Why was much of North America underwater during the Ordovician Period?

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Ordovician paleogeography — Ordovician Period The distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Ordovician Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

During the Ordovician Period, much of North America was submerged under shallow, epicontinental seas primarily due to high global sea levels.1

This rise in sea level was driven by rapid seafloor spreading that occurred following the breakup of the supercontinent Rodinia. This geological activity created tall oceanic ridges that effectively raised the average elevation of the seafloor, displacing water and causing it to flood the interiors and margins of continents. Consequently, during the Early and Late Ordovician epochs, nearly the entire North American continent was covered by these vast, shallow seas.12