The history of the earth

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Transcript The history of the earth

Or how did you arrive here….
The formation of the Solar
 The most accepted model is the “nebular
hypothesis”: the formation of the SS
originated from a nebula of dust and gas.
Earth’s formation
 Accretion of fragments made of mainly
 The heat generated by the collision of
elements, melted the planet and the denser
materials moved and formed the core.
Silicates formed mantle and crust.
The age of the Earth
 17th century: 4004 years
 19th century: 40 million years
 End of 19th century: 100 million years
 Currently accepted: 4550 million years
Dating the record
 RELATIVE DATING: consists of ordering the
strata or geological events recorded in one or
more stratigraphic series chronologically.
 ABSOLUTE DATING: consists of assigning a
specific date to each stratum or geological
event recorded in a series, indicating how
long ago it was formed or occurred.
-Radiometric methods (rate of decay of
The strstigraphic record
 The stratigraphic record is the information
about the past contained in the strata of
sedimentary rocks.
 All sedimentary basins originated at a specific
time and place and, from their origins to the
present day, they have undergo a series of
processes that affect the strata formed in
1. Filling in with sediments
2. Diagenesis ( transformation of sediments
into sedimentary rocks)
3. Alteration of the sedimentary basin
(lithospheric dynamics: erosion,
deformations, etc)
The information contained in
the strata.
 The main characteristics of a stratum are its
extension, lithological composition (types of
rocks and environmental conditions), its fossil
content (biodiversity in that period, index
fossils provide more info) and its
arrangement ( geological processes).
The basic principles of
1. Principle of uniformitarism: the geological
processes that acted in the past are the
same and have the same effects on rocks as
2. Priniple of the succession of geological
events: events occurred after the formation
of the rocks
3.- Principle of the arrangement of the strata:
1. Original horizontality: strata are formed
2. Lateral continuity: a stratum has the same
age trhoughout its extension
3. Superposition: each layer is older than the
one above it
 4.-Principle of the succession of fossils: Strata
containing the same fossil have the same age.
Index fossils are important.
 Index fossils: had a large geographical
distribution but only during a specific period.
 Facies fossils: can only live in certain
 Fossils provide temporal and paleoecological
Comparing strata
Natural strata
Fold - syncline
Folds: anticline
Folds: pliegues
Fold elements
Faults: fallas
Reverse fault
Fossil formation. Step 1
 Fossilisation only happens in the rarest of cases,
when a plant or animal dies in the right
circumstances. Animal corpses are usually eaten
by something, or bacteria rots them away before
fossilisation can occur, and even hard parts like
bones and shells are eventually destroyed
through erosion and corrosion. The trick to
becoming a fossil is to die in a location where
your body - or bits of it - are protected from
scavengers and the elements. This means
getting buried in sand, soil or mud and the best
place for that is on the seabed or a river bed.
Step 1
Step 2
 Only in very rare cases do the soft parts of
animals - the flesh, skin and internal organs become fossils. Even when buried under mud
or soil, decay still takes place, though lack of
oxygen does slow it down. If a skeleton is dug
up at this stage, it will still be made of bone.
Remains like these that haven't truly
fossilised yet are sometimes called 'subfossils'.
Step 3
 As more time passes, sub-fossils become buried
deeper and deeper. What was mud or sand
becomes compressed on its way to becoming
rock. But even safely sealed away underground,
time doesn't stand still. Chemicals and minerals
percolate through the sediment and the original
bone or shell gradually recrystallizes. In extreme
cases, the entire thing can dissolve away, leaving
a hollow where it once was. If palaeontolgists
find a hollow like this, they can pour liquid rubber
in to make a fossil cast, or put it in a medical
scanner to see what the original looked like.
Step 4
 In other cases, minerals from the rocks gradually
impregnate the bone, shell or wood, changing its
chemical composition and making it capable of
surviving for as long as - or sometimes longer
than - the rock enclosing it. In cases where the
original has dissolved away, the minerals can
gradually fill the hollow to create a natural cast
of the original. So sometimes a fossil doesn't
contain anything of the original creature except
its shape. Even that shape can take a battering! If
the rocks are distorted and squeezed by
geological forces, then the fossils within them
will be too.
Step 5
 Even rocks have a finite lifespan. Eventually
the rock enclosing a fossil is eroded away,
and the fossil is revealed on the surface of the
ground. With luck, a sharp-eyed fossil
collector will spot and excavate it. Otherwise
the elements will continue to batter it, until it
- along with the rocks around it - is reduced
once more to sand, silt or mud.
The Earth’s history
 Geological time spans more than 4500 million
years. This period has been divided into
intervals; the largest units are eons, divided
into eras, divided into periods.
History of life on Earth
The history of life on Earth began about 3.8 billion years ago, during the
Archean era, initially with single-celled prokaryotic cells, such as
bacteria. Multicellular life evolved over a billion years later and it's only
in the last 570 million years that the kind of life forms we are familiar
with began to evolve, starting with arthropods, followed by fish 530
million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals
didn't evolve until 200Ma and our own species, Homo sapiens, only
200,000 years ago. So humans have been around for a mere 0.004% of
the Earth's history.
 The Tree of Life
 The Tree of Life illustrates how different species arise from previous
species via descent with modification, and that all of life is connected.
The diagram below shows the relationship between the major biological
groups. The centre represents the last universal ancestor of all life on
earth, the outer branches the major biological groups.
The tree of life
Diagram of eons and eras
4550-3800 million years ago
 Layers of Earth formed
 First atmosphere formed
 Meteorites bombarded the Earth
 Moon formed
 At the end of the Hadean eon primitive life
3800-540 million years ago
 Only microcontinents (Archean era)
 Crust grew
 Supercontinents and litospheric movements
 Meterorite bombardement stopped
 Tectonic plate movement
 Oxygen appeared in atmosphere
 Iron oxide rocks
 Ice! The coldest period ( Cryogenian)
 First supercontinent: Rodinia
540-250 million years ago
 Rodinia broke up and reunited as
 Paleozoic
250 -65 million years
 Mesozoic
 Pangea separated in present-day continents.
 Half of the continents were submerged and
covered with limestone and plankton (
65 million years ago –
present day
 Cenozoic
 Collision of continents
 Climate cooled: 18 glacial periods