Earth 1

The Earth

Origin, Evolution and Cycles

Chapters 16 & 17

Outline

  1. Evolution of Earth and Solar System (Hadean Earth)
  2. Nebular Hypothesis/Theory
  3. Formation of the Earth per se
  4. Origin of the Moon
  5. Close of the Hadean – a Transition to the Archean
  6. Structure of the Solid Earth – Differentiation
  7. Evolution and Structure of the Atmosphere
  8. Earth’s Internal Heat Engine
  9. Earth/Plate Tectonics
  10. Dynamic and Catastrophic Process/Events on the Earth’s Surface
  11. Cycles
  12. 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

  1. 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”

  1. As the Earth cooled, the upper crust stabilized into protocontinents interspersed with

liquid water, and the origins of life followed soon thereafter

  1. 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

  1. The transition between the Hadean to Archean is a very critical period in the

evolution of the Earth

  1. The story behind the acceptance of the theory of plate tectonics is an interesting

example of how science proceeds in a revolutionary sense

  1. 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

  1. The principle of seafloor spreading and convection cells is integral to understanding

the major processes of plate tectonics

  1. 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

  1. Evolution of Earth and Solar System (Hadean Earth)
  2. 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)

  1. 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)

  1. 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

Atmosphere

0-30 Km in height

Composed of CO2, little O2 and liquid water

  1. 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

  1. 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

  1. 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

Key is that a liquid core (or mantle) loses the S waves while the P waves pass on through

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

  1. 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)

  1. 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)

  1. Earth/PlateTectonics

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

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)

Breaking up of Pangaea into smaller continents

Evidence in support of Wegener’s theory of continental drift

  1. Fit of the continents

Remarkable similarity in coastlines of South America and Africa

Draw images

Idea vigorously refuted by US scientists

  1. 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

  1. Rock type and structure

If continental drift is accurate, rock types should show a match between

adjacent land masses

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

  1. 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

  1. 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

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

  1. 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

Keys:

1. Convection cells (this is a key and relate to boiling pot)

2. Conveyor belt

3. Origin of molten rock in mantle

4. Disposition of new rock as it cools

5. Deep trenches as new rock subsides

  1. 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

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

  1. On rush of new data from multiple disciplines
  2. Revision of theory
  3. 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

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 pasteach

other (shear force)

  1. 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

  1. Cycles
  1. Earth Science’s Tool Bag