365 Days of Climate Awareness 24 - Supercontinents

 

365 Days of Climate Awareness

24 - Supercontinents

On the assumption that earth's continents have been roughly constant in volume over the earth's history--notwithstanding some sediment loss to subducting ocean plates--geological research and theory holds that at several times during earth's 4.5 billion year existence, the continental masses were all, or nearly all, gathered into one large land mass. As with climate, the definition of "supercontinent" can be a bit arbitrary, but one holds that a supercontinent must contain at least 75% of all land mass. (For this reason the earlier continent Gondwana is not always considered a supercontinent.)

Reconstruction of early continents is a matter of radiometric dating and examining the magnetic patterns of successively earlier rock masses. The age can be combined with the inclination of the magnetic particles in the rock--aligned with the earth's magnetic field--to estimate their latitude on formation, and so reconstruct paleogeography. Geologists have reconstructed up to ten (depending on the definition) prior supercontinents.

These reconstructions can be used to help model paleoclimates, particularly involving the fluid dynamics of the ocean and atmosphere at the time. We will revisit weather several times in future posts, with the additional information gained along the way. But the arrangement of continents can have massive climatic effects worldwide.

Pangea and later continental formations.

Continental masses generally experience more extreme temperatures than the ocean, because of the high thermal capacity of water and the fact that it moves, and transfers heat with great efficiency. So a land mass, like Antarctica, at the pole tends to become colder than ocean like the Arctic. And an equatorial continental mass like Africa or South America will be hotter than the surrounding sea.

Other factors matter too, such as the aridity vs vegetation of the landmasses. One huge continental mass is likely to be arid throughout much of its interior (think of the Gobi or Sahara Deserts), lowering the amount of weathering, and limiting the amount of carbon dioxide drawdown, leading to higher global temperatures. Smaller continents, particularly in the mid- to lower latitudes, are likely to have wetter weather, with increased weathering and greater carbon dioxide drawdown, contributing to lower global temperatures (think Indonesia).

This chain of events appears to have occurred with the breakup of the equatorial continent Rodinia, which existed from about 1.3 billion to 750 million years ago, into many smaller equatorial continents. The climate cooled significantly when the large dry continent split up into smaller, wetter pieces. The most recent supercontinent which preceded our current geography is Pangea. North and South America split away with the rifting which began around 175 million years ago and has created the Atlantic Ocean.

No supercontinent now exists (Eurasia plus Africa--a good chunk of old Gondwana--doesn't make the cut) and most land mass is in the northern hemisphere. A significant amount exists as islands and small continental masses, with one continent (Antarctica) covering a pole. The earth is currently in an icehouse (not snowball, which is 100% frozen) state.

Tomorrow: a brief history of the planet.

Be well!