Download Formation of the Crust and Continents

The Early Earth
Objectives
• Describe the evidence used to determine the age
of Earth.
• Understand why scientists theorize that the early
Earth was hot.
Vocabulary
– zircon
– asteroid
– meteorite
The Early Earth
Earth’s “Birth”
• For about the first 4 billion years of Earth’s
4.6-billion-year existence, most of the life-forms
that inhabited Earth were unicellular organisms.
• In 1996, the announcement that a meteorite
from Mars might contain microscopic fossils of
bacteria rekindled scientific interest in the search
for life elsewhere in the universe.
• It may be possible to identify clues to the
possible existence of life on other planets
through rocks from those planets.
The Early Earth
Earth’s “Birth”
• There is evidence of life’s
beginnings on Earth in
Precambrian rocks.
• Most of Earth’s history is
contained within the
4 billion years that make
up the Precambrian.
The Early Earth
How old is Earth?
• We know that Earth must be at least as old as the
oldest rocks in the crust.
– The age of the oldest rocks on Earth is between
3.96 to 3.8 billion years.
– Evidence of 4.1- to 4.2-billion-year-old crust exists in the
mineral zircon that is contained in metamorphosed
sedimentary rocks in Australia.
– Zircon is a very stable mineral that commonly occurs in
small amounts in granite.
The Early Earth
How old is Earth?
– Meteorites have been radiometrically dated at
between 4.5 and 4.7 billion years old.
– The oldest rock samples from the Moon are
approximately 4.6 billion years old.
– Scientists commonly agree that the age of Earth is
4.6 billion years.
The Early Earth
Earth’s Heat Sources
• Earth was most likely extremely hot shortly after
it formed, and there were three likely sources of
this heat.
– The first source was radioactivity.
• Radioactive isotopes were more abundant during
the past.
• One product of radioactive decay is energy, which
generates heat.
The Early Earth
Earth’s Heat Sources
– The second source of Earth’s heat was the impact of
asteroids and meteorites.
• Asteroids are metallic or silica-rich objects that are
1 km to 950 km in diameter.
• Meteoroids are small asteroids or fragments
of asteroids.
• Meteorites are meteoroids that fall to Earth.
• Evidence suggests that collisions, which generate
a tremendous amount of thermal energy, were
much more common throughout the early solar
system than they are today.
The Early Earth
Earth’s Heat Sources
– The third source of Earth’s heat was gravitational
contraction.
• As a result of meteor bombardment and subsequent
accumulation of meteorite material on Earth, the
size of Earth increased.
• The weight of the material caused gravitational
contraction of the underlying zones, the energy of
which was converted to thermal energy.
• The new material also caused a blanketing effect,
which prevented the newly generated heat from
escaping.
The Early Earth
Section Assessment
1. How are meteorites evidence of Earth’s age?
The Early Earth
Section Assessment
2. What are were the three likely sources of heat
on Earth shortly after it formed?
The Early Earth
Section Assessment
3. Identify whether the following statements are
true or false.
______ The Precambrian represents about half of
Earth’s existence.
______ The oldest rocks on Earth are between 3.96
and 3.8 billion years.
______ Meteorites are asteroids that fall to Earth.
______ A meteorite could possibly provide evidence of
life on another planet.
Formation of the Crust and Continents
Objectives
• Explain the origin of Earth’s crust.
• Describe the formation of the Archean and
Proterozoic continents.
Vocabulary
– differentiation
– Precambrian shield
– Canadian Shield
– microcontinent
– Laurentia
Formation of the Crust and Continents
Formation of the Crust and Continents
• Early in the formation of Earth, the planet was
molten, and numerous elements and minerals
were mixed throughout the magma.
• Over time, the minerals became concentrated in
specific zones and Earth became layered.
• As the magma reached the surface and cooled,
landmasses began to form.
Formation of the Crust and Continents
Formation of the Crust
• When Earth formed, iron and nickel, which are
dense elements, concentrated in its core.
• Lava flowing from the interior of Earth
concentrated the less-dense minerals near the
surface of Earth over time.
• The denser minerals, which crystallize at higher
temperatures, concentrated deeper within Earth
and formed the rocks that make up Earth’s mantle.
Formation of the Crust and Continents
Formation of the Crust
Differentiation is the
process by which a
planet becomes
internally zoned when
heavy materials sink
toward its center
and lighter materials
accumulate near its
surface.
