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Chapter 4: Overview • Earth Structure, Plate Tectonics, & Paleomagnetism • Plate position and climate – Polar Position Hypothesis – Pangaea climate, test of GCM • Plate movement and climate – Seafloor Spreading Rate and CO2 – Continent-Continent Collision Mountains and CO2 Earth’s Layered Structure Plate Tectonic Map Plate Boundaries Activities at different Plate Boundaries Seafloor Magnetic Stripes used to reconstruct ancient plate positions Plate Positions and Climate Polar Position Hypothesis 1. Continental scale glaciers only form on continents in high latitudes This is the case today, and appears to match geologic data from the past 2. When continents drift over polar regions the world is thrown into “icehouse” world conditions This idea is not support by geologic data from the past. Gondwana was near the South Pole for several 100 my, but only during part of that time was the world in “icehouse” conditions. Conclusion – Plate position is not enough to control global climate – greenhouse gas variations are still need. Pangaea •Existed in a fairly stable way from 250 – 180 mya •Stretched from 70N to 70S •Symmetric, climate similar in both hemispheres – perfect for GCM testing Pangaea of a GCM test Model Inputs • 70N – 70S • Sea Level about like today • Topography – simplified (1000M plateau) • Sun 1% weaker; so CO2 higher • Fossil tell us it was warmer than today • CO2 of 1650ppm used (280ppm pre-industrial) Model Outputs • Agreement of model and data – arid interiors (evaporites) – humid near coasts • monsoons: summer rains and dry winters • Disagreement of model and data – Model predicts wide temp variation summer to winter – Data shows not so cold winters – Do we need more CO2? Plate Movements and Climate 1. Seafloor Spreading Rate Hypothesis •Faster spreading – more CO2 output at mid-ocean ridge •Leads to faster subduction – temporarily put C back into Earth (sediments subducted) •Arc volcanoes become more active – releasing more CO2 •Question – would increased weathering of rising volcanic mountain remove CO2 fast enough to stop the building up of CO2 in atmosphere??? •Unfortunately, ocean floors only 175 my old – not enough data to really test the model Plate Movements and Climate 2. Uplift – Weathering Hypothesis Plate Movement and Climate • • • • • • Uplift Weathering Hypothesis Continent-Continent Collision creates the largest mountain ranges on Earth No subduction, so no volcanic activity Therefore no CO2 into atmosphere Just lots of fresh rock for chemical weathering (hydrolysis) to act on That removes CO2 from atmosphere Should push Earth into “icehouse” conditions because there is no feedback to stop the mountain formation Plate Movements and Climate Uplift Weathering Hypothesis (does it fit the data?) • Recent Glaciations (Icehouse Worlds) – 35 mya to today, matches Himalayas forming when India collided with China – 325 – 240 mya, matches formation of Pangaea with multiple collisions – 440 mya, doesn’t really match a collision, but it was a brief and unusual glaciation. • Uplift Weathering and Spreading Rate (maybe?) hypotheses are both solid ideas