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Earth’s History
Earth’s History
Earth’s History
Earth probably formed from an accumulation
of rock, dust, and gases drawn together by
its own gravity about 4.6 billion years ago.
 The rocks of Earth’s crust preserves clues
that help us unravel the
mystery of our changing
planet, its environments,
and the development
of terrestrial life.

Uniformitarianism
Geologists think that the forces that they
observe today are similar to processes
that occurred throughout Earth’s history.
 “The present is the key to the past.”

Law of Superposition

The rocks at the bottom of an undisturbed
exposure are usually the oldest.
– There are occasional exceptions to this law.
Original Horizontality
A rock is always older than
the process that changed it.
 Sediments are usually
deposited in layers.

– When we see sedimentary
layers, we usually assume that
these layers were deposited
level, and that they were tilted
after they had turned into
sedimentary rock.
Igneous Extrusions

An Extrusion occurs when molten rock flows
onto Earth’s surface, where it crystallizes to
form igneous rock.
– An extrusion is younger than the rock below it,
but older than the rock that will form on top.
– The rock below the extrusion will show a zone of
contact metamorphism
where the hot lava baked it.
Igneous Intrusions

An Intrusion is an internal process where
magma squeezes into or between layers
of pre-existing rock.
– The hot molten rock changes the surrounding
rock immediately
above and below
and next to it by
contact
metamorphism.
Folds and Faults
Folds are bends in rock layers produced by
movements of Earth’s crust, generally related
to Earth’s tectonic plates.
 Faults are breaks in the rock where movement
has occurred often associated with
earthquakes.
 Offset layers are indications of faulting.

– Folds and
Faults occur
after the rock
has formed.
Fossils

The preserved remains or traces of living things.
– Can reveal a great deal about past life forms and
environments.
– Can also provide clues about the past geological events
or relative ages of rock layers.

Trace Fossils do not contain the remains of the
organisms that produced them.
– Include the impressions of shells,
dinosaur footprints, oddly shaped
formations from sediments filling
in animal burrows and
petrified drippings.
– Trace fossils reveal much about an
organisms behavior and relationship to
its living and nonliving environment
Correlation of Rocks

Geologists try to
match similar rock
strata in different
locations to see if
they formed at the
same time or under
similar conditions.
– Color, texture,
composition.
– Compare index fossils in the strata.
Correlation of Rocks Example

Which layers are the same?

Of the rock layers E and F,
which is the oldest?

What is the correct
sequence of rock layer from
oldest to youngest?

An unconformity (buried
erosional surface) is
represented by the interface
between which two layers?

What type of rock is layer A?
Geologic
Time Scale
Based on rock
formations that
contain
characteristic fossil
groups and on
changes in the kinds
of organisms that
inhabited Earth.
 The scale is divided
into eras, periods,
and epochs.

Evolution of Life

Geologists believe that life forms existed
in the Precambrian Period.
– They had no hard parts , therefore few left
fossils.
– Precambrian fossils are very rare.

More complex organisms
developed as time went on.
– Some disappeared (went extinct)
from the fossil record .

Within each species there
are variations in size, shape
and other traits.
Evolution of Life
The Evolution of Life
(Charles Darwin) states that
individuals that have traits
that better adapt them to
their environment will
survive longer and have
more offspring to pass on
these desirable traits.
 This process of Evolution
of Life, leads to the
extinction of some species
and formation of new ones.

Evolution of Life

Paleontologists (geologists
who study fossils) have found
remains of a large variety of
plants and animals that lived in
many different environments.
– Some still exist, but most have
become extinct.

Most organisms decompose or
are consumed by other
organisms after they die, only
a very small percentage leave
any fossil remains.
– Because of this, many forms of
life will never be known.
Life and The Atmosphere

Microscopic
organisms that
developed
about 2.2
billion years
ago changed
the mixture of
gasses in our
atmosphere.
Life and The Atmosphere
About 3.8 billion years ago, the atmosphere
probably consisted of a mixture of carbon
monoxide, carbon
dioxide, hydrogen,
nitrogen, ammonia,
and methane.
 The atmosphere today
is 78% nitrogen
and 21% oxygen.

Past Geologic Events
No single location shows a complete record of
the geologic past.
 If an area was above sea level for a while, it is
likely that sediments were not deposited and
older rocks have been destroyed by erosion.
 When a new rock layer is layered on a surface
left by erosion, it forms a buried erosion
surface (unconformity).

Radioactive Dating

Measurements of natural radioactivity in the
rocks have allowed the geologic time scale to
become an absolute time scale.
– One that gives the
absolute age
(numerical age) of an
object (measured
in years).

Chemical elements
often have several
forms (isotopes) that
differ in the number of
neutrons in their
atomic nuclei.
Radioactive Dating
– If the nucleus of an isotope has more or fewer
than the normal number of neutrons, the
isotope may be radioactive.
– A radioactive isotope will break down naturally
into a lighter element called a decay product.
– In the process, it gives off radioactivity.
– A sample of a radioactive element contains
millions of atoms, from which we can predict a
rate of decay.
Half-Life

The rate of decay of a radioactive element is
measured by its half-life.
– Different radioactive elements have different
half-lives.

A half-life is the time required for half of an
element’s atom in a
sample to change to
the decay product.
– At the end of one half-life,
a sample contains equal
amounts of the radioactive
element and its
decay product.
Half-Life
In each succeeding half-life, half of the
remaining atoms decay.
 As the element decays,
fewer radioactive atoms
remain in the sample,
and more decay
product accumulates.

– The higher the ratio of
decay product to
radioactive element,
the older the sample.
Decay-Product Ratio
The ratio between the mass of a radioactive
element and its decay product in a sample.
 After we determine this ratio, we can
calculate how many
half-lives have
gone by since
the sample was
formed and then
determine its age.

Selecting the Best Radioactive
Element for Dating a Sample



The sample to be dated must contain a measurable
quantity of a radioactive element and its decay
product.
A sample containing the remains of living
organisms is likely to contain radioactive carbon-14.
The sample’s age must also be considered.
– Carbon-14 can only
date samples no older
than about 50,000 years.
– Uranium-238 can measure
samples of the oldest
rocks on our planet.