collapse. It forms the Negros de Aras surface northwest of the volcano and the El Cenizal surface due north where it has a hook-like surface distribution; the name "Negros de Aras" was given to the deposit before it was known that it had been formed by a landslide. The thickness of the deposit varies, with thin segments in the extreme southeastern and southwestern parts being less than thick and the central parts reaching . The deposit spreads to a maximum width of and is bounded by levees higher than , which are less prominent on the eastern side. As later parts of the collapse overrode the earlier segments, they formed a northeast-trending scarp in the deposit, across which there is a striking difference in the surface morphology of the collapse. The landslide deposit has been stratigraphically subdivided into two units, the Monturaqui unit and the El Cenizal unit. The first unit forms most of the surface and consists itself of several subunits, one of which includes basement rocks that were integrated into the collapse as it occurred. Likewise, the El Cenizal unit included basement rocks as well, such as playa deposits. The amount of basement material is noticeably large and might form as much as 80% of the landslide volume; the topography of the northwestern side of the volcano may have prevented the mass failure from being localized along the basement-edifice surface area, explaining the large volume of basement involved. Further, the basement-derived material was probably mechanically weak and thus allowed the landslide to move over shallow slopes. This basement material forms part of the white surfaces in the landslide deposit; other bright areas are formed by fumarolically altered material. The basement material was originally considered to be pumice. The landslide deposit contains large blocks, so called toreva blocks, which were torn from the mountain and came to a standstill unmodified, forming ridges up to several high; the largest such blocks are long and wide and their total volume is about . These blocks form an almost closed semicircle at the mouth of the collapse amphitheatre and in part retain the previous stratigraphy of the volcano. Such toreva blocks are far more frequent in submarine landslides than subaerial ones and their occurrence at Socompa may reflect the relatively non-explosive nature of the collapse and material properties of the collapsed mass. Aside from the toreva blocks, individual blocks with sizes of