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. 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 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 Granitic-andesitic composition 30–70 km thick 30–70 km thick 1/3 of Earth surface 1/3 of Earth surface Complex structures Complex structures Up to 4.0 Ga old Up to 4.0 Ga old

Three basic structural components of continents Shields Shields Stable platforms Stable platforms Folded mountain belts Folded mountain belts

Shields (e.g., Canada) Low elevation and relatively flat Low elevation and relatively flat ”Basement complex" of metamorphic and igneous rocks ”Basement complex" of metamorphic and igneous rocks Composed of a series of zones that were once highly mobile and tectonically active 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 Shields covered with a series of horizontal sedimentary rocks Sandstones, limestones, and shales deposited in ancient shallow seas Sandstones, limestones, and shales deposited in ancient shallow seas Many transgressions, regresssions caused by changes in spreading rate Many transgressions, regresssions caused by changes in spreading rate

Mountain belts Relatively narrow zones of folded, compressed rocks (and associated magmatism) Relatively narrow zones of folded, compressed rocks (and associated magmatism) Formed at convergent plate boundaries Formed at convergent plate boundaries Two major active belts: Cordilleran (Rockies-Andes), Alps-Himalayan Two major active belts: Cordilleran (Rockies-Andes), Alps-Himalayan Older examples: Appalachians, Urals 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

Southern Valley and Ridge Northern Valley and Ridge

Valley and Ridge in Pennsylvania

Valley and Ridge in Tennessee

Stages in the formation of the Southern Appalachians Fig

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 ITCZ shifts with seasons Circulation driven by solar heating Circulation driven by solar heating Circulation affected by seasonal heat transfer between tropical ocean and land Circulation affected by seasonal heat transfer between tropical ocean and land Heat capacity and thermal inertia of land < water Heat capacity and thermal inertia of land < water

Atmospheric Circulation Atmosphere has no distinct upper boundary Atmosphere has no distinct upper boundary Air becomes less dense with increasing altitude Air becomes less dense with increasing altitude Air is compressible and subject to greater compression at lower elevations, density of air greater at surface Air is compressible and subject to greater compression at lower elevations, density of air greater at surface What drives atmospheric circulation? What drives atmospheric circulation?

Free Convection Atmospheric mixing related to buoyancy Atmospheric mixing related to buoyancy Localized parcel of air is heated more than nearby air Localized parcel of air is heated more than nearby air Warm air is less dense than cold air Warm air is less dense than cold air Warm air is therefore more buoyant than cold air Warm air is therefore more buoyant than cold air Warm air rises Warm air rises

Water Vapor Content of Air Saturation vapor density Saturation vapor density Warm air holds 10X more water than cold Warm air holds 10X more water than cold

General Circulation of the Atmosphere Tropical heating drives Hadley cell circulation Tropical heating drives Hadley cell circulation Warm wet air rises along the equator Warm wet air rises along the equator Transfers water vapor from tropical oceans to higher latitudes Transfers water vapor from tropical oceans to higher latitudes Transfers heat from low to high latitudes Transfers heat from low to high latitudes

Summer Monsoon Air over land heats and rises drawing moist air in from tropical oceans 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 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 Uplift Weathering Hypothesis Chemical weathering is the active driver of climate change Chemical weathering is the active driver of climate change Rate of supply of CO 2 constant, rate of removal changes Rate of supply of CO 2 constant, rate of removal changes Global mean rate of chemical weathering depends on availability of fresh rock and mineral surfaces 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 Rate of tectonic uplift controls/enhances exposure of fresh rock surfaces

Source of Greenhouse Gases Input of CO 2 and other greenhouse gases from volcanic emissions Input of CO 2 and other greenhouse gases from volcanic emissions

Is Volcanic CO 2 Earth’s Thermostat? If volcanic CO 2 emissions provide greenhouse, has volcanic activity been continuous through geologic time? No, but… If volcanic CO 2 emissions provide greenhouse, has volcanic activity been continuous through geologic time? No, but… Carbon input balanced by removal Carbon input balanced by removal Near surface carbon reservoirs Near surface carbon reservoirs Stop all volcanic input of CO 2 Stop all volcanic input of CO 2 Take 270,000 years to deplete atmospheric CO 2 Take 270,000 years to deplete atmospheric CO 2 Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y -1 ) Surface carbon reservoirs (41,700 gt) divided by volcanic carbon input (0.15 gt y -1 ) Rate of volcanic CO 2 emissions have potential to strongly affect atmospheric CO 2 levels on billion-year timescale Rate of volcanic CO 2 emissions have potential to strongly affect atmospheric CO 2 levels on billion-year timescale