Formation of the Crust and Continents
Formation of the Crust
Formation of the Crust and Continents
Formation of the Crust
• Earth’s earliest crust most likely formed as a
result of the cooling of the uppermost mantle and
was similar to basalt.
• As sediment-covered slabs of the crust were
recycled into the mantle at subduction zones, the
slabs partly melted and generated magmas with
different mineral compositions.
• These magmas crystallized to form the first
granitic continental crust, which was rich in
feldspar, quartz, and mica.
Formation of the Crust and Continents
Formation of the Crust
• The formation of the majority of crustal rocks was
completed by about 2.5 billion years ago.
• As less-dense material has a tendency to float on
more-dense material, continental crust “floats” on
top of the mantle below it.
• Basaltic crust is more dense than granitic crust,
and therefore, it does not float as high on the
mantle.
Formation of the Crust and Continents
Formation of the Crust
Formation of the Crust and Continents
The Cores of the Continents
A Precambrian shield is a core of Archean and
Proterozoic rock that forms the core of each
continent.
• Buried and exposed parts of a shield together
compose the craton, which is the stable part of a
continent.
The Canadian Shield is the name for the
Precambrian shield in North America because
much of it is exposed in Canada.
Formation of the Crust and Continents
The Cores of the Continents
Formation of the Crust and Continents
Growth of Continents
Microcontinents, which were small pieces of
continental crust that formed during the Archean,
began to collide as a result of plate tectonics early
during the Proterozoic.
• At each of these collision sites, the Archean
microcontinents were sutured or fused together
at orogens.
• These orogens are belts of rocks that were
deformed by the immense energy of the colliding
continents.
Formation of the Crust and Continents
Growth of Continents
Laurentia, the
ancient continent
which was
assembled
1.8 billion years
ago, would
become the core
of modern-day
North America.
Formation of the Crust and Continents
Growth of Continents
• Near the end of the Early Proterozoic, between
1.8 and 1.6 billion years ago, volcanic island arcs
collided with the southern margin of Laurentia.
• The final phase of Proterozoic growth of Laurentia,
the Grenville Orogeny, occurred between 1.2
billion and 900 million years ago.
• By the end of the Proterozoic, nearly 75 percent of
present-day North America had formed.
Formation of the Crust and Continents
Growth of Continents
• By the end of the Proterozoic, all of the major
masses of continental lithosphere had formed.
• As the lithospheric plates moved around, they
periodically collided and sutured together to form
Rodinia, the first supercontinent.
• Rodinia began to break apart at the end of the
Proterozoic and continued to do so during the
Early Phanerozoic.
Formation of the Crust and Continents
Growth of Continents
Formation of the Crust and Continents
Section Assessment
1. Match the following terms with their definitions.
___ differentiation
___ Precambrian shield
___ microcontinent
___ Laurentia
A. the process by which a planet
becomes internally zoned
B. a core of Archean and
Proterozoic rock that forms the
core of continents.
C. the ancient continent that
formed the core of modern-day
North America
D. small pieces of continental
crust that formed during the
Archean
Formation of the Crust and Continents
Section Assessment
2. What is the Canadian Shield?
Formation of the Crust and Continents
Section Assessment
3. Why do the rocks of the earliest crust no
longer exist?
Formation of the Atmosphere and Oceans
Objectives
• Describe the formation of Earth’s atmosphere and oceans.
• Identify the origin of oxygen in the atmosphere.
• Explain the evidence that oxygen existed in the
atmosphere during the Proterozoic.
Vocabulary
– cyanobacteria
– stromatolite
– banded iron formation
– red bed
Formation of the Atmosphere and Oceans
Formation of the Atmosphere and Oceans
• Earth’s early atmosphere was nothing like what
it is today.
• The oxygen that early forms of algae produced
through the process of photosynthesis affected
the development of life on Earth in two very
important ways.
– It changed the composition of the atmosphere and thus
made life possible for oxygen-breathing animals.
– It produced the ozone layer that filters ultraviolet (UV)
radiation.
Formation of the Atmosphere and Oceans
The Precambrian Atmosphere
• Hydrogen and helium probably dominated
Earth’s earliest atmosphere but probably
escaped into space due to their small masses.
• Gases that have greater masses, such as
carbon dioxide and nitrogen, cannot escape
Earth’s gravity.
