Historical Geology Chapter Vocabulary
Chapter 8-Precambrian-Archeon Eon
based upon Historical Geology-by Wicander and Monroe


The ARCHEON eon represents nearly 46% of Earth's geologic history from 4.6 bya to 2.5 bya.
It is the earliest portion of the Precambrian.

The oldest rocks found on Earth are 3.96 billion years old.  The very earliest portion of the Earth's history has
no remaining evidence.

Eoarchean goes from 4.6 bya to 3.6 bya.
This period was dominated by large meteorite impacts.  The early Earth might have resembled our Moon.
The early Earth rotated faster (a 10 hour day) and the Moon was much closer.

Earth's original crust was probably basaltic (MAFIC).
Weathering of surface rocks  and island arcs tended to increase silica content.
The original continents started out quite small but slowly grew larger (CONTINENTAL ACCRETION).

Every continent today has a large area of exposed Precambrian rock (called a PRECAMBRIAN SHIELD).
Precambrian rocks underlie Paleozoic strata (platforms).
Platforms and shields make up CRATONS.

The Precambrian shield in North America is the CANADIAN SHIELD (mostly exposed in Canada).
It is made of smaller bodies of land that have been squeezed together. 
We can see the rocks because of non-deposition and erosion by glaciers.

Fig. 8.5 shows the distribution of Precambrian rocks.

Rock types found in Precambrian shields
GRANITE-GNEISS complexes mostly (intrusions in island arcs?)
GREENSTONE BELTS interesting (greenish color due to chlorite)-pillow lavas present
(rocks formed underwater-ocean floor caught and squeezed between small continents
during accretion (Figure 8.9)? or due to rifting (Figure 8.10)?)

By the end of the Archean, 30% to 40% of present continental crust existed.

Atmosphere and Hydrosphere
Today: 78% Nitrogen, 21% Oxygen, 0.04% Carbon Dioxide, 1% Argon
Original Atmosphere: Hydrogen and Helium-blown away by SOLAR WIND
DEGASSING of Earth released Carbon Dioxide, Water, Hydrogen, Sulfur, Chlorine (but no Oxygen)
(comets may have also contributed Water, Methane, Ammonia)
Oxygen formed later by DISSOCIATION and PHOTOSYNTHESIS

Origin of Life
chemical evidence in 3.8 billion year old rocks
fossil evidence in 3.3 to 3.5 billion year old rocks
only bacteria and archaea are known from Archean rocks.
ABIOGENESIS (how life was created from non-living materials)

p. 160 experiment by Stanley Miller: creation of amino acids by electrical spark  with early gases.
Some proteinoids can be created by heating amino acids (POLYMERIZATION)
Proteinoids aggregate uinto microspheroids with cell-like membranes.

Submarine hydrothermal vents (smokers) may have been influential in the formation of life.

PROKARYOTE CELLS-lack a sack around the genetic material (no nucleus)
EUKARYOTE CELLS (now most common) have a nucleus

STROMATOLITES -3.5 billion years ago (slime on a rock)

ANAEROBIC (life that does not use free oxygen)
HETEROTROPHIC (use external materials for food) vs. AUTOTROPHIC (create food-plants)

Mineral Resources
gold, silver. copper, zinc, nickel, cobalt, chromium