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2014 GSA PRESIDENTIAL ADDRESS
Figure 5. Geologic map of Vesta, the second-most massive asteroid, based on data from the Dawn orbiter. Modified from Williams et al. (2014).
analyses on rocks in the laboratory, not to mention the geochro-
nology provided by analyses of radiogenic isotopes, strengthens
the characterizations of mapped geologic units and the interpreta-
tions of geologic history.
Figure 6. A portion of a geologic traverse map through the Columbia Hills in WHAT PLANETARY GEOSCIENCE HAS WROUGHT GSA TODAY | www.geosociety.org/gsatoday/
Gusev crater. The yellow line marks the route of the Spirit rover. For a larger
version with a key of geologic units, see Crumpler et al. (2011). The geologic exploration of planetary bodies, along with the
analysis of extraterrestrial samples, has demonstrated that the
tried-and-true tools and methods of geology can be exported to
other worlds. Like Earth, our planetary neighbors are geologic
experiments conducted at a grand scale, but carried out with
different starting compositions and under different physical
conditions. From the study of other bodies, we can test the gener-
ality of the geologic processes we have worked so hard to under-
stand on our own planet. And in some cases, we gain
fundamentally new insights. A few examples are
• The early terrestrial planets, including Earth, had magma
oceans, formed by heat from the decay of short-lived radionu-
clides and collisions with other bodies. Global-scale melting
had profound implications for the differentiation into cores,
mantles, and crusts, and for the geochemical partitioning of
elements required by modern industries that fuel the world’s
economies.
• Plate tectonics dominates terrestrial geology, but Earth’s
moving plates are unique among solar system bodies. One-plate
planets lose their internal heat in novel ways, and stagnant-lid
tectonics allows a bewildering array of geologic structures.
• Magmatism on Earth occurs mostly at plate boundaries, so
melting mechanisms on other planets are different. Basalts,
albeit with distinctive compositions, are ubiquitous on all
rocky bodies, but the pathways and extents of magma evolu-
tion differ, making granitic rocks virtually unrepresented
outside our own planet.
• Impact cratering is the most significant geomorphic process on
other planets and must have been on the early Earth as well.
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