Ocean Drilling Program (ODP) Hole 990A on the Southeast Greenland margin (Fig.1) provides a unique chance to study the physical and magnetic properties of blocky aa and ropy pahoehoe lava flows. This superficial surface structure of lava flows has its origin in the volumetric flow rate, the eruption and cooling rate, and other parameters that are also reflected by distinct differences in the interior of the flows. Thus, the flow type tells us something about the physical volcanological process (Pinkerton and Sparks, 1976; Peterson and Tilling, 1980; Rowland and Walker, 1990; Hon et al., 1994; Ho and Cashman, 1997; Katz, 1997).

During Leg 163 at Hole 990A, we drilled into a suite of lava flows beneath a sedimentary cover (Fig.1). The first igneous unit was found in Section 163-990A-5R-1 at 212 meters below seafloor (mbsf). Altogether, 13 igneous units were recognized down to the bottom of the hole at 342.7 mbsf. The different lava flows could be clearly differentiated from each other by the presence of weathered or vesicular flow tops (Duncan, Larsen, Allan, et al., 1996; ODP Leg 163 Shipboard Scientific Party, 1996) (Fig. 2). The top of the volcanic sequence at Site 990 is deeply weathered and oxidized, and all of the lava flow units have oxidized flow-tops. In combination with the recovery of soil horizons, this shows that the eruption and emplacement of the lava flows occurred under subaerial conditions. Subaerial emplacement is also indicated by the physical properties with characteristic low density and velocity values at the flow _tops, which is a function of the high degree of subaerial flow-top alteration. In contrast to these subaerial lava flows, submarine lava flows (pillow lava) do not show these characteristic flow _tops and bottoms.

Detailed descriptions of the variation of physical properties within subaerial basalt piles in different areas are given by Aubele et al. (1988), Planke (1994), Delius et al. (1995), Bücker et al. (1998), S. Planke et al. (unpubl. data). The latter two are dealing with Hole 990A data. All reveal characteristic low-density and low-velocity values in the flow-tops. A comprehensive petrologic and structural summary of the volcanic succession in Hole 990A is given by Duncan, Larsen, Allan, et al. (1996). These lava flows can be subdivided into three types: aa, pahoehoe, and a transitional form.

This paper summarizes the physical and magnetic properties and the flow textures of aa, pahoehoe, and transitional form flows.