Crack patterns in ice and rock
First a little warning to all volcanoholics: The only direct connection this post has to volcanoes, is the crack pattern of Katla’s cauldrons!
In July 2011 geothermal heating caused local melting of Katla’s icecap. Internet soon brought aerial footage of the formation of circular depressions on the ice surface, the cauldrons – or in Icelandic, “sigkatlar”.
Katla cauldron on 10 July 2011, picture by the crew of TF Life. From: http://www.dv.is/frettir/2011/7/10/myndir-fra-eftirlitsflugi-landhelgisgaeslunnar/
The remaining ice “roofs” above the molten areas were not longer strong enough to carry their own weight, and not plastic enough to bend. So they had no choice, cracked / broke and dropped until a new state of equilibrium was reached. Each cauldron shows a circulair pattern of cracks around a center.
Ever since this images appeared I have wondered if rock would fracture in the same way under similar circumstances. A sudden subsidience or uplift of rock layers because of magma movements might be a comparable situation.
Then one day I ran into this seismic picture of the Silverpit crater. In the first place it fascinates me because its pattern very much resembles the pattern of the cracked ice of the Katla cauldrons.
Silverpit crater seismic map. Credit and copyright: Phil Allen (Production Geoscience Ltd) and Simon Stewart (BP). From: http://en.wikipedia.org/wiki/Silverpit_crater
The Silver Pit is known as a submarine valley in the North Sea, about 45 km east of Spurn Head of East Yorkshire, England. It is thought to be a tunnel valley formed by subglacial erosion and once, when the sea level was much lower than now, being a part of the Wash river. The Silver Pit is currently covered by sediments up to 1500m thick. Fishermen are familiar with the Silver pit fishing ground.
Search for natural gas deposits in the North Sea led to the discovery of a crater-like structure in the Silver Pit. Geoscientists studied three-dimensional seismic reflection data and discovered unusual concentric rings with a diameter up to 20km.
The scientists think the structure is the result of a 60–65 million years old meteor impact. In 2002 the discovery and the impact theory were published in the Nature journal. Abstract here.
If the crater actually is a meteor impact and the age is correct, the impact could be part of the Cretaceous–Paleogene extinction event which led to mass extinction about 65.5 million years ago.
Cross section of the Silverpit crater. From: Journal of the Geological Society; 2008; v. 165; issue 4; jgs.lyellcollection.org
A fact that speaks for the impact theory is the presence of an uplift, a central peak in the middle of the crater.
The impact theory is however disputed. Salt withdrawal in the underground has also been suggested as a possible reason for the crater-like subsidience. A proof for the impact theory might be the finding of molten host rock at the crater bottom, caused by the extreme heat of a meteor.
But whatever the final conclusions about the structure will be, its crack pattern has at least been visualized and scientifically documented.
Just to compare: A picture of a well preserved impact structure on Jupiter’s moon Europa:
Image from NASA / JPC Planetary Photojournal