Understanding of the Earth ca. 1700 (the end of the ‘scientific revolution’) Earth proven to be an oblate spheroid (Newton’s predictions, confirmed by

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 supposed 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.

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

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

An Indonesian volcano following a large eruption

Catastrophies are central to ‘Catastrophism’

Mt. Pinatubo, 1991

Catastrophies are central to ‘Catastrophism’

The Catastrophists view of the North Atlantic

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.

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

Mass wasting on an Italian hillside

Gradual processes are central to ‘gradualism’

Cross-bedded sands in the Namibian desert

Cross-bedded sandstone, Arizona

The equivalence of modern and ancient sedimentsis central to ‘uniformitarianism’

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)

Logical principles of stratigraphy - the tools for measuring relative time

(1) 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.

• 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

‘up’ side of younger layer

‘up’ side of older layer

‘down’ side of younger layer

‘down’ side of older layer


(2) Original horizontality: Sedimentary strata are originally deposited in horizontal sheets;

i.e., deposition is sensitive to gravity

• 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

Pre-existing rock = ‘basement’ or ‘bedrock’

oldest

youngest

middle

Deposits

Mud flats on the Trinity river, Texas

Whoever thought up original horizontality was a genius

Dry lake bed, Death Valley



Sand dunes in Namibia

Original horizontality is stupid


Badwater alluvial fan, Death Valley

Stromatolites in Shark Bay, Austrialia



(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

Common types of truncations• Faults• Erosional surfaces• Igneous intrusions (silicate liquids migrating through crust)

2

12

1

Depositional contact between 1 and 2

Fault

i.e., faulting post-dates deposition of unit 2 on unit 1

Compaction faulting in Holocene sands, Pakistan

Worked problem using stratigraphic principles

Siccar Point, ScotlandThe cathedral of Uniformitarianism

Age = Volume of valley

Rate of sediment discharge

Charles Darwin and the age of the Chalk Downs (central England)

~ 45 billion yearsi.e., the Chalk Downs in central England took 4x the currently-understood

age of the Universe to be carved by erosion

Measure from topography

Collect sediment in buckets hungIn stream

Unformitarianism trips on its own shoe laces: ‘measurements’ of absolute age

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

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

This debate can still be found in a newspaper or web site near you

A canyon in six days!"that most complex of godless movements spawned by the pervasive and powerful system of evolutionary uniformitarianism"

Lord Kelvin’s response to uniformitarianism+catastrophism

• 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

Lord Kelvin looking into a box

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.


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


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

Case 2: Sun’s accretion, continued:

Age ≤ 0.5MsxV2

3.8x1026 J/s

Age ≤ 1015 s ~ 30 Million years

Potential energy = -GMimj

Rji

Total mass M at center-of-masslocation, i

Component particle mass mat 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 equalto the escape velocity from the Sun today:

(plus any contained in rotationor other motion of cloud)

V = (2GMs/R)0.5 = 618 km/s

i0.5miv2 = 0.5Ms(6.18x105)2


Take 3: directly determine age of the Earth by inverting the conductive temperature profile observed in its outer few km of crust

Measurements from a geothermal area in Iceland The archetype for the outer 300 km of the Earth

dT/dz ~ 1˚/40 meters, on average, near Earth’s surface


Take 3: directly determine age of the Earth by inverting the conductive temperature profile observed in its outer few km of crust

Q.E.D.: Physicists rule; geologists drool

T (˚C)

Radial distance

1500

‘pinned’ by radiative balanceof surface

t0t1

t2

0

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

Melting point of rock

Documents

Understanding of the Earth ca. 1700 (the end of the ‘scientific revolution’) Earth proven to be an oblate spheroid (Newton’s predictions, confirmed by