Geologic time
Absolute dating: determination of a numerical date for
a rock or an event, only became possible in the XX century
Relative dating: placing rocks and events in order from oldest
to youngest or vice versa
Relative dating is based on
1. Law of Original Horizontality- Sedimentary strata and lava flows are deposited in horizontal sheets. If
these layers are not horizontal, subsequent movements have occurred.
2. Law of Lateral Continuity- Strata and lava flows extend laterally in all directions and pinch out at the
edge of their deposition.
3. Law of Superposition- In an undisturbed sequence of strata or lava flows; each layer is older than the one
above and younger than the one below.
4. Law of Inclusions- Inclusions are rock & mineral fragments contained in another rock type. Any inclusion
is older than the rock that contains it.
5. Law of Cross-Cutting – Any feature that cuts across a rock or sediment must be younger than the rock or
sediment through which it cuts. Examples: faults, unconformities and igneous intrusions.
6. Law of Unconformities- Unconformities represent gaps in geologic time when layers were not deposited
or when erosion removed layers.
a. Disconformity-Unconformity between parallel layers
b. Angular Unconformity-Tilted strata are overlain by younger more parallel strata.
c. Nonconformity-Sedimentary layers deposited on igneous or metamorphic rock.
7. Faunal Succession- Fossil organisms succeed one another in a definite, irreversible, and determinable
order.
8. Index Fossils- Used to correlate rock sequences from different locations.
a. Must have lived for a short geologic period of time.
b. Must have had a wide geographic range.
c. Must be easily identified.
9. Correlation- matching of rocks of similar age in different regions.
Established three principles
of relative dating:
- Superposition
- Original horizontality
- Lateral continuity
(born Dutch, 1638-1686)
Famous geology teacher, founder of Neptunism
All rocks were formed in the ocean and precipitated
from water. Major controversy involved the formation
of igneous rocks that he believed also took place in the
ocean.
Abraham Gottlob Werner
(1749 or 1750 – 1817)
Germany
trained in law and mining
Scottish geologist, ‘gentlemen-farmer’,
the father of uniformitarianism and plutonist
School in geology
Established that geologic time was very long,
defined unconformity and cross-cutting
relationships.
James Hutton
(1726-1797)
trained as lawyer and
doctor
Hutton also proposed that the interior of the
Earth was hot, and that this heat was the engine
which drove the creation of new rock: land was
eroded by air and water and deposited as layers
in the sea; heat then consolidated the sediment
into stone, and uplifted it into new lands.
Criticized neptunist point of view that all rocks
were formed from water
Professor and later secretary of a National
Institute, held important government positions
during both Napoleon and Bourbons.
In 1826 he was made grand officer of the
Legion of Honour; and in 1831 he was raised by
Louis Philippe to the rank of peer of France, and
was subsequently appointed president of the council
of state. In the beginning of 1832 he
was nominated to the ministry of the interior, but in
May he died in Paris of cholera after a brief illness.
Georges Cuvier (1769-1832),
father of catastrophism and vertebrate
paleontology, established extinction as a fact
Charles Lyell
The Principles of Geology:
Being an Attempt to Explain the Former
Changes of the Earth's Surface, by Reference
to Causes now in Operation (3 vol., 1830-33).
(1797-1875)
Lyell advocated a uniformitarian view of geology. This assumed first of all the
constancy of natural laws (except as regarded the origin of new species which
was left rather vague). The kinds of causes which affected the earth in the past
must be assumed to have been exactly those we see in operation today
(such as erosion, sediment deposition, volcanic action, earthquakes etc.)
Irish- Scottish mathematician, physicist and inventor
who contributed to many branches of physics.
Thomson also calculated the age of the earth (100my)
from its cooling rate and concluded that it was too
short to fit with Lyell’ s theory of gradual geological
change or Charles Darwin's theory of the evolution
of animals though natural selection.
Kelvin, lord William Thompson
(1824-1907)
1. Law of Original Horizontality- Sedimentary strata and lava flows are
deposited in horizontal sheets. If these layers are not horizontal, subsequent
movements have occurred.
2. Law of Lateral Continuity- Sedimentary strata and lava flows extend laterally in all
directions and pinch out at the edge of their deposition.
3. Law of Superposition- In an undisturbed sequence of strata or lava flows;
each layer is older than the one above and younger than the one below.
5
4
3
2
1
younger
older
Rocks of Grand Canyon
Turbuck & Lutgens, 2002
4. Law of Inclusions- Inclusions are rock or mineral fragments contained in
another rock type. Any inclusion is older than the rock that contains it.
Inclusions, e.g. pieces of granite in sandstone
5. Law of Cross-Cutting – Any feature that cuts across a rock or sediment
must be younger than the rock or sediment through which it cuts. Examples:
faults, erosional surfaces (unconformities) and igneous intrusions.
6. Law of Unconformities- Unconformities represent gaps in geologic time when
layers were not deposited or when erosion removed layers.
a. Disconformity- unconformity between parallel layers
b. Angular Unconformity- tilted strata are overlain by younger more parallel strata.
c. Nonconformity- sedimentary layers deposited on igneous or metamorphic rock
Angular unconformity
Disconformity
Non-conformity
igneous or met. rock
There are two types of contact
between igneous and sedimentary
rocks:
- “baked contact” when surrounding
sedimentary rock was changed
by heat from igneous intrusion;
indicates that igneous rock is
younger than sedimentary;
- Erosional contact that indicates
non-conformity and that sedimentary
rock was formed after igneous rock;
Unconformities of Grand Canyon
Turbuck & Lutgens, