LO 9 :

Geologic History of the Geosphere

What is Geologic Provinces ?
• A large region of Earth’s surface with common geologic history
like the common ( Folds , Faults , Earthquakes , Volcanos and
orogens ) happened in a certain area in certain time

• Here is an example for

one of the geologic provinces
made for the world :
Earth’s History
• The oldest oceanic crust is not older than 200 million years where the
continental crust of rocks is much younger than that .
• The Earth was formed about 4.6 billion years ago , back then Earth
was probably fairly small , it was third its size now . The growth of this
planet is due to the attraction of the debris of the solar system .
Examples of the debris of solar system is ( asteroids , meteors and
comets ) .
• Earth was a hostile place back to 500 million years where it wasn’t a
place that is sustainable for life this is due to the presence of much
volcanos and the spontaneous hit of meteorites in Earth’s surface .
• The spontaneous hit of many meteorites provided Earth internal heat which
made the young Earth’s surface molten .

• The heat in this young Earth was 3 times of the internal heat of Earth now . The
very young Earth is called Hadean . Named for Hades the god of the underworld
in the Greek mythology .

• After while , the hit of meteorites and eruption of volcanos were slowed down .
That resulted in the cooling down of OUTSIDE of Earth .

• In that time , A few minerals begun to crystalize and something like scum
patches begun to form in the molten surface and they were able to move around
a lot which resulted in the destruction of the unstable surface of the Hadean by
melting .
• Also this melting was an impact of meteorites . Some of the
melted material was recycled into the mantle .
• Because of the high temperatures in the upper mantle , most of
the oceanic and continental crust was in the form of BASALT
which had the ability to stay stable in high temperatures like
those who crystalize in the mid-ocean ridges ( explained in the
upcoming slide ) .
• Due to the melting and remelting of basalt rock , GRANITE was
formed as recycled rocks .
The oldest material on Earth
• Zircon is the oldest continental material in Earth’s surface , its
chemical name is zirconium silicate . It is hard and durable .
• It was originally formed as igneous rock which means that it was
formed due to cooling of magma but due to the weathering
process it was recycled to younger sedimentary rocks .
• Its age is nearly 4.4 billion years old .
• Its image :
Mid-ocean Ridges
• It is something related to plate tectonics but here is a quick hint
for what is it helpful for and why does it happen :

• The mid-ocean ridge is a continuous range of undersea

volcanic mountains that encircles the globe almost entirely
underwater
• they occur along the kind of plate boundary where new ocean
floor is created as the plates spread apart. Thus the mid-ocean
ridge is also known as a "spreading centre " or a "divergent plate
boundary."
Plate Tectonics
• Here is the link :

https://youtu.be/3ZpDjdFzQUM
Recapping
• The plate tectonics happened due to the conventual currents that
happened due to the transmission of the heat restored in the
core after it the mantle
• Plate tectonics got 3 different types :
1-CONVERGENT plate boundary
2-TRANSFORM plate boundary
3-DIVERGENT plate boundary
Convergent Plate Boundary
• Continental crust and Continental crust , this forms high
mountains like Himalaya Mountains which happened due to the
Collison of 2 continental plates ( Eurasian and Indian plates )
• Continental crust and Oceanic crust , this forms subduction zones
where these zone happens due to Collison of a continental crust
with an oceanic crust making the oceanic crust bulks under the
continental crust because oceanic crust got high density , this
results in volcanos in the sides of the continental crusts .
• Oceanic crust and Oceanic crust , this forms island arcs where this
formed due to the Collison of 2 oceanic crusts , one of them will
buckle under the other forming these arcs ( orogeny )
Divergent Plate Boundary
• Continental crust and Continental crust , this forms the rifts we
see in the deserts where this happens due to the diverges of both
of 2 continental crusts .
• Oceanic crust and Oceanic crust , this forms the rifts in the sea
where this happens due to the diverges of both 2 oceanic crust .

