Transcript ppt
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 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.