4. Contents:
1. Earth History:
2. How Old is the Earth?
3. What is geochronology?
4. principle of Uniformitarianism:
5. Types of Geological Dating:
6. Principal involve in relative age:
7. Numerical Dating:
i. Atomic number
ii. Mass number
iii. Isotopes
iv. Radioactive decay
v. Nuclie of radioisotopes may change in three way
vi. Half life
vii. Radioisotopes for the age of earth
8. Isostacy:
i. Isostatical adjustment
ii. Hypoyhesis by J H Pratt And G B Airy
5. Earth History
Distance from Sun: 150 million kilometers
(93.2 million miles)
Orbital period: 365.256 days
Rotational period: 23.9345 hours
Tilt of axis: 23.45 degrees
Diameter: 12,756 kilometers (7,973 miles)
Mean density: 5.515 g/cc
Mean surface temperature: 15°C
Atmospheric pressure: 1.013 bars
Atmosphere composition: 77% N, 21% O and
2% other.
6. How Old is the Earth?
Estimated age for the Earth and the rest of the
solar system is about 4.6 billion years ago.
The oldest Earth rocks: 3.8 to 3.9 billion years .
Oldest Earth minerals (zircons): 4.2 billion years.
Oldest Moon rocks: 4.44 billion years ago.
3.8 Billion Year Ago, life develops.
7. WHAT IS GEOCHRONOLOGY?
The study of earth , Age
of earth , Age of rock ,
And some other
formation is known as
Geochronology.
the branch of geology
concerned with the
dating of rock
formations and
geological events.
8. principle of Uniformitarianism
The geologic processes
going on today, such as
weathering, erosion,
volcanism, and
earthquakes, also went
on in the past.This
concept is known as
the principle of
uniformitarianism.
9. Types of Geological Dating
Many geologic events that Earth Scientists
study occurred millions of years ago. The ages
of these events can be determined in two
different ways.
1. Relative age
2. Numerical age
Both of these dating methods are needed to
accurately record geologic time chronologically
and to organize the geologic rock record.
10. Relative Age & Absolute Age
Relative age:
Does not identify their
actual date of occurrence.
Does not involve
numbers, but rather a
sequencing of events by
comparing the events.
Can’t tell us how long ago
something happened,
only that it followed one
event and preceded
another.
Numerical age:
Identifies the actual date of
an event.
Pinpoints the exact time in
history when something
took place.
Involves numbers to count
or calculate from a known
event.
11. 1.Relative age:
we can find the age of rock by
comparing with the other rock.
We use common sense in relative age.
Used to determine if one thing is
younger or older than another.
The relative age of a rock or event is the
age as compared to other rocks or
events.
12. Principle involve in relative age
1. Original horizontality:
2. Law of superposition:
3. A younger rock may contain pieces of older rock:
4. Cross-Cutting Relationships:
5. Igneous clues:
6. Stream & terraces:
7. Desert pavement:
8. Gap in geological records:
9. The fossil to infer the age of rock:
10.Law of faunal succcessions:
11.Index fossils:
12.Geological time scale:
13. 1) Original horizontality:
The principle of original horizontality states that
sediments are deposited in horizontal layers that
are parallel to the surface on which they were
deposited.
This implies that tilted or folded layers indicate that
the crust has been deformed.
14. 2) Law of superposition:
Within sequence of
undisturbed
sedimentary or
volcanic eruption the
older rock line below
then the younger rock.
This phenomena is
known as Law Of Super
Position
15.
16. The idea that rock and fossil found in lower layer
are older than the rock and fossil found in the top
layer.
17. 3) A younger rock may contain pieces of
older rock:
When the bed rock was erode which was
already present and erosion caused by river
and when it deposited on bed rock.it was
already formed there the new rocks will
produced which was younger then bed rock.
18. 4) Cross-Cutting Relationships:
The Fine grains sediments was erupted on
surface but when large sediments come
from the earth surface they may also erupt
in surface.
