Download Chapter 7: An Extraordinary Beginning: The Hadean and Archean

Chapter 7 – An Extraordinary Beginning: Hadean and Archean
7.1 Introduction
 Considering the beginning of the universe relates to the question of equilibrium
vs. directional change in Earth history
o The origin of the universe led to the succession of worlds on the evolving
Earth
7.2 Origin of Universe
 Early Observations and Theories
o The first plausible hypothesis for the origin of the universe stems from the
nebular hypothesis of Kant and Laplace, which suggested the origin of the
solar system
o The nebular hypotheses also explained the fact that the planets all move in
the same direction around the sun, that most of the planets rotate in the
same direction as Earth, and that the orbits of the planets are nearly
circular
o A weakness of the nebular hypothesis is that it predicts that the sun should
spin more rapidly than it actually does
 The Big Bang: From Hypothesis to Theory
o Lemaître proposed the “Big Bang” hypothesis, that all the matter in the
universe was initially compressed into a singularity
o Objections to the Big Bang model included its apparent violation of
physical laws (such as the law of gravity); arguments held for a steady
state universe rather than the expanding universe suggested by the Big
Bang
o The astronomer Hubble demonstrated that the universe was actually much
large than previously imagined, based on the Doppler effect (or red shift),
and expanding
o Further astronomical discoveries, such as the radio galaxies, quasars, and
cosmic background radiation supported the Big Bang hypothesis
 The Inflationary Universe
o To understand the origin of the universe according to the Big Bang
hypothesis, one must abandon everyday notions of matter, energy, space
and time
o The Big Bang was not the expansion of matter into space, but the
expansion of what we know as matter, energy, space and time into a foamlike structure
o During the Big Bang, matter moved with space, not through it
7.3 Origin of Matter and Forces of Nature
 In the brief early stages of the Big Bang, the universe was incredibly hot and
infinitesimally small
 After 300,000 years, the universe had cooled and expanded to about the present
size of our galaxy; it was composed of white-hot gas
 After 1 billion years, stars and galaxies were present; the first stars were 100 to
1000 more massive than our sun
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
Today the universe consists of 75% hydrogen and nearly 25% helium; much of
this probably arose during the Big Bang, but heavier elements formed later inside
the early stars
o The process of fusion of atomic nuclei to form heavier elements is
nucleosynthesis
o When these stars ultimately exploded as supernovae, the heavy elements
(such as iron) were distributed throughout their galaxies and beyond
o As old stars dies, newer stars formed that contained higher concentrations
of metals
o The presence of metals in the next generation of stars allowed stars with
smaller masses to form, thus accelerating the rate of star formation
7.4 Formation of the Solar System
 By 10 billion years ago our solar system began to form through a modern version
of the nebular hypothesis
 The solar nebula model is supported by some basic observations
o The major planets lie in or very close to the plane of the ecliptic (the plane
of Earth’s orbit around the sun)
o All the planets rotate counter clockwise in nearly circular orbits
 As the planets formed, they differentiated into two groups
o The inner or terrestrial planets (Mercury, Venus, Earth, and Mars) consist
mostly of silicate minerals and metals, so are much denser
o The outer or Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are
composed of gas and are larger than the terrestrial planets
 As the planets were coalescing, so was the sun
o It reached the temperatures and pressures at which hydrogen fuses to form
helium
o It reached an equilibrium when the outward pressure from the interior
began to counteract the gravitational collapse of matter toward its center
 The asteroids represent remnants of the formation of the solar system
o Most orbit the sun in a belt between Mars and Jupiter
o Jupiter’s gravitational field may have disrupted the planet-forming process
in that region of the solar system
o The asteroids are thus the remains of small planets, or planetesimals;
fragments of them are found as meteorites of various types
o Comets also represent material left over from the formation of the solar
system; they originate in the Oort cloud, far beyond the orbit of the planets
7.5 The Hadean: Origin of Earth and Moon
 Earth’s Earliest Evolution
o The first phase of Earth history is known as the Hadean Eon (named for