Removal of Atmospheric CO 2 Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Slow chemical weathering of continental rocks balances input of CO 2 to atmosphere Chemical weathering reactions important Chemical weathering reactions important Hydrolysis and Dissolution Hydrolysis and Dissolution

Hydrolysis Main mechanism of chemical weathering that removes atmospheric CO 2 Main mechanism of chemical weathering that removes atmospheric CO 2 Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Reaction of silicate minerals with carbonic acid to form clay minerals and dissolved ions Summarized by the Urey reaction Summarized by the Urey reaction CaSiO 3 + H 2 CO 3  CaCO 3 + SiO 2 + H 2 O CaSiO 3 + H 2 CO 3  CaCO 3 + SiO 2 + H 2 O Atmospheric CO 2 is carbon source for carbonic acid in groundwater Atmospheric CO 2 is carbon source for carbonic acid in groundwater Urey reaction summarizes atmospheric CO 2 removal and burial in marine sediments Urey reaction summarizes atmospheric CO 2 removal and burial in marine sediments Accounts for 80% of CO 2 removal Accounts for 80% of CO 2 removal

Dissolution Kinetics of dissolution reactions faster than hydrolysis Kinetics of dissolution reactions faster than hydrolysis Dissolution reaction neither efficient nor long term Dissolution reaction neither efficient nor long term Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean Dissolution of exposed limestone and dolostone on continents and precipitation of calcareous skeletons in ocean CaCO 3 + H 2 CO 3  CaCO 3 + H 2 O + CO 2 CaCO 3 + H 2 CO 3  CaCO 3 + H 2 O + CO 2 Although no net removal of CO 2 Although no net removal of CO 2 Temporary removal from atmosphere Temporary removal from atmosphere

Atmospheric CO 2 Balance Slow silicate rock weathering balances long-term build-up of atmospheric CO 2 Slow silicate rock weathering balances long-term build-up of atmospheric CO 2 On the million-year time scale On the million-year time scale Rate of chemical hydrolysis balance rate of volcanic emissions of CO 2 Rate of chemical hydrolysis balance rate of volcanic emissions of CO 2 Neither rate was constant with time Neither rate was constant with time Earth’s long term habitably requires only that the two are reasonably well balanced 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 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 Times of continental collision coincide with times of glaciations Uplift weathering hypothesis consistent with geologic record Uplift weathering hypothesis consistent with geologic record

Earth’s High Topography Only a few regions with elevations above 1 km Only a few regions with elevations above 1 km Most young tectonic terrains Most young tectonic terrains Exception is E. African plateau Exception is E. African plateau

India-Asia Collision Formation of Tibetan Plateau Formation of Tibetan Plateau Large geographic region elevated Large geographic region elevated Initial collision about 55 mya Initial collision about 55 mya Uplift continues today Uplift continues today No large continental collisions between mya No large continental collisions between mya

Elevation on Earth Most high elevation caused by subduction of oceanic crust and volcanism Most high elevation caused by subduction of oceanic crust and volcanism Mountain ranges associates with subduction common throughout geologic time Mountain ranges associates with subduction common throughout geologic time Deep-seated heating and volcanism Deep-seated heating and volcanism East African plateau East African plateau Mechanism of uplift not unique to last 55 my Mechanism of uplift not unique to last 55 my Existence of uplifted terrains like the Tibetan Plateau Existence of uplifted terrains like the Tibetan Plateau Not common through geologic time Not common through geologic time Conclude – amount of high elevation terrain is unusually large during last 55 my 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? Does the amount of high elevation terrain result in unusual physical weathering? Most likely given 10 fold increase of sediment to the Indian Ocean Most likely given 10 fold increase of sediment to the Indian Ocean Steep terrain along southern Himalayan margin Steep terrain along southern Himalayan margin Presence of powerful South Asian monsoon Presence of powerful South Asian monsoon

Tuesday Video: Winds of Change Homework #5 Due

Next Thursday Exam #2 (March 9 th )