The Volcanoes that Killed a Continent, Shaped Civilizations, and Transformed the World
The bottom of the world holds a dark secret
If I had to point to a single geological feature of the Earth that had the greatest consequence on life in the post-dinosaur world, that is, on our entire global ecosystem as encountered today, I would choose the undersea volcanic spreading ridges that presently encircle Antarctica. This little-known ring of cracks in the ocean bottom has drastically influenced the geographical positioning of continents and oceans around the Earth, which in turn, has forever altered the flora and fauna of each of the biogeographical realms. The resulting motion of continents has galvanized biodiversity in many regions around the globe while isolating (and so protecting) certain plants and animals in others. And this system of ridges has killed as well—leading to the death of nearly all vertebrate life in Antarctica. Indeed, the resulting arrangement of continents has even had an enormous effect on the fates of human civilizations, contributing to the great differences in technological progress among different populations. All this has been the byproduct of the seafloor spreading ridges that currently surround our southernmost continent.1 Indeed, it is the very reason that the Southern Hemisphere is so oceanic while the Northern Hemisphere is disproportionately continental.
Spreading ridges are the cracks in the surface of the Earth that snake throughout all oceans and mark the divergent boundaries between tectonic plates. That is, they are the gaps where hot magma flows up from the mantle and solidifies to form new seafloor along the edges of two plates moving away from each other. They are the mostly hidden-from-view mechanism for continental drift for which Wegener searched but could not find—the place where ocean basins are born, pushing continents apart as they grow.
Spreading ridges often begin as continental rift valleys, like that currently running through Eastern Africa from the southern tip of the Red Sea in the north to Mozambique in the south. This long fissure that is slowly carving the African continent is strewn with volcanoes, rift lakes, geysers, and hot springs—an elongated strip of geothermal Yellowstone-type features, all gurgling and boiling as the Earth’s thin outer shell cracks open and exposes the effects of the hot furnace below.
Just such a rift region started splitting the Pangean continental configuration more than 150 million years ago, forming a long natural fissure along the eastern edge of the Americas and the western edge of Europe and Africa. As the supercontinent continued to fracture, the salty rift lakes along this depression started to congregate to form small seas. Eventually, as the rift continued to widen, the global seawater system poured into it. This was the birth and nonage of the Atlantic Ocean.2
Figure: The History of the Atlantic Ocean. The rift system that carved the matching outlines of the Americas, Eurasia, and Africa currently runs through the middle of the Atlantic Ocean. This is the crack where Atlantic seafloor first began to erupt onto the surface of the Earth in the Early Jurassic, as the New World separated from Old, and this is where Atlantic seafloor continues to form today. As shown in the figure, the reddish bands of crust all formed within the last 20 million years (m.y.) As you move away from the ridge in either direction, the age of the seafloor continues to increase until you reach the oldest stretches of Atlantic seafloor abutting the continents (green and blue patches). These ancient swathes run parallel to the continental outlines and fit together neatly when one removes the younger crust between them.
The system of mid-ocean ridges that currently surrounds Antarctica helped continue the disintegration of Pangaea by breaking up its once contiguous southern block—an event that significantly transformed the history of life on Earth. Antarctica was the only Gondwanan continent that touched all the other ones—South America, Africa, Indo-Madagascar, Australia, and New Zealand. But the formation of the young ocean all around Antarctica has pushed all these tectonic plates—and the Pacific Plate as well—northward toward the equator, leaving Antarctica alone at the bottom of the world (see figure below). In other words, an entire unbroken latitudinal band of crust (including seafloor), just north of Antarctica, is moving northward toward the equator.
