Earth 1
The Earth
Origin, Evolution and Cycles
Chapters 16 & 17
Outline
I.
II.
III.
IV.
V.
Evolution of Earth and Solar System (Hadean Earth)
a. Nebular Hypothesis/Theory
b. Formation of the Earth per se
c. Origin of the Moon
d. Close of the Hadean – a Transition to the Archean
e. Structure of the Solid Earth – Differentiation
f. Evolution and Structure of the Atmosphere
g. Earth’s Internal Heat Engine
Earth/Plate Tectonics
a.
Dynamic and Catastrophic Process/Events on the Earth’s Surface
Cycles
Earth Science’s Tool Bag
Take Home Messages
1. Early evolution of the solar system: the Nebular Hypothesis
2. Earth’s internal structure is largely a function of early molten state followed by slow
cooling leading to a very distinct pattern of differentiation
3. The chemical structure of the Moon is very comparable with that of the Earth’s
mantle, which suggests that the Moon originated from a “big splash”
4. As the Earth cooled, the upper crust stabilized into protocontinents interspersed with
liquid water, and the origins of life followed soon thereafter
5. The tool bag for investigation the structure and dynamics of the Earth is an
interesting story in itself and shows how multiple techniques and observations can
clearly resolve between competing proposals and help address pressing applied
problems
6. The transition between the Hadean to Archean is a very critical period in the
evolution of the Earth
7. The story behind the acceptance of the theory of plate tectonics is an interesting
example of how science proceeds in a revolutionary sense
8. The theory of continental drift has several key pieces of evidence including the
geological “fit” of the continents, fossils, rock type and structure, paleoclimate
and paleomagnetism
9. The principle of seafloor spreading and convection cells is integral to understanding
the major processes of plate tectonics
10. Cycles on the earth are a re-occurring phenomenon and play a critical role; the
features of cycles are important in understanding the dynamics of the
atmosphere, hydrosphere (oceans), biosphere and geosphere
Earth 2
I. Evolution of Earth and Solar System (Hadean Earth)
A. Nebular Hypothesis/Theory
Cloud of dust and gas collected in solar system – 99% H and He
Gravity: nebulae slowly collapsed and began to spin
As spinning/rotation became faster, two processes emerged
1. flat disk in outer part of nebulae (like a pizza)
2. planets all gravitationally form on same plane with precursor of sun in
center (99% of the nebulae is sun)
Planetismals = accumulation of mass resulting in gravity centers and further
accumulation of mass
Temperature profile in nebulae
Inner set of planetismals where temperature affected evolution of planets –
terrestrial planets – (Mars, Venus, Earth and Mercury)…solid planets
Outer set of planetismals where temperature did not play a role in evolution of
planet –called the Jovian planets – (Jupiter, Uranus, Saturn, Neptune and
Pluto)
Composition is the same as that of the original nebulae (H and He)
4.58 Billion years ago to ~4.0 Billion – Hadean Era (Greek for “hell”)
Extremely hot, widespread volcanisms, multiple debris impacts (same for Earth,
Mars and Venus)
B. Formation of the Earth per se
Mechanisms
1. Gravitational attraction of planetismals
2 x 107 g debris/y now, so 4 B years ago it was probably orders of
magnitude higher)
2. Kinetic energy + mass + gravitational settling + sun’s temperature
Earth largely molten and the consequences were:
1. Dense materials (Fe and Ni) sank to interior
2. Lighter materials rose to the top
This process is called differentiation and the effect is one of layering from the
inside out
Core @ center
3500 Km in diameter
High pressure (so high that Fe and Ni would normally be molten
but the pressure is so great they are solid
At the outer edge of the core, less pressure and FE and Ni are
liquid/molten
Mantle
Depth of 2900 Km
Composed of silica rich minerals plus O, Mg (Magnesium) and Fe
Pressures were high (relative to surface)
Crust
Lightest material of all (mostly Aluminum)
10-70 KM in depth
Earth 3
Atmosphere
0-30 Km in height
Composed of CO2, little O2 and liquid water
C.
Origin of the Moon
Three hypotheses to explain the origin of the moon
1. Captured as a “fly by” from intercellular space
2. Formed at the same time as the earth (part of nebular hypothesis)
3. Collision of interstellar body with earth and subsequent “ejection” of part of
earth’s crust and mantle into orbit around the Earth…”Big Splash Theory”
Data from moon (Apollo Mission) show the following attributes
 Density and composition is very different from the Earth as a whole
 Density and composition are very similar to that of the Earth’s mantle
 Data support the Big Splash theory
D. Close of the Hadean – a Transition to the Archean (~4 B years ago)
Fall off of debris impacts as planets swept most of debris
On Earth, stabilization of liquid water
Build up of crustal fragments (protocontinents)
Note: probably when life first arose
E. Structure of the Solid Earth – Differentiation
Observations of solid Earth
Restricted to drilling of only a few Km into the crust and yet Earth is 6000 KM
Yet we know quite a bit about the structure and composition of the core and
mantle (analogous to using techniques to “decipher” composition of
distant stars and galaxies using other techniques)
How do we know the internal structure?
