Geology Reference
In-Depth Information
So it was that post-Theia Earth settled into a turbulent time of cooling and self-organiz-
ation. What was that nascent world like? Geologists have dubbed Earth'sfirst five hundred
million years the Hadean Eon, in recognition of the hellish conditions that must have pre-
vailed. Informed speculation paints a spectacular picture of Earth's Hadean Eon: sulfurous
volcanic exhalations, rivers of glowing lava, and a steady bombardment of asteroids and
comets incessantly disrupted Earth's surface. We are nevertheless severely challenged to
know Earth's first few hundred million years in any detail, for we are completely lacking
in tangible evidence.
Of Earth's origin, we have the rich record of the Solar System—the Sun and the myriad
objectsboundtoitbygravity.Tensofthousandsofmeteoritesprovidethemostintimatein-
terior glimpses of the age of planetesimals. Details of the Moon's origin are found in every
Moon rock and soil. But nothing at all survives from Earth's earliest days, at least nothing
known on Earth itself. Not a fragment of rock nor a grain of mineral.
Remarkably, such evidence might still exist in the form of meteorites that were ejected
from Earth's earliest surface during giant impacts billions of years ago and subsequently
landedbackonEarthoronthenearbyMoon.Suchspecimensmustexist,perhapsinabund-
ance, some virtually unaltered in all that time. Indeed, the quest for Earth's earliest relics
has been cited as one of many scientific rationales for going back to the Moon. Exacting
geological surveysofthelunarsurfacemightbeluckyenoughtofinderrantHadeanstones
and thus reveal truths about Earth's inaccessible past.
But as nice as it would be to hold a piece of Earth's first hardened surface, we aren't
completely stymied. For while Earth has changed over and over again, the laws of chem-
istry and physics have not. Four and a half billion years ago, those laws of chemistry and
physics prevailed, as they always do, but without any more really big thwacks or other
planet-size complications.
Elemental Inevitability
The early evolution of Earth was a consequence of two intertwined chemical realities:
cosmochemistry(makingelements)andpetrochemistry(makingrocks).Firstcamecosmo-
chemistry and the stellar production of all the heavier elements: everything in the periodic
table beyond hydrogen and helium, which are elements one and two in the first row. In
our universe, several of those chemical elements were destined to become dominant: oxy-
gen, silicon, aluminum, magnesium, calcium, and iron far outweigh all other heavy ele-
ments, particularly in the rocky terrestrial planets. Those six elements make up 98 percent
of Earth's mass, as they do the mass of Earth's Moon and of Mercury, Venus, and Mars.
Each of these “big six” elements has a distinctive chemical story to tell. Each, in its own
way, helped make Earth the way it would inevitably be after the Big Thwack. The key is
