Pockmark

Pockmarks are crater-like or basin-shaped depressions in underwater sediments. They range from small features metres across to large formations hundreds of metres wide and many metres deep. They are documented in seas, oceans and lakes, across different geological environments. Their shape, size, distribution and internal structure vary; a field may contain individual depressions or extensive patterns on the submerged landscape.

Pockmarks have often been associated with fluids escaping from beneath the seabed, including gas or water, which can mobilize sediment and leave a depression. They can therefore indicate areas of fluid movement, but the presence of a pockmark alone does not establish an active seep or identify a unique origin. Geological context and direct evidence, such as sediment cores, acoustic imagery, chemical measurements and observations of bubbles or seep-related communities, are needed to test interpretations.

Research on the Sur Pockmark Field offshore Central California illustrates why local evidence matters. Surveys identified more than 5,000 depressions at water depths of roughly 500 to 1,500 metres. Researchers reported no evidence of high methane concentrations in the investigated sediments, while cores and subsurface images showed layers of fine sediment interrupted by sandy turbidites. The study interprets these layers and erosional surfaces as evidence that episodic, unconfined sediment gravity flows helped maintain and shift some depressions over at least 280,000 years. The researchers did not determine exactly how the features initially formed. This regional explanation should not be generalized to every pockmark field.

Pockmark fields help scientists investigate seabed fluid flow, sediment transport and submarine geological hazards. Some occur alongside gas hydrates, slope failures or other features of interest, but associations are not proof that a pockmark signals an imminent hazard or earthquake. High-resolution seafloor mapping, remotely operated vehicle observations, sediment sampling and subsurface imaging can help distinguish competing explanations. Understanding local geology is also relevant when assessing seabed infrastructure, including cables and offshore energy installations.