Understanding of the Earth ca. 1700 (the end of the ‘scientific revolution’)

1. Understanding of the Earth ca. 1700 (the end of the ‘scientific revolution’) • Earth proven to be an oblate spheroid (Newton’s predictions, confirmed by French geographers). Thus, earth is a fluid at large scale • Earth density ~ 5 g/cc; moment of inertia less than for a homogeneous sphere • Existence and approximate locations of nearly all major land masses known • Liebniz distinguishes igneous from sedimentary rock and postulates their origins. Notes some special process needed to place sediments at high altitude in mountain belts • 1st-order dichotomy of land vs. sea floor noted as important clue to earth structure and history • Interior of the earth is inferred to be hot These facts are the foundation of our understanding of the earth’s structure and dynamic behavior, but 18th century scientists failed to grasp the big picture, leaving geology far behind physics, chemistry and parts of biology. Why? Time is the key to earth science, and it was deeply misunderstood.

2. Natural Philosophy (‘catastrophism’ to its detractors) Thomas Burnet; 1680’s • History has a knowable beginning and end • Natural history is made up of a sequence of unique events • These events follow natural laws, although most interesting events were unusually grand in scale • Scientists can unravel all of Earth’s history based on a correct reading of an extant record. • That record is the Old Testament. Key observations for Natural Philosophy • Distinction between continents and oceans • Water can ‘create’ and ‘destroy’ land (deposition and erosion) • There are lithified sediments on high mountains • The bible tells us of a great flood

3. Natural Philosophers were also called ‘Neptunists’ • Water moving across the surface of the earth is the agent of all significant geological change • Water will eventually wear down the highs and fill in the lows; thus history is an arrow pointing toward a flat, static earth A wave-carved British coastline

4. Catastrophies are central to ‘Catastrophism’ An Indonesian volcano following a large eruption

5. Catastrophies are central to ‘Catastrophism’ Mt. Pinatubo, 1991

6. The Catastrophists view of the North Atlantic

7. Actualism, or Gradualism (‘uniformitarianism’, originally to its foes; now to all) James Hutton, 1780; Charles Lyell, 1830 • ‘No vestige of a beginning, no prospect of an end’ • History is comprised of an effectively endless number of similar cycles • The processes that drive those cycles are subtle but can be seen all around you with careful observation • The ‘ghost in the machine’ that keeps things from running down is unseen heat and movement at depth (made to respond to thermodynamicists and their pesky third law) • A trained eye can understand the workings of these cycles by observing the ‘geological’ record. 

8. Uniformitarianists were also called ‘plutonists’ • Molten rock, steam and thermal expansion/contraction are agents of geological change • History is a cycle, with processes of mountain building and erosion renewed from below

9. Gradual processes are central to ‘gradualism’ Mass wasting on an Italian hillside

10. The equivalence of modern and ancient sediments is central to ‘uniformitarianism’ Cross-bedded sands in the Namibian desert Cross-bedded sandstone, Arizona

11. Out of place; use much later in lecture, just before Kelvin Uniformitarianism is usually described as the ‘winner’ of this debate, but it contained some ideas we now find bizarre: • Uniformity of natural law (e.g., gravity; this one’s o.k.) • Uniformity of process (nothing has ever happened that can’t be seen happening in the world today) • Uniformity of rate (‘gradualism’) • Uniformity of state; non-progression (I.e., the world has always looked more or less like it does today)

12. Logical principles of stratigraphy - the tools for measuring relative time Steno’s law: When a layer of sediment or lava is deposited, it has an ‘up’ and a ‘down’ side. Any younger sediment or lava deposited on a pre-existing, older one must be laid down with the younger one’s ‘down’ surface contacting the older one’s ‘up’ surface. ‘up’ side of younger layer ‘down’ side of younger layer ‘up’ side of older layer ‘down’ side of older layer • This is an obvious logical necessity for any simple depositional process • May be wrong for some igneous rocks (melt) or precipitates from fluids, which might be injected between or across pre-existing layers

13. Logical principles of stratigraphy - the tools for measuring relative time (2) Original horizontality: Sedimentary strata are originally deposited in horizontal sheets; i.e., deposition is sensitive to gravity Deposits youngest middle oldest Pre-existing rock = ‘basement’ or ‘bedrock’ • This is just a good idea, not a logical necessity • Many high-energy environments (sand dunes, gravel fans at the mouths of canyons) violate original horizontality

