National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2005

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

Wheat, C.G., M.J. Mottl, A.T. Fisher, D. Kadko, E.E. Davis, and E.T. Baker

Geochem. Geophys. Geosyst., 5(12), Q12006, doi: 10.1029/2004GC000700 (2004)

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 <20 m below the summit, corresponding to a height that is 30 m above the surrounding turbidite plain. These elevated fluid temperatures at shallow depths provide thermal buoyance to drive seafloor seepage from the outcrop.

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