Download Our Earth

Our Blue-Green Earth!
Topics
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Core and Structure
Plate Tectonics
Atmosphere
Magnetosphere
Tides
Basic Structure
• Mass = ~ 6x1024kg
• Average Density ~
5500kg/m3
• Surface Water!
• Surface rocks
~ 2000-4000kg/m3
Hence Dense Core
Crust, Mantle, Outer Core and Inner Core
Seismic Activity
Earthquakes send
shock waves
through the Earth.
Probe of Earth’s
internal structure!
Primary and Secondary Waves
P-Wave and S-Wave Behaviour
• P- Waves travel ~ 5-6km/s
• Travel through both
liquid and solid
• Pressure wave
like sound (Longitudinal)
• S-waves travel slower
~ 3-4 km/s
• Absorbed in Liquid
• Shear waves (Transverse)
Internal Structure
• Note Jump in density at the
Core/Mantle boundary.
• Dense core rich
in iron and nickel.
• Metallic core, Rocky
(silicon-based) mantle.
• Crust is light – “floats”!
Basalts (3000-3300kg/m3)
Granites (2700-3000kg/m3)
Cooling
Planet
• “Differentiation” – not
uniform density
• Slow cooling from outside in
• Radioactive heating – Rock
acts as insulator – hot core.
Planet’s History
• Debris, dust and rock coalesce.
• Heat generated by external
Bombardment melts surface, not the core.
• Radioactive heating – trapped by insulating
rocks - melts iron-rich rocks.
• Gravity causes heavy iron to sink to centre.
• Crust is cooling ‘skin’ on the surface.
Seismic Studies
Reveal…
More Surprises!
• Core is rotating faster than the planet!
By ~0.67 sec/day = ¼ turn/century.
• Evidence of solid iron ‘crystal’ for the
inner core. – P wave speed depends on
orientation.
Surface Activity
Earthquakes and Volcanoes
….Ancient and Modern
Internal stresses
caused this terrain to
be folded upwards.
Erosion by wind and
rain have exposed the
different constituents.
Plate Tectonics
Spots indicate sites of major volcanoes and
earthquakes….taken together outline ‘plates.
Plate Tectonics
• Still drifting today.
• Plates ≠ Continents
• Move ~cm/year (like fingernail growth!)
Seems small but moves ~4000km over ~200
million years (5% of Earth age)
Continental Drift
Estimates of
previous positions
of plates given
current drift rates
and directions.
Some
Plates
collide!
They don’t stop…
but build major
mountain ranges…
or deep ocean
trenches
Some
Plates
Move
Apart!
Seafloor
spreading
in the
Mid-Atlantic
Ridge
What Drives Plate Tectonics?
• Probably gigantic thermal convection cells that
drag the surface with its motion.
• ‘Soft’ hot rock takes millions of years to cycle.
Magnetic Field
Direction Reversal
• Age increases with
distance from Ridge.
• Magnetic field in rock
at the time it cooled
maps Earth’s field.
• Field reversal every ~
0.5 million years!
Colour = Same Direction
The Earth’s Atmosphere
N2 ~78%,
O2 ~21%
Ar ~0.9%,
CO2 ~0.03%
Water (variable)
– 0.1-3%
• Layers defined
by temperature
inversions.
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Atmospheric Layers
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Ionosphere
Mesosphere
Stratosphere
Troposphere
Complex Convection Cells
Winds driven
by Sun’s heat
and Earth’s
rotation.
Origin and Evolution of the Atmosphere
• Primary atmosphere: light gases H2, He, CH4 (methane), NH3 (ammonia)
• Secondary Atmosphere: Outgassed via
volcanoes: H2O, CH4 , SO2, NO, N2, NH3
• Hot light gases escaped + UV ionisation.
• Temperature cooled - Water condensed.
• CO2 and SO2 dissolved in rocks.
• Life produces oxygen....and O3 (ozone) layer.
The Greenhouse Effect
• Clouds reflect and
absorb sunlight.
• Sunlight heats up
surface.
• Hot bodies radiate
energy.
• CO2 and H2O absorb
heat efficiently…
“Greenhouse Gases”.
Global
Warming
• CO2 concentration
increased by ~16%
since 1958.
• Temperatures
predicted to rise by
~0.9-3.5o by 2100.
• Long time variations
in orbit parameters
contribute….
…..So do humans.
Saw-tooth is due
to seasons
Warning Signs…
…for Climate Change
• Deforestation
• Ozone Depletion
• El Niño Effect
~50%
Reduction,
~10-20%
in midlatitudes
~20,000km2 per year
We need
to take
this
seriously!
Why is the Sky Blue?
• Scattering of
light by dust
particles.
• Scattering
depends on
particle size.
• Blue ~10x
more likely
to scatter.
• Red Sunsets.
Earth’s Magnetic Field
• Magnetic Poles
aligned ~11o from
rotation axis
• Drift ~ 10km / year
• Created by electric
currents in liquid
iron core: Dynamo =
Convection + rapid
rotation.
• North is South!
Magnetosphere
• Discovered in
1950’s by
satellites.
• Protects our
atmosphere
from solar
wind.
Van Allen Belts
Spiral
motion
traps ions
in
converging
magnetic
fields lines.
• Two main doughnut-shaped regions - pronounced
at equator: ~3000km protons, 20,000km electrons.
• New belt (1993) = O, N and Ne ions.
Aurora Borealis & Australis
UV image - 1996
Views from Space
Earth’s Tides
Daily, monthly and yearly cycles.
Consider Just Earth-Moon System
• Gravity ∝ 1/r2
• Lunar gravity:
Pull is greatest on
side closest Moon.
• Variations –
compared to that
at Earth centre.
Differential or
Tidal force ∝ 1/r3.
Result: Tidal Bulge
High and Low tides twice a day.
Solar and Lunar Tides
• New/Full Moon
enhances Tides.
= SPRING
• 1 & 3 Quarter
Moons reduced
tides = NEAP
Sun partially
cancels
Moon’s effect
Earth’s
Tidal
Bulge
• Earth rapid spin causes a tidal bulge – oceans
depth offset from Earth-Moon line by ~10°.
• WHY? Friction – Rotating Earth ‘drags’ tidal
bugle around with it.
• RESULT? – Slowing Earth down – longer days!
Furthermore: Moon Drifting Away
• Laser Pulses bounced of the
Moon show it is receding at
a rate of ~3.8cm/year.
• Eventually…
……Earth ‘day’ will be 47x
longer than now and Moon
will stay fixed in sky at
~1.43x present distance…
….in billions of years time.