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VIZUALIZING EARTH HISTORY
By Loren E. Babcock
Chapter 8
Archean World
Overview of Archean Time
Major geologic and biologic events of the Precambrian.
The “Precambrian,” or the time of the Archean Eon plus the
Proterozoic Eon, was a time in which many crucial events in
Earth’s physical, chemical, and biological evolution took place.
Precambrian - Initial formation of the Earth, solidification of the
crust, development of the cratons (cores of continents), initiation
of plate tectonic activity, formation of the oceans and
atmosphere, the first prokaryotic and eukaryotic life, and early
icehouse-greenhouse cycles including glacial episodes.
The Precambrian rock record is fossil poor.
Overview of Archean Time
The Phanerozoic is better understood than the Archean.
Archean Eon - The unit of geologic time beginning with Earth’s
formation, perhaps 4.56 billion years ago, and ending at the
beginning of the Proterozoic Eon, 2.5 billion years ago.
Proterozoic Eon - The unit of geologic time beginning 2500
million years ago (2.5 billion years ago) and ending at the
beginning of the Phanerozoic Eon, 542 million years ago.
The Archean Eon begins with this planet’s formation,
4.56 billion years ago. The end of the Archean is defined
by the beginning of the next eon, the Proterozoic, a point
arbitrarily chosen as 2.5 billion years ago.
Overview of Archean Time
The Phanerozoic is better understood than the Archean.
Craton - The core of a continent— the part of the Earth’s
continental crust that has attained relative stability and
received little deformation for at least 1 billion years.
Origin of Earth and the Solar System
Using meteorites and moon rocks to decipher
early Earth history
We have access to at least two extraterrestrial sources of
information about the age of Earth: meteorites and Moon
rocks. Meteorites are rocks that have fallen to Earth
(or some other planet or moon) from space.
Meteorite - Relatively small rock that falls
to a planetary surface from interplanetary space.
Origin of Earth and the Solar System
Using meteorites and moon rocks to decipher
early Earth history
Rocks brought back from the lunar surface by US astronauts
during the Apollo missions of the 1960s and 1970s are silicate
rocks of basaltic (mafic) and ultramafic composition.
They are comparable in composition to stony meteorites
(mostly ordinary chondrites) and to Earth’s mantle.
Origin of Earth and the Solar System
Using meteorites and moon rocks to decipher
early Earth history
Radiometric analyses of stony meteorites and Moon
rocks using uranium-lead, uranium-thorium, potassiumargon, and rubidium-strontium dating methods provide ages
that cluster between 4.5 and 4.6 billion years.
Origin of Earth and the Solar System
Birth of the Universe and the Solar System
Astronomers refer to the formation of the universe as the
“big bang.” Initially, all matter is assumed to have been
concentrated at a single point. Upon explosion, matter shot
out in all directions. Eventually gravitational attraction caused
its assembly into galaxies, which are disk-shaped clusters of
stars.
The universe is thought to be 15 to 18 billion years old
based on calculations of the wavelengths of light
radiating from distant stars.
Origin of Earth and the Solar System
Early evolution of the Earth and Moon
The hypotheses for the origin of the Moon most accepted
today is that the moon formed after the Earth was impacted
by a body about the size of Mars.
About 4.5 billion years ago, the magma ocean of the Moon
had solidified to form a crust, and the Earth’s second magma
ocean had solidified to form its second basaltic crust.
Origin of Earth and the Solar System
Describe how the Earth’s core, mantle, and
crust were formed.
The Earth shows layers of differing composition and zones
of differing rock strength. The compositional layers consist
of an iron-nickel-rich core, a silicate-rich mantle, and a silicate
crust. The crust is thicker beneath the continents than it is
under the oceans.
The boundary between the crust and the mantle is called the
Moho (or Mohorovičić discontinuity). Zones of differing rock
strength are the cool, rigid lithosphere (on the outside of the
planet), the hot, more plastic asthenosphere,
and the hot but strong mesosphere.
Origin of Earth and the Solar System
Describe how the Earth’s core, mantle, and
crust were formed.
Origin of the Atmosphere and Ocean
Explain how the Earth’s atmosphere was initially developed.
The early atmosphere must have formed after the Earth had
coalesced and was large enough to retain gases in its
gravitational field. While in the molten state, volatiles
easily escaped to the surface in a process called
outgassing (or degassing).
Outgassing - The process of releasing gases, including
water vapor, from magma.
Origin of the Atmosphere and Ocean
Explain the sources of water for Earth’s early ocean, and
explain the source of salts dissolved in it.
Earth’s outgassing vapors condensed to form liquid
water of the ocean. Comets and volcanoes also
release salt-free water.
The source of the ocean’s salt comes from the chemical
weathering of rocks, particularly on land and at
the shoreline.
