Foundations of Physical Science Workshop: Variations in the Heating and Cooling of Earth.
Plate Tectonics 1 September 10, 13,700,002,008. The earth history of our planet for the last 4.2 Ga:...
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Transcript of Plate Tectonics 1 September 10, 13,700,002,008. The earth history of our planet for the last 4.2 Ga:...
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Plate Tectonics 1
September 10, 13,700,002,008
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The earth history of our planet for the last 4.2 Ga:
slow coolingSlow cooling of the
earth
mantle temperatures have declined
from 1580 degC
to 1375 degC
in the last 4.5 billion years
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Sources of heat
initial heatradioactive decaygravitational settling
All sources are dying away …
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Evolution of earth in last 4 Ga
Is primarily consequence of the cooling process
Key character of cooling is
“Convection”
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Convection
cold cold cold cold coldcold
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Conditions Necessary for Convection
1) Gravity.
2) A fluid that experiences changes in density as its temperature changes.
3) A heat source within or at the bottom of the fluid.
cold cold cold cold cold
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Results of Convection
1) Heat is transferred from bottom to top of fluid (flux of heat)
2) The fluid circulates
3) regions of upwelling & downwelling, divergence and convergence (i.e. flux of fluid)
cold cold cold cold cold
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Convection with Lithosphere
1) Lithosphere is cold, stiff, brittle region at top
2) Subduction zone: downwelling, convergence of plates
3) Ridge: upwelling, divergence of plates
cold cold cold cold cold
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Lithosphere in 3D
700 km
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Comparison of Convection in Atmosphere and Solid Earth
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Phenomenon Atmosphere Mantle
Material Air Mg-Fe silicate rock“Peridotite”
Driving force Bouyancy Bouyancy
What changes Temperature Temperaturedensity & water content & depletion
Heat source Sunlight Radioactive decay
Rate of heat 1000 watts/m2 0.1 watts/m2
transsport
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Phenomenon Atmosphere Mantle
Speed 1 m/s3x109 cm/year 1 cm/year
Lapse rate 6 deg-C/km 1 deg-C/km
Coriollis important Yes No
Phase change Water condenses Peridotite meltsto make rain to make magma
After phase change Dry air Depleted peridotiteis heavier is lighter
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Phenomenon Atmosphere Mantle
Effects of flow Wind makes Flowing mantleocean waves moves continents& sand dunes
Small scale Tornadoes Mantle plumesFeatures
Human viewpoint From the bottom From the top
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Comparison of Convection in Atmosphere and Solid Earth
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Magma is Solid Earth’sversion of “rain”
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Rain vs. magma
rain …
rising moist air
adiabatically cools
crosses vapor-liquid phase boundary
water droplets form
are negatively bouyant
and fall down as rain
Leaves dry air behind
magma …
rising undepleted mantle
adiabatically cools
crosses solid-liquid phase boundary
liquid rock droplets form
are positively bouyant
and rise up as magma
Leaves depleted mantle behind
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Lithosphere – brittle top of part of the earth – is a key feature of our planet
that makes its style of convection different than “bubbling soup”
Lithosphere: about 100 km thick
asthenopshere: about 600 km thick; vigorous convection
Deeper mantle is more viscous and convects more slowly
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Brittle Materialcracks or faults instead of flows
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Please memorize
these fault configurations and names
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Crustal Movementsmeasured through GPS
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Schematic earth
Schematitic GPS velocities of the ground
10 mm per year
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Schematic map
“Plates”
Really spherical caps
Large sections of the lithosphere sliding over the lithosphere
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Schematic map
The plate boundaries are regions of active tectonism
Plate 1
Plate 2
Plate 3
1-3 boundary
2-3 boundary
1-2
boun
dary
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Schematic map
All motions are a type of rotation or spinning of the cap about a pole of rotation
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Lithosphere is organized into plates that rigidly move
Measured with GPS Inferred from geology
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Four Plates in this Diagram
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Divergent Plate Boundaries
plates are moving apart
Is a gap created as they move apart?
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Convergent Plate Boundaries
plates are colliding
What happens to the excess material?
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Transform Plate Boundaries
plates are sliding past one another
Is a gap created as they move apart?
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Plate Motion in the Past
http://www.ucmp.berkeley.edu/geology/anim1.html
Earth Scientists have worked out the history of the earth’s plate motions for the most recent a billion years or so of earth history
but the farther back in timethe more crudely the plate configuration is known
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Why Melting?
(The Mantle’s Rain)
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Geological evidence for extension is very common at divergent plate boundaries
on the sea floor at a divergent boundary
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Geological evidence for volcanism is very common at divergent plate boundaries (e.g. this
sea floor hot spring)
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Divergent plate
boundary in the ocean is
called a mid-ocean
ridge
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Cre
styo
ung
& h
ot
Fla
nkol
d &
coo
l
Fla
nkol
d &
coo
l
5000 km
0 Ma 200 Ma
200 Ma
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Thermal ContractionRidge Crest: Hot Lithosphere: 1300CRidge Flank: Cold Lithosphere: 800CChange in temperature: 500C
Thickness of lithosphere 100 km = 105 m
Thermal contraction coefficient of rock: 310-5 per C
Thermal contraction: 310-5 105 500 = 1500 m
Ridge flank should be about 1500 meters below the crest
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Depth-Age Relationship
Shape of ridge is explained by cooling of initially hot lithosphere emplaced at ridge axis
If you know the age of a patch of sea floor, you can predict its depth to amazing accuracy!
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Ridge crests – young – little sedimentFlank – old – thick sediment
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Mid-Atlantic Ridge
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Divergent Plate Boundary on a Continent
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