The relation between Time & Geology

Time & Geology
Ayon Saha
Department of Geology & Mining
University of Barishal

Table of Contents
 Relative Age
 Principle used to Determine Relative Age
 Unconformities
 Correlation
 The Standard Geologic Time Scale
 Numerical Age
 Isotopic Dating
 Uses of Isotopic Dating
 Age of the Earth
 Comprehending Geologic Time

Relative Age
 The age of a rock or fossil compared to another rock
or fossil.
 Example: Rock a is OLDER than Rock d.
 An actual age in years is not determined.
Youngest
Oldest

Relative Age cont.
 Principle used to Determine Relative Age
• original horizontality
• superposition,
• lateral continuity,
• cross-cutting relationships,
• inclusions
• fossil Succession

Principles of Relative Age
Law of Superposition : When sedimentary rock layers are
deposited, younger layers are on top of older deposits.

Law of Original Horizontality : Sedimentary rock layers are
deposited horizontally. If they are tilted, folded, or broken, it
happened later.

Law of lateral continuity : Sedimentary rock layers extend in
all directions horizontally unless a river erodes them or an
earthquake moves them

Law of Cross-Cutting Relationships: If an igneous intrusion
or a fault cuts through existing rocks, the intrusion/fault is
Younger than the rock it cuts through.

The Law of Inclusions : If one rock body contains fragments of
another rock body it must be younger than the fragments of rock
it contains.
Inclusions of B are older than C.Inclusions

The Law of Fossil Succession : The fossils present in a rock
exposure or in a core hole can be used to determine the ages of
rocks very precisely. Some fossils have a short, well-known time
of existence. These useful fossils are called index fossils.

Unconformities
• An unconformity is a break in the rock record
produced by erosion and/or nondeposition of
rock units
• The rock record is incomplete
• Types of unconformities
–Angular unconformity
–Disconformity
–Nonconformity
–Paraconformity

Types of unconformities
Angular unconformity - When the underlying (older) rocks
and overlying (younger) rock strata show some angle to
one another.

Disconformity : When the underlying (older) and
overlying (younger) sedimentary rock strata are parallel
and the contact plane is an erosional surface.

Nonconformity : when the underlying rocks are Igneous or
Metamorphic (i.e. unstratified) and the overlying rocks are
sedimentary. (stratified)

Paraconformity:which strata are parallel; there is little
apparent erosion and the unconformity surface resemble
s a simple
bedding plane. Also known as nondepositional unconfor
mity; pseudoconformity.

Correlation
The determination of the time-equivalency of rock units
 Physical continuity –
• physically tracing a continuous exposure of a rock unit
 Similarity of rock types -
• assumes similar sequences of rocks formed at same time
• can be inaccurate if very common rock types are involved
 Correlation by fossils -
• fossil species succeed one another through the layers in a predictable
order

The Standard Geologic
Time Scale
 Composed of standard stratigraphic divisions based on rock sequences and
is calibrated in years measured by relative age.

Numerical Age
 Puts absolute values (e.g., millions of years) on the ages of
rocks and geologic time periods
• uses radioactive decay of unstable isotopes
• only possible since radioactivity was discovered in 1896
 Radioactive isotopes change (decay) into daughter isotopes
at known rates.
 Rates vary with the isotope :
e.g., U235 , K40 , C14 , etc.

Half-life
Scientists measure the length of time it takes for one-half of the
original isotope to decay
After one half-life, a sample contains 50 percent parent and 50
percent daughter
After two half-lives, the sample contains 25 percent parent and
75 percent daughter.

Uses of Isotopic Dating
 Estimate the age of the sample and choose an isotope with an appropriate
range.
 First find out what minerals are in the sample. The minerals in the rock need
to have the element which is wanted to use for dating.
 Carbon-14 can only be used to date
samples that were once living (organic)
 Ex: Wood, bone, cloth, paper

Age of the Earth
 Numerical dating gives age for Earth of ~ 4.56 billion years
• oldest age obtained for meteorites, believed to have been
unchanged since the formation of the solar system
• Earth and rest of solar system very likely formed at this time
 Geologic (deep) time is vast
- a long human lifetime represents
only about 0.000002% of
geologic time

Geologic time scale
 The geologic time scale is a “calendar” of Earth’s 4.5 billion
year history
 Subdivides geologic time based on fossil assemblages
 Subdivides geologic history into units for easy reference
 Originally created using relative dates
 Absolute dates later applied with development of radiometric
dating techniques

Geologic time scale cont.
 Divided into eons, eras, periods , and epochs
• Eon – the greatest expanse of time
• Era – subdivision of Eon
• Period – subdivision of Era
• Epoch – subdivision of Period
Eons Eras Periods Epochs
Smaller divisions of time

 - Phanerozoic (“visible life”) –
the most recent eon, began
about 545 million years ago
 - Proterozoic
 - Archean
 - Hadean – the oldest eon
 Eons – The largest unit of Geological Time
Scale

 Era – subdivision of an eon
 Eras of the Phanerozoic eon
 Cenozoic (“recent life”)
 Mesozoic (“middle
life”)
 Paleozoic (“ancient
life”)

 Period – subdivision of an era
 Names derived from:
 “Type” localities (e.g. Jurassic,
named after Jura Mountains)
 Rock characteristics (e.g.
Carboniferous, coal-rich rocks in
the UK)
 From various whims (e.g. Silurian,
named after Celtic tribe of Wales)

 Epochs are normally separated by
significant changes in the rock
layers they correspond to.
 We are currently living in
the Holocene Epoch of the
Quaternary Period.
 Epoch – subdivision of the geologic timescale that is longer
than an age and shorter than a period

The relation between Time & Geology

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