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
Nova Scotia is a Provinces and territories of Canada, province of Canada, located on its east coast. It is one of the three Maritime Canada, Maritime provinces and Population of Canada by province and territory, most populous province in Atlan ...
, Canada in the late 1960s by Lew King and Brian McLean of the
Bedford Institute of Oceanography, using a new
side scan sonar developed in the late 1960s by
Kelvin Hughes. 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
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...
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 prese ...
. 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
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Cold seeps
A cold seep (sometimes called a cold vent) is an area of the ocean floor where seepage of fluids rich in hydrogen sulfide, methane, and other hydrocarbons occurs, often in the form of a brine pool. ''Cold'' does not mean that the temperature o ...
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Limnic eruption
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Demersal fish
Demersal fish, also known as groundfish, live and feed on or near the bottom of seas or lakes (the demersal zone).Walrond Carl . "Coastal fish - Fish of the open sea floor"Te Ara - the Encyclopedia of New Zealand. Updated 2 March 2009 They oc ...
Bibliography
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References
Sedimentology
Oceanography
Lakes
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