Pockmarks are concave, crater-like depressions on seabeds that are caused by fluids (liquids and gasses) escaping and erupting through the seafloor. They can vary in size and have been found worldwide.
Pockmarks were discovered off the coasts of
Nova Scotia, Canada in the late 1960s by Lew King and Brian McLean of the
Bedford Institute of Oceanography, using a new
side scan sonar
Side-scan sonar (also sometimes called side scan sonar, sidescan sonar, side imaging sonar, side-imaging sonar and bottom classification sonar) is a category of sonar system that is used to efficiently create an image of large areas of the se ...
developed in the late 1960s by
Kelvin Hughes
Hensoldt UK, formerly Kelvin Hughes, is a British company specialising in the design and manufacture of navigation and surveillance systems and a supplier of navigational data to both the commercial marine and government marketplace.
The company ...
. Before the two researchers King and McLean used the side scan sonar, they had noticed 'notches' on echo sounder and shallow seismic records in the seafloor off Nova Scotia. They believed these notches to represent gullies and curvilinear troughs in the muddy seafloor. However, they could never work out how to join these notches from one survey line to the next. It was, therefore, not before they surveyed with the area-coverage system, Side scan sonar, that they realized the notches were in fact closed depressions (craters) and not curvilinear features. This was a great surprise, because there are very few craters on the Earth's surface.
Although pockmarks were first documented and published 50 years ago, they are currently still being discovered on the ocean floor and in many lakes, the world over. Spatial delineation and morphometric characterisation of pockmarks in the central North Sea seabed have been achieved by semi-automatic methods.
The craters off Nova Scotia are up to in diameter and deep. Pockmarks have been found worldwide. Discovery was aided by the use of high-resolution multibeam acoustic systems for bathymetric mapping. In these cases, pockmarks have been interpreted as the morphological expression of gas or oil leakage from active
hydrocarbon system or a deep overpressured
petroleum reservoir
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations.
Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...
. Specifically, long term deep fluid flow resulting in pockmarks is linked to undersea methane gas escape under pressure.
[Chand et al (2017) Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea Geo-Mar Lett (2017) 37:215–227 DOI 10.1007/s00367-016-0487-x]
See also
*
Cold seeps
A cold seep (sometimes called a cold vent) is an area of the ocean floor where hydrogen sulfide, methane and other hydrocarbon-rich fluid seepage occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature of the seep ...
*
Limnic eruption
*
Demersal fish
Bibliography
*
*
*
References
Sedimentology
Oceanography
Lakes
{{geology-stub