During the Silurian , the Earth witnessed many changes in the way in which landmasses were distributed around the globe. Although there were no major volcanic events, a deglaciation and rise in sea levels occurring at that time produced varying periods of continent coverage and exposure. The variation of ocean levels occurred alongside the process of continental fragmentation and grouping that occurred from the Cambrian to the present.
At that time, the continents were distributed very differently than they are today. The Silurian world consisted of a vast north polar ocean and a south polar supercontinent (Gondwana) with a ring of approximately six continents. By the Silurian period, a large portion of the Rodinian landmass had become fragmented, and those fragments migrated toward the equatorial region. Most of these fragments were eventually assembled by a series of plate collisions into the super-continents of Laurussia and Laurasia. The modern Philippine islands were most likely inside the Arctic Circle, while Australia and Scandinavia resided in the tropics; South America and Africa were probably over the South Pole.
There was no major volcanic activity during the Silurian; however, the period is marked by major orogenic (mountain-building) events in eastern North America and in northwestern Europe, resulting in the formation of the mountain chains there. This was called the Caledonian Orogeny. In other areas, large igneous rock formations of the Middle Silurian arose, such as those in Central Europe, as well as light sedimentation throughout the Baltic region. While not characterized by dramatic tectonic activity, the Silurian world experienced gradual continental changes that would be the basis for greater global consequences in the future, such as those that created terrestrial ecosystems.
The Silurian oceans are also of particular interest for activity between the regions known as Laurentia, Baltica and Avalonia. The ocean basins between these areas substantially closed together, continuing a geologic trend that had begun much earlier. The new marine habitats produced by these profound changes in the Silurian seas provided the framework for significant biological events in the evolution of life. Coral reefs, for example, made their first appearances in the fossil record during this time.
The Silurian period was a time when the earth underwent considerable changes that had important repercussions for the environment and the life within it. The Silurian witnessed a relative stabilization of the world's general climate, ending the previous pattern of erratic climatic fluctuations. One significant feature of these changes was the melting of large glacial formations. This contributed to a substantial and significant rise in the levels of the major seas, creating many new marine habitats.
The Silurian period's condition of low continental elevations with a high global stand in sea level can be strongly distinguished from the present-day environment. This is a result of the flood of 65% of the shallow seas in North America during the Llandovery and Wenlock times. The shallow seas ranged from tropical to subtropical in climate. Commonly present in the shallow seas were coral mound reefs with associated carbonate sediments. Due to reduced circulation during the Ludlow and Pridoli times, the process of deposition of evaporites (salts) was set in motion. Some of these deposits are still found in northern Europe, Siberia, South China and Australia.
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