Transform Plate Boundary

• Happens when the 2 slides transform from each other as
u can see in the video .
Crustal Evolution
• The oldest known rock on Earth is about 4.1 billion years old. It is a granodiorite from
Canada’s Northwest Territories. It formed below the surface as an igneous rock. Its
chemistry is somewhere between that of granite and diorite. Granodiorite is usually
formed at subduction zones .
• As illustrated in the video and in #11 , when 2 plates collides together , they forms
island scars
• When plates joined together it is called accretion
• Southwest Greenland contains the oldest preserved crust of a continental landmass.
It is 3.8 billion years old. It is Earth’s oldest surviving accretionary orogen. Here,
oceanic lithosphere was subducted, and new continental crust
was produced. However, it is highly fragmented and metamorphosed. This makes it
difficult to study.
 What is Orogen ?
• a linear or arc-shaped region that has been subjected to
intense folding and other deformation during the tectonic
cycle.
• Continental collisions result in subduction that melts the leading
edges of plates. As a result, andesitic magma forms. This type of
magma has a lower density than surrounding crustal rocks.
• It takes the circular to oval shape this make it less dense and
thick. This increases the buoyancy which is the ability of the
body to float in certain areas
• They remain on the surface where they are subjected to billions
of years of erosion. The thicker crust is also better insulated
from the upper mantle and becomes more stable. These areas
of the crust form continental shields.
What is Continental Shield ?
• Continental shield is a group of rocks that was the initial rocks
during the young Earth , they say that the old of the rocks of shields
is older than 3.7 billion years .
• They formed the roots of continents besides that makes the Earth’s
surface stable .
• They were produced due to the plate tectonics and erosion , it was
formed from Precambrian crystalline rocks .
• They commonly have a gently convex surface and are
surrounded by sediment-covered platforms.
• One of rocks surrounding shields is greenstones
 • Shields have grown by the addition of rocks , these rocks contain dark
minerals and basaltic compositions from the sea floor .
Greenstones
• They are rocks formed around the shields . They are green
colored metamorphic rocks formed from dark igneous rock that
often occurs in belts within Precambrian shields
• There name comes from the green mica-like minerals they
contain .
• Belts of greenstone contains metamorphosized lava flows and
sediments from chains of volcanic island . Volcanic island is often
meant to be between island arc and the colliding continent
• Their mineral composition is controlled by the chemistry
of magma. It also depends on the temperature at which
the magma cooled.
• One of the most unique greenstone belts in the world is
called The Barberton Supergroup .
• Supergroup is a very large stratigraphic unit consisting of
many important smaller groups .
Greenstone Belts
• Plumes of granodiorite are common in greenstone belts.
They cause shields to dome upward. Trapped between the
rising domes, the greenstones became strongly folded and
metamorphosed.
• A high-grade rock was formed due to the strongly folded
and metamorphized shield which is GNEISS .
At the same time , high domes was formed from ancient
blocks of crusts with greenstone-granodiorite-granite which
we call craton .
• From that we could differentiate between the old and later
formed continental crust .
What is Craton ?
• a large stable part of continental crust that has undergone
little deformation for a long period.
• Shields is considered as Cratons .
• A craton is an old and stable part of the continental crust that
has survived the merging and splitting of continents and
supercontinents for at least 500 million years. Some are over
2 billion years old. Cratons are generally found in the
interiors of continents and are characteristically composed of
ancient crystalline basement crust of lightweight felsic
igneous rock such as granite. They have a thick crust and
deep roots that extend into the mantle beneath to depths of
200 km. More information about BASMENT CRUST IN #28
Diagram showing how greenstones was
formed :
Weathering and Rock Cycle
• Three-billion-year-old sedimentary rocks form the top part of
the
Barberton Supergroup. The rock is about 2.5 km thick. These
rocks indicate that Earth’s young, very small continents were
subject to weathering and erosion. At this time , there was no
soil or plants on land. The atmosphere had yet to develop free
oxygen. It was mostly water vapor, carbon dioxide, and
nitrogen .
• The atmosphere back then was more powerful at decomposing
rocks than today. The surfaces of the young continents were
rapidly stripped of weathered particles which means that there
wasn’t any weathered particles all of them was decomposed .
• Rains washed their rocky surfaces. Sediment was
transported by rivers. It was deposited around the edges of
the continents .
• Coarse sediments were deposited in shallower water. They
formed conglomerates and sandstones.
• Ripples and bedding structures also formed. They were like
those seen in the ocean today.
• Ripples are sedimentary structures , it is the same shape
when you throw a rock in the water , the rock will move
away from its point of its entry in a circle or ring shape .
These shapes always happens in high current flow of water .
• Finer sediments were transported into deeper water . They
formed mudstone .
a) Ripple marks b) Bedding structure
Development of Magnetosphere
• The loss of heat from Earth’s exterior allowed its interior structure to develop. As
molten iron solidified, it created the dense, solid inner core at the centre of the
planet. The core has remained solid because of enormous pressures. The outer
core is liquid .
• Motion in the iron-rich outer core continuously generates electrical currents.
• Because of rotating the magnet through an electric field , a magnetic field is
formed , this is the same way in the Earth’s core . It is called geodynamic effect , it is
self-sustaining which means that it could maintain itself .
• The magnetic field shields Earth from the solar wind. Without the
magnetic field, the constant stream of particles from the Sun would
have eroded Earth’s atmosphere. It would be lost into space.
• When igneous rock cool , magnetic mineral record the direction and strength of the
magnetic field . Earth’s magnetic field moves in complex patterns where they
identified that the magnetic dipoles changed its poles many many times . The last
time it reversed its poles was 780,000 years ago .
The Development of North American
continent
• Large fragments of the crust are called terranes. The
various terranes of the North American continent show
how continents grow largely by accretion. The huge
continental mass is the North American Craton.
• Its basement , like the oldest rocks in most continents, is
now covered by younger sedimentary rocks. Drilling
through the crust reveals various basement rocks. These
rocks vary in age.
• Each terrane was added by being welded to the
continent during the process of subduction.
What is Terranes ?
• a region of crust added to a craton from a tectonic plate
as a result of accretion (joining of 2 plates) .
• Terranes vary greatly in composition. In general they are
made up of blocks of oceanic crust (basalts and
greenstones) and overlying sedimentary rocks
(limestones, cherts, shales, and sandstones)
What is Basement crust ?
• The lowermost units of rocks that was happened due to
the splitting of the continents and making it more
stable . The units are often igneous and metamorphic.
The few slides coming after is cropped
images from Earthcomm for the illustration of
the Formation of Appalachian Mountains
please pay attention to them !!!
The Appalachian Mountains formation
• The Appalachian Mountains formed when the rocks were trapped in the
basins between the North American and African continents. The rocks were
squeezed together to fold and fault .
Intense deformation produced high-grade metamorphic rocks .
• This is one way that crust is welded to a continent by accretion along its
margin. The Appalachian mountain chain continues northward into Nova
Scotia. Other parts of the chain were separated by the opening of the Atlantic
Ocean. They are found in Ireland, Western England, Scotland, and Norway.
• The young Appalachians may have been as high as the Himalayas are today.
The mountains have since been worn down by erosion. As the mountains are
eroded, the debris is transported by rivers to the coast. The terrane becomes
buried by a thick sedimentary cover. The sedimentary deposits cause the
continent to grow more in size.
The Development of the North American
Cordillera
• The western edge of North America was also built up by a
succession of accreted terranes.
• They are the youngest of the major tectonic units of the
continent. They are less than 500 million years old. The
accreted terranes on the West Coast are made of sediments.
• Coast are made of sediments. These sediments were scraped
off the subducting Pacific Ocean crust. This took place as the
oceanic crust was thrust under the North American Plate.
Enrichment Information !!!!!
• The Pacific Northwest was not always a subduction zone.
Prior to the Triassic Period, North America was part of
Pangea. At that time, the coast of North America was
along the west side of Idaho. Pangea split apart 225 to
200 million years ago. As it did, North America began to
move west into the neighbouring oceanic plate. New
terrane was welded onto the west coast of the continent
by accretion. Subduction formed volcanic mountains.
Also, stresses in the crust folded and faulted rocks to
form mountain ranges.
Geologic Province of North
American coast 🡪🡪🡪🡪🡪🡪
• subduction resulted in a huge amount of magma. This
magma rose to the surface.
• Different generations of coarse-grained, pale granitic
rocks were added to the crust. This took place for more
than 100 million years. Scientists think that around 90
million years ago, the zone of accretion shifted west to
the coastal ranges.
• The supercontinent called Gondwanaland split. It
became Europe, Asia, North and South America, Africa,
Australia, and Antarctica. As the crust thinned and rifted,
huge outpourings formed continental flood basalts.
What is Flood basalts ?
• accumulation of mostly horizontal basalt formed by multiple
eruptions from fissures.
• Flood Basalts are high volume eruptions that flood vast areas of the
Earth, covering broad regions with flat lying lava surfaces. They are
said to be the result of mantle convection through hot spots, which
occur sporadically in time and place.
• Basaltic lava has a low viscosity. It may have flowed at speeds of 5
km/h. Sections of this gigantic deposit reached 190 km in length.
• The overlying
mass of gigantic sill of basalt caused the crust to subside. It formed
the
Columbia Basin (Plateau).
Seismic Anatomy Map
• The figure illustrated above , shows the seismic velocities of the
North American continent .
• Blue colored areas are the areas with low velocities.
• Red colored areas are the areas with high velocities.
• If you stand in active subduction zone , the seismic velocity will be
higher .
Geosphere , Hydrosphere , Atmosphere and
Biosphere
Please pay attention to watch this 12 minutes
video :

