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 Volcanic activity is still active on this planet but no longer active on Mars and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 (Outgassing). 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).
d. Large deposits of iron-rich Precambrian sedimentary rocks called banded
iron formation provide evidence of our early atmosphere (oxygen in the
atmosphere)
4. Precambrian Rocks
 are buried deeply and can be seen in places like the Grand Canyon and
mountain ranges where rock is extensively eroded.
 make up the bulk of continental crustal which are called shields because it
roughly resemble a warrior’s shield in shape
a) Fossils
 Plants and large animals 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
 In broad terms are fossil evidence of Prokaryotes
b) Most actual organisms preserved in Precambrian rock are microscopic
c) Many of the ancient fossils are preserved in chert (hard dense chemical
sedimentary rock). These fossils are the most primitive organism called
PROKARYOTES. The most advanced are called EUKARYOTES and evolved
later.
d) Many are Trace fossils that are not fossils of the animals themselves but rather
impression of their activities, such as trails and worm holes
e) Much of what we know about Precambrian rocks comes from ores (iron,
nickel&gold) mined from shields