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
Catastrophies are central to ‘Catastrophism’
An Indonesian volcano following a large eruption
Catastrophies are central to ‘Catastrophism’
Mt. Pinatubo, 1991
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
Gradual processes are central to ‘gradualism’
Mass wasting on an Italian hillside
The equivalence of modern and ancient sediments
is central to ‘uniformitarianism’
Cross-bedded sands in the Namibian desert
Cross-bedded sandstone, Arizona
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.
‘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
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
Whoever thought up original horizontality was a genius
Mud flats on the Trinity river, Texas
Whoever thought up original horizontality was a genius
Dry lake bed, Death Valley
Whoever thought up original horizontality was a genius
Original horizontality is stupid
Sand dunes in Namibia
Original horizontality is stupid
Badwater alluvial fan, Death Valley
Original horizontality is stupid
Stromatolites in Shark Bay, Austrialia
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
1
2
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)
Compaction faulting in Holocene sands, Pakistan
Worked problem using
stratigraphic principles
Siccar Point, Scotland
The cathedral of Uniformitarianism
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
Age =
Volume of valley
Rate of sediment discharge
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
The ambiguity of landforms: uniformitarianists and catastrophists each have their ‘own’ valleys
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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!
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
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.
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
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
Case 2: Sun’s accretion, continued:
Total mass M at center-of-mass
location, i
Potential energy =

-GMimj
Rji
(plus any contained in rotation
or other motion of cloud)
Rji
Component particle mass m
at location j
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
Age ≤
0.5MsxV2
3.8x1026 J/s
Age ≤ 1015 s ~ 30 Million years
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
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
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