https://youtu.be/6LkmD6B2ncs
 LO 10 :
The release of gases from earth’s interior
Gases and liquids in Mantle rocks
• Everyone of us must observed that when you open a carbonated
beverage (Coca-Cola) which contains dissolved gas in it , the gas flow
away this is the reason for the reduction of pressure .
• The reason you do not see bubbles in an unopened bottle is
because the pressure of the outside of the bottle restricts
the movement of the gas in the liquid. This prevents the gas
from escaping .
• Heating the opened bottle increases the rate at which the gas
escapes.
• There are some gases dissolved in earth’s hot mantle which are:
carbon dioxide, nitrogen, and water vapor.
• Mantle is subjected to very high pressures , this pressure could range
between 250,000 to 1.7 million times of the atmospheric pressure .
• This high pressure make the dissolved gases (carbon dioxide , nitrogen
, water vapor) remain in equilibrium in their environment . Remember
the example of the carbonated beverage
• During volcanic activity, molten rock rises through the crust. The
environmental conditions change. As the depth below the surface
decreases, the mass of the overlying crust decreases. Therefore, the
pressure on the rising magma also decreases . With a decrease in
pressure, gases dissolved within the magma are released.
• Earth’s rocky lithospheric plates contain special minerals.
These minerals can hold water within their crystals. Mineral
matter in Earth’s mantle also contains water.
• According to several experiments done by scientists in Japan
in order to know about the relation between the pressure
and the ability for crystals to carry water in it , They found
that at this great pressure the rocks in the mantle are able to
contain huge volumes of dissolved water vapor.
• There is debate that the mantle contains volume of water
greater than all the water in Earth’s ocean .
• Many crystals in Earth’s crust begin to form in some kind of
fluid. As these crystals grow, they often trap tiny bubbles of
the fluid. This provides important information about the
conditions under which the crystals formed. By heating the
bubbles, scientists can find the temperature and pressure at
which the crystals formed. One instrument used for this was
designed by the USGS. It is extremely sensitive. It is capable
of detecting amounts as low as 8 parts per billion in samples
as small as 0.01 mm in diameter.
A microscopic fluid inclusion within a quartz
grain
• Gases dissolved in Earth’s mantle are released into the atmosphere during
volcanic eruptions. These can occur when a plate descends downward
into the mantle. The plate can get so hot that any water or gas it contains
rises into the overlying mantle. The overlying mantle melts into magma.
The magma is less dense than the solid rock that surrounds it. It then rises
through the mantle to the overlying crust. As it nears Earth’s surface,
there is less overlying rock. The pressure on the magma decreases. At the
surface, the magma is under atmospheric pressure. Gases such as carbon
dioxide, nitrogen, sulphur dioxide, and water vapor dissolved in the magma
are released into the atmosphere .
• Various volcanic processes release dissolved gases from Earth’s mantle.
These include continental and oceanic volcanic eruptions, outpouring of
lava flows, and rifting. Also included are rising plumes at hot spots and at
mid-ocean ridges. During these processes, hot material from the deep
mantle reaches Earth’s surface. As this takes place, any dissolved gases it
contains are released.
Plate which
descends down
The Evolution of Earth’s Fluid spheres
• Earth’s interior has been releasing gases since its formation 4.6 billion
years ago. These gases were trapped within the solid rock particles that
came together under the influence of gravity to form Earth. Over time,
Earth mas increased by more and more rocky debris was pulled in from
the solar system. As Earth’s mass increased, its gravity grew. The
pressure increased on its interior , then its interior heated and melted.
• Meteorites bombarded the surface of the young Earth. This caused its
primitive crust to melt and rift. As this occurred, large amounts of gases
were released through fissures in Earth’s surface. As this occurred, large
amounts of gases were released through fissures in Earth’s surface.
• Earth’s gravity kept these gases from being stripped away by the
solar wind and blowing off into space. The process by which huge
volcanic eruptions transfer matter from the mantle to the
atmosphere is called outgassing.
Outgassing
• The release of gases from Earth’s interior that formed the
primordial
atmosphere and continues today.
• Outgassing early in Earth’s history produced primoradial atmosphere
(largely water vapor and carbon dioxide). There were also lesser
amounts of carbon monoxide, hydrogen, and hydrogen chloride.
• In contrast to the atmosphere today, oxygen was mostly absent.
• During the early Hadean, Earth’s surface temperature was extremely
hot. It was too hot for water to exist as a liquid at the surface. As a
result, there were no oceans, lakes, rivers, or groundwater.
• As the young Earth cooled, outgassing formed a new atmosphere. It is
consists of (methane, hydrogen, nitrogen, and water vapor, with
smaller amounts of noble gases and carbon dioxide). Noble gases like
helium.
• Earth’s cooler surface radiated less heat into the atmosphere. Further
cooling enabled water vapor to condense. It fell to the surface as rain.
Much of the first rains would have fallen on hot volcanic rock and
evaporated. As the crust cooled more, rain began to collect in low-
lying areas. This formed bodies of water on the surface. Eventually,
enough water was released from Earth and condensed to form the
world’s oceans .
• On the primitive continents, Earth’s first river networks formed. The
rivers flowed and transported rock particles worn from the
continents. These sediments built up in the oceans. They formed early
marine sedimentary rocks .
• The outgassing of carbon dioxide produced rainfall and oceans that
were more acidic than today: Because carbon dioxide, when
combined with water, forms carbonic
` acid. For instance the pH of
early Haden ocean 5.8 and the pH of the ocean now nearly 7.
• Some of the dissolved chemicals would later become limestone and
the shells of marine organisms. Today, the oldest organic marine
limestones are found in western Australia. They are in rocks 3.5 billion
years old.
Formation of oceans and atmosphere