Dike and sill may shows cross cutting
relationship.
Rock fracture or faults may occur which are
younger than the orignal rock formation.
20. Igneous Intrusion
When magma
forces its way
into cracks or
crevices in crustal
rock and
solidifies, it forms
a mass of igneous
rock called an
intrusion.
21. 5) Igneous clues:
The magma is in molten state that was
present in sub surface.
When magma cooled at the surface it may
become lava.
The cooling state of lava is known as
extrusion.
The age of extrusion rock is younger then
the rock formed beneath earth surface.
22. Igneous Intrusions and Extrusions
When magma forces its way into cracks or
crevices in crustal rock and solidifies, it forms
a mass of igneous rock called an intrusion.
When lava solidifies at the surface it forms a
mass of igneous rock called an extrusion.
Since the rock that the magma moved
through, or over, existed prior to the
intrusion, (or extrusion), it must be older.
23. 6) Stream & terraces:
The terraces were formed
before the river flows to
its present level.
The age of stream terraces
is older then the modern
channels.
The upper channel are
younger that the lower
channels so on.
24. 7) Desert pavement:
Remaining material of
fine grains when mixed
with water then they
collide up to form
desert pavement.
It may have ten
thousand year to
formed desert
pavement.
25. 8) Gap in geological records:
Unconformity is a surface that may shows gap in
geological records.
A missing interval of time that may represent gap.
Unconformity is formed when the layer of rock are
missed due to erosions.
26. 9) The fossil to infer the age of rock:
Fossils are imprints or remains of animals and
plants that are preserved in rocks or sediments.
Fossils are used to find the age of rock.
The fossil that are found in upper side are younger
then the fossil that are found at the bottom.
We can find the age of one rock and compare it
from the other rock by fossil.
27. 10) Law of faunal succcessions:
Fossil changes upwards from older sedimentary rocks to
younger sedimentary rocks.
This systamatic change of fossil with age is known as law
of faunal succession.
The fossil that was found in younger sedimentary rocks
cannot found or meet in older sedimentary rocks.
Petrologist have find the thousand fossils of species and
have be indentified the sequence in which they exist.
28. Fossils:
Naturally occurring
preserved remains or
impressions of living things.
Generally only hard parts
get preserved e.g Bones,
teeth, shells.
Hard parts are replaced by
naturally occurring minerals.
Eurypterus
NewYork State Fossil
Silurian index fossil
29. Fossils:
Generally found in sedimentary rock layers .
Fossils can provide information about
ancient environments.
Fossils preserved in the rocks provide
evidence that many kinds of animals and
plants have lived on Earth in the past under
a variety of different conditions.
30. 11) Index fossil:
Very short lived geologically wide spread species
is known as index fossil.
Exist during specific periods of geological times.
A single index fossil co relative the rock in which
that fossil meets.
31. 12) Geological time scale:
A great variety of
plants, animals, and
simpler life forms
have lived on Earth in
the past .
Most life forms of the
geologic past have
become extinct.
32. Geologic time is subdivided divided into units based
on fossil evidence.
There are 4 major divisions:
Precambrian – represents the first 85% of Earth’s
history (mostly devoid of fossils).
Paleozoic Era – represents ~ 8.5% of Earth’s history
(invertebrates, fishes, amphibians, vertebrates and
land plants first appear).
Mesozoic Era – rep. ~ 3.5% of Earth’s history
(dinosaurs, earliest birds, and mammals).
Cenozoic Era – rep. ~ 1.4% of Earth’s history
(humanoids show up late ~0.04% of history).
33. NUMERICAL DATING
To assigned the age of
rock in thousand
million and billion of
years.
34. Atomic number:
the number of proton of an atom is the element
atomic number.
Element: element is the number of proton in its nucleus.
Mass number:
the number of proton and neutron is known as
mass number O16.
Isotopes:
isotopes of an element which may contain same
number of proton and different number of neutrons.