• Considerable volcanic activity during the Early
Precambrian released tremendous amounts of
gases into the atmosphere through the process
of outgassing.
Formation of the Atmosphere and Oceans
The Precambrian Atmosphere
• The most abundant gases vented from volcanoes
are water vapor (H2O), carbon dioxide (CO2),
nitrogen (N2), and carbon monoxide (CO).
• Many geologists hypothesize that outgassing
formed Earth’s early atmosphere.
• In addition, the early atmosphere most likely
contained methane (CH4) and ammonia (NH3).
• Argon (Ar) also began to accumulate during the
Early Precambrian.
Formation of the Atmosphere and Oceans
Oxygen in the Atmosphere
• There was no oxygen in the atmosphere during
the Precambrian.
The oldest known fossils, which are about 3.5
billion years old, are the remains of tiny,
threadlike chlorophyll-bearing filaments of
cyanobacteria.
• Ancient cyanobacteria used photosynthesis to
produce the nutrients they needed to survive,
giving off oxygen as a waste product.
Formation of the Atmosphere and Oceans
Oxygen in the Atmosphere
Oxygen Producers
– The abundance of cyanobacteria increased throughout
the Archean until they became truly abundant during
the Proterozoic.
– Stromatolites, which are large mats and mounds of
billions of cyanobacteria, dominated the shallow
oceans of the Proterozoic.
Formation of the Atmosphere and Oceans
Oxygen in the Atmosphere
Evidence in the Rocks
– Iron oxides are identified by their red color and provide
undeniable evidence of free oxygen in the atmosphere.
– Evidence indicates that there was little or no free oxygen
in the atmosphere throughout most of the Archean.
– Near the end of the Archean and by the beginning of the
Proterozoic, photosynthesizing stromatolites in shallow
marine water increased oxygen levels in localized areas,
which caused banded iron formations to form.
Formation of the Atmosphere and Oceans
Oxygen in the Atmosphere
Evidence in the Rocks
– Banded iron formations are deposits which consist of
alternating bands of chert and iron oxides.
– Red beds are sedimentary rocks that are younger than
1.8 billion years and rusty red in color.
– The presence of red beds in rocks that are Proterozoic
and younger is strong evidence that the atmosphere by
this time contained free oxygen.
Formation of the Atmosphere and Oceans
Importance of Oxygen
• Oxygen is important because most animals
require it for respiration and it provides protection
against UV radiation from the Sun.
• Earth is naturally protected from this radiation by
ozone (O3) molecules that are present in the
lower part of Earth’s upper atmosphere.
• Oxygen in Earth’s atmosphere that was
produced mainly through photosynthesis also
contributes to the ozone layer.
• Nearly all the oxygen that is present was
released into the atmosphere by photosynthesis.
Formation of the Atmosphere and Oceans
Formation of the Oceans
• Oceans are thought to have originated largely
from the same process of outgassing that formed
the atmosphere.
• As the early atmosphere and the surface of
Earth cooled, the water vapor condensed to
form liquid water.
• During the Archean, tremendous amounts of rain
slowly filled the low-lying, basalt-floored basins,
thus forming the oceans.
Formation of the Atmosphere and Oceans
Formation of the Oceans
• Dissolved minerals made the oceans of the
Precambrian salty just as they make the oceans
salty today.
• A recent hypothesis suggests that some of
Earth’s water may have come from the
bombardment of microcomets, or small comets
made of frozen gas and water.
Formation of the Atmosphere and Oceans
Oxygen in the Atmosphere
Oxygen Causes Change
– The Precambrian began with an oxygen-free
atmosphere and simple life-forms.
– This oxygen added by cyanobacteria not only enabled
new life-forms to evolve, but it also protected Earth’s
surface from the Sun’s UV rays.
– Oceans formed from abundant water vapor in the
atmosphere and possibly from outer space.
– Earth was then a hospitable place for new life-forms
to inhabit.
Formation of the Atmosphere and Oceans
Section Assessment
1. Match the following terms with their definitions.
___ cyanobacteria
___ stromatolite
A. sedimentary rocks that are
younger than 1.8 billion years
and are rusty red in color
___ banded iron
formation
B. large mats and mounds of
cynobacteria
___ red bed
C. deposits that consist of
alternating bands of chert and
iron oxides
D. chlorophyll containing bacteria
that may be responsible for the
addition of oxygen to Earth’s
early atmosphere
Formation of the Atmosphere and Oceans
Section Assessment
2. Why is Earth’s current atmosphere rich in
carbon dioxide and nitrogen?