Earth’s Oldest Rocks
Early continental crust
The cores of continents are amalgamated from large podlike
rock bodies welded along metamorphic zones called greenstone
belts. The podlike bodies are mostly high-grade metaigneous
rocks representing the felsic crust of Archean protocontinents,
and the greenstones connecting them are metavolcanic and
metasedimentary rocks rich in chlorite, a green mineral formed
under low-grade metamorphic conditions.
Earth’s Oldest Rocks
Early continental crust
Earth’s Oldest Rocks
Greenstone belt - Elongate area within an Archean shield
containing metamorphosed and deformed volcanic and
sedimentary rocks, and characterized by abundant
chlorite-rich greenstone.
Mobile crust phase - Interval of Earth history during
which amalgamation of the continental crust occurred.
The oldest known block of continental crust is
in the Acasta Formation, part of the
Slave Craton, which is now part of northern
Canada (3.8 to 4.0 billion years old).
Earth’s Oldest Rocks
Banded iron formation Sedimentary rock composed of
thin chert (quartz) bands
interlayered with iron oxide
minerals.
One banded iron formation,
from the Itsaq gneiss complexIsua greenstone belt of
Southwest Greenland, was
deposited 3.7 to 3.8 billion years
ago.
Archean Life Forms
The earliest organisms: The prokaryotes.
The earliest fossils are of simple microbes preserved as
molds with carbon in Archean chert. Chert, which is
cryptocrystalline quartz, solidifies from a gel of silica.
Cyanobacteria - Blue-green eubacteria, most of which
are photosynthetic.
Photosynthesis - The process by which plants, algae, and
some bacteria create organic molecules from carbon
dioxide and water using energy from sunlight.
Archean Life Forms
The earliest organisms: The prokaryotes.
Archean Life Forms
The earliest organisms: The prokaryotes.
Autotrophy - “Self-feeding” by means of either harvesting
light energy from the Sun or from oxidation of inorganic
compounds to make organic molecules.
Heterotrophy - A means of obtaining nutrients by ingesting
or breaking down organic matter.
Archean Life Forms
The earliest evidence of life
Stromatolite - A thinly layered biogenic-sedimentary structure
resulting from the trapping and binding of fine sediment
in layers by photosynthetic cyanobacteria.
These thinly layered features are persistent in Archean and
Proterozoic sedimentary environments. The oldest known
stromatolite is from the Fig Tree Group of South Africa, where
they have been dated at about 3.2 billion years.
Archean Life Forms
Abiotic synthesis of amino acids and the earliest life forms
Amino acids, composed of carbon, hydrogen, oxygen,
and nitrogen, are the chemical building blocks of life.
In 1953, Stanley Miller and Harold Urey published results
of a simple laboratory experiment demonstrating that
amino acids could be created under abiotic conditions
without the availability of free oxygen. Their experiment
was designed to imitate the conditions under which
life was thought, in the early 1950s, to have arisen on
Earth more than 3 billion years ago.
Archean Life Forms
Abiotic synthesis of amino acids and the earliest life forms
Archean Life Forms
Abiotic synthesis of amino acids and the earliest life forms
Photochemical dissociation - The splitting of molecules into
their components by means of energy from sunlight or other
light sources.
In order for life to have arisen abiotically, it must have first
developed under anoxic, aqueous conditions. It is possible
that life arrived to this planet aboard a meteorite or a comet.
The more likely option is that life arose on Earth. If so, it must
have evolved in a place where contact with free oxygen of the
atmosphere was limited. This “primordial soup” could have
developed in small isolated ponds, lakes, and a range of
marginal-marine settings.
Archean Life Forms
Simple organic molecules can be created abiotically.
One anoxic environment where amino acids could organize
into more complex genetic and organic materials is along
mid-ocean ridges. Earth’s early crust was probably rich in
spreading centers and subduction zones.
Hydrothermal vent - Opening in the Earth’s crust, usually
associated with magmatic activity, where hot water, often
enriched in ions, is released.
Archean Life Forms
Simple organic molecules can be created abiotically.
Archaebacteria - Organisms belonging to the domain
Archaea, including the methanogenic, halophilic, and
thermoacidophilic prokaryotes.
Some archaebacteria not only survive under the extreme
conditions that exist along mid-ocean ridges, but require
them for their metabolism. They obtain energy through
chemosynthesis, in which naturally occurring chemical
reactions are allowed to occur within their cells.
Archean Life Forms
Does life exist beyond Earth?
Whether life exists elsewhere in the Solar System, and whether
the early Earth may have been “seeded” with life forms arriving
from beyond our planet are questions that remain speculative.
In the search and test for fossilized life forms from elsewhere
in our Solar System or elsewhere in the universe, we can use
the same criteria for meteorites or rocks collected directly from
extraterrestrial bodies as we use in the study of Earth’s earliest
life forms.