge. In addition to the two main types of flows, numerous undifferentiated flows are present around Alba Mons that are either too degraded to characterize or have hybrid characteristics. Flat-topped ridges with indistinct margins and rugged surfaces, interpreted as lava flows, are common along Alba's lower flanks and become less sharp in appearance with increasing distance from the edifice. In high resolution images, many of the flows on the volcano's upper flanks originally characterized as sheet flows have central channels with levee-like ridges. The morphology of lava flows can indicate properties of the lava when molten, such as its rheology and flow volume. Together, these properties can provide clues to the lava's composition and eruption rates. For example, lava tubes on Earth only form in lavas of basaltic composition. Silica-rich lavas such as andesite are too viscous for tubes to form. Early quantitative analysis of Alba's lava flows indicated that the lavas had low yield strength and viscosity and were erupted at very high rates. Alba's unusually low profile suggested to some that extremely fluid lavas were involved in the volcano's construction, perhaps komatiites, which are primitive ultramafic lavas that form at very high temperatures. However, more recent work on the tube-