Formation of the Atmosphere and Oceans
Section Assessment
3. Identify whether the following statements are
true or false.
______ Stromatolites currently exist on Earth.
______ Free oxygen is released during outgassing.
______ Around half of the oxygen that we breathe
today was released into the atmosphere
through photosynthesis.
______ There was little free oxygen in the atmosphere
during the Archean.
Early Life on Earth
Objectives
• Describe the experimental evidence of how life
developed on Earth.
• Distinguish between prokaryotes and eukaryotes.
• Identify when the first multicellular animals appeared in
geologic time.
Vocabulary
– amino acids
– eukaryote
– hydrothermal vent
– Varangian Glaciation
– prokaryote
– Ediacara fauna
Early Life on Earth
Origin of Life on Earth
• Fossil evidence indicates that life existed on
Earth about 3.5 billion years ago.
• Earth probably could not have supported life
until about 3.9 billion years ago because
meteorites were constantly striking its surface.
• This places the origin of life somewhere
between 3.9 and 3.5 billion years ago.
Early Life on Earth
Origin of Life on Earth
Experimental Evidence
– Molecular biologists in the 1920s also suggested that an
atmosphere containing abundant ammonia and methane
but lacking free oxygen would be an ideal setting for the
“primordial soup” in which life may have begun.
– Stanley Miller and Harold Urey set up an apparatus that
contained a chamber filled with hydrogen, methane, and
ammonia to simulate the early atmosphere.
– Sparks from tungsten electrodes simulated lightning in
the atmosphere.
Early Life on Earth
Origin of Life on Earth
Experimental Evidence
– Their atmospheric chamber was connected to a lower
chamber that was designed to catch any particles that
condensed in the atmospheric chamber.
– Only one week after the start of the experiment, the
lower chamber contained organic molecules such as
cyanide (CN), formaldehyde (H2CO), and four different
amino acids.
– Amino acids are the building blocks of proteins,
the basic substances from which life is built.
Early Life on Earth
Origin of Life on Earth
Experimental Evidence
– Continued experiments showed that 13 of the 20 amino
acids known to occur in living things could be formed
using the Miller-Urey method.
– Further experiments demonstrated that heat, cyanide,
and certain clay minerals could cause amino acids to
join together in chains like proteins.
– Miller and Urey demonstrated that however life first
formed, the basic building blocks of life were most
likely present on Earth during the Archean.
Early Life on Earth
Origin of Life on Earth
The Role of RNA
– The nucleic acids RNA and DNA are the basic
requirements for reproduction, an essential
characteristic of life.
– In modern organisms, DNA carries the instructions
necessary for cells in all living things to function.
– RNA ribozymes, unlike DNA, can replicate without the
aid of enzymes, and may have been the first replicating
molecules on Earth.
– An RNA-based world may have been intermediate
between an inorganic world and the DNA-based organic
world that followed.
Early Life on Earth
Origin of Life on Earth
Hydrothermal Vents and the Beginnings of Life
– Life on Earth may have originated deep in the ocean,
near active volcanic seafloor rifts.
– Hydrothermal vents are the openings where hot water
rises and is expelled from the ocean floor.
– All of the energy and nutrients necessary for the
origin of life are present at these deep-sea
hydrothermal vents.
– Some scientists hypothesize that during the Archean,
near hydrothermal vents, amino acids joined together
on the surfaces of clay minerals to form proteins.
Early Life on Earth
Proterozoic Life
• The only evidence of life-forms that existed
before the Proterozoic is the fossilized remains
of unicellular organisms called prokaryotes.
– A prokaryote is an organism that is composed of a
single cell, which does not contain a nucleus and is the
simplest kind of cell.
– A eukaryote is an organism that is composed of a cell
or cells that contain a nucleus.
Early Life on Earth
Proterozoic Life
The Varangian Glaciation was a widespread
glaciation event that occurred between 800 and
700 million years ago that played a critical role in
the extinction of many members of a group of
possible eukaryotes, the acritarchs.
• Shortly after the ice retreated toward the poles,
700 million years ago, multicellular organisms
first appeared in the fossil record.