• Some scientists believe that the process of outgassing did not form
the atmosphere and oceans. They argue that the young Earth’s gravity
pulled in chemicals released by the Sun.
• Other scientists think that Earth’s water came from gigantic comets
several kilometers long, passing by Earth. They suggest that Earth’s
gravity pulled in ice and rocky material from these comets.
Interaction between Fluid Sphere and Young
Earth
• Many scientists believe that Earth’s young atmosphere was richer in
greenhouse gases. This kept the planet from freezing over. As the Sun
became brighter, the concentration of greenhouse gases declined.
They eventually reached modern levels. This is known as the faint
young Sun paradox.
• The interaction between the geosphere and the fluid spheres was
very important to the development of the granite rocks that now
make up the continents.
• Earth’s young crust was largely made of basalt weathered from the
crust had more silica. Silica is resistant to chemical and physical
attack. Silica-rich sedimentary rocks built up in the oceans. They later
melted in subduction zones. As a result, the melted basaltic oceanic
crust became richer in silica from the added melted sedimentary
rocks.
Fluid Spheres on Other planets and Moons
• Earth is not the only planetary body in the solar system with fluid
spheres. Venus and Mars both have atmospheres. Like Earth. The
mass of Earth’s atmosphere is about 100 times greater than Mars. It is
100 times less than that of Venus.
• One of Jupiter’s moons, Io, has an atmosphere that is strongly
affected by the Jupiter’s gravitational field. Europa, another of
Jupiter’s moons, has a thin oxygen-rich atmosphere.
• Titan, Saturn’s largest moon, is icy and rocky. It has an atmosphere
rich in nitrogen with clouds of methane.
• Earth’s fluid spheres are unique among the other planets. Earth is the
only planet with abundant free oxygen.
• Venus and Mars also have oxygen, but it is tied up in carbon dioxide
molecules.
• Earth also has abundant water in all three states
Questions
 LO.11:
The Biosphere and The Evolution of The
Atmosphere
How scientists detected the Early Atmosphere ?

• It is easy to understand that scientists couldn’t directly sample the

Earth's early atmosphere from billions of years ago . Instead , they
looked for clues , Some of these clues are in the chemistry of rocks ,
most of these rocks were formed in the ancient marine environment .
• Iron oxide is one compound in these rocks where scientists knew
about the early atmosphere from where it told them the amount of
oxygen in Earth’s ancient atmosphere .
Iron Oxide
• It is an Inorganic chemical compounds composed of iron and
oxygen. Many varieties exist, including the minerals
magnetite (Fe O ) and hematite (Fe O ).
3 4 2 3

• Oxidation is the combination of a substance with oxygen.

Iron oxidized, or combined with oxygen, to form iron oxide.
This process usually happens in water .
• If you put an iron tablet in a beaker of tap water , You most likely
observe the water change to reddish brown as the iron tablet
dissolved. Gradually, reddish-brown particles formed in the
water. This gave it a cloudy appearance.
• As more iron combined with oxygen, larger and larger iron-oxide
particles formed. They sank to the bottom of the container. This
removed the iron from the solution. If you do not add new iron,
the concentration of iron in the solution decreases thus if you
left the water for few days and then come back you will find that
water came back to its original appearance .
• Paint or rust proofing is used to protect the iron in these items
from rusting. They prevent the iron from coming into contact
with moisture
and oxygen. This prevents iron oxide from forming.
Banded Iron Formations
• rocks, more than two billion years old, that consist of oxides,
sulfides , or carbonates of iron, thinly interbedded with chert , a silica
rich rock.
• they were made up of alternating dark and light layers of rock. Some
of the layers contain iron-rich minerals. The minerals are hematite
and magnetite which are common.

