THE LAST PAPER OF WARREN B. HAMILTON
Earth’s Archean was the era of internally mobile crust. TTG crust formed by hydrous melting of mafic protocrust leaving dense, depleted, garnet-rich residue that delaminated and sank, beginning re-enrichment of the mantle. Archean granite and greenstone crust has no modern analogue.
Proterozoic dynamics, driven by vertical variations in density, reflect the deposition and collapse of basins. Paleomagnetic data cast doubt on the existence of a magnetic field at that time. Only in the late Proterozoic did a weak asthenosphere develop over which lithospheric plates could slide.
The Phanerozoic has been the era of plate tectonics. Lithosphere motions documented by palaeomagnetism suggest Earth’s magnetic field may have developed at ~600 m.y. The Cambrian explosion in which all modern phyla evolved may have required its shielding against radiation.
In modern Earth, hinges migrate oceanward as subducting slabs sink to 660 km beneath overriding plates. Plate boundaries change radically, incompatible with plume-driven tectonics.
Earth, Mars, Venus and Moon fractionated by 4.5–4.4 b.y. Bombardment saturated surfaces with craters and pools of impact-melted mafic protocrust that fractionated into layered igneous complexes. Venus, Mars and Moon retain impacted surfaces. They lack asthenospheres or liquid cores, and cannot sustain plate tectonics.
Variants of plume theory have been inappropriately exported to other planets. Martian “volcanoes” do not resemble Hawaii, but single-event impacts. Much of Venus’ surface is saturated with rimmed circular basins that would elsewhere be recognized as craters. Moon’s petrology and landscapes rule out a long-lived magma ocean.
Volatiles arrived with icy bombardments starting ~4.0–3.9 b.y. Ocean remnants survived for 2–3 b.y. on Venus, and ~1 b.y. on Mars. Only Earth was internally hot and active enough to circulate volatiles downward enabling hydrous melting, slow re-enrichment of the upper mantle and, at ~600 m.y., plate tectonics and rapid biological evolution.