The Hadean Eon: Before the Rock Record

Hadean Eon

James Hutton, the 18th-century Scottish geologist who was the first to provide convincing evidence that the Earth was very old, could not measure Earth’s age directly, and indeed speculated that there may be “no vestige of a beginning.” But isotopic dating studies conducted in recent decades have shown that it is possible, in fact, to assign a numerical age to our planet’s formation. Specifically, dates obtained for a class of meteorites thought to be remnants of the planetesimal cloud out of which the Earth formed yield an age of 4.57 Ga. Geologists currently take this age to be the Earth’s birth date. But the oldest whole rock yet found is only 4.03 Ga, and a clear record of Earth history, as recorded in continental crustal rocks, does not begin until about 3.85 Ga. Geologists refer to the mysterious time interval between the birth of Earth and 3.85 Ga as the Hadean Eon (from Hades, the Greek god of the underworld) because during this interval the Earth’s surface was, at times, like an inferno. Many major events happened during the Hadean. By about 4.5 Ga, the Earth underwent internal  differentiation gravity pulled molten iron down to the center of the Earth, where it accumulated to form the core, leaving a mantle composed of ultramafic rock. Researchers suggest that soon after or perhaps during differentiation, a Mars-sized protoplanet collided with the Earth. The energy of this collision blasted away a significant fraction of Earth’s mantle; the resulting debris formed a ring of silicate-rock debris orbiting the Earth. This ring then coalesced to form the Moon, which, when first formed, was less than 20,000 km away. (By comparison, the Moon is 384,000 km from Earth today.)

This speculative painting depicts the Hadean Earth's surface as a magma ocean pummeled by meteorites. The Moon was much closer then, but might not have been visible through the dense atmosphere.
In the wake of differentiation and Moon formation, the Earth was so hot that much of its surface was an ocean of seething magma (figure above). Rafts of solid rock formed temporarily on the surface of the magma ocean, but these eventually sank and remelted. This stage lasted at least until about 4.4 Ga. After that time, the amount of radioactive heat generation decreased (because elements with short half-lives had decayed), so the Earth might have become cool enough for solid rocks to form at its surface. The evidence for this statement comes from western Australia, where geologists have found 4.4-Ga grains of a durable mineral called zircon. During the Hadean Eon, outgassing of the Earth’s mantle began to take place. This means that volatile (gassy) elements or compounds originally incorporated in mantle minerals were released and bubbled out of volcanoes, along with lava. The gases accumulated into a toxic atmosphere of water (H2O), methane (CH4), ammonia (NH3), hydrogen (H2), nitrogen (N2), carbon dioxide (CO2), sulfur dioxide (SO2), and other gases. Some researchers speculate that gases from comets colliding with Earth may have contributed additional gases to the early atmosphere. If the Hadean Earth’s surface was sufficiently cool for an extensive solid crustal rock to form beginning at 4.4 Ga, then the first oceans may have accumulated soon thereafter, when water in the atmosphere condensed and fell as rain. Though mineral grains as old as 4.4 Ga exist, the oldest whole rock yet found on Earth has an age of only 4.03 Ga, and most rocks are younger than 3.85 Ga. What destroyed most, if not all, of the pre-3.85-Ga rock (and oceans, if they existed) of the Earth? The answer comes from studies of cratering on the Moon. These studies suggest that the Moon and, therefore, all inner planets of the Solar System underwent intense meteor bombardment between 4.0 and 3.85 Ga. Researchers speculate that this bombardment would have pulverized and/ or melted almost all crust that had existed on Earth at the time, and would have destroyed the existing atmosphere and ocean. Only after the bombardment ceased could long-lasting crust, atmosphere, and oceans begin to form. The discovery of 3.85-Ga marine sedimentary rocks in Greenland suggests that the appearance of land and sea happened quite soon after bombardment ceased. What did the Earth’s surface look like at 3.85 Ga? An observer probably would have found small, barren landmasses, spotted with volcanoes, poking up above an acidic sea. But both land and sea would have been obscured by murky, dense (CO2- and SO2-rich) air.
Credits: Stephen Marshak (Essentials of Geology)
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