The mud volcanoes in the estuaries of Bay of Bengal. Credit: C. De
A geologist from Kolkata has identified a biological tool that can immediately label a beach as unstable. This eliminates the need for complicated physical processes like drilling to ascertain beach condition before coastal engineering operations — construction of dams, bridges or roads.

The key to identifying an unstable beach that looks perfectly stable from the top are the little mud volcanoes formed by the mud-loving amphibious crab Uca marionis.

Chirananda De of the Geological Survey of India, who reported the interesting find recently1 while working in the estuary mouths along the Bay of Bengal says that burrow-head mud volcanoes are perfect biogenic signals that the beach will erode fast.

"The top layer of the coast is sandy and one can drive a vehicle over it easily. It might be mistaken for a stable beach. However, below this layer are the muddy layers. This is where the crabs burrow into," De told Nature India.

The mud volcanoes appear in very specific areas on either side of the estuaries where soft muddy bottom sediments of the coastal mudflats are covered by a thin blanket of relatively rigid beach sands that provide a false sense of substrate stability.

The author in the estuarine delta and (inset) the mud loving crab Uca marionis.
The shallow-seated mud layers allow the crabs to burrow and consequently the mud slur from within oozes out to form conical heaps of mud resembling volcanic cones. If the thin sand cover is removed, it triggers very fast erosion of the bottom mud. "This results in widening of the estuary mouth and very fast erosion of the beach," De explains. A surge of waves is enough to wash away the sand cover exposing the unstable beach.

The crabs can't penetrate below the local water level since it would mean their burrows would be flooded. They try to confine their burrow to within 1.5 metres of depth. The mud volcanoes, "by virtue of limited power of vertical penetration down to required muddy substrate and requirement of special substrate conditions and geomorphic position" are therefore considered unique ichnological tools to precisely demarcate highly unstable beach sectors.

The crabs are omnipresent on the beach and form another kind of burrow structures elsewhere in the study area located in the western downdrift coastal plains of the Sundarban Delta Complex, West Bengal. These are the chimney shaped burrows.

The chimneys, however, do not signal an unstable beach condition since they indicate semi-consolidated mud underneath.

Mud volcanoes, therefore, are a ready solution to replace many engineering and geotechnical planning operations such as continuous beach profiling, satellite imagery and remote sensing, De says.