Download Chapter 4: Overview

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
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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