Chapter 13 Precambrian Time Vast and Puzzling Time
Precambrian encompasses immense geological time, from
Earth’s distant beginning 4.56 billion years ago until the
start of the Cambrian period.
1. comprises about 88 % of the geologic time scale.
2. most Precambrian rocks do not contain fossils.
3. many are metamorphic and deformed. Extremely
eroded and hidden by over laying strata.
Earth Forms
 Gravity pulled together dust, rock, and ice in space.
 High velocity impact of rock form space caused the
planet to melt causing iron and nickel to sink toward
Earth’s center.
 Less dense materials such as silicates floated forming
layers (mantle and crust).
 Over several hundred million years the crust and
mantle cooled and hardened forming rock.
Scientists study other planets to give us clues about our
planet.
1. Earth was bombarded by objects from space in its
early history as was Mars and Venus.
2. Volcanic activity is still active on this planet but
no longer active on Mars and Venus.
3. The Earth is still tectonically active but not Mars
or Venus. The cores of the planets Mars and
Venus cooled to the point that plate tectonics
ceased
4. Saturn’s moon, Titan, has an atmosphere with
rain and oceans of liquid methane as well as
water ice.
Earth’s ATM Evolves
1. Our present ATM is a stable mixture of nitrogen,
oxygen, a small amount of argon, and trace gases like
carbon dioxide and water vapor. Scientists think as
Earth’s crust cooled, gases that had been dissolved in
molten rock were gradually released.
2. Earth’s original ATM was made up of gases similar to
those released in volcanic eruptions today-water vapor,
carbon dioxide nitrogen, and several trace gases, but no
oxygen.
a. Planet cools
b. H2 O vapor condense to form clouds
c. Great rains begin
d. Rainwater evaporated in the hot air before
reaching the ground.
e. Evaporation cools Earth’s surface
f. Oceans form from continue Torrential rains.
g. Oceans help reduce the water vapor in air and
amount of CO2 (CO2 dissolved in water)
h. Nitrogen remains in ATM.
3. Oxygen is released in ATM
a. Primitive organisms, primarily Cyanobacteria use
CO2 and release oxygen.
b. Oxygen content in ATM increases.
c. Free oxygen did not remain free because it
combined with the iron in the ATM and in the
rocks to form iron oxides (rust). Once all the iron
has reacted then oxygen begins to accumulate in
the ATM (about 2.5 billion years ago).
4. Precambrian Rocks
a) Rocks are buried deeply and can be seen except
in places like the Grand Canyon and mountain
ranges where rock is extensively eroded.
b) Shields are deformed Precambrian metamorphic
rocks that dominate the surface of some
continents. It is called a shield because they
roughly resemble a warrior’s shield in shape.
c) Fossils
 Precambrian had not yet evolved
 most common are stomatolites which are not
the remains of actual organisms but are
layered mounds or columns of calcium
carbonate deposited by algae.
 Stomatolites are indirect evidence of algae
because they closely resemble deposits
make by modern algae
d) Most actual organisms preserved in Precambian
rock are microscopic
e) Many of the ancient fossils are preserved in
chert-are hard dense chemical sedimentary rock.
These fossils are the most primitive organism
called PROKARYOTES. The most advanced are
called EUKARYOTES and evolved later.
f) Many are Trace fossils that are not fossils of the
animals themselves but rather impression of their
activities, such as trails and worm holes