Supercontinent Cycle - Effects On Sea Level

Effects On Sea Level

It is known that sea level is generally low when the continents are together and high when they are apart. For example, sea level was low at the time of formation of Pangaea (Permian) and Pannotia (latest Neoproterozoic), and rose rapidly to maxima during Ordovician and Cretaceous times, when the continents were dispersed. This is because the age of the oceanic lithosphere provides a major control on the depth of the ocean basins, and therefore on global sea level. Oceanic lithosphere forms at mid-ocean ridges and moves outwards. As this happens, it conductively cools and shrinks. This cooling and shrinking decreases the thickness and increases the density of the oceanic lithosphere, and the result is the general lowering in elevation of the seafloor away from mid-ocean ridges. For oceanic lithosphere that is less than about 75 million years old, a simple cooling half-space model of conductive cooling works, in which the depth of the ocean basins in areas in which there is no nearby subduction is a function of the age of the oceanic lithosphere . In general,

where is the thermal diffusivity of the mantle lithosphere (~8*10−7 m2/s), is the effective thermal expansion coefficient for rock (~5.7*10−5 C° −1), is the temperature of ascending magma compared to the temperature at the upper boundary (~1220 °C for the Atlantic and Indian Oceans, ~1120 °C for the eastern Pacific) and is the depth of the ridge below the ocean surface. After plugging in rough numbers for the sea floor, the equation becomes:

for the eastern Pacific Ocean, and:

for the Atlantic and Indian Oceans, where is in meters and is in millions of years, so that just-formed crust at the mid-ocean ridges lies at about 2,500 m depth, whereas 50 million-year-old seafloor lies at a depth of about 5000 m.

As the mean level of the sea floor decreases, the volume of the ocean basins increases, and if other factors that can control sea level remain constant, sea level falls. The converse is also true: younger oceanic lithosphere leads to shallower oceans and higher sea levels if other factors remain constant.

Area A can change when continents rift (stretching the continents decreases A and raises sea level) or as a result of continental collision (compressing the continents leads to an increase A and lowers sea level). Increasing sea level will flood the continents, while decreasing sea level will expose continental shelves.

Because the continental shelf has a very low slope, a small increase in sea level will result in a large change in the percent of continents flooded.

If the world ocean on average is young, the seafloor will be relatively shallow, and sea level will be high: more of the continents are flooded. If the world ocean is on average old, seafloor will be relatively deep, and sea level will be low: more of the continents will be exposed.

There is thus a relatively simple relationship between the Supercontinent Cycle and the mean age of the seafloor.

• Supercontinent = lots of old seafloor = low sea level
• Dispersed continents = lots of young seafloor = high sea level

There will also be a climatic effect of the supercontinent cycle that will amplify this further:

• Supercontinent = continental climate dominant = continental glaciation likely = still lower sea level
• Dispersed continents = maritime climate dominant = continental glaciation unlikely = sea level is not lowered by this mechanism