Chert
• a sedimentary rock made of tiny quartz crystals formed from watery
solutions rich in silica , often grey in colour.
Example for Example for
Branded Iron Formation Chert
• Branded Iron Formations are commonly associated with greenstone
belts. They were moved onto the continents by accretion.
• Accretion is the process by which the cores of ancestral continents
grew by the subduction of adjacent oceanic crust and the welding of
crust to the continental margin which means that accretion happens
when there is island arc in the oceanic crust and at the same time the
oceanic crust is being subducted by the continental crust so that island
arc resist the oceanic crust to be subducted resulting in the accretion
of the both of the plates .
• Scientists think that during the late Hadean the oceans contained vast
amounts of iron and silica. One source of the iron seems to have been
underwater volcanoes. The iron was released in fluids from
hydrothermal vents. There, the material from the upper mantle mixed
with seawater
• Another source for the iron and silica in the oceans was Earth’s
crust.
Earth’s young basaltic crust was rich in minerals that contain iron
and
silica. Earth’s new crust was exposed to the warm primitive
atmosphere. This caused rocks to weather. The weathering process
released iron and silica from the crust. They found their way into
rivers in solution and in mineral fragments. The rivers eventually
deposited the iron and silica into the oceans.
• Iron was being added to seawater for several hundred million years.
However, it was not being removed. Also, at that time, there were no
organisms that required silica to form shells. As a result, the ocean
water became saturated with silica as well. Iron and silica are the
chemicals found in the iron-rich and chert layers that form banded
iron formations
How banded iron formed ?

• Between about 3.5 and 2 billion years ago, the iron and silica
began to
react with oxygen dissolved in Earth’s ancient ocean. Iron
reacted to form iron-oxide minerals. They built up in layers on
the ocean bottom. Silica in the water also reacted with the
oxygen. As a result, layers of silica-rich chert were formed.
• Some scientists think that the layering of banded iron
formations reflects the competing influences of hydrothermal
processes and continental fluxes of material where volcanism
released iron and silica
From where did oxygen come from ?

• Scientists thought that the oxygen must have come from ancient
cyanobacteria. Recall that cyanobacteria are phototrophs. They use
photosynthesis to convert energy from the Sun into food. Oxygen is a
by-product of photosynthesis. The population of cyanobacteria on
Earth increased. As a result, 3.5 to 2.5 billion years ago, more and
more oxygen was released into Earth’s atmosphere.
• Cyanobacteria is considered the first prokaryotic cell to be formed
• The atmosphere and oceans readily exchange gases. As the oxygen
content of the atmosphere increased, the dissolved oxygen content
of
the surface waters in the oceans also increased. Oxygen reacted
with the dissolved iron and silica in the ocean. This resulted in the
layers of the banded iron formations. Until 2.3 billion years ago,
oxygen was drawn down into the oceans. It replaced oxygen used
up in forming the banded iron formations. This kept the
concentration of oxygen in the atmosphere to low levels. The levels
were 1–2 percent of what they are today.
• the deposition of banded iron formations began to slow down.
There was less and less dissolved iron in the oceans. The rate at
which iron was resupplied to the oceans had decreased. By 1.8
billion years ago, there was very little dissolved iron in ocean water.
The banded iron formations stopped forming altogether.
• The cyanobacteria flourished. They continued to produce oxygen.
However, the iron was no longer there to act as a sink for oxygen in the
oceans. Less oxygen was drawn from the atmosphere into the oceans. As
a result, oxygen began to build up in the atmosphere.
• The soils indicate that atmospheric oxygen at the time was 15 percent
that of the modern level. Current levels of oxygen were reached probably
about 600 million years ago .
• Along with a buildup of oxygen, came an increase in ozone levels (O ). The
3

increase in oxygen and ozone had important implications for life on

Earth.
Ozone absorbs much of the UV radiation that strikes the atmosphere. It
formed a protective layer around Earth. This made the continents a more
hospitable area for organisms to live.
Questions
LO 12,13 :
Geologic Times
What is Geologic Time ?
• It is the interval of time since the formation of Earth and there is a scale for
it .
• The geologic time scale is the division of geologic time into units based on
evolutionary events of geologic history.
• Some parts of Earth had more unique events than others. Some events
occurred only once in Earth’s past. As The formation of Earth , The
development of the core , The mantle , The crust occurred , Development
of the atmosphere.
• Other events are cyclical. Mountain formation , The eruption of volcanoes ,
and Earthquakes , which means that it forms regularly in repeated cycles .
• In order to detect the geologic time scale , scientists begun to use the
history of the biosphere.
What is Biosphere ?
• It is relatively thin life-supporting stratum of Earth’s surface, extending
from a few kilometres into the atmosphere to the deep-sea vents of
the ocean. The biosphere is a global ecosystem composed of living
organisms (biota) and the abiotic (non-living) factors from which they
derive energy and nutrients. Fossils lies in the biosphere .
• Fossils are evidence of once-living things. These organisms are
preserved in sediments and rocks.
• Fossils indicate when Different species of organisms have appeared
and disappeared over time.
• Extinct species do not reappear later. Scientists use the arrival and
extinction of different fossils as markers.
Eons