the hot condition on Earth)
o Sources of heat included radioactive decay (which continues to drive plate
tectonics), the accretion of matter through gravitational attraction,
bombardment by meteors, and accretion of Earth’s core
o Earth’s metallic core was completed within 100 million years after Earth
began to form
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o As Earth grew, its mantle started to differentiate; a 400-km deep magma
ocean formed due to the intense heat
o During its existence, the magma ocean fed volcanoes that spewed lava
onto Earth’s surface and gases into its atmosphere; this “outgassing”
accumulated in a primitive atmosphere
o The specific composition of the early atmosphere is still controversial, but
it is thought to resemble the present atmospheres of Mars and Venus
 Origin of the Moon
o Moon rocks collected by space missions are as old as 4.5-4.6 billion years,
suggesting that the Moon formed as the early Earth was coalescing and
differentiating
o The lunar highlands consist of anorthosite (rich in plagioclase), and the
maria are huge flood basalts that fill large impact basins
o Early hypotheses for the formation of the Moon included fission,
simultaneous accretion, and capture as possibilities
o The currently favored model is the impact hypothesis, which argues that a
planetesimal sideswiped Earth, and the resulting debris coalesced as the
Moon
7.6 The Archean: Beginnings of a Permanent Crust
 Shields and Cratons: Cores of Continents
o Shields are large areas of igneous and metamorphic rock dating from the
Archean and Proterozoic
o Undeformed sedimentary rocks overlie the rocks of the shield; together the
shield and its sedimentary cover constitute the cores of the continents,
called the cratons
o Continents become enlarged by orogenesis at their margins; this is clearly
seen in the case of North America, but the pattern is less distinct for other
cratons
 Gneiss Terranes
o The Archean rocks of the shields are of two main types: gneiss terranes
and greenstone belts
o The oldest crustal rocks on Earth are 3.9 billion years old (Acasta Gneiss);
still older mineral grains (4.4 billion year old zircons) occur as detrital
grains in younger sedimentary rocks
 Greenstone Belts
o Greenstone belts consist of extrusive igneous rocks followed upward by
sedimentary rocks
o Different hypothesis have been proposed to account for the origin of
greenstone belts (intracontinental rift basin and back-arc basin); it is
possible that greenstone belts may have formed in each of the two settings
 Microplate Tectonics and Differentiation of the Early Crust
o Archean continents probably grew by collision and accretion along
convergent plate boundaries
o Today Earth’s outer shell is composed of a doen large plates, but in the
Archean it may have been composed of 100 or more microplates
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o Eventually the Archean microplates became sutured to produce larger
continents
o The basins between the microplates contain pillow basalts and sediments;
when the protocontinents collided, they were deformed and
metamorphosed to produc the greenstone belts
o Studies of rare earth elements (REEs) support the gradual differentiation
of Earth’s crust through time through such processes as subduction,
melting, and magmatic differentiation; by comparison, the Moon’s crust
indicates little or no differentiation
7.7 Climatic Evolution of the Inner Planets
 Habitable Zone
o The region within the solar system where life may exist is known as the
habitable zone
o If Earth were closer to the sun it might have experienced runaway
greenhouse conditions; if farther from the sun, runaway glaciation
o Earth is the only planet to fall within the habitable zone throughout its
existence
o Venus has probably had runaway greenhouse throughout its existence;
Mars now has runaway glaciation, but may have been warm enough for
ocean to exist early in its history
 Faint Young Sun
o Computer models indicate that early in Earth’s history, the sun was only
about 75% as bright as today
o Nevertheless, there is no evidence for runaway glaciation during Earth’s
first 3 billion years of existence; large amounts of greenhouse gas are
thought to have counteracted the faint young sun
 Weathering and Tectonism on the Inner Planets
o Smaller planets are likely to lose their internal heat faster than larger ones,
and thus not to develop plate tectonics; thus Mercury and Mars (and the
Moon) have rigid, one-plate lithospheres
o Venus is close to Earth’s size, but its surface features indicate that plate
tectonics has largely if not entirely ceased there; its dryness would have
inhibited the melting of plates during subduction
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