Use of network of sensors on surface – seismometers (also on the Moon)
Waves from earthquakes travel through the Earth and on the surface
Two wave types
P or compressional waves: travel through solid and liquid minerals
S or shear waves: travel through solid but not liquid
P waves
liquid
S Waves
Example of a block of wood
Example of the Earth
Key is that a liquid core (or mantle) loses the S waves while the P waves pass on
through
Earth 4
Data from seismometers suggest two prominent features
P and S waves behave very differently has they travel through the solid Earth
There are marked physical and chemical transitions that occur at the area
between the (1) crust and mantle and (2) mantle and core
F. Evolution and Structure of the Atmosphere
Early in the Hadean Ear, the atmosphere was largely silicate (Si) vapor (high
temperatures)
As Earth cooled, Si condensed out and formed rock on the crust
Permanent atmosphere resulted from two principal processes
1. Outgasing of gases trapped in the rocks (not unlike outgasing today from
volcanoes) including hydrogen sulfide, CO2 and H2O
2. Meteorites…i.e., addition of gases from interstellar material
Contribution form meteorites thought to be very significant
H2O sufficient to yield all H2O in oceans
Organics and panspermia (origins of life from interstellar material)
H2O, organics, carbon monoxide, CO2 and ammonia (NH3)
As Earth cooled, H2O in atmosphere condensed to form oceans
Stabilization of the Earth’s crust
Transition for Hadean to Archean and oldest known rock (~4 B years) are from
this period (Canadian Shield)
Keys to transition
Growth of continents
Stabilization and liquefaction of H2O
Decreasing rate of impacts of meteorites
Increasingly predictable environment with more hospitable conditions
(more benign)
G. Earth’s Internal Heat Engine
Earth’s temperature increase with depth-geothermal gradient
In crust, temperature increases rapidly @20-300C/Km
Mantle temperature – 12000C
Core-Mantle Interface – 45000C
Core – 67000C
Process generating the heat
1. Radioactive decay
2. Heat from original bombardment during the Archean Era (meteorite impacts)
II. Earth/Plate Tectonics
Unique scientific story (worth exploring as the manner in which scientific progress is
made)
Dominance of crustal antiquity (permanence of the oceans and landforms in
Earth 5
antiquity) and analogy to dried and shriveled fruit
Since 1960’s – vast new data dramatically transformed the discipline of earth
sciences
“Scientific Revolution” comparable to that in physics in the 20-40’s, biology with
Darwin, and in biology with discovery of DNA
Start was 1920’s with idea of mobile Earth, which was distasteful to US scientists
By 19678, revolution had folded together continental drift and sea floor
spreading into a new theory called plate tectonics
A.
Stages and History of the Revolution
1. Continental Drift – Idea ahead of its time
Alfred Wegener –1915 German scientist
Concept of Pangaea as supercontinent (200 M years ago)
Diagram on board (and listen to students giggle)
South
America
Africa
Breaking up of Pangaea into smaller continents
Evidence in support of Wegener’s theory of continental drift
a. Fit of the continents
Remarkable similarity in coastlines of South America and Africa
Draw images
Idea vigorously refuted by US scientists
b. Fossil evidence
Fossilized remains of plants and animals best explained by a land bridge
connecting S America and Africa
Aquatic snaggle-toothed reptile
Found only in western Africa and eastern South America
Fossil fern
Same distribution
c. Rock type and structure
If continental drift is accurate, rock types should show a match between
adjacent land masses
Earth 6
Mountain belts that terminate on one coastline and re-appear on another
coast line (best example is the Appalachians that are comparable in
structure and rock type to mountains in British Isles and
Scandinavia
d. Paleoclimate (define as ancient climate…paleo….climate)
Observation: dramatic climate changes 200-300 M years ago in many
Locations
1. ice sheets covered extensive areas of Southern Hemisphere,
including S. Africa, S. America and Australia
2. glacial till all of the same age on all continents (above)
3. striated rock (grooves in rock) all aligned as of continents
were part of Pangaea
Best explanation in light of all above data sets:
Landmasses fit together as one supercontinent (Pangaea) with S. Africa
centered over the South Pole
Despite compelling data, most US scientists rejected idea until the 1960’s
“utter damn rot” W.B. Scott, American Philosophical Society
Why was the revolution stalled with continental drift?
