The "bomb rocks" of Charnwood Lodge National Nature ReserveĀ are exposures of coarse and fragmented volcanic rocks, arranged chaotically as intermingled blocks.
They are a feature of great interest and importance to those studying the Charnwood Forest Geopark and were were named so by pioneering writer of Charnwood geology Watts (1947), because he thought the large fragments fell as volcanic bombs.
Today, this is not considered to be quite the truth. Such a classification would require them to have fallen as molten lava, ejected directly from the vent. True bombs carry the physical indicators of a viscous, semi fluid object which was shaped in flight.
These rocks do not carry this signature and also lack the bubble structures created by escaping gases.
They are formed of a grey andesite with phenocrysts of plagioclase feldspar, which is similar to other surrounding rock complexes, with a particular affinity drawn to the Grimley Andesite of Whitwick. Theirchaotic arrangements and tendency to rounding of corners suggests they formed part of a pyroclastic debris flow down the flanks of the Charnwood volcanoes, colliding together as they moved.
Volcanos here tended towards a fracture and collapse style of eruption caused by pressure build up of viscous (rather than liquid) lava domes in the vents. True lava flows weren't really a thing and that's why we don't see the likes of basalt and pumice.
As such, the Charnian geology is marked mainly by breccias and marine deposited dust sediments, although some magmatic elements (like the south and north charnwood diorites) have intruded their way through in places!
In conclusion, they are more accurately determined to be a volcanic breccia.
And THAT is a coarsely grained rock made up of angular fragments of volcanic material, cemented together in a matrix of finer sediments. It is an Italian word and its translation back to English should give you all the context you need to understand the concept.
"Rubble".
The folding and crushing you can see on some planes is part of the typical Charnian cleavage plane formed as the whole was folded in later years into Charnwoods overall anticline ( this refers to an arch like shape)
Comments