Monday Musings: The Beginnings of Mountains
The Silurian Period started at the end of the Ordovician Period 443.8 Ma and ended at the beginning of the Devonian Period 419.2 Ma. This was a time of rising seas as the Ordovician glaciers melted. If you recall, these glaciers caused a mass extinction. Because sea level rose so much, new marine environments were created including many warm shallow seas as deep ocean basins closed because continents were moving closer together. These warm shallow seas allowed the first coral reefs to form.
At the beginning of the Silurian Period, a large ocean sat over the north pole while Gondwana was still over the south pole. Fragmented landmasses such as Laurentia, Avalonia, and Baltica hovered around the equator.
There wasn’t a huge variety in topography but there was a significant mountain-building event throughout the entire period. The Caledonian Orogeny, named for the Latin name of Scotland, was the convergence of Avalonia, Baltica, and Laurentia. This happened in a series of phases: the Grampian Orogeny, the docking of Eastern Avalonia with Baltica, the Scandian phase and the Acadian Phase.
The Grampian Orogeny was the result of Laurentia (North America) colliding with an ocean island arc (Scotland and northern/western Ireland). This collision actually occurred in the middle Ordovician at about the same time as the Taconic Orogeny, an early mountain-building event in New England that began the Appalachians.
At the end of the Ordovician, eastern Avalonia and Baltica collided in a more gentle convergence called a docking. This created what is called the Trans-European Suture Zone from the North Sea close to Denmark, through part of the Baltic Sea and Poland, then follows the Eastern Carpathian Mountains of Ukraine to the Black Sea.
During the early Silurian until the mid-Devonian, the Scandian phase occurred. This was the result of the Fennoscandian peninsula colliding with Greenland. It also created thrusting in the Northern Highlands of Scotland and the Outer Hebrides.
The last phase occurred during the Devonian Period. The Acadian Orogeny was the final mountain-building event for the Appalachian Mountains. It also involved connecting England and Wales to the rest of Great Britain (they were part of Avalonia rather than the island arcs).
Altogether, this orogeny was the start of building the great Central Pangean Mountains that would exist from the Carboniferous through the Triassic.
 Thanks for coming to today’s lesson and tune in tomorrow to participate in some trivia! Fossilize you later!
