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A Gondwana Rodinia Nuna Kenor Granulite facies 2000
UHT metamorphism 1500
4000 Pangea
Eclogite-HP granulite
3000 metamorphism

2000 HP-UHP Gradient (oC/GPa)
change in scale metamorphism
1500
Number 1000

1000

500

500 Zircon record (n = 100,445)

B0 0
80 Modern passive margins 100%
60 80% Normalized 87Sr/86Sr curve of
40 river runoff vs. mantle influence
change in scale 60%

20 40%

Number 15 Seawater 20%
Passive margins
10

5

C 0 0%
15 8
7
Mean εHf (t) in detrital zircon 10 Oxygen 6

5 δ18O(‰) zircon
0

-5 5

-10 Hf 4

-15 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
0
Age (Ga)

Figure 1. (A) Histogram of more than 100,000 detrital zircon analyses showing several peaks in their U-Pb crystallization ages
over the course of Earth’s history (Voice et al., 2011), which are very similar to the ages of supercontinents. Also shown is the
apparent thermal gradient versus age of peak metamorphism for the three main types of granulite-facies metamorphic belts
(Brown, 2007, 2014). UHT—ultrahigh temperature; HP—high pressure; UHP—ultrahigh pressure. (B) Histogram of
the ages of ancient and modern passive margins (Bradley, 2008) and normalized seawater 87Sr/86Sr curve (Shields, 2007).
(C) Running mean of initial �Hf in ~7000 detrital zircons from recent sediments (Cawood et al., 2013) and moving average
distilled from compilation of ~3300 �18O analyses of zircon versus U-Pb age (Spencer et al., 2014).

THE GEOLOGICAL RECORD distributed about a series of peaks and troughs (Fig. 1; and see GSA TODAY | www.geosociety.org/gsatoday
also Bradley, 2011). The record is incomplete (e.g., Holmes, 1965;
Earth’s history is one of continuous activity in the generation of Hutton, 1788; Raup, 1972), and incomplete records tend to result
igneous, metamorphic, and sedimentary rocks. Yet in terms of in biases, not least because the preservation of the records of
stratigraphy, most of geological time is represented by gaps (in different environments and tectonic settings is highly variable.
unconformities and disconformities) and not by the rocks them-
selves. Moreover, the geological record is only what remains— The implications of bias are well established in the discussions
what is preserved. Few rocks from the oceans survive, and of the fossil record (e.g., Smith and McGowan, 2007). Particular
preservation is shaped by tectonic setting, the development of sedimentary facies dominate the rock record, and two-thirds of
mountain belts, and erosion. Thus, while geological activity may extant animal species have no hard parts that would lend them-
be continuous, the distribution of different rock units is heteroge- selves to being easily fossilized. Most fossils come from lowland
neous in both space and time; the ages of igneous crystallization, and marine habitats where the conditions for fossilization and
metamorphism, continental margins, and mineralization are preservation are most prevalent, and although only 20% of extant

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