The conjunction of the Icelandic eruption with my visit to England (see my last Blog) caused me to wander again in tectonic realms.
My starting point was the matter of volcanic island arcs that are created by the tectonic subduction of oceanic plates. In Himalayan Portfolios I discussed the Kohistan-Ladakh arc that became the area between the greater Himalayan chain and the Karakoram. This topic awakened me to the presence of an arc nearer home. In my April Blog I mentioned the remains of an island arc which runs south to north in Connecticut to the east of the Connecticut valley where it is known as the Bronson Hill formation. It continues up through Massachussets, New Hampshire and northeast across Maine. This arc formed in the early to mid Ordovician period in what geologists call the Iapeatus Ocean. This was a time before there was any Atlantic Ocean and before a diversity of oddly composed continents assembled into the super continent Pangaea. The arc formed off the coast of Laurentia (later to become North America). Continuing plate motion thrust the arc against the mainland around 450 million years ago producing a substantial "Taconic" mountain range. (This is the simple account; the details of the process are exceedingly murky.) The Taconic mountain-building process was an early stage in the formation of the northern Appalachian Mountains. The Scottish Grampian mountain building process took place about this time (see below.)
Also in the Ordovician period, but thousands of miles away on the other side of the Iapetus Ocean, another subduction of the Ocean floor began the process that created the English Lake District. Between 460 and 450 million years ago subduction generated the Eykott Volcanic Group, and, later, the larger Borrowdale Volcanic Group. A large island caldera, similar to the Greek island of Santorini, may have repeatedly built up and then erupted. These volcanic islands eventually docked with the coast of a mini continent known to geologists as Avalonia. Lava flows and vast amounts of ash covered the area and the mountains created formed the core of the Lake District. This European end of Avalonia later became the underlying layers of England and Wales, southeast Ireland, most of the Isle of Man and part of Belgium. Not far away was another slightly larger continent: Baltica (Norway etc.)
The Suture: In the Ordovician period there were no land plants or land animals, but the island volcanoes must have made life very unpleasant for local trilobites and graptolites. These had distinct forms according to whether they resided on the Avalonian or Laurentian side of the Iapatus Ocean. Scotland along with New England was on the Laurention side of the ocean far away from Avalonia and Baltica (hence the Grampian and Taconic connection.) This public-domain map from Wikipedia shows the Iapetus Suture in red and the fosil distribution after the Iapetus Ocean closed about 400 million years ago in the late Silurian and Early Devonian.
The Iapetus closure generated the Acadian round of mountain building on both the Laurentian and Avalonian sides of the suture. [Getting the nomenclature straight for the various mountain building events and rest periods is quite difficult (see Figs 1 and 2, McKerrow, Niocaill & Dewey). The term Caladonian mountain bulidng has been applied in many different ways, somtimes as a part of the Acadian process. In Avalona in its independent period prior to the formation of the Borrowdale Volcanics there was (Late Cambrian and early Ordovician) a mountain building process associated with Penobscot in Maine and with North Wales.
The Split: The process of continental assembly continued into the Permian with the addition of Gondwana (Africa etc) to form Pangea. More mountain building took place in the central and southern Appalachians as a result (Alleghenean mountain building). When Pangea split up in the Triassic and Jurassic period there was a false start creating the Connecticut rift valley, but finally the split came to the east. The magma spewing ridge that created the Atlantic Ocean ignored the Iapetus suture. It sliced through Avalonia leaving part attached to Laurentia and left Scotland attached to England. It also created the Icelandic volcanoes -- which brings us full circle.
An Avalonian Trek. Connecticut hosts the end of the non European end of Avalonia. The start of the terrain seems to be the granitic gneiss at lighthouse point by New Haven harbor. A little down the rocky coast is the Stony Creek quarry that provided the granite for the base of the statue of Liberty. The Bedrock Geological Map of Connecticut shows that Avalonian rocks underthrust the gneiss fieldstone in the Eastern Highlands, however, a small area of Avalonian rocks is exposed in the middle of the Eastern region near Willimantic. A walk along the Avalonian terrain would begin in Connecticut, continue through Massachusetts, Maine, New Bruswick, Nova Scotia and southern Newfoundland. It would continue through southern Ireland. The suture line in passing between Ieland and the border between Scotland and England clips the western tip of the Isle of Man. Most of the island is Avalonia derived; the short costal strip of Laurentia is known as the Dalby group. The Avalonia trek could wind up in the Lake District and North Wales.
The Noble Cause: Having expounded the case for an independent Avalonia it remains to suggest that Avalonia, less fictional than Ruritana, is clearly in need of a national anthem. There are precedents. Any reasonable suggestion will be posted. It could begin: Rocks, rocks, rocks, rocks/ You gotta learn to take your knocks. / Take em, take em, in your stride/ With the old volcanic pride...
