No. 100: Jul-Aug 1995
[The following long, initially dull (?) discussion leads unerringly to the Bermuda Triangle via a Fortean phenomenon!]
Unrecognized until just a few years ago (SF#100*), sea-bed pockmarks are remarkable geological features. They occur worldwide on the floors of all of the oceans and even some lakes. They are found in shallow waters and at depths of thousands of meters. In diameter, these roughly conical depressions may span 350 meters or more and be up to 35 meters deep. No trivial phenomenon, some pockmark fields exceed 1,000 km2. Like the curious abyssal ridges (SF#97), sea-bed pockmarks are rarely discussed despite their great geological and economic importance.
Recent issues of Geology contain three fascinating papers relating to giant sea-bed pockmarks. In Ref. 1, J.T. Kelley et al describe a pockmark field in Belfast Bay, Maine. Here, the density of the pockmarks reaches 160 per km2, and they are apparently the largest pockmarks yet discovered. The Belfast Bay field is "fresh" and "active" in the sense that the pockmarks are sharply defined and methane bubbles still stream up from buried organic matter.
Natural-gas plume rising from the sea-floor off the Carolina coast.
Lastly, in Ref. 3, C.K. Paull et al report on the release of plumes of methane bubbles from the Carolina continental rise at a depth of 2167 meters. Here, the sediments are riddled with methane hydrate.
Why all this fuss over fizzy ice, and what is the connection between methane hydrate and pockmarks?
Ref. 1. Kelley, Joseph T., et al; "Giant Sea-Bed Pockmarks: Evidence for Gas Escape from Belfast Bay, Maine," Geology, 22:59, 1994.
Ref. 2. Vogt, Peter R., et al; "Methane-Generated (?) Pockmarks on Young, Thickly Sedimented Oceanic Crust in the Arctic: Vestnesa Ridge, Fram Strait," Geology, 22:255, 1994.~ Ref. 3. Paull, Charles K., et al; "Methane-Rich Plumes on the Carolina Continental Rise: Association with Gas Hydrates," Geology, 23:89, 1995.
SF#xx = Science Frontiers #xx. SF#25 and SF#73 are also printed in the book Science Frontiers. To order, see here.