Hole 504B is located about 200 km south of the Costa Rica Rift and constitutes the reference section for the structure of the upper oceanic crust. Compared to core, the continuous electrical resistivity (at m scale) and the high-resolution electrical images (at cm scale) recorded in Hole 504B, provide a continuous and detailed lithostratigraphic description of the effusive section at Site 504. Flow thicknesses measured from cm scale FMS images average 0.5 (+ or -0.1) m. The massive units, known to bound fluid circulation at large scale into the crust, are constituted with a series of 20 to 50 individual flows. If Site 504 was created over two volcanic cycles, each volcanic cycle allows the emplacement of [0.60 (+ or -0.30)]X10 (super 6) m (super 3) of magma per m along the ridge axis. This computation leads to an estimate of magma volume for a single eruption of [0.003 (+ or -0.001)]X10 (super 6) m (super 3) per m along the ridge axis, and eventually, a gradient in magma pressure within the magma chamber lens of 52(+ or -26) MPa, appropriate for one eruption.