• The longest unit of time and division of geologic time that contains
two or more eras. So far, there have been four eons. Each has its own
unique story.
• The Hadean Eon is the first part of Earth’s history. This part of
Earth’s history lasted 800 million years. During this time, the Moon
formed , and meteorites bombarded the geosphere.
• Toward the end of the Hadean, simple life had gained a foothold on
Earth.
Scientific Notation for Geological time

• Using the scientific notation for geological time, that is

1 Ma for mega annum or millions of years
1 Ga for giga annum or billions of years
More Eons
• The Archean Eon , lasted from 3.8 to 2.5 Ga. It contains evidence
of early cellular life and photosynthesis.
• Next came the Proterozoic Eon 2.5 Ga–542 Ma . It is marked by
the appearance of early life forms. However, it was the time before
abundant life. The First major glaciation (Formation of glaciers) took
place 2.3 Ga .
• Presently, Earth is in the Phanerozoic Eon. It has lasted about
542 million years so far. It is a time of abundant life. This is about an
eighth of geologic time.
• During the Phanerozoic Eon, evolution has led to large-scale
changes in the biosphere. Scientists have used these to further define
the geologic time scale.
 Hadean

Archean

Proterozoic

Phanerozoic
Eras
• Eons are divided into shorter units of time called eras which is a major
division of geologic time that contains two or more periods.
• The Phanerozoic Eon is divided into three eras.
The Paleozoic Era is the era of ancient life. It lasted about
290 million years.
The Mesozoic Era is the era of middle life. It lasted about
185 million years.
The Cenozoic Era is the current era of modern life. It has lasted
about 65 million years so far.
• Changes in the mix of animals and plants in the biosphere define the
beginning and end of each era.
• For example, the Mesozoic Era is called the Age of Dinosaurs. It
ended when most of life on Earth became extinct.
• The next is called the Cenozoic Era . It included the evolution of
mammals. Many of these mammals live on Earth today.
Periods
• Eras are divided into shorter units of time called periods. Each period is
named by the scientists who discovered it.
• Sometimes the names are from a particular region like the Cambrian,
Ordovician, and Silurian are names of three periods . They are named after
Welsh tribes.
• Each period lasts tens of millions of years. They subdivide geologic time in a
way similar to how hours divide a day.
• Each one also tells a unique part of Earth’s history. For example, the
Cretaceous Period was when large dinosaurs were abundant. It lasted about
80 million years. After the Cretaceous Period ended, the number of
dinosaurs decreased.
• A new period began. This was called the Paleogene Period. Scientists have
defined this period according to the appearance of a large number of
mammal fossils.
Epoch

• It is part of the geologic time scale. That help define time even more
precisely. This is much like minutes within an hour. Epochs are often
determined by special events in other parts of the Earth system.
Dating Rocks by using Radioactive decay
• Scientists use geologic evidence for many events in Earth’s history.
• Mountains provide evidence of plate collisions or volcanic activity. The
shields that form each continent tell about how Earth’s crust evolved.
• Special iron-rich rocks formed in the oceans tell about how the
atmosphere was formed. Changes in fossil groups indicate changes in
past climates.
• Younger layers of sediments are deposited on top of older layers. This
gives a relative age of a layer of rock. You can identify a rock layer as
being younger or older than the layers next to it.
• Relative age: a date given for a sample expressed as younger than or
older than another rock or fossil.
• Knowing the exact ages of rocks helps them to unravel further some of
the secrets of Earth’s history.
• It can help them to answer questions like: -
1-How old is Earth?
2- When did the first continents form?
3-How long does it take for ocean crust to be recycled in the mantle?
• To determine the absolute age of a rock, scientists look for radioactive
minerals.
• Absolute age: a date given for a sample expressed in years.
• These minerals contain radioactive elements. The nuclei of these
elements are unstable. As a result, they break apart over time. This
process is known as radioactive decay.
• Radioactive decay: the process by which an atomic nucleus of an
unstable atom loses particles
• As particles are released, the original element changes into a different
one. The new element has slightly lighter properties. The atom that
undergoes decay is called the parent atom. The product is called the
daughter atom.
• Parent atom is the atom that undergoes radioactive decay in a
nuclear reaction.
• Daughter atom is the product atom from the radioactive decay of a
parent atom.
• Scientists look at the rate at which a radioactive element in a mineral
decays to determine the age of the rock in which it is contained. The
time it takes for half of the parent atoms to decay into daughter
atoms of a different element is called a half-life.
• Half-life : the length of time it takes for half of a radioactive
substance to decay
• Knowing the half-life of an element and the fraction of parent atoms
left, a decay graph is used to determine when the parent atoms were
fully intact. The date when the rock was formed can then be
determined.
• Example for decay graph :
• The radioactive isotope of rubidium has a very long half-life. It takes
about 48.8 billion years for half of it to change to strontium. Elements
such as this, with very slow rates of decay, are good for finding the
ages of very old rocks.
• Using radioactive decay, scientists have found Earth’s oldest mineral
to be zircon
• Zircon contains small traces of the radioactive element uranium.
(Uranium decays to form lead.)
• Zircon samples have been found that date as far back as 4 billion
years.
• Zircon is highly resistant to weathering and erosion.
Zircon
Radioactive Dating
• Radioactive dating is done mostly on igneous rocks.
• Radioactive elements are trapped in certain minerals when magma
cools and hardens to form rock.
• There are a few types of sedimentary rocks that contain radioactive
elements.
• The radioactive “clock” is reset in new minerals that form when rocks
are deformed. These minerals give a younger age for the rock than
when it originally formed.
Radiocarbon Dating
• Radiocarbon dating is used to find the age of once-living materials between
100 and 50,000 years old. This range is especially useful for determining
ages of human fossils and habitation sites. The atmosphere contains three
isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is
radioactive , it has a half-life of 5,730 years. The amount of carbon-14 in the
atmosphere is tiny and has been relatively stable through time. Plants
remove all three isotopes of carbon from the atmosphere during
photosynthesis. Animals consume this carbon when they eat plants or other
animals that have eaten plants. After the organism’s death, the carbon-14
decays to stable nitrogen-14 by releasing a beta particle.
• The nitrogen atoms are lost to the atmosphere, but the amount of carbon-
14 that has decayed can be estimated by measuring the proportion of
radioactive carbon-14 to stable carbon- 12. As time passes, the amount of
carbon-14 decreases relative to the amount of carbon-12.
Potassium-Argon dating
• Potassium-40 decays to argon-40 with a half-life of 1.26 billion years.
Argon is a gas so it can escape from molten magma, meaning that any
argon that is found in an igneous crystal probably formed as a result
of the decay of potassium-40. Measuring the ratio of potassium-40 to
argon-40 yields a good estimate of the age of that crystal. Potassium
is common in many minerals, such as feldspar, mica, and amphibole .
With its half-life, the technique is used to date rocks from 100,000
years to over a billion years old. The technique has been useful for
dating fairly young geological materials and deposits containing the
bones of human ancestors.
Uranium-lead dating
• Two uranium isotopes are used for radiometric dating.
Uranium-238 decays to lead-206 with a half-life of 4.47 billion years.
Uranium-235 decays to form lead-207 with a half-life of 704 million years.
• Uranium-lead dating is usually performed on zircon crystals When zircon
forms in an igneous rock, the crystals readily accept atoms of uranium but
reject atoms of lead. If any lead is found in a zircon crystal, it can be
assumed that it was produced from the decay of uranium. Uranium-lead
dating is useful for dating igneous rocks from 1 million years to around 4.6
billion years old. Zircon crystals from Australia are 4.4 billion years old,
among the oldest rocks on the planet.
 Questions:
(these slides are animated make sure to
get in the Fullscreen mode)
 LO 14:
The fossil record
Food Chains and Food Webs:

• Plants use energy from the Sun to make food through the photosynthesis
process.
• Producers are the organisms that make their own food.
• Consumers are the organisms that rely on plants for food to obtain energy.
• The food chain is a kind of flowchart to show how organisms are connected
to each other by the food they eat. It shows how energy and matter are
transferred from producers to the next levels of consumers.
• The relationships between organisms are
shown in the form of a food web. (as
shown in the figures)
• Decomposers are a special group of
consumers who obtain the matter and
energy they need from wastes and dead
plants and animals.
Fossils:
• Fossils are any evidence of past life preserved in sediments or rocks.
• There are two types of fossils:
1- Body fossil 2- Trace fossils

1-Body fossils:
• They are any remains or imprints of actual organic
material from a creature or plant that has been
preserved in the geologic record (Bones, teeth, shells,
and other hard body parts).

Dinosaur bones
• They may become broken, worn, or even dissolved before they might be
buried by sediment.
• Soft bodies organisms are hard to preserve unless under specific conditions
(such organisms fall into a muddy sea bottom in quiet water and are fossilized)
• For the specific conditions, the fossil record of soft-
bodied organisms is far less well known
• There is a strong bias in the fossil record. Some
organisms rarely have the chance of becoming
fossilized. Under very specific circumstances, however,
even these can become part of the fossil record.
• Bias is a purposeful or accidental distortion of observations,
data, or calculations in a systematic or nonrandom manner
Jellyfish fossil
2-Trace fossil:
• It is a fossilized track, trail, burrow, tube, boring, tunnel or other remnant
resulting from the life activities of an animal.

• Trace fossils are useful for geologists and

paleontologists because certain kinds of
organisms, which live in specific
environments, make distinctive traces.
• Paleontologist is a scientist who studies the
fossilized remains of animals and/or plants.
Feeding trails and burrows
• young sediments and rocks, the actual body parts of an organism are often
preserved.

• In older rocks, the body parts are usually dissolved away, recrystallized or
replaced by another kind of mineral but the imprints are still preserved
which can be studied if the rock splits apart in the right place and the right
orientation .

• Scientest collect rocks and put them on special mechanical splitting devices
to try to find at least a few fossils.
Fossilization:
• only a very small part of what once lived is spared being a meal for some
other organism.

• any organism on Earth will be either consumed by another organism or

decomposed by microorganisms following death.

• Decay affects soft body parts and some of the harder, more resistant body
parts.

• Each plant and animal that lives in the forest eventually ends up on the
forest floor in some form.
• Soft tissues of animals, leaves, and
flowers are used by decomposers as they
decay within several weeks or are used by
some other organism as a food source.

• The most resistant body parts (insect

exoskeletons, vertebrate bones, wood,
leaf cuticles, seeds, pollen, and spores)
may remain on the forest floor for many
years or even centuries depending upon
the physical and chemical conditions of
the soil.
• For an organism or body part to become a fossil, certain conditions must
occur. It must either live within or be moved to a place where it can be
buried. However, being buried does not guarantee that a fossil will form.

• all organisms will decay but there must be other factors at work during or
right after burial to slow or stop decay.

• The conditions necessary for fossilization do not exist everywhere all of the
time but only in a few places and for only a tiny bit of the time.
• huge numbers of organisms have
become fossilized because of the extent
of geologic time.
• It is difficult to imagine how long a
million years is. Yet physical, chemical,
and biological processes have been
operating on Earth not just for millions
of years, but for billions of years.