35. Radioactive decay:
It is the process in which unstable isotopes
may spontaneously change into the stable isotopes is known
as radioactive decay process.
Nuclei of radioisotopes may change in
three way:
1. alpha decay
2. Beta decay
3. Electron capture
36. Alpha decay:
Ejects of two proton in two neutron from a nucleus .
Atomic number reduce by two.
Atomic mass reduce by four.
Beta decay:
Release of an electron from a nucleus.
If the electron is emitted from the neutron during radio active
decay the neutron become a proton and the atomic number is
increasing by one.
Electron Capture:
Proton is the neutron capture on orbiting electron .
The proton become a neutron.
An atomic number one less then parent isotopes.
Not change an atomic mass.
37. Half life:
The half life of radio active element is the time of take
for half of the atom of the unstable parent element to
decay into the atom of the more stable daughter
element.
Atom of parent 100 = 50 - 25
Atom of daughter 50 - 75
38. Radioactive isotopes commonly used to
determined the age of earth
Parent isotopes Half life of parent
isotopes
Daughter product Mineral &
rock data
Potassium-40 1.3 billion year Argon-40 Muscovite
Biotite
Hornsland
Gloconite
Uranium-238 4.5 Billion year Lead-206 Zircon
Uranite
Uranium 235 713 million year Lead-207 Zircon
Uranite
Thorium-230 14.1 billion year Radium-226 Zircon
Uranite
Rubidium-87 49 billion year Strontium-87 Zircon
Uranite
Carbon-14 5730 years Nitrogen-14 Muscovite
Biotite
Potasium
Feldspar
Or
Igneous rock
39. The amount of K-40 in that rock was first
determined by chemical anylysis.
Amount of daughter product Ar-40 must also be
added adding the two values gives us how much
Kr-40 is present.
By knowing how much KR-40 is orignally is present
and how much remain in the rock.we can calculate
the rock age on the basis of half life.
Age of rock=3.323T log(1+Nd/Np)
T is half life of radioactive isotopes
Np remains for half life of Parent isotopes
Nd remains for Half life of daughter isotopes
mass spectrometre is used to determined it.
40. The isotopes that decay quickly can be used to date
young rocks.
K-Ar:
K-Ar method is used to date volcanic
rock.volcanic rock is cool on surface .the age of
this rock is like the age of eruption.
41. Rb-Sr:
method provided the some of the first ages for
old granites and metamorphic rocks was given the key
in demostrating the same rock on earth and some
meteorites were billions of year old.
U-Th:
possibly occur in same minerals it is possible to
demostrate two age in one mineral.
U-Pb:
less commonly used then the Ar-K and Rb-Sr
Method used in rare minerals commonly used as
major constituents.
42.
43. Isostacy:
In 1889 scientist C.E Dutton suggested that.
A gravitational balance exist between continents
and ocean basins.
Isostacy is balance or equilibrium of adjacent block
of lithosphere floated on the asthenosphere of high
density.
The mountain stand high because they are
composed of rocks of low densiy
The ocean basins are low because of the rock of
high density.
44. Isostatical ajustments:
The vertical movement of the crystal of rocks to
reach the equilibrium the sediments from higher
part of crust deposited on the lower part due to
move rapid erosion
Decrease in weight from higher part of crust cause
into rise.
Increase in weight from lower part of crust cause
into sink.
Isostatic adjustments take place when large volume
are eroded from or deposited on a part of crust.
46. 1) J H pratt:
It has been assumed that
the boundry b/w the upper
light material and lower
dense rocks are at a
uniform depth.
This observation made by
the help of seismic studies.
He said that blocks of
different density located at
different height but there
base is uniform
47. 2)G.B Airy:
All the blocks are made up of
same material.
According to this there is a
change in the density of rocks
at depth the upper higher
material floated on the dense
layer.
The depth of this change may
varies from place to place.
Airy said that ice berg is
same on upside as in lower
side.
