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Kjg Johnson Granite Porphyry
94 Kcp Cathedral Peak Granodiorite 94
Half Dome Granodiorite
Khd
stippled where porphyritic
Kuna Crest Granodiorite (east)
Kkc/Kga
tonalite of Glen Aulin (west)
92 N 92
38˚00’N
10 km Cathedral Half Kuna
Age, 90 Kcp JGP Peak Dome Crest
Ma 90
Kga Kjg

88 Khd 88
Kkc
37˚45’N

86 Glen Half Cathedral Khd 86
Aulin Dome Peak JGP 119˚35’W 119˚15’W
Outer Inner Inner Outer

Figure 2. Summary of precise U-Pb zircon ages for the Tuolumne Intrusive Suite divided between its western (left) and eastern (right) sides. Ages are plotted
at their positions relative to the inner and outer contacts of a given unit. Our ages as of 2001 (white symbols) showed eastward/inward younging of the
western side of the suite; we tested and confirmed this pattern in 2001 by dating samples from the eastern side. Ages determined in other labs since then
(gray symbols) have also confirmed this pattern and the 10-m.y. age span. Symbol sizes roughly indicate precision of ages and placement within each unit.
Ages in gray from summary in Paterson et al. (2016). JGP—Johnson granite porphyry.

results again spanned 10 m.y., and our CELEBRATED CASES WHERE plow through it. The problem lay not in the
minds started to open to the possibility that GEOLOGY GOT THE BEST OF field observations but in the mechanism used
the standard model could not account for PHYSICS (APPARENTLY) to explain them. Development of the plate-
this anomaly. We hypothesized that if the The history of geology involves well- tectonics hypothesis and the recognition that
TIS had been intruded over 10 m.y., then known cases where field interpretations continents are carried by relatively strong
corresponding units on the eastern side that were initially ruled out by physical lithospheric plates reconciled field geology
(Fig. 2) should become younger westward analyses were later shown to be correct. An with physics (the details of the continental
over the same time span. New samples col- example is Kelvin’s (1863) estimate that the drift controversy are much more complex;
lected in 2001 to test this prediction con- Earth is <400 m.y. old, based on a thermal e.g., Oreskes, 1999).
firmed the pattern, as did later U-Pb geo- calculation that assumed conductive heat Field geology triumphed over physics in
chronology by a number of labs (Fig. 2). loss from an initially molten Earth. these cases not because the physics was
The more-precise ages revealed another Compilation of an immense amount of field wrong, but because incorrect physical mod-
failed prediction of the nested-pulse hypoth- evidence and actualistic reasoning about els were used. Once a correct model was
esis: Most age variation occurs within the process rates led geologists to insist that the identified, the conflict evaporated.
plutons rather than between them; i.e., across Earth was much older, and that inference
mapped contacts. For example, dates from was ultimately vindicated by geochronol- WHEN WHAT IS CLEAR TO THE EYE
the Half Dome Granodiorite span ~4 m.y., ogy. England et al. (2007) demonstrated IS NEVERTHELESS UNLIKELY TO
but those from near its margins differ from that the flaw in Kelvin’s argument was the BE TRUE
adjacent TIS units by <1 m.y. These results assumption of conductive heat transfer, and In geology there are many examples where
led to a new hypothesis: Plutons in the TIS that recognition of mantle convection rec- seemingly incontrovertible field interpreta-
were amalgamated from small increments onciled thermal calculations with geochro- tions turn out to be controvertible after all.
whose boundaries are difficult to see nologic evidence that the Earth formed at A few examples follow.
(Coleman et al., 2004; Glazner et al., 2004). ca. 4.5 Ga.
We thus began to explore the implications In another case, the continental-drift Resolved Controversies
of incremental assembly of plutons rather hypothesis neatly explained continuity of In Yellowstone National Park, Iddings
than trying to fit our data into the standard continental geology and paleofaunal prov- (1899, p. 430) examined the complex con-
model. This was challenging because many inces across the Atlantic Ocean and did away tact relations of Pleistocene basalt and rhyo-
of the processes assumed to operate in plu- with the need for sunken land bridges and lite lavas (Fig. 3) and stated, “It is evident…
tonic magma chambers, such as convection, other speculative means of accounting for that the rhyolite fused the basalt.” Fenner
crystal settling, sidewall crystallization, and the evidence. However, physicists had cor- (1938, p. 1458) agreed and stated that “…
stoping, cannot occur at the pluton scale if rectly argued that continental drift, as origi- relations that are so plainly revealed hardly
only small parts of a pluton are substan- nally proposed, was implausible because the permit doubt” of Iddings’s interpretation.
tially molten at any given time. mantle is far too viscous for continents to Fenner knew that this interpretation was

6 GSA TODAY | October 2022