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From Plate Tectonics to Climate Prof. Eric Calais Purdue University - EAS Department [email protected] http:/www.eas.purdue.edu/~ecalais What did you think of Bonnie Schneider’s speech? A. I understood most of it. B. I got some of it. C. I have no clue what she was saying The presence of land reptile fossils in Permian-Triasic freshwater shales (~200 million years) in several continents suggests that they were once connected. Ancient positions of the Earth’s magnetic poles, recorded in rocks, are different for North America and Europe from 400 to 140 million years … unless they formed a unique continent. Continents have moved with respect to each other 1. Global Positioning System (GPS) satellites broadcast a radio signal towards the Earth 2. GPS antenna and receivers record the radio signal and convert it into satelliteantenna distances 3. Three distances => solve for latitude, longitude, elevation Space geodesy, in particular GPS, provides positions with high precision. For $100: precision 10 m, for $10,000: precision 1 mm Repeated GPS measurements show that the longitude of Algonquin (Canada) is changing at a rate of 1.5 cm/yr Continents show consistent pattern of displacement They move at speeds ~few cm/yr = the speed your fingernails grow… Earthquake distribution is not random: very narrow deforming zones (= plate boundaries) versus large areas with no earthquakes (= rigid plate interiors) Tectonic plates and their boundaries today -- continents are embedded in the plates and move with them Ocean-ocean subduction ⇒ island arc Transform fault ⇒ strike-slip motion lithosphere Oceanic spreading center ⇒ creation of new oceanic crust Ocean-continent subduction ⇒ volcanism Continental rift ⇒ break-up of a continent lithosphere viscous mantle viscous mantle Lithospheric plates float on a viscous mantle. Deformation (e.g., earthquakes) occur at their boundaries: divergent (spreading centers), convergent (subductions), or strike-slip Venting of hot fluids along the EPR Pacific plate Cocos plate Pillow lavas basalts along the EPR Sea-floor topography showing the East Pacific rise (EPR). Central America is in the background. Oceanic ridges are actually “spreading centers” along which oceanic lithosphere is continuously created by volcanism Eurasian plate Pacific plate Philippines Sea plate Oceanic trenches correspond to “subduction zones” along which oceanic lithosphere continuously disappears into the mantle 6 cm/yr This coral head was below sea-level before the earthquake: it has been uplifted by ~1 m. Indian plate Before the earthquake: Long-term accumulation of elastic strain The Earth behaves elastically: plate motions load the rubber band… Indian plate During the earthquake: Sudden release of elastic strain … until the load exceed the strength of the rubber band, whish snaps = an earthquake Indian plate Earthquake = release of elastic energy Subduction earthquake => seafloor move up A lot of water… Seafloor displacement triggers a tsunami wave A lot of water… Amplitude growth as water depth decreases Two plates are moving past each other at a speed of 1 cm/yr along a major seismic fault. The last major earthquake on that fault occurred in 2000 and resulted in 2 m of instantaneous displacement (magnitude 7). When is the next earthquake of similar size due? A. B. C. D. E. 2020 2050 2200 There is no solution to the problem I don’t know. Magma in subduction zones: • Produced thanks to dehydration of oceanic crust that lowers melting point of mantle rocks • Very viscous (high silica content) and contains large amounts of dissolved water. As magma rises (because more buoyant than surrounding rocks), pressure decreases => dissolved water form steam bubbles. • Low magma viscosity => bubbles float to top of magma, then released into atmosphere (degassing) => magma reaches surface with low gas content • High magma viscosity => bubbles are trapped inside magma as it rises => magma reaches the surface with high gas content at high pressure w.r.t. atmospheric pressure => explosion, release of large amounts of gas at once Subduction zones are associated with large explosive volcanoes The 1991 Mount Pinatubo Eruption, Philippines Quantity of aerosols before (top) and following the June 15, 1991, Mount Pinatubo eruption (blue = less, red - more). • Between June 12 and June 16, 1991, Pinatubo released 30 Mt of sulfur dioxide into the atmosphere => converted into very fine particulates (sulfate aerosols). • Aerosols scatter and absorb incoming sunlight => cooler temperature worldwide. Explosive volcanoes affect climate The oldest piece of oceanic floor is ~180 My old while the age of Earth is ~4.6 Gy old. This is because: A. Oceans are a recent feature (geologically speaking) on Earth B. Oceanic lithosphere continuously disappears at subduction zones C. We have not yet explored all the oceans and the oldest piece of ocean has not yet been discovered. The relative position of continents since ~500 My is known 300 My: close ocean => Appalachians, 220 My: Pangea, 150 My: Gondwana+Laurentia and Thethys ocean, 100-90 My: Atlantic opens, India takes off, 40-30 My: Thethys ocean closes => Alpine chains Mount Viso, French Alps Old seafloor rocks are now found in mountain belts. Mountain belts result from the collision of continents: e.g. Himalayas and Tibet result from India-Eurasia collision 40 My ago radiolarites Pillow basalts The 5,000 m-high Tibetan Plateau acts like a gigantic exposed brick, absorbing summer heat and heating the atmosphere above it. Hot air rises, and cool, moist air drawn in from over surrounding oceans rushes in to replace it. That moist air is the source of monsoon rains India Rapid uplift of Tibet starts ~20-17 My ago => strengthen monsoon: ⇒ Increased weathering => chemical reactions removes CO2 from atmosphere ⇒ Oceanic upwellings => increase in nutrients => increased phytoplancton productivity => increase burial of organic carbon => decrease of atmospheric CO2 ⇒ Global cooling (growth of Antarctic ice sheets, cf O2 isotopes) ⇒ Drier climate in Africa => forest replaced by grassland => decline of great apes and emergence of Homo? (details highly debated!) Plate tectonics influences climate Absolute sea-level is derived from measurements of satellite-to-seafloor distance and satellite-toEarth center distance (derived from its orbit) Global sea level is rising at ~3 mm/yr. Result of steric effect (volume increase with temperature, between 0.4 and 1.9 mm/yr) and eustatic effect (melting of ice sheets) Space geodesy (satellite altimetry) can measure sealevel changes globally Take home messages • The rigid outer shell of planet Earth (~100 km thick): – Constitutes (lithospheric) plates – Floats on top of a viscous mantle – Moves at speeds on the order of a few mm to cm per year. • Plate tectonics links with climate because it is responsible for: – The formation of major topographic features (continents/oceans, mountains/lowlands) – Explosive volcanism. • Space geodesy allows us to measure: – Current motions of tectonic plates – Current sea-level change – Shape and rotation of the Earth, function of climate Continental drift in action