DEFINITION OF HADEAN

The first of the four major eons of geological

time: the Hadean, Archean, Proterozoic, and Phanerozoic.

Some time classification schemes use an alternative division

of early time, in which the Hadean is considered the earliest

part of the Archean. As the earliest phase of Earth’s evolution,

ranging from accretion to approximately the age of

first rocks (4.55 to 4.0 Ga [Ga = giga annee, or 109 years]),

it is the least known interval of geologic time. Only a few

mineral grains and rocks have been recognized from this

eon, so most of what we think we know about the Hadean

is based on indirect geochemical evidence, meteorites, and

models.

Although the universe formed about 14 billion years

ago, it was not until 5 Ga ago that the Earth started forming

in a solar nebula, consisting of hot solid and gaseous matter

spinning around a central protosun. As the solar nebula

spun and slowly cooled, the protoearth swept up enough

matter by its gravitational attraction to have formed a small

protoplanet by 4.6 Ga. Materials accreted to the protoearth

as they sequentially solidified out of the cooling solar nebula,

with the high-temperature elements solidifying and

accreting first. The early materials to accrete to the protoplanet

were rich in iron (which forms solids at high temperatures),

whereas the later materials to accrete were rich in H,

He, and Na (which form solids at lower temperatures and

would not accrete until the solar nebula cooled). Heat

released by gravitational condensation, the impact of late

large asteroids, and the decay of short-lived radioactive isotopes

caused the interior of the Earth to melt, perhaps even

forming a magma ocean to a depth of 310 miles (500 km).

This melting allowed dense iron and nickel that was accreted

during condensation of the solar nebula to begin to sink to

the core of the planet, releasing much more heat in the process,

and causing more widespread melting. This early differentiation

of the Earth happened by 4.5 or 4.4 Ga and caused

the initial division of the Earth into layers, including the

dense iron-nickel rich core and the silicate rich mantle. The

outer layer of the Earth was probably a solid crust for most

of this time since it would have cooled by conductive heat

transfer to the atmosphere and space. The composition of

this crust is unknown and controversial, since none of it is

known to be preserved. Some models would have a dense

ultramafic crust (komatiite), whereas others suggest a lighter

anorthositic (made of essentially all plagioclase feldspar)

crust. Still other models suggest that the early crust resembled

modern oceanic crust. In any case, the crust was a conductively

cooled rigid layer capping a hot, convecting

magma ocean. By analogy with magma lakes that form in

calderas such as Hawaii, this crust was probably also moving

with currents in the underlying molten magma and

showing early plate tectonic behavior. On magma lakes and

on the early Earth, the outer crust moves apart at divergent

boundaries, where molten material from below wells up to

fill the open space, then cools to become part of the surface

crust. This crust is broken into numerous rigid plates that

slide past each other along transform faults and converge at

several types of convergent boundaries. Subduction zones

form where the crust of one plate slides below another, and

collision zones form where the two crustal plates deform

each other’s edges. A third type of convergent boundary not

known on the present Earth has been recognized on magma

oceans. In these cases, the crust of both converging plates

sinks back into the magma ocean, forming a deep V-shaped

depression on the surface where they join and sink together.

This early form of plate tectonics would begin to mature as

the magma ocean crystallized, and crustal slabs began to

partially melt yielding buoyant magmas of silicic composition

that would rise, crystallize, and form protocontinents.

These protocontinents would gradually coalesce, the magma

ocean would solidify as the mantle but continue to convect,

and the continental crust would rapidly grow. This style of

plate tectonics led into the Archean.

Between 4.55 Ga and 3.8 Ga, the Earth was bombarded

by meteorites, some large enough to severely disrupt the

surface, vaporize the atmosphere and ocean, and even melt

parts of the mantle. By about 4.5 Ga, it appears as if a

giant impactor, about the size of Mars, hit the protoearth.

This impact ejected a huge amount of material into orbit

around the protoearth, and some undoubtedly escaped. The

impact probably also formed a new magma ocean, vaporized

the early atmosphere and ocean (if present), and

changed the angular momentum of the Earth as it spins and

orbits the Sun. The material in orbit coalesced to form the

Moon, and the Earth-Moon system was born. Although

not certain, this impact model for the origin of the Moon is

the most widely accepted hypothesis, and it explains many

divergent observations. First, the Moon orbits at 5.1° from

the ecliptic plane, whereas the Earth orbits at 23.4° from

the ecliptic, suggesting that some force, such as a collision,

disrupted the angular momentum and rotational parameters

of the Earth-Moon system. The Moon is retreating

from the Earth, resulting in a lengthening of the day by 15

seconds per year, but the Moon has not been closer to the

Earth than 149,129 miles (240,000 km). The Moon is significantly

less dense than the Earth and other terrestrial

planets, being depleted in iron and enriched in aluminum,

titanium, and other related elements. These relationships

suggest that the Moon did not form by accretion from the

solar nebula at its present location in the solar system. The

oxygen isotopes of igneous rocks from the Moon are the

same as from the Earth’s mantle, suggesting a common origin.

The age of the Moon rocks shows that it formed at 4.5

Ga, with some magmatism continuing until 3.1 Ga, consistent

with the impactor hypothesis.

The atmosphere and oceans of the Earth probably

formed from early degassing of the interior by volcanism

within the first 50 million years of Earth history. It is likely

that our present atmosphere is secondary, in that the first or

primary atmosphere would have been vaporized by the late

great impact that formed the Moon, if it survived being

blown away by an intense solar wind when the Sun was in a

T-Tauri stage of evolution. The primary atmosphere would

have been composed of gases left over from accretion, including

primarily hydrogen, helium, methane, and ammonia,

along with nitrogen, argon, and neon. However, since the

atmosphere has much less than the expected amount of these

Title Post:
Rating: 100% based on 99998 ratings. 99 user reviews.
Author:

Terimakasih sudah berkunjung di blog SELAPUTS, Jika ada kritik dan saran silahkan tinggalkan komentar