GEOG20009 Lecture Notes - Lecture 14: Milankovitch Cycles, Plate Tectonics, Continental Drift
15 May 2018
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Professor
LECTURE 14 (p.1): PLATE TECTONICS AND EARTH HISTORY
• Continental drift: Wegner proposed idea that the continents had moved other time, based on evidence from many
disciplines
• Crust: upper layer of the Earth – floats on top of asthenosphere, moves over it
o Oceanic Crust: thin, dense, young (7km)
o Continental Crust: thick, buoyant, mostly old (10-70km)
• Plate tectonics: 12 major plates slide around Earth, either diverging, converging or transforming (converging),
hotspots
• Earthquakes and volcanism is mostly focused at plate margins
• Mantle convection: drag/push and pull – convection currents
Plate Tectonic Super Cycles
• Supercontinent Cycles: Continuous formation and breaking up of supercontinents – very long time scale
• Assembly of supercontinent – species could move freely across landmass, global fragmentation reduced gene flow,
leading to rapid speciation and radiation
• Wilson Cycle: Closure and opening of ocean basins by plate tectonic processes
• Divergence = high sea level, spreading = high CO2 – greenhouse effect
• Convergence = low sea level, little spreading, orogeny and weathering = lowCo2 = icehouse
LECTURE 14 (p.2): QUATERNARY CLIMATE CHANGE
• Distribution of life on Earth highly influenced by climate – mainly solar radiation
• Global atmospheric and ocean circulation:
o Main mechanisms for distribution of heat from equatorial to polar regions
• Climate changes on different timescales: historical, tectonic, orbital
• Up to 30% land surface covered by ice during glacial cycles in the Quaternary
• Our knowledge of past climates is derived from ‘natural archives’
o Ice cores, tree rings, corals etc.
• Deep sea cores have given us understanding of past 2.7Ma climate cycles
o Record shows that global ice-volume changes have dramatically decreased
• Oxygen isotope geochemistry: difference between O16 and O18 allows us to understand
whether the climate was glacial or interglacial
Glacial Period Characteristics
• Lower temperature – greater drop in mid to high latitudes
• Lower atmospheric water content (lower rainfall)
• Higher thermal gradients (wind speed)
• Growth of continental ice sheets, expansion of alpine glaciers and permafrost
• Falls in sea level, increases in land area
• Contraction of wet tropics, expansion of arid/semi-arid belt
Orbital Forcing of glacial-interglacial cycles
• Eccentricity: rotation around sun is elliptical
• Tilt: 23.5-degree tilt, accountable for seasonality
• Precession: Earth axis wobbling, affecting insolation reaching Earth
• Scientists calculate these, and record the total amount of insolation reaching certain latitudes
• Milankovic Cycles: ice sheets grow during periods of Northern Hemisphere summer insolation at 65-degree N is low
and decay when NH summer insolation is high
o Theory of glacial cycles
• Do not fully account for glacial cycles – must consider complex climate feedbacks
BIOGEOGRAPHIC RESPONSES TO QUARTERNARY CLIMATE CHANGE
• Species responses:
o Adaption to a new local environment
o Dispersal (migration) with their optimal habitat as it changed location (latitude or altitude)
o Range reduction and/or extinction
• Studying past vegetation changes: pollen or charcoal – easy to reconstruct vegetation cover using these
Topics
• Climate changes in
Quaternary (last
2.7Ma)
• General
characteristics of
glacial-interglacial
cycles
• How do we know the
climate changed?
• What are the causes?
• Biogeographical
response to past
climate changes
Document Summary
Lecture 14 (p. 1): plate tectonics and earth history. Continental drift: wegner proposed idea that the continents had moved other time, based on evidence from many disciplines. Crust: upper layer of the earth floats on top of asthenosphere, moves over it: oceanic crust: thin, dense, young (7km, continental crust: thick, buoyant, mostly old (10-70km) Plate tectonics: 12 major plates slide around earth, either diverging, converging or transforming (converging), hotspots. Earthquakes and volcanism is mostly focused at plate margins: mantle convection: drag/push and pull convection currents. Plate tectonic super cycles: assembly of supercontinent species could move freely across landmass, global fragmentation reduced gene flow, Convergence = low sea level, little spreading, orogeny and weathering = lowco2 = icehouse. Lecture 14 (p. 2): quaternary climate change: distribution of life on earth highly influenced by climate mainly solar radiation, global atmospheric and ocean circulation, main mechanisms for distribution of heat from equatorial to polar regions.
