Heat flow through a basaltic outcrop on a sedimented young ridge flank

One hundred seven thermal gradients were measured in shallow surface sediments using the submersible Alvin within a 0.5 km2 area on and around the 65‐m‐high, mostly sediment‐covered Baby Bare outcrop located on the eastern flank of the Juan de Fuca Ridge. Heat flow values range from 0.35 W m−2, which is close to the average value (0.27 W m−2) for the sediment‐buried 3.5‐Myr‐old crust surrounding the outcrop, to as high as 490 W m−2. Some measurements are purely conductive, whereas others are consistent with upward fluid seepage through this sediment layer. Highest heat flow values are found roughly 10 m below the summit along a ridge‐parallel fault where shimmering water, springs, and communities of clams were found. Heat flow values surrounding a second fault are elevated to a lesser extent (maximum of 9.2 W m−2). The total power output from this 0.5 km2 area is 1.5 MW, about 10 times greater than the conductive power output predicted for a commensurate area of 3.5‐Myr crust. Much of this heat loss is conductive (∼84%), consistent with an independent estimate of the convective heat flux from Rn/heat anomalies in spring fluids and in the water column above the springs. Calculations suggest that the 64°C isotherm, which is the temperature in the surrounding upper crust, is