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Transcript
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)