14. Whoever thought up original horizontality was a genius Mud flats on the Trinity river, Texas

15. Whoever thought up original horizontality was a genius Dry lake bed, Death Valley

16. Whoever thought up original horizontality was a genius

17. Original horizontality is stupid Sand dunes in Namibia

18. Original horizontality is stupid Badwater alluvial fan, Death Valley

19. Original horizontality is stupid Stromatolites in Shark Bay, Austrialia

20. Logical principles of stratigraphy - the tools for measuring relative time (3) Truncation rule (‘cross-cutting relations’): Geometric truncation of one planar or linear geologic feature (e.g., a sedimentary bed) by another generally indicates that the truncated feature is older Fault 2 2 1 Depositional contact between 1 and 2 1 i.e., faulting post-dates deposition of unit 2 on unit 1 Common types of truncations • Faults • Erosional surfaces • Igneous intrusions (silicate liquids migrating through crust)

21. Compaction faulting in Holocene sands, Pakistan

22. Worked problem using stratigraphic principles

23. Siccar Point, Scotland The cathedral of Uniformitarianism

25. Unformitarianism trips on its own shoe laces: ‘measurements’ of absolute age Charles Darwin and the age of the Chalk Downs (central England) Measure from topography Volume of valley Rate of sediment discharge Age = Collect sediment in buckets hung In stream ~ 45 billion years i.e., the Chalk Downs in central England took 4x the currently-understood age of the Universe to be carved by erosion

26. The ambiguity of landforms: uniformitarianists and catastrophists each have their ‘own’ valleys

27. This debate can still be found in a newspaper or web site near you "that most complex of godless movements spawned by the pervasive and powerful system of evolutionary uniformitarianism" A canyon in six days!

28. Lord Kelvin’s response to uniformitarianism+catastrophism Lord Kelvin looking into a box • First quantitative estimates of the ages of celestial objects based on ‘modern’ physical theory (I.e., Newtonian physics, thermodynamics, Fick’s laws and the kinetic theory of gases). • Engaged a mature scientific community and discredited ‘lax’ logic of Uniformitarian dating • Arguments of this kind are still made to date astrophysical events, processes on other planets, and poorly sampled geologic events

29. Lord Kelvin’s measurement of the age of the earth Take 1: a proof was presented in his Ph.D. thesis, but he burned his writings on this work after his thesis defense. It has never been recovered or reproduced.

30. Lord Kelvin’s measurement of the age of the earth Take 2: determine the age of the Sun using principles of gravitation and thermodynamics; infer this to be the maximum age of the Earth. I: Measure flux of energy at earth’s surface (best above atmosphere directly facing sun) =1340 Js-1m-2 II: Integrate over area of a sphere with radius equal to distance from earth to sun (assumes sun emits energy isotropically) area = 4π(1.5x1011)2; power = 3.8x1026 Js-1 If dJ/dt is a constant: (dJ/dt)xAge ≤ mass of sun x initial energy content (‘E’, in J/Kg)) Age ≤ (2x1030 Kg)/(3.8x1026) x E Age ≤ 5000 x E

31. Lord Kelvin’s measurement of the age of the earth Take 2, continued: Age of sun ≤ 5000 x initial energy content of sun in J/Kg Case 1: If sun’s radiance is driven by a chemical reaction, like combustion, then it’s highest plausible initial energy content is ~ 5x107 J/Kg If the sun is a ball of gasoline, it is ≤ 2.5x1011 s, or 8000 years, old Case 2: Sun’s radiance is dissipating heat derived from its initial accretion: Potential energy of pre-accretion cloud… converts to kinetic energy when cloud collapses… turns into heat if collisions between accreting material are inelastic

32. Case 2: Sun’s accretion, continued: Total mass M at center-of-mass location, i -GMimj Rji Potential energy =  (plus any contained in rotation or other motion of cloud) Component particle mass m at location j Rji Solution depends on the distribution of mass and velocity in the cloud before its collapse to form the sun One simple solution supposes all constituent masses arrived at the sun with a velocity equal to the escape velocity from the Sun today: V = (2GMs/R)0.5 = 618 km/s i0.5miv2 = 0.5Ms(6.18x105)2 0.5MsxV2 3.8x1026 J/s Age ≤ Age ≤ 1015 s ~ 30 Million years

33. Lord Kelvin’s measurement of the age of the earth Take 3: directly determine age of the Earth by inverting the conductive temperature profile observed in its outer few km of crust The archetype for the outer 300 km of the Earth Measurements from a geothermal area in Iceland dT/dz ~ 1˚/40 meters, on average, near Earth’s surface

34. Lord Kelvin’s measurement of the age of the earth Take 3: directly determine age of the Earth by inverting the conductive temperature profile observed in its outer few km of crust Melting point of rock 1500 t1 t0 t2 T (˚C) ‘pinned’ by radiative balance of surface 0 Radial distance dT/dt = k d2T/dx2 k = thermal diffusivity ~ 5x10-3 cm2/s (= ‘conductivity’/(densityxCv)) Solution not simple, but is approximated by x = (kt)0.5, where x = distance from surface to mid-point in T profile. x ~ 30 km; t ~ 20 million years Q.E.D.: Physicists rule; geologists drool