THE HISTORY OF THE EARTH
The formation of the Solar System is explained by several hypotheses that from the “nebular
hypothesis”. This establishes that the solar system originated from a nebula of gas and dust
produced by a supernova (explosion of a star). The different variations of this hypothesis explain the
origin and development of features that are characteristic of the Solar System. These evidences are
obtained from meteorites and through telescopes.
Planets move around the Sun in the same direction and approximately the same elliptic plane.
The Sun revolves in the same direction as the planets.
The inner planets are rich in high melting point metals (silicates) and the outer planets of mass
of gases and volatile substances (they
just have small amounts of iron and
silicates).
FORMATION OF THE SOLAR SYSTEM
1. The nebula that started the Solar
System probably came from a
supernova.
2. The material from the nebula began
to come together as it rotated in
the same direction as the Sun and
planets do today. It flattened into a
disc shape.
3. The temperatures rose due to
particles colliding. In the center,
nuclear fusion began which led to
the creation of the Sun.
4. The temperatures began to fall and
the vaporized materials condensed:
the most refractory nears the Sun
and the most volatile far from the
Sun.
5. The condensed fragments collided and stuck together. As they got bigger, gravitational
attraction pulled the fragments together until the planets were formed.
Two thirds of the history of the universe had already passed when the Earth was created 4550
million years ago. The Earth is so old that it is measured in millions of years (Ma).
There had been several hypotheses about the age of our planet:
1. James Ussher (17th C) was an archbishop which based his theory on the Bible and historical
sources. He concluded that the Earth had been created in the year 4004 B.C
2. Lord Kelvin (19th C) suggested that the Earth was molten 40 million years ago, and since then
it had been cooling.
3. John Joly (end of the 19th C) though that the Earth’s first oceans were composed by fresh
water and that they had become saline over time, so by measuring the salinity of the oceans
he could date the Earth’s age. He estimated that the Earth was formed 100 million years ago.
Now, with the discovery of radioactivity we know that the Earth was
formed 4550 million years ago.
The Earth was formed by the accretion of fragments made of
different elements (silicates and metals mainly).
The heat generated by the collision and the decay of radioactive
elements melted the planet. The metallic materials, which were
denser, moved together and formed the core. The silicates formed the mantle and the
primitive crust.
METHODS OF DATING
Dating consist of estimating the age of an event or object by placing it in a specific time or period.
There are two type of dating:
Absolute or direct dating: dates events or materials by determining how million years old
they are. Normally we use radiometric dating for dating rocks and materials. This method
consists in comparing the number of specific radioactive isotopes it contains.
PROCESS:
1. Radioactive decay: Atoms in certain unstable chemical elements (for example
Uranium 238) lose particles from their nucleus in a process of disintegration.
2. This transforms them into other stable elements (Uranium 238  Lead 206),
emitting radiation.
This process happens at a fixed speed it can be used in dating and it is expressed by as a half-life (T).
A half-life is the time it takes for half of a given quantity of a radioactive isotope to decay into
another isotope. We use a radiometric device to detect rocks with isotopes that are decaying.
Example: Carbon 14. Carbon isotopes are: C 12 (6 p+, 6 n); C 13 (6p+, 7 n) and C 14 (6 p+, 8 n).
Where does C 14 come from?
1. We have N 14 (7p+; 7 n). Ionic solar radiation + N 14  less protons  isotope C 14 .
2. With time, C 14 emits radiation because it disintegrates and
forms again N 14.
The radiation emitted by C 14 while disintegrating is measured to date the
material we are dating.
1.
2.
3.
4.
5.
There are small percentages of Carbon 14 in the air.
Plants perform photosynthesis and absorb this Carbon 14.
Animals eat plants, and finally we eat the animals.
Now we have the same Carbon 14 that there is in the atmosphere.
However, when we die, we no longer eat, so we stop having the same
amount of C 14 in our bodies than in the atmosphere.
6. Our corps emits radiation (Beta particles) from C 14 transforming into N 14.
7. Depending on how many Beta particles the corps emits, we can measure when did the corps
dies. (15 B particles/ min: 0 half-lives, 7.5 B particles/ min: 1 half-life)
Relative or indirect dating: to put rocks, fossils or events in chronological order without
specifying exact dates. It is measured by the deposition of sediments, which forms layers.
This happens periodically, so scientist just have to put these stratus in order, taking in
account the following principles:
1. Each layer is younger that the one below ir and older
than the one above it.
