‫‪Geologic time‬‬
‫)الزمن الجيولوجي(‬
Determining geological ages
Relative age dates (‫ – )التأريخ النسبي‬placing
rocks and geologic events in their proper
sequence
of formation
 Numerical dates (‫ – )التأريخ المطلق‬define the
actual age of a particular geologic event
(termed absolute age dating)

Steno’s Laws

Nicolaus Steno, 1669:
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Principle of original horizontality
Principle of superposition
Principle of original continuity
Powerful tools for:
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Analysis of outcrops.
Principle of Original Horizontality
(‫)قانون التأفق المبدئي‬

Sedimentary rocks are formed in layers
(strata) which were originally horizontal
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Flat strata are probably undisturbed by
folding and faulting
Tilted strata have been affected by tectonics.
Principle of Original Horizontality
Principle of Superposition
(‫)قانون تعاقب الطبقات‬
 Developed
 In
by Nicolaus Steno in 1669
an undeformed sequence of
sedimentary or volcanic rocks, oldest
rocks at base; youngest at top
Principle of Superposition

Oldest sedimentary
rocks at the bottom: C
deposited first

Younger sedimentary
rocks on top: B then
A.
Superposition illustrated by strata
in the Grand Canyon
Principle of cross-cutting relationships
(‫)قانون التقاطع‬
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Magma under pressure squeezes into nearby rock
whenever fractures form in the rock
Then the magma slowly cools and becomes rock itself.
This newly formed igneous rock is, of course, younger
than the rock it intruded
Younger features cut across older ones.
When a fault cuts through other rocks, or when magma
intrudes and crystallizes, we can assume that the fault or
intrusion is younger than the rocks affected.
Principle of cross-cutting
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Cross cutting igneous
rocks are younger
than what they
intrude.

Faults are younger
than what they cut.
Principle of Inclusion
(‫)قانون الجذة القاطعة‬
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Inclusions are pieces of one rock
unit that are contained within
another.
Erosion surfaces exist in the
rock record...
Fragments within strata above
are derived from the older
strata below  they are older
than the strata containing them.
The strata containing the
fragments are younger than the
strata the fragments came from
Included Fragments
If a rock contains a fragment or particle of
another rock, then the fragment is older than
the rock.
 Example
A pebble that is part of a conglomerate rock is
older than the rock. The pebble had to exist
first in order to become part of the
conglomerate when it formed.

Conglomerate (sedimentary)
Breaks in stratigraphic record
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Sometimes the sedimentation stops and the strata become
exposed at the surface
The beak in deposition may result from a regional uplift or sea
level falling
The strata exposed at the surface are subjected to weathering
and erosion
Sedimentation restarts when the area becomes submerged in
water again
The breaks (gaps) are marked by a rough and eroded surface
called unconformity
Unconformity is an indication of environmental conditions
which cause the sedimentation to stop.
Unconformities (loss of rock record)
(‫)عدم التوافق‬
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An unconformity is a break in the rock
record produced by erosion and/or
nondeposition
Types of unconformities
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Angular unconformity – tilted rocks overlain by flatlying rocks (‫)عدم التوافق الزاوي‬
Disconformity – strata on either side of the
unconformity are parallel
Nonconformity – sedimentary rocks deposited above
metamorphic or igneous rocks (basement)
Conformable Contact
(‫)متوافقة‬
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Layers of rock that
have been deposited
without any
interruption.
No gaps in time.
No missing record
due to erosion, nondeposition, etc.
Formation of an angular
unconformity
An angular unconformity at Siccar Point, England
Unconformity
Cross Cutting Relationships in strata
Development of a Nonconformity
Nonconformity in the Grand Canyon - Strata
deposited over Schist
Disconformity
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A break in the rock
record across which there
is little change in stratal
orientation.
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Often just a pause in
deposition (subtle).
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May also be obvious
erosion surface.
Correlation of rock layers

Matching strata of similar ages in different
regions is called correlation
Correlation of strata in
southwestern United States
Sections are
incomplete
Correlation of rock layers with fossils

Correlation relies upon fossils
of fossil succession – fossil
organisms succeed one another in a
recognizable order - thus any time period is
defined by the type of fossils in it
 Principle
Determining the ages of
rocks using fossils
Geologic time scale

The geologic time scale – a “calendar” of
Earth history
 Subdivides
geologic history into units
 Originally created using relative dates

Structure of the geologic time scale
 Eon
– the greatest expanse of time
Geologic Timescale
Divisions based on fossils
Geologic time scale

Structure of the geologic time scale
 Names
of the eons
Phanerozoic (“visible life”) – the most recent
eon, began about 540 million years ago
 Proterozoic
 Archean
 Hadean – the oldest eon

Geologic time scale

Structure of the geologic time scale
– subdivision of an eon
 Eras of the Phanerozoic eon
 Era
Cenozoic (“recent life”)
 Mesozoic (“middle life”)
 Paleozoic (“ancient life”)

 Eras
are subdivided into periods
 Periods are subdivided into epochs
Geologic time scale

Precambrian time
 Nearly
4 billion years prior to the
Cambrian period
 Not divided into small time units because
the events of Precambrian history are not
know in detail
 Immense space of time (Earth is ~ 4.5 Ga)
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Radioactivity (Used to age date rocks)
Spontaneous changes (decay) in structure of
atomic nuclei
Types of radioactive decay
 Alpha emission
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Beta emission
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Emission of 2 protons and 2 neutrons (an alpha
particle)
 Mass no.= +4
 Atomic no.=-2
An electron (beta particle) is ejected from the nucleus
 Mass no.= constant
 Atomic no.=+1
Electron capture
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An electron is captured by the nucleus
The electron combines with a proton to form a neutron
 Mass no.= constant
 Atomic no.=-1
Neutron capture (A)
and Beta emission (B)
Using radioactivity in dating
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Parent – an unstable radioactive isotope
Daughter product – isotopes resulting from
decay of parent
Half-life – time required for one-half of the
parent isotope in a sample to decay
A radioactive decay curve

Dating with carbon-14 (radiocarbon dating)
 Half-life
only 5730 years
 Used to date very young rocks
 Carbon-14 is produced in the upper
atmosphere
 Useful tool for geologists who study very
recent Earth history
Using radioactivity in dating

Importance of radiometric dating
 Allows
us to calibrate geologic timescale
 Determines geologic history
 Confirms idea that geologic time is immense