Download From Plate Tectonics to Climate

From Plate Tectonics
to Climate
Prof. Eric Calais
Purdue University - EAS Department
[email protected]
http:/www.eas.purdue.edu/~ecalais
What did you think of Bonnie
Schneider’s speech?
A. I understood most of it.
B. I got some of it.
C. I have no clue what she was saying
The presence of land reptile
fossils in Permian-Triasic
freshwater shales (~200 million
years) in several continents
suggests that they were once
connected.
Ancient positions of the Earth’s
magnetic poles, recorded in rocks,
are different for North America and
Europe from 400 to 140 million
years … unless they formed a
unique continent.
Continents have moved with respect to each other
1. Global Positioning
System (GPS) satellites
broadcast a radio signal
towards the Earth
2. GPS antenna and receivers
record the radio signal and
convert it into satelliteantenna distances
3. Three distances => solve for
latitude, longitude, elevation
Space geodesy, in particular GPS, provides positions with high
precision. For $100: precision 10 m, for $10,000: precision 1 mm
Repeated GPS measurements show that the longitude of Algonquin
(Canada) is changing at a rate of 1.5 cm/yr
Continents show consistent
pattern of displacement
They move at speeds ~few
cm/yr = the speed your
fingernails grow…
Earthquake distribution is not random: very narrow deforming zones
(= plate boundaries) versus large areas with no earthquakes (= rigid
plate interiors)
Tectonic plates and their boundaries today -- continents
are embedded in the plates and move with them
Ocean-ocean
subduction
⇒ island arc
Transform
fault
⇒ strike-slip
motion
lithosphere
Oceanic spreading
center
⇒ creation of new
oceanic crust
Ocean-continent
subduction
⇒ volcanism
Continental rift
⇒ break-up of a
continent
lithosphere
viscous mantle
viscous mantle
Lithospheric plates float on a viscous mantle.
Deformation (e.g., earthquakes) occur at their boundaries: divergent
(spreading centers), convergent (subductions), or strike-slip
Venting of hot fluids along the EPR
Pacific
plate
Cocos
plate
Pillow lavas basalts along the EPR
Sea-floor topography showing the East Pacific
rise (EPR). Central America is in the background.
Oceanic ridges are actually “spreading centers” along which
oceanic lithosphere is continuously created by volcanism
Eurasian plate
Pacific plate
Philippines
Sea plate
Oceanic trenches correspond to
“subduction zones” along which
oceanic lithosphere continuously
disappears into the mantle
6 cm/yr
This coral head was below sea-level before
the earthquake: it has been uplifted by ~1 m.
Indian plate
Before the earthquake: Long-term
accumulation of elastic strain
The Earth behaves elastically: plate
motions load the rubber band…
Indian plate
During the earthquake:
Sudden release of elastic strain
… until the load exceed the strength of the
rubber band, whish snaps = an earthquake
Indian plate
Earthquake = release of elastic energy
Subduction earthquake => seafloor move up
A lot of water…
Seafloor displacement triggers a tsunami wave
A lot of water…
Amplitude growth as
water depth decreases
Two plates are moving past each other at a speed
of 1 cm/yr along a major seismic fault.
The last major earthquake on that fault occurred
in 2000 and resulted in 2 m of instantaneous
displacement (magnitude 7).
When is the next earthquake of similar size due?
A.
B.
C.
D.
E.
2020
2050
2200
There is no solution to the problem
I don’t know.
Magma in subduction zones:
• Produced thanks to dehydration of oceanic crust that
lowers melting point of mantle rocks
• Very viscous (high silica content) and contains large
amounts of dissolved water.
As magma rises (because more buoyant than surrounding
rocks), pressure decreases => dissolved water form steam
bubbles.
• Low magma viscosity => bubbles float to top of
magma, then released into atmosphere
(degassing) => magma reaches surface with low
gas content
• High magma viscosity => bubbles are trapped
inside magma as it rises => magma reaches the
surface with high gas content at high pressure
w.r.t. atmospheric pressure => explosion, release
of large amounts of gas at once
Subduction zones are associated with large explosive volcanoes
The 1991 Mount Pinatubo Eruption, Philippines
Quantity of aerosols before (top) and following the June 15, 1991,
Mount Pinatubo eruption (blue = less, red - more).
•
Between June 12 and June 16, 1991, Pinatubo released 30 Mt of
sulfur dioxide into the atmosphere => converted into very fine
particulates (sulfate aerosols).
•
Aerosols scatter and absorb incoming sunlight => cooler
temperature worldwide.
Explosive volcanoes affect
climate
The oldest piece of oceanic floor is ~180 My old
while the age of Earth is ~4.6 Gy old. This is
because:
A. Oceans are a recent feature (geologically
speaking) on Earth
B. Oceanic lithosphere continuously disappears at
subduction zones
C. We have not yet explored all the oceans and the
oldest piece of ocean has not yet been discovered.
The relative position of continents since ~500 My is known
300 My: close ocean => Appalachians, 220 My: Pangea, 150 My: Gondwana+Laurentia and Thethys ocean, 100-90 My:
Atlantic opens, India takes off, 40-30 My: Thethys ocean closes => Alpine chains
Mount Viso, French Alps
Old seafloor rocks are now found in mountain belts.
Mountain belts result from
the collision of continents:
e.g. Himalayas and Tibet
result from India-Eurasia
collision 40 My ago
radiolarites
Pillow basalts
The 5,000 m-high Tibetan Plateau acts
like a gigantic exposed brick, absorbing
summer heat and heating the atmosphere
above it. Hot air rises, and cool, moist air drawn in from over surrounding oceans rushes in to replace it. That moist air is the
source of monsoon rains
India
Rapid uplift of Tibet starts ~20-17 My ago => strengthen
monsoon:
⇒ Increased weathering => chemical reactions
removes CO2 from atmosphere
⇒ Oceanic upwellings => increase in nutrients =>
increased phytoplancton productivity => increase
burial of organic carbon => decrease of
atmospheric CO2
⇒ Global cooling (growth of Antarctic ice sheets, cf
O2 isotopes)
⇒ Drier climate in Africa => forest replaced by
grassland => decline of great apes and
emergence of Homo?
(details highly debated!)
Plate tectonics influences climate
Absolute sea-level is derived from measurements
of satellite-to-seafloor distance and satellite-toEarth center distance (derived from its orbit)
Global sea level is rising at ~3 mm/yr.
Result of steric effect (volume increase with
temperature, between 0.4 and 1.9 mm/yr) and
eustatic effect (melting of ice sheets)
Space geodesy (satellite altimetry) can measure sealevel changes globally
Take home messages
•
The rigid outer shell of planet Earth
(~100 km thick):
– Constitutes (lithospheric) plates
– Floats on top of a viscous mantle
– Moves at speeds on the order of a
few mm to cm per year.
•
Plate tectonics links with climate
because it is responsible for:
– The formation of major topographic
features (continents/oceans,
mountains/lowlands)
– Explosive volcanism.
•
Space geodesy allows us to
measure:
– Current motions of tectonic plates
– Current sea-level change
– Shape and rotation of the Earth,
function of climate
Continental drift in action