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Ben Black
EPS 131
Prof. Tziperman
Summary: Changing Ocean Geometry over the Past Billion Years
Ocean geometry has changed vastly and repeatedly over time, through the
mechanism of plate tectonics.
Plate tectonics is the movement of plates—pieces of Earth’s crust—driven by
subduction and sea-floor spreading. As the continents and plates move around, the low
basins filled by oceans move around as well. Oceanic plates are much heavier than
continental plates, and thus they do most of the sinking. Continental plates are too light to
subduct; instead they float on the surface of the earth, always accreting in size as new
light continental matter is formed through vulcanism and deposition. Rifting can occur in
the middle of a continental plate, but the newly formed crust is usually denser (and thus
subsides, creating an ocean basin).
This is what happened in the Atlantic, in a famous example of changing ocean
geometry. Laurentia and Gondwanaland were originally joined a few hundred million
years ago in Pangea. As they broke apart through rifting, the Atlantic formed in the midst
of the two continents. Wegener suggested this, building the similarities between South
America and Africa into a case for continental drift. This theory was eventually
developed (through the discovery of sea-floor spreading) into modern plate tectonics.
On a time scale of millions to billions of years, the continental material often
collides, sometimes forming connected aggregates of essentially all the continents in the
world. These are called supercontinents. There have been many of them that formed and
broke apart.
And now a summary of the ways in which the ocean geometries have actually
changed over the past billion years:
1. About a Billion years ago, the supercontinent Rodinia began to fragment,
forming the Pacific Ocean to the West of Laurentia (future North America) around 800
Ma, and also forming what would become Gondwanaland. For the next couple hundred
million years, these were the two major continental regions: Gondwanaland and
Laurentia. They collided and parted several times, creating and destroying long-lost seas
each time.
2. The Gondwanaland and Laurentia formation was fairly stable and lasted about
200 My, from 500 Ma to 300 Ma. But in the meantime, Gondwanaland drifted over the
South Pole. The formation of massive glaciers on top of Gondwanaland lowered sea level
by at least 165 feet. A tremendous marine mass extinction occurred: one of many for
similar glacial reasons in subsequent epochs.
3. Pangea was a true supercontinent, formed from the collision of Laurasia and
Gondwanaland. It lasted from roughly 255 Ma to 180 Ma. There was even a massive
inland sea, the Paleo-Tethys Ocean. It was not until after the breakup of Pangea in the
middle Jurassic that the Atlantic began to form.
4. The formation of the modern configuration. It all began with the fragmentation
of Pangea in the Jurassic. In particular, North America and South America rifted apart
from Africa and Europe in the late Cretaceous, about 90 Ma, and the Atlantic and even
proto-Caribbean began to be recognizable around this time. By about 14 Ma, the present
pattern had emerged clearly.
See below for a page with animations of the past 750 Ma:
http://www.ucmp.berkeley.edu/geology/tectonics.html
The Research Paper: “Paleogeographic reconstructions and basins development of the
Arctic”
This paper was written by Golonka, Bocharova, Ford, Edrich, Bednarczyk,
Wildharber and published in 2003, in Volume 20 of the journal “Marine and Petroleum
Geology.”
It was a major reconstruction project including 31 maps, data from geology,
paleomagnetic data, and stratigraphy fed into a plate tectonic model of about 300 plates
to model the evolution of the Arctic Basin.
The paper had several interesting findings. The researchers reported that a major
Ocean—the Iapetus—existed roughly where the Arctic is now relative to other plates
from 482-438 Ma. This ocean then closed up around 200 Ma—reappearing as the Arctic
around the same time as the Atlantic (~163 Ma) at the north pole. The rifting of the
Arctic was caused by Anui-Anvil Ocean subduction zones—now gone, but around
present-day Iceland. Also, curiously enough, it was a very active region tectonically,
with lots of volcanoes (though we should remember that many areas of the Arctic are still
very active tectonically). Until 133 Ma, there was still restricted circulation resulting in
organic-rich shale deposits—but there was also strong upwelling, helping biologic
activity. By 58 Ma, the present form of the Arctic was easily recognizable, as encircling
North America, Greenland, and Eurasia broke apart. It officially became the Arctic
around 55 Ma. It was only in the Eocene (55-34 Ma) that sea-floor spreading shifted from
the West to the East of Greenland.