Early Life on Earth
Ediacara Fossils
Fossils collectively referred to as the Ediacara
fauna are the impressions of soft-bodied
organisms that were discovered in Late
Proterozoic rocks in the Ediacara Hills of
southern Australia.
Early Life on Earth
Ediacara Fossils
• It is generally agreed that these fossils represent
animals that were composed of different types of
eukaryotic cells.
• Scientists are unsure, however, whether the
Ediacara fauna are relatives of modern animal
groups or whether they were completely
different types of organisms.
• The Ediacara fauna seem to provide fossil
evidence of an ancestral stock of complex
Proterozoic animals.
Early Life on Earth
Ediacara Fossils
• Some scientists consider the similarity in shape to
animals in other phyla coincidental and that the
Ediacara fauna represents a virtual dead end.
• Ediacara fossils have been found in all parts of
the world.
• These organisms seem to have flourished
between 670 and 570 million years ago until an
apparent mass extinction.
Early Life on Earth
Section Assessment
1. Match the following terms with their definitions.
___ prokaryotes
___ eukaryotes
___ amino acids
___ Ediacara fauna
A. organisms that are composed
of cells that contain a nucleus
B. an organism that is composed
of a single cell, which does not
contain a nucleus
C. fossils of soft-bodied organisms
that were discovered in Late
Proterozoic rocks
D. the building blocks of proteins
Early Life on Earth
Section Assessment
2. When did life most likely develop on Earth?
Early Life on Earth
Section Assessment
3. Identify whether the following statements are
true or false.
______ RNA can be easily synthesized under
conditions that likely existed at the surface of
the Archean Earth.
______ Ediacara fauna may not represent an ancestral
stock of any modern group.
______ Life is currently being synthesized at
hydrothermal vents.
______ Cyanobacteria are examples of prokaryotes.
Section 22.1 Study Guide
Section 22.1 Main Ideas
• Geologists have used radiometric dating to show
that Earth must be at least 4.2 billion years old.
• Because the solar system formed all at the same
time, Moon rocks and meteorites that are
approximately 4.6 billion years old suggest that
Earth is also 4.6 billion years old.
• The early Earth was a very hot place because of
abundant radioactive isotopes, bombardment by
meteorites, and gravitational contraction.
Section 22.2 Study Guide
Section 22.2 Main Ideas
• Earth’s early crust formed by the cooling of the
uppermost mantle. This early crust weathered
and formed sediments.
• Sediment-covered slabs of this early crust were
subducted and generated magmas that
contained granitic minerals.
• During the Archean, microcontinents collided
with one another throughout the Proterozoic and
formed the cores of the continents. By the end of
the Proterozoic, the first supercontinent,
Rodinia, had formed.
Section 22.3 Study Guide
Section 22.3 Main Ideas
• Earth’s early atmosphere and the oceans
formed mainly by the process of outgassing.
• Nearly all of the oxygen in the atmosphere is a
result of photosynthesis.
• Certain minerals oxidize, or rust, in the presence
of free oxygen. Proterozoic red beds are
sedimentary rock deposits that contain oxidized
iron. They are the evidence that there was free
oxygen in the atmosphere during the Proterozoic.
Section 22.4 Study Guide
Section 22.4 Main Ideas
• All the ingredients were present on the early
Earth to form proteins, the building blocks of life.
Amino acids, the molecules that make up
proteins, were likely abundant on the surface of
the early Earth.
• Prokaryotic cells are generallly small and
contain no nuclei. Eukaryotic cells contain nuclei
and are generally larger and more complex than
prokaryotic cells.
• The first evidence of multicellular animals is
fossils of 2.1 billion year old eukaryotic algae.
Chapter Assessment
Short Answer
6. What is the process of differentiation?
Chapter Assessment
Short Answer
7. When and where did the Ediacara organisms
flourish?
Chapter Assessment
True or False
8. Identify whether the following statements are
true or false.
______ During the Varangian Glaciation, the glacial ice
advanced almost to the equator.
______ Asteroids are usually smaller than 1 km across.
______ Processes which modify a system are known
as feedback.
______ DNA can replicate without the help of enzymes.
______ By the end of the Proterozoic, all of the major
masses of continental lithosphere had formed.
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Chapter 22 Images
Image Bank
Chapter 22 Images