Larsen, H.C., Duncan, R.A., Allan, J.F., Brooks, K. (Eds.), 1999 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 163 41 5. DATA REPORT: PHYSICAL AND MAGNETIC CHARACTERIZATION OF AA AND PAHOEHOE FLOWS: HOLE 990A 1 C.J. Bücker, 2 K.V. Cashman, 3 and S. Planke 4 ABSTRACT The overall good core recovery in the basaltic sequence and the dense recording of physical and magnetic properties in Hole 990A are well suited to investigate the lava pile in detail, although no wireline measurements could be achieved during the entire Leg 163. The goal of the investigation is to show the different lava flow types with their physical and magnetic prop- erties, based on reliable measurements of density and susceptibility with a sampling interval of 5 cm, and velocity and magnetic intensity with a sampling interval of 10 cm. 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. This 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. The lava pile was divided into aa, pahoehoe, and a transitional form as a result of optical core observations as well as by groundmass crystal texture measurements. Highest density and velocity values are found in the aa flows; in particular the velocities of the pahoehoe flows are smaller than in the aa flows. In general, the pahoehoe flows show the highest magnetic intensities, whereas the magnetic intensities for the aa flows are 3.5 times smaller. The magnetic susceptibility shows an opposite trend. INTRODUCTION 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; Peter- son 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). Al- together, 13 igneous units were recognized down to the bottom of the hole at 342.7 mbsf. The different lava flows could be clearly differ- entiated from each other by the presence of weathered or vesicular flow tops (Duncan, Larsen, Allan, et al., 1996; ODP Leg 163 Ship- board Scientific Party, 1996) (Fig. 2). The top of the volcanic se- quence at Site 990 is deeply weathered and oxidized, and all of the lava flow units have oxidized flow tops. In combination with the re- covery of soil horizons, this shows that the eruption and emplacement of the lava flows occurred under subaerial conditions. Subaerial em- placement is also indicated by the physical properties with character- istic low density and velocity values at the flow tops, which is a func- tion 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 with- in 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 Dun- can, 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. DATABASE Because of bad weather conditions, no downhole measurements could be achieved during Leg 163. But the high recovery of basalt (70% overall core recovery) from Hole 990A enabled a dense sam- pling of physical properties measurements, including P-wave veloc- ity, bulk density, natural gamma ray, magnetic susceptibility, and natural remanent magnetization. Gamma-ray attenuation porosity evaluator (GRAPE) density, natural gamma-ray, and magnetic sus- ceptibility were measured with the shipboard multisensor track (MST) system on full-round cores with a sampling interval of 2 cm prior to sawing. As soon as possible after core retrieval, P-wave ve- locity was measured on seawater-saturated half-round cores with the Hamilton Frame velocimeter at an average sampling interval of 5 cm. Additionally, bulk density and velocity were measured on discrete samples (minicores with 1-in diameters and 1-in heights). The magnetic intensities were measured every 10 cm with the ship- board cryogenic magnetometer. Prior to the measurement, an alternat- ing field demagnetization at 30 mA was applied to remove secondary remanences, like drilling-induced magnetization (Y. Nakasa, pers. comm., 1996). All measurements were checked by an intensive quality control, using the visual core descriptions (VCDs) (Duncan, Larsen, Allan, et al., 1996) for comparing and editing the data. During the MST mea- surements, data were taken at each predefined interval, whether there 1 Larsen, H.C., Duncan, R.A., Allan, J.F., Brooks, K. (Eds.), 1999. Proc. ODP, Sci. Results, 163: College Station, TX (Ocean Drilling Program). 2 Joint Geoscientific Research of the State Geological Surveys, Stilleweg 2, D- 30655 Hannover, Federal Republic of Germany. c.buecker@gga-hannover.de 3 Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403- 1272, U.S.A. 4 Department of Geology, University of Oslo, PB 1047, Blindern, N-0316 Oslo, Norway. 1 Larsen, H.C., Duncan, R.A., Allan, J.F., Brooks, K. (Eds.), 1999. Proc. ODP, Sci. Results, 163: College Station, TX (Ocean Drilling Program). 2 Joint Geoscientific Research of the State Geological Surveys, Stilleweg 2, D- 30655 Hannover, Federal Republic of Germany. c.buecker@gga-hannover.de 3 Department of Geological Sciences, University of Oregon, Eugene, OR 97403- 1272, U.S.A. 4 Department of Geology, University of Oslo, PB 1047, Blindern, N-0316 Oslo, Norway.