The Web of Life (20-5) Sea Level

 Sea Level

The dynamics of ocean depth and planetary tectonics are intertwined, with ocean depth limited by the critical properties of water. Mid-ocean ridges are typically submerged to a depth at which seawater pressure is close to critical: the water is almost as dense as physically possible.

Ocean depth optimizes plate tectonics and the circulation of water by the plate system into subduction zones and out of andesite volcanoes. Intricate dynamics of plate ingassing and degassing balance ocean depth.

Little continental flooding occurred during the formation of the supercontinents Pannotia and Pangea. Conversely, the sea-level rise during the Cambrian is attributed to new ocean ridges formed from the breakup of Pannotia. Sea levels rose during the Cretaceous, in the wake of Pangea’s dispersal.

Sea level is generally low when continents converge and high when dispersed. This owes to the dynamics of the oceanic lithosphere, which controls the depth of ocean basins by conductive cooling and shrinking – decreasing the thickness and increasing the density of the oceanic crust, lowering the seafloor away from mid-ocean ridges. As the sea floor drops, the volume of the ocean basins increases.

The age of the sea floor reflects sea level. Hence, there is a relatively simple relationship between the supercontinent cycle and average sea floor age. With a supercontinent, much of the seafloor is old. Sea level is low.

Conversely, new seafloor is created at mid-ocean ridges during continental breakup, which characterizes the world today. Generating new sea floor lifts sea level.

Because continental shelves have a shallow slope, a small rise in sea level causes considerable continental flooding.

The area of global landmasses has varied considerably throughout Earth’s history, as continental flooding submerges crust from time to time.