2. In case the original position has been altered (like
faults): an event, such as a fault or fold, is younger than
the rocks it affected and older than the rocks it didn’t
affect.
DIVISION OF EARTH’S HISTORY: The geological time of
Earth’s history is divided according to great changes as a result of
tectonic activity, climate change, varying sea levels and the mass
extinction of life forms. They are divided in eons, which are divided into
eras, which are divided into periods. At the present we are in the
Phanerozoic eon, the Cenozoic era and the Quaternary period.
PRECAMBRIAN EON
The Precambrian eon starts from the creation of the Earth 4.5 billion years ago to 542
million years ago.
It lasts 87% of the Earth’s history.
HADEAN EON
It started 4.5 billion years ago and ended 3.8 billion years ago.
The Earth was forming: meteorites collided with the Earth, which
prevented the Earth from solidifying in the surface (it was molten). Heavy elements
like iron descended to the center and formed the core. There was a mantle too, and
the lighter elements like silicon rose to the surface and formed the primitive crust.
Nearly all the crust has been subducted so there are few rocks
preserved from this eon.
High volcanism.
The atmosphere was formed: there was dust and gases. It was
composed of hydrogen, methane, helium, nitrogen, ammonia and
water vaporThe first oceans were formed too. It was thought this happened
due to the condensation of water vapor and the formation of
rain. Also, it is thought, comet impacts could have supplied the
planet with additional water vapor, forming
clouds and rain that formed oceans.
The Moon was formed.
ARCHAEAN EON
It started about 3.8 million years ago and ended 2.5 billion years ago.
It was a geological period: there are some rocks that remain form this eon
There were only microcontinents.
The atmosphere had great amounts of CO2 and few of free O2. There was a
greenhouse effect that warmed the Earth sufficiently to prevent the
development of glaciations.
Almost all the Earth was covered by oceans. These were created by the
condensation of water from the release of gases from the volcanoes.
PROTEROZOIC
It started 2. 5 billion years ago and ended 542 million years ago.
The continental crust grew quickly as it is more or less our crust
nowadays.
Meteorite bombardment stopped and tectonic plate movement
began.
The oxygen-poor atmosphere of the Archaean transformed into an
oxygen rich one, which began to accumulate in the atmosphere.
The first supercontinents were formed. Many small portions of continents collided
and joined into one large supercontinent. This happened two times until Rodinia
formed in the middle Proterozoic (ap- 1100 million years ago). Then the Panthalassic
Ocean formed and Rodinia was split into two parts. While this happened
they were been covered with ice (the Cryogenian period). This is the
coldest period of the planet’s history (even the equator was covered with
ice) .By the end of the Precambrain (600 million years ago) these two
parts collided and formed the supercontinent Pannotia.
LIFE
Hadean: it is thought that in the very end of this period the earliest primitive life-forms
appeared. They were prokaryotic organisms.
Archaean: Then came the cyanobacteria (bacteria and blue algae) , which appeared
(3.5 billion years old). They were prokaryotes unicellular organisms which performed
photosynthesis ,reducing the amount of CO2 in the atmosphere and releasing oxygen
Proterozoic: the presence of oxygen caused the evolution of the first organisms that
performed aerobic respiration. Then, the first eukaryotic (unicellular) organisms
appeared. They became established in the
environment.
At the end of this eon, the first multicellular
organisms appeared: the Ediacaran fauna, soft- bodied
organisms which were the precursors of organisms
with skeletons. They depended on oxygen to live.
PALAEOZOIC ERA
It goes from 542 million years ago to 251 million years
ago. It is divided into 6 periods:
1. Cambrian: 570 million years ago to 438
million years ago.
2. Ordovician
3. Silurian
4. Devonian
5. Carboniferous
6. Permian
It is characterized by the formation of Pangaea
and the diversification of life.
TECTONIC PLATES: Due to tectonic plate
movement Pannotia was separated and
Pangaea was formed.
Early Palaeozoic: Gondwanaland was
formed.
Middle Palaezoic: North America and
Europe collided; Australia was
subducted.
Late Palaeozoic: first stage of Pangaea.
MOUNTAIN RANGES FORMED:
Urals
Appalachians
CLIMATE:
The atmosphere was similar to the actual one: there was nitrogen and hydrogen, which were
fundamental for life to diversify.