This has had a rather fascinating geometrical consequence. The equator is obviously the largest latitude—the east-west slice of the Earth where it is fattest, wrapping around the full circumference. None of the other latitudes describe the full circumference of the Earth. Instead, as you approach the poles, these parallel latitudinal rings become smaller. The circle of latitude at 60° South is half the size of the equator (0°). (This is not true of longitudes, which are not parallels but all meet at the north and south poles. All longitudes are the same great size—describing the full circumference of the Earth in the north-south direction just as the equator does in the east-west direction.) The result of this northward motion was that the southern continents have had to spread apart as they moved to larger latitudes, radiating away from each other.3
Figure: As the Gondwanan continents moved northward, away from Antarctica, they steadily became more dispersed along more latitudinally expansive stretches of the globe. They, in a sense, radiated outward from a center as if following the spokes of a wheel. The resulting effect of this Southern Hemisphere spread on all things biological—including eventually the history of human civilizations—would be difficult to overestimate.
This is why the Southern Hemisphere is so young and oceanic. The circular bands of the Earth’s crust that occupied the smaller circumferences near Antarctica have been pushed northward toward the larger latitudinal circumferences—and these southern bands of crust simply did not have enough material to occupy these regions.4 What filled in the missing space? New seafloor crust in the Pacific, Atlantic, and Indian Oceans. Surveys of seafloor age confirm that over the last 80 million years, the most prolific ocean-floor production in the world has occurred in the Southern Hemisphere—particularly in the east-west direction between the Gondwanan regions (see video below). Much of this new oceanic material was needed to fill the gaps created by the plates moving north toward the equator.
Thus, the circum-Antarctic seafloor ridges are the reason why the bottom of the globe is so blue—the reason why the Southern Hemisphere has such massive Southern Pacific, Indian, and Atlantic Oceans, broken by the occasional beiges and greens of the isolated continents. It is impossible to have a full understanding of the current organic complexion of any of these regions—Antarctica, Australia, New Zealand, Indo-Madagascar, and South America—without referring to their former Gondwanan grouping and the resulting isolation that followed the development of the circum-Antarctic ridges. North America and Eurasia did not suffer such fractionalization, and today, they remain in essence a single great continent, connected via a now-flooded arm that stretches beneath the Bering Sea and Strait. But what had once been a single land mass below the equator, a tight cluster of continents that shared many plants and animals, has now divided into many diverse and stunning realms. Indeed, Antarctica, Madagascar, Australia, and New Zealand have all become islands—and for a while South America was as well.
Perhaps, the most obvious biological consequence of the development of the circum-Antarctic ridge is also the most disturbing. As the surrounding ridge systems steadily isolated Antarctica, all of its non-flying and non-marine animals became trapped on a continent that was about to be overtaken by one of the most destructive forces in Earth history: ice. But that’s a story for another post—as is the explanation for how this arrangement of continents has shaped human history.5
Video accompanying my article: Dennis McCarthy “Geophysical explanation for the disparity in spreading rates between the Northern and Southern hemispheres,” Journal of Geophysical Research, 112, B03410, doi:10.1029/2006JB004535, 2007, American Geophysical Union.
Figure originally appeared in Dennis McCarthy “Geophysical explanation for the disparity in spreading rates between the Northern and Southern hemispheres,” Journal of Geophysical Research, 112, B03410, doi:10.1029/2006JB004535, 2007, American Geophysical Union.
Figure of ages of ocean crust in the Atlantic from Mr. Elliot Lim and Mr. Jesse Varner, CIRES & NOAA/NCEI. Data source: Muller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest 2008. Age, spreading rates and spreading symmetry of the world's ocean crust, Geochem. Geophys. Geosyst., 9, Q04006, doi:10.1029/2007GC001743
ODSN, 1999. ODSN Plate tectonic reconstruction service, Research Center for Marine Geosciences/ Kiel and the Geological Institute of the University of Bremen,
Dennis McCarthy (2007) “Geophysical explanation for the disparity in spreading rates between the Northern and Southern hemispheres,” Journal of Geophysical Research, 112, B03410, doi:10.1029/2006JB004535) See paper for a more detailed analysis of the geometric consequences of the northward motion of continents away from Antarctica.
Adapted from a chapter in "Here Be Dragons" (Oxford University Press, 2009)