Download Changes in Ocean Geometry Over the Past Billion Years

Changes in Ocean Geometry
Over the Past Billion Years
Ben Black. EPS 131 2004. Professor Tziperman.
What I’m talking about
 Through the machinery of plate
tectonics, continents and oceanic plates
move around the surface of the globe.
 The geometry of the oceans has
changed vastly and repeatedly over
time, as the plates move.
A Brief Explanation of the
Mechanisms of Change
 Plate movement
driven by subduction
and spreading—
mainly oceanic
plates.
 Continental plates
are too light to
subduct.
 Continents as scum
floating on a pond.
Changing Ocean Geometry
Explains a Lot.
 Perhaps one of the
most famous
examples of
changing ocean
geometry is the
growth of the
Atlantic ocean. The
Atlantic began to
form about 150 Ma,
in the Mesozoic.
Wegener and Plate Tectonics
 Wegener used
similarities between
South America and
Africa to postulate
continental drift.
 Combined with theories
of subduction and seafloor spreading, this
became modern plate
tectonics.
And Now, the Meat: Ocean Geometry
Over the Past Billion Years
 As we said before, continental material doesn’t
subduct. Thus it has been accreting for billions of
years. The first continents would have accreted from
volcanic islands rising out of a global ocean.
Precambrian Collisions: 1 Ba to 544 Ma
 There were many major
supercontinents that
formed and broke apart.
 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.
Gondwanaland: 500 Ma to 300 Ma
 The Gondwanaland
and Laurentia
formation was fairly
stable and lasted
about 200 My. But in
the meantime,
Gondwanaland
drifted over the
South Pole.
The Significance of High Latitude Continents
 The formation of
massive glaciers on top
of Gondwanaland
lowered sea level by at
least 165 feet.
 A tremendous marine
mass extinction
 Shifts in currents and
locations of deep water
formation—little known.
Pangea—255 Ma to 180 Ma
 Pangea was a true supercontinent, formed from the
collision of Laurasia and Gondwanaland. 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.
The Breakup of Pangea and the creation of the Modern Globe
750 Ma to the Present:
Research Paper: “Paleogeographic reconstructions and
basins development of the Arctic”
 By Golonka, Bocharova, Ford, Edrich,
Bednarczyk, Wildharber
 Published 2003, in Volume 20 of the journal
“Marine and Petroleum Geology”
 Major reconstruction project including 31
maps. Data from geology, stratigraphy fed
into a plate tectonic model of about 300
plates to model evolution of Arctic Basin
 Verified by independent regional analysis
Research Paper: Paleogeographic Reconstructions of the Arctic
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A Plate tectonic model tracing the evolution of the Arctic Ocean from
500 Ma to present
A major Ocean—the Iapetus—existed roughly where the Arctic is now
relative to other plates from 482-438 Ma.
Up to 200 Ma, the ocean then closed up—reappearing around the
same time as the Atlantic (~163 Ma) at the north pole.
Rifting of Arctic caused by Anui-Anvil Ocean subduction zones—now
gone, but around present-day Iceland.
A very active region tectonically, with lots of volcanoes
Several oceanic plates disappear (Izanagi) and appear (Kula) here.
Until 133 Ma, still restricted circulation resulting in organic-rich shale
deposits—but there is also strong upwelling, helping biologic activity.
Thus some thermohaline circulation also possible in Barents?
By 58 Ma, present form of Arctic easily recognizable, as encircling
North America, Greenland, and Eurasia break apart. It officially
becomes the Arctic around 55 Ma. It was only in the Eocene (55-34
Ma) that sea-floor spreading shifts from the West to the East of
Greenland.