Earth Science 13.1 Precambrian Time
Earth’s
History:
Precambrian
Time
Precambrian Earth
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Geologists travel to Isua,
Greenland to study some of
Earth’s oldest rocks.
Radiometric dating shows that the
rocks of Isua greenstone belt are
about 3.8 billion years old.
Geologists think that the Isua
greenstone belt began as volcanic
rock that formed underwater.
Over billions of years, heat and
pressure changed the rock to
greenstone, a metamorphic rock.
Precambrian Earth
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Geologists know little about early
Precambrian time. Most rocks
from this time have been eroded
away, subducted, or greatly
metamorphosed.
Relative dating of Precambrian
rocks is difficult because the
rocks rarely contain fossils. Yet
key geological events occurred
during Precambrian time.
Earth formed about 4.56 billion
years ago. During Precambrian
time (4.56 billion to 542 million),
the atmosphere and oceans
formed and plate tectonics began
to build up continental landmasses.
Earth forms
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Scientists hypothesize that
Earth formed as gravity pulled
together dust, rock, and ice in
space.
As Earth grew, it’s gravity
increased, pulling in more of
these materials.
The high-velocity impact of
rocks from space caused the
planet to melt
Radioactive decay was also
generating much more heat
than at the present time.
Earth forms
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Melting permitted the most
dense materials, such as iron
and nickel, to sink towards
Earth’s center.
Less dense materials, such as
silicates, floated, forming
layers that became the mantle
and crust.
Over several hundred million
years, the crust and mantle
cooled and hardened , forming
rock.
The Atmosphere Evolves
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Today, the air you breath is a
stable mixture of nitrogen,
oxygen, a small amount of argon,
and trace gases like carbon
dioxide and water vapor.
Our planets atmosphere several
billion years ago was far
different.
Earth’s original atmosphere was
made up of gases similar to those
released in a volcanic eruption:
water vapor, carbon dioxide,
nitrogen and several trace gases,
but no oxygen.
Scientists think that as earth’s
crust cooled, gases that had been
dissolved in molten rock were
gradually released.
The Atmosphere Evolves
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Some of Earth’s earliest life
forms radically changed the
makeup of Earth’s atmosphere
by using carbon dioxide and
releasing oxygen.
Slowly the atmosphere’s oxygen
content increased.
The Precambrian rock record
suggest that much of this
oxygen combined with iron. Iron
combines with oxygen to form
iron oxides, rust.
Large deposits of iron-rich
Precambrian sedimentary rocks,
called banded iron formations,
provide evidence of this
process.
The Oceans Form
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Earth’s oceans formed as the
planet continued to cool.
The water vapor condensed to
form clouds, and the great rains
began.
At first, the rainwater evaporated
in the hot air before reaching the
ground or quickly boiled or
evaporated when it did reach the
ground.
This evaporation sped up the
cooling of Earth’s surface.
The Oceans Form
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Torrential rains continued and
slowly filled low areas, forming
the oceans.
This reduced not only the water
vapor in the air but also the
amount of carbon dioxide, which
became dissolved in the water.
A nitrogen-rich atmosphere
remained.
Continents develop
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Geologists think that small
continents began to form about
500 million years after Earth’s
formation.
As on Earth today, mantle
convection drove the processes of
plate tectonics.
Tectonic forces caused early
continents to grow by accretion
and to split apart by rifting.
Continents develop
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At the same time, the rock cycle
began.
The early continents were made
up of granitic igneous rock that
was less dense than the volcanic
rock of the oceanic crust.
Weathering and erosion led to the
formation of sedimentary rocks.
Tectonic forces and volcanic
activity in the crust helped to
form metamorphic rock.
Continents develop
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Today, if you view Earth from
space, you will find very little
Precambrian rock left exposed.
As we learned from the Law of
Superposition, most Precambrian
rocks are buried beneath more
recent rocks.
But we can still find Precambrian
rocks exposed where younger rock
layers are extensively eroded.
Examples include the Grand
Canyon and some mountain ranges.
Continents develop
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There are however large core
areas of Precambrian rocks that
make up the surface of some
continents.
These areas are called shields
because they roughly resemble
the a warrior’s shield in shape.
The Canadian Shield forms the
core of the continent of North
America.
Most shields are made up of
deformed metamorphic rock.
Canadian Shield
Precambrian Life
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Most Precambrian fossils are so small
they are called microfossils.
To study microfossils, geologists make
very thin slices of rock that can be
viewed under a microscope.
Using microfossils and other evidence,
geologists have begun to reconstruct
life’s early history.
The earliest life probably evolved in
the oceans.
Precambrian Life
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Later one-celled organisms evolved
that used light energy to produce
food through photosynthesis.
These organisms consisted of simple
cells called prokaryotes.
Slowly, more complex cells, called
eukaryotes, evolved.
Later in the Precambrian, multicelled
organisms with soft bodies developed.
The Earliest Life:
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Scientists do not know when life first
appeared on Earth or what it may have
looked like.
Because water is needed for most life
processes of living things, the earliest
life probably evolved in the oceans.
Some scientists think the first
organisms might have resembled
bacteria found near hydrothermal
vents in the deep ocean.
The Earliest Life:
Photosynthetic Organisms
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Precambrian rocks from about 3.5
billion years ago contain fossils of
one-celled organisms.
These organisms resemble modern
cyanobacteria. Cyanobacteria are onecelled organisms that make their own
food through photosynthesis.
In photosynthesis, organisms use
carbon dioxide and the energy of
sunlight to make food in the form of
hydrocarbons.
Photosynthesis releases oxygen into
the atmosphere.
The Earliest Life:
Photosynthetic Organisms
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Geologists have found traces of
cyanobacteria from Precambrian time.
The fossils are stromatolites.
Stromatolites are layered mounds of
calcium carbonate deposited by
cyanobacteria.
Stromatolites closely resemble similar
deposits made by modern
cyanobacteria. Today, stromatolites
are relatively rare.
They occur in warm, shallow water
along sheltered coastlines. Today,
geologists infer that Precambrian
stromatolites formed in similar
conditions.
stromatolites
The Earliest Life:
Prokaryotes and Eukaryotes:
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Precambrian time, two different
groups of organisms evolved;
prokaryotes and eukaryotes.
Cyanobacteria belong to a group of
simple one-celled organisms called
prokaryotes.
A prokaryote is a cell that lacks a
nucleus.
Earth’s earliest organisms were
prokaryotes.
The Earliest Life:
Prokaryotes and Eukaryotes:
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About 1.8 billion years ago, cells
evolved that were larger and more
complex than prokaryotes.
These cells, called eukaryotes, had
nuclei.
Geologists have found fossil
eukaryotes; red and green algae that
were are about one billion years old.
Many early eukaryotes were onecelled organisms. Unlike prokaryotes,
eukaryotes can also be made up of
many cells.
Eukaryotic fossil
The Earliest Life:
Multicelled Organisms:
 The development of eukaryotes led to
dramatic evolutionary change. Fossil
evidence shows that the earliest animals
had evolved in the late Precambrian
Period.
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In the 1940s, geologists found strange
looking fossils in the Ediacara Hills is
southern Australia.
The fossils were about 600 million years
old. The Ediacara fossils were molds and
casts of soft bodied animals. They
resemble worms, jellyfish, and corals
living in the oceans today.
To e preserved as fossils, these animals
must have been rapidly buried in fine
sediment.