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Dynamic Earth Class 16 2 March 2006 The Flow of the Continents (Chapter 5) Building Mountains: New Zealand and Tibet Deformation of the Continental Crust Deformation of continental crust Since continents are not destroyed by subduction, we look here for the ancient history of Earth. orogeny: sum of the tectonic forces (i.e., deformation, magmatism, metamorphism, erosion) that produce mountain belts Mountains and Mountain Building Mountains are one part of the continuum of plate tectonics—the most evident one. Example: Limestones at the top of Mount Everest. Structures of continents 1) Continents are made and deformed by plate motion. 2) Continents are older than oceanic crust. 3) Lithosphere floats on a viscous layer below (isostasy). Age of the Continental Crust Blue areas mark continental crust beneath the ocean Continental characteristics • Granitic-andesitic composition • 30–70 km thick • 1/3 of Earth surface • Complex structures • Up to 4.0 Ga old Three basic structural components of continents • Shields • Stable platforms • Folded mountain belts Shields (e.g., Canada) • Low elevation and relatively flat • ”Basement complex" of metamorphic and igneous rocks • Composed of a series of zones that were once highly mobile and tectonically active Stable platforms • Shields covered with a series of horizontal sedimentary rocks • Sandstones, limestones, and shales deposited in ancient shallow seas • Many transgressions, regresssions caused by changes in spreading rate Mountain belts • Relatively narrow zones of folded, compressed rocks (and associated magmatism) • Formed at convergent plate boundaries • Two major active belts: Cordilleran (Rockies-Andes), Alps-Himalayan • Older examples: Appalachians, Urals Mountain types Folded—Alps, Himalaya, Appalachians Fault block—Basin and Range Upwarped—Adirondacks Volcanic—Cascades Stacked Sheets of Continental Crust Due to Convergence of Continental Plates Volcanic Origin, e.g. Cascades Upwarped with Reverse Faults, e.g. Central Rocky Mountians Tilted Normal Fault Blocks, e.g. Basin and Range Province Folded Rocks, e.g. the Appalachian Ridge and Valley Uplift Formed by Removal of Ice Sheet Uplift Caused by Heating Subsidence Caused by Cooling Uplift Caused by Heating Subsidence Caused by Extension Uplift Caused by Rising Mantle Plume Building fold mountains (1) Building fold mountains (2) The Applachians Northern Valley and Ridge Southern Valley and Ridge Valley and Ridge in Pennsylvania Valley and Ridge in Tennessee Stages in the formation of the Southern Appalachians Fig. 17.30 Overlapping Thrust Faults, e.g. the Himalayas Tibet—not just mountains, a huge plateau too India has collided with Asia Continent–Continent Convergent Boundary Indian plate subducts beneath Eurasian plate 60 million years ago Indian subcontinent collides with Tibet 40–60 million years ago Accretionary wedge and forearc deposits thrust northward onto Tibet Approximately 40–20 million years ago Main boundary fault develops 10–20 million years ago Exotic terranes Faults galore… …and earthquakes Himalayan collision ideas A complicated explanation emerges The drooling lithosphere So now we think we have figured it out Indian climate before Himalayas Monsoons – Circulation in ITCZ ITCZ shifts with seasons Circulation driven by solar heating Circulation affected by seasonal heat transfer between tropical ocean and land Heat capacity and thermal inertia of land < water Atmospheric Circulation Atmosphere has no distinct upper boundary Air becomes less dense with increasing altitude Air is compressible and subject to greater compression at lower elevations, density of air greater at surface What drives atmospheric circulation? Free Convection Atmospheric mixing related to buoyancy Localized parcel of air is heated more than nearby air Warm air is less dense than cold air Warm air is therefore more buoyant than cold air Warm air rises Water Vapor Content of Air Saturation vapor density Warm air holds 10X more water than cold General Circulation of the Atmosphere Tropical heating drives Hadley cell circulation Warm wet air rises along the equator Transfers water vapor from tropical oceans to higher latitudes Transfers heat from low to high latitudes Summer Monsoon Air over land heats and rises drawing moist air in from tropical oceans Winter Monsoon Air over land cools and sinks drawing dry air in over the tropical oceans Monsoon Climate: Tibet heats up and rises Moist Indian Ocean air sucked in Clouds form as moist air blocked by mts Uplift Weathering Hypothesis Uplift Weathering Hypothesis Chemical weathering is the active driver of climate change Rate of supply of CO2 constant, rate of removal changes Global mean rate of chemical weathering depends on availability of fresh rock and mineral surfaces Rate of tectonic uplift controls/enhances exposure of fresh rock surfaces Source of Greenhouse Gases Input of CO2 and other greenhouse gases from volcanic emissions Is Volcanic CO2 Earth’s Thermostat? If volcanic CO2 emissions provide greenhouse, has volcanic activity been continuous through geologic time? No, but… Carbon input balanced by removal Near surface carbon reservoirs Stop all volcanic input of CO2 Take 270,000 years to deplete atmospheric CO2 Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y-1) Rate of volcanic CO2 emissions have potential to strongly affect atmospheric CO2 levels on billion-year timescale Removal of Atmospheric CO2 Slow chemical weathering of continental rocks balances input of CO2 to atmosphere Chemical weathering reactions important Hydrolysis and Dissolution Hydrolysis Main mechanism of chemical weathering that removes atmospheric CO2 Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Summarized by the Urey reaction CaSiO3 + H2CO3 CaCO3 + SiO2 + H2O Atmospheric CO2 is carbon source for carbonic acid in groundwater Urey reaction summarizes atmospheric CO2 removal and burial in marine sediments Accounts for 80% of CO2 removal Dissolution Kinetics of dissolution reactions faster than hydrolysis Dissolution reaction neither efficient nor long term Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean CaCO3 + H2CO3 CaCO3 + H2O + CO2 Although no net removal of CO2 Temporary removal from atmosphere Atmospheric CO2 Balance Slow silicate rock weathering balances long-term build-up of atmospheric CO2 On the 1-100 million-year time scale Rate of chemical hydrolysis balance rate of volcanic emissions of CO2 Neither rate was constant with time Earth’s long term habitably requires only that the two are reasonably well balanced Tectonic Uplift and Weathering Uplift causes several tectonic and climatic effects that affects weathering by fragmenting fresh rock Testing the Hypothesis Times of continental collision coincide with times of glaciations Uplift weathering hypothesis consistent with geologic record Earth’s High Topography Only a few regions with elevations above 1 km Most young tectonic terrains Exception is E. African plateau India-Asia Collision Formation of Tibetan Plateau Large geographic region elevated Initial collision about 55 mya Uplift continues today No large continental collisions between 100-65 mya Elevation on Earth Most high elevation caused by subduction of oceanic crust and volcanism Mountain ranges associates with subduction common throughout geologic time Deep-seated heating and volcanism East African plateau Mechanism of uplift not unique to last 55 my Existence of uplifted terrains like the Tibetan Plateau Not common through geologic time Conclude – amount of high elevation terrain is unusually large during last 55 my Physical Weathering High Does the amount of high elevation terrain result in unusual physical weathering? Most likely given 10 fold increase of sediment to the Indian Ocean Steep terrain along southern Himalayan margin Presence of powerful South Asian monsoon Tuesday Video: Winds of Change Homework #5 Due Next Thursday Exam #2 (March 9th)