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Transcript
The Plate Tectonics Revolution A paradigm shift in the Earth Sciences Topics: Review of last class Why isn’t the Earth expanding? Plate boundaries, earthquakes and volcanoes What causes plate tectonics? How fast do plates move? Earth evolution: the supercontinent cycle The Snowball Earth hypothesis Alfred Wegener Key evidence for continental drift: coastline fit fossil record similarity of rock assemblages on different continents Development of Plate Tectonic Theory Harry Hess Key evidence for “seafloor spreading” Young age of oceans relative to continents Mid-ocean ridges Development of Plate Tectonic Theory The Vine-Matthews-Morley Hypothesis Oceanic Magnetic Stripes = Seafloor Spreading + Magnetic Field Reversals Key evidence: Discovery of magnetic field reversals Magnetic mapping of oceans Development of Plate Tectonic Theory J. Tuzo Wilson 1. Hotspot hypothesis • Linear volcanic island/seamount chains 2. Transform faults • Earthquake activity on oceanic fracture systems 3. Wilson Cycle • Geology of mountain belts Development of Plate Tectonic Theory Why isn’t the Earth expanding? Theory of Plate Tectonics 1. Earth’s surface is divided into rigid lithospheric plates 2. New oceanic lithosphere is created at mid-ocean ridges 3. Oceanic lithosphere is destroyed at deep-sea trenches (subduction zones) 4. This process largely explains the global distribution of earthquakes and volcanoes Development of Plate Tectonic Theory Tectonic plates Types of plate boundaries Types of plate boundaries Characteristic Faulting Example: Iceland Type of Volcanic Activity Fissure eruptions at mid-oceanic ridges Types of plate boundaries Continent-Continent Ocean-Ocean Ocean-Continent (Phillipines) (Chile) (Tibet) Characteristic Faulting Type of Volcanic Activity Stratovolcanoes Types of plate boundaries Example: California Characteristic Faulting San Andreas Fault What causes plate tectonics? Heat from deep within the Earth causes mantle convection Decompression melting produces midocean spreading Plate motion is assisted by slab pull How fast do plates move? Fastest: East Pacific Rise (15 cm/year) Slowest: Arctic Ridge (2.5 cm/year) How do plates change with time? California has changed from a convergent boundary to a transform boundary in the last 30 million years Mount Everest India has been colliding with Asia for the past 50 million years The plate-tectonic rewind button: Age of oceanic crust Earth evolution: The supercontinent cycle About 225 million years ago, a single supercontinent existed: Pangea (all lands) The ancient world ocean is called Panthalessa (all seas) Earth evolution: The supercontinent cycle About 200 million years ago, Pangea split into Laurasia and Gondwanaland The ocean created by the split is called the Tethys Sea. This is now the Mediterranean Sea Earth evolution: The supercontinent cycle About 180 million years ago, the Atlantic ocean began to form Continued spreading has produced the current configuration of continents The supercontinent cycle The formation and breakup of supercontinents seems to happen on a ~ 500 million year cycle The previous supercontinent (~ 700 million years ago) is called Rhodinia Another supercontinent will most likely form in Earth’s distant future The Snowball Earth hypothesis Some scientists believe that 600-700 million years ago, Earth experienced a series of global ice ages This occurred at a time when Rhodinia was at a high latitude 1. Ice caps extend toward equator 2. Snowball Earth: H20 and carbon cycles shut off 3. Supergreenhouse effect from volcanoes 4. Greenhouse Earth The Snowball Earth hypothesis These climate shocks appear to have triggered the evolution of multicellular animal life Snowball Earth is challenging long-held assumptions regarding the limits of global change.