1. Data in support of the hypothesis
2. Matched by data not in support of the hypothesis (not presented above)
3. No understanding of the Earth’s mechanics (key missing ingredient)
4. No knowledge of the oceans (70% of the surface) and yet no observations
form the ocean floor
2. Revolution Unfolds
A. Continental Drift and Paleomagnetism
Explain term paleomagentism (paleo….magnetism)
Earth’s magnetism
Magnetic field of Earth with north and South Pole
Analogous to bar magnet
BUT not necessarily aligned with geographical areas of today
Magnetism of ancient rocks
Some rocks contain minerals that serve as “fossilized compasses”
e.g., iron rich rock
At specific temperature (580 0C), the effect of the existing
magnetic field of the Earth orients the iron minerals so they
are “frozen” in place when the minerals solidify…they
point to true North at the time of cooling
These rocks are paleomagnetic
Observations on paleomagnetic rocks
1. 1950’s: based on rocks solidified over last 500-M years,
position of pole wandered from Hawaii through Siberia and
Earth 7
on to present location
Two hypotheses:
1. Poles wandered or
2. Continents drifted apart (data supported this
hypothesis)
2. Polar wandering curves from North America and Europe
curves in rock were identical except offset by 240 longitude
Two hypothesis:
1. Two poles migrating in parallel (very unlikely)
2. Two continents were together at one time and
then diverged 240 in longitude
B. Continental Drift and Sea Floor Spreading – 1960’s
Whole host of data poured forth in 1960’s
1. Ocean floor dredging and gage of rocks on ocean floor to 160
M (ocean is a mere babe relative to that of the continental
crust of 4 B)
2. Tectonic activity in deep ocean trenches
3. Global ocean ridges discovered
Mechanistic Understanding of the Dynamics of the Earth’s Surface
offered
SEAFLOOR SPREADING by John Hess at Princeton
Proposed two key parts
1. Ocean ridges
2. Upwelling of molten rock in mantle at the ridges
Discuss significance of having a sound mechanism to underpin the
observations
Mechanism
Ocean
Keys:
1. Convection cells (this is a key and relate to boiling pot)
2. Conveyor belt
3. Origin of molten rock in mantle
Earth 8
4. Disposition of new rock as it cools
5. Deep trenches as new rock subsides
C. Sea Floor Spreading and Geomagnetic Reversal of Ocean Rocks
Discovery of alternating stripes of magnetism (poles) in rocks on ocean
floor using magnetometers dragged behind ships
Mid Oceanic
Ridge
Key to data
1. Stripes of magnetism (normal and reverse polarity)
2. Polarity was a mirror image on either side of the mid-oceanic
ridge
Conclusion: paleomagnetic rock on opposite sides of ridge was
formed at the same time (consistent with sea floor
spreading)
These paleomagnetic data from sea floor are the most convincing data
in support of the theory of sea floor spreading
Recapitulation of a Scientific Revolution
1. Started with observations and an idea by Wegener
2. He formulated a general hypothesis
3. He collected data in support of his hypothesis as others collected data
against his hypothesis
4. Floundering of positions for at least several decades
5. Missing a general theory
6. Technological break through: ability to analyze ocean floor using new
techniques
7. On rush of new data from multiple disciplines
8. Revision of theory
9. Development of a general mechanism for sea floor spreading
D. Modern Theory of Plate Tectonics
1. Merger of continental drift, and sea floor spreading yields plate
Earth 9
tectonics
Key points to modern version:
1. Strong, rigid crust (lithosphere) including uppermost mantle
plus the crust
2. Weaker region of the mantle (asthenosphere) that is molten
(location of convection cells)
3. Lithosphere broken into numerous segments or plates that are in
motion and continually changing sin size and shape
Seven major plates including North America, South
America, Africa, Eurasia, Pacific, Australia and
Antarctica
Key is that plates are NOT synonymous with continents but
include ocean floors as well (e.g., North America
plate runs from Pacific Ocean across North America
and out to mid-Atlantic Ridge)
4. Lithosphere moves very slowly – 5 cm per year
5. Titanic grinding of lithospheric plates generates earthquakes,
volcanoes and mountain ranges
6. Source of energy for movement of plates is ultimately the heat
of the core, which in turn has its principal origin from the
early Archean Era!!!!
2. Structural Features of Plate Tectonics
Plate Boundaries
Divergent boundaries
Plates diverge from one another (mid oceanic ridge)
Convergent Boundary
Plates converge resulting in one plate being
subducted below the other
Transform Fault
Plates converge at an angle and neither plate is
subducted but rather they grind past each
other (shear force)
III. Dynamic and Catastrophic Process/Events on the Earth’s Surface
Plate tectonics explains the global distribution of earthquakes (e.g., Pacific Rim)
Discuss subduction zones and origin of magma
Absence of quakes along mid-oceanic ridges
Note that text does a good job of describing dynamics of surface process and
recommend that the students read that material
IV. Cycles
V. Earth Science’s Tool Bag
Earth 10