• The geologic time scale. It indicates

when various kinds of organisms are first
seen in the fossil record.
 https://youtu.be/sC9iqGb94hc
• This Video May help you in memorizing the geologic time scale
illustrated above
Fossilization rocks:
• Fossiliferous: A rock that contains fossils.
• Not all sedimentary rocks contain fossils.
• Some kinds of sedimentary rocks contain more
fossils than others. Limestones are the most
fossiliferous sedimentary rocks as they are made up
in part, or entirely, of the body parts of shelly
marine organisms.

Fossiliferous limestone
• Some shales are fossiliferous as well because certain organisms like to
live on muddy seafloors.
• Sandstones usually contain much fewer fossils than limestones.
• Fewer kinds of organisms can tolerate the strong currents and shifting
sand beds that are found in areas where sand is being deposited.
• For the same reason, conglomerates are the least fossiliferous of
sedimentary rocks.
 LO 15:
Mass Extinction
Textbook:
EarthComm chapter 8 section 9
The Extinction of Species
• It is impossible to predict the success or failure of one species . Many
physical and biological factors interact in complicated ways , all these
factors determine the success or failure of a species.
• Also, the data from the fossil record seem to indicate that different kinds
of organisms have different rates of success.
• It is found that there are species that lasted for tens of millions of years ,
others lasted for hundreds of million of years . Other life spans for
species only lasted for a few million years.
• The 2 main reasons for Extinction are loss of habitat and loss of genetic
variation
The Extinction of Species at the end of Mesozoic
• There was big difference in the landscapes of the Mesozoic Era and
the Cenozoic Era. (The terms come from the Greek meso-, meaning
middle and kainos-, meaning new.)
• The group of animals that dominated the Earth for nearly 130 million
years during Mesozoic suddenly became extinct , the extinction
process mostly took place almost overnight. This sudden extinction
affected some plants and many groups of animals that lived on land. It
affected much of the food web in the oceans as well.
• Groups from phytoplankton to top carnivorous disappeared from
fossil record , they were never be seen again except as a fossil
material.
• Phytoplankton are small photosynthetic organisms, mostly algae and
bacteria, found inhabiting aquatic ecosystem.
• Carnivorous are an organism that eats mostly meat, or the flesh of
animals, sometimes carnivores are called predators.
• The post boundary biosphere was very different in nature. It was
established early in the Paleogene. It took several million years for the
plant and animal groups known to exist now to evolve. It took a long
time to fill all the ecological spaces opened by this extinction event.
• The Paleogene is one of periods in tertiary period of Cenozoic Era.
• The post-boundary fossil record shows that changes in the kinds of
animals now extinct are linked to food source(s). They are related to
the appearance and disappearance of these food sources.
• The fossil record shows that when evolution comes and changes the make up of
plants in a community, dependent organisms must find a new food source. They
must change how they process food for nutrition as well. If they are not able to do
so, they face extinction.
• Until the Mid-Cenozoic, there was no evidence in the fossil record for grazing
animals. About this time grasslands appeared . Following this, many new groups of
animals are found for the first time in the fossil record. The diets of these animals
include the plants of the grasslands like grazing animals .
• In North America, such animals include camels, rhinoceroses, and horses. Also
included are many other mammals that are now known to be extinct. The extinction
of a few species now and then appears to be a normal phenomenon. Scientists refer
to the appearance and disappearance of a few species at any time as background
extinction.
• Background extinction is normal extinction of species that occurs as a result of
changes in local environmental conditions.
Biodiversity & Mass Extinction
• It is the diversity of different biologic species and/or the genetic
variability among individuals within each species.
• Fossil record sometimes show a change in the biodiversity
• A mass extinction event is when species vanish much faster than they
are replaced. This is usually defined as about 75% of the world's
species being lost in a 'short' amount of geological time - less than 2.8
million years.
• The major types of diversities in the biodiversity are genetic diversity,
species diversity, ecological diversity and functional diversity
• There have been five major ones throughout the history of life. During
these, up to 90 percent of the known biodiversity was lost. One such event
took place at the end of the Palaeozoic Era. This was between the Permian
and the Triassic Periods.
• it was more devastating to life on Earth than extinction at the end of
Mesozoic.
• Geologic time is divided into the Palaeozoic, Mesozoic, and Cenozoic
Eras. The specific places the time is divided and the time each era last
has a reason. The reason is the size and abruptness of the extinctions.
Causes of Mass Extinction
• A scientist named Luis Alvarez and his colleagues proposed a hypothesis. It
stated that the extinction was caused by the collision of a huge asteroid
with Earth. They based this on studies of sections through sedimentary
rocks. These rocks were found at the Mesozoic– Cenozoic boundary. In
several sections, they found geochemical evidence. Where amounts of
element iridium in this time was many times greater than normal amounts
of the element iridium. The evidence pointed toward a catastrophic
collision.
• Iridium is very rare on Earth’s crust. Large amounts of iridium are
found on meteorites and comets. Therefore, the iridium was
introduced into the Earth system during certain impacts.
Another Evidence of the collison
• Another important piece of evidence was found in the Yucatán
Peninsula of Mexico. It was the remnants of a colossal impact
structure. This structure is called the Chicxulub crater. The collision
placed so much dust and ash into the atmosphere that the climate
became much cooler. It is thought that light could not reach Earth’s
surface. Earth’s ecosystems were stressed for a long time after the
collision. This led to widespread extinction of many species. Evidence
of sediment movement and deposition by a gigantic sea wave could
be seen in the Gulf of Mexico. This evidence, likely caused by the
impact, has strengthened the hypothesis.
Another Evidence
• There is also good evidence for increased volcanic activity
around this time.
• A few scientists like an alternative hypothesis for the great
extinction. It has to do with climate change induced by the eruption
of the volcanoes. Evidence is still being gathered. There are still details
to resolve about the extinction and its causes.