The temperature was becoming similar to nowadays one. Glaciers formed
430 Ma.
FLORA AND FAUNA:
During the first period marine algae spread. The first land plant was the fern.
Then there were arthropods and reptiles.
EXTICTIONS:
The first great extinction took place 438 Ma, during the Ordovician-Silurian period. Many marine
animals became extinct. The principal theory for this extinction is:
A supernova took place (gamma rays and X-rays killed the animals)
The second greatest extinction (and the biggest ever) took place at the end of the Palaeozoic era, 252
Ma ago. The principal theories are:
A glaciation.
The collision of plate tectonic plates: lava covered the world’s sky in ashes.
As the Pangaea was forming, coast areas were scarce. This caused the temperatures not to
be balanced inland so there were extreme temperatures. This led to the extinction of many
animals species. It is the most reliable theory.
MESOZOIC ERA: THE AGE OF REPTILES
It is the era when the dinosaurs appeared. It is divided into 3 periods:
1. Triassic (smallest one): 248 Ma to 206 Ma ago
2. Jurassic : 206 Ma to 144 Ma ago
3. Cretaceous: 144 Ma to 65 Ma ago.
CLIMATE
Triassic: hot and dry.
Jurassic: from hot and dry to warm
(there was no polar ice) with
flooded areas.
Cretaceous: Low seasonality. The
sea level raised due to the broke up
of Pangaea. Climate was warm all
over the globe and there were no
glacial periods).
FAUNA AND FLORA
Triassic: little dinosaurs and
mammals.
Jurassic: gigantic dinosaurs and big reptiles. Dinosaurs diversified (flying and aquatic reptiles
appeared) and the first birds. Gymnosperms plants dominate the Earth’s surface).
Cretaceous: not so big dinosaurs and new plants (angeosperms).
CONTINENTS:
Triassic: Pangaea
Jurassic: Pangaea
broke up into Gondwanaland
and Laurentia.
Cretaceous: volcanic activity led to more separation.
Early Triassic:
Late Jurassic:
Late Cretaceous:
EXTINCTIONS:
Permo-Triassic (P-T)
Cretaceous-Triassic (K-T): it is thought that was caused by the impact of a meteorite in the Gulf of
Mexico
CENOZOIC ERA: means “recent life”.
It goes from 65 million years ago to now. It is divided into 2 periods:
Tertiary (divided into Paleogene and Neogene)
Quaternary
CONTINENTS
Continents we know were formed from the rupture of the Pangaea. Continental collision orogeny
formed the Alps, Carpathian, Atlas, Himalayas and Rocky Mountains (this rose without faulting, there
was steep inclination of rivers and the Grand Canyon of the Colorado River).
Tertiary period:
GLACIATION AND INTERGLACIATION PERIODS
Quaternary period:
The Earth started to cool about 50
Ma. There were lots of glacial and
interglacial periods (like in the actual
moment) that led to:
Changes in global sea levels
Extensive glacial moraines.
FAUNA AND FLORA
Mammals and flowering
plants substituted reptiles
and gymnosperms.
Mammals’ evolution:
mammals colonized land and
water, birds dominated air.
Mollusks and reef-breeding corals diversified.
Major extinction event (Ice Age animals appeared due to the climate change).
HUMAN EVOLUTION
67 million years ago. All hominids were born in Africa except homo neanthertalensis and
heilderbergensis which came from Europe.
Australopithecus: walked upright.
Homo Habilis : used rudimentary tools and instruments made of stone.
Homo Erectus: carved stones and controlled fire.
Homo heilderbergensis.
Homo neanthertal: made a great variety of specialized tools from stone
Homo sapiens: had same physical aspect as present day humans. Created cave paintings and
stone carvings.
Humans are bipedal (they walk with two legs). They may have done this because they were forced to
stop living in trees because of climate change (forest were colder and had to go to the sabbanah) This
caused a series of changes in human anatomy:
1. Spinal cord (upright position). Bones changed due to the environment, which led to an
increase in side of the vertebrae. Standing upright caused many problems.
2. Pelvis: the channel of birth makes an angle of 90º 3. Limbs (arms and legs): arms were no so long.
4. Cranium
The fact of standing upright made several improvements in the human’s body:
1. Vision amplified: sight improved, which allowed them to see their prey and predators, but small
angle of vision.
2. They used the hands (that were free because they did no longer use them to climb, run…). Hands
evolved: they had a thumb that was opposed to the rest of the fingers that let them make precise
movements. This led to the:
3. Development of the brain: the occipital band disappeared , the cranium was bigger.
4. Mastication: it depends on the diet. Tooth changed and the skull too, as muscles inserted in the jaw
in different ways depending on the diet.
FOSSILS
They are the remains of a living organism (the inorganic matter of it transformed into rock).
They are studied by paleontologists. Fossils can have formed thousands or even million years ago.
Fossils are an indispensable source of information. They provide two types of information:
Temporal information: species evolve and change. A fossil species will only appear in rocks
for a specific period of time in the Earth’s history. This means that the species or rocks can be
dated.
Palaeoecological information: living things adapt to specific type of environments, so
paleontologists can use fossils to learn about environmental conditions of a particular age.
This information can be:
Age of the layer of the land was the fossil was found. Fossils are found in different
layers of rock. Each layer of rock represents a specific interval of geologic time.
Over time, layers of rock and sediment are laid down one on top of the other. The
oldest rock layers (and therefore the oldest fossils) are always on the bottom, and
the youngest are on top. This is referred to as superposition (super = above).
Climate changes: Organisms from the past lived in ecosystems with particular
kinds of climates .One factor is temperature .Some fossils give us clues about the
temperatures of terrestrial ecosystems. We’ll take in example two leaves that
have very different shapes. Whether the leaves are smooth-edged or toothed
can provide clues about temperature. (In
forests that grow in areas of warm
temperatures, most of the plants have
smooth, non-toothed leaves. Leaves with
toothed edges are found on trees in forests
that grow in cool areas).
Past geological events: For example, the
building of mountains. Finding marine fossils
on mountain tops may be evidence of a past uplift event. Finding marine fossils
and sediments at the top of mountains shows that they were once the ocean floor.
Continental shift: The distribution of certain fossils provides evidence that
the continents have moved. E.g.: This is
a leaf from the fossil plant Glossopteris.
Fossils of Glossopteris leaves have been
found in rocks that were formed 320 to
210 million years ago. These plants were
very common in some ecosystems,
especially around 250 million years ago.
When Gondwanaland split apart into
separate continents, the plants were
simply carried along with the land. Eventually the continents moved further apart
to their present locations
If they were extinct:
Though many organisms
of the past are extinct
their relatives are often alive. The fossil record gives us
evidence about past extinction events. The extinction of one
species can affect other organisms: Fossils tell about past and
present biodiversity, about past relatives and extinctions
Features of the living organism (evolution): how organisms
are related. By studying features that are shared among
organisms, we can learn about how they are related.
Behavior. Family behavior (where eggs and nest are placed ), or social behavior (if there
are a lot of fossils of the same type of animal means that they were in herds.
Interaction: Some common interactions between organisms include: predator and
prey and dispersal (moving to a new location) . This fossil seed has tiny hooked
spines on it that were used to “grab” onto the coat and skin of passing animals. As
a result, the animal might carry it to new locations.
Environment: A complete ecosystem composed of abiotic (non-living) factors (like the ,
sea level, the sand, andthe temperature of the water ) and of biotic (living) factors (such
as the plants ,animals and their interactions)
Diet.
FOSSILIZATION PROCESS
1. The animal dies. The corps has to be insulated with air from the
environment.
2. Sediments deposit (sand, water, mud) and protects the organic matter
from bacteria.
3. Structures undergo chemical degradation. Minerals replace tissues.
CaCO2 , Fe, SiO, FeS erosion: produce an exact reproduction.
4. Fossil remains the same shape and size since it was living. It is
composed of rock.
5. Pro-mineralization: fossilization, like carbonization: tissues are made of C: all the remains
consist in carbon.
It is found all over the world, but it is only
6. All living thing’s fossil have remained the shape and
found in rocks formed 54 to 35 million years
structure since the first time they were created.
ago during a period of time called the Eocene.
INDEX FOSSILS: Some fossils help us to date the age of rocks.
Each of these fossils are found all over the world BUT they
come from organisms that existed for only a specific period
of time. Since they were alive only during a specific period of
time, that means that their fossils are found in very specific
layers of rock. A fossil found in specific layers of rock all over
the world is called an index fossil.
Every time we find index fossils, we can figure out the age of
the rocks they are preserved in.
These fossils appeared and extinct in a short period of time, but spread all over the Earth. They
provide relative dating.
There are no fossils before the Proterozoic eon because organisms didn’t had skeletons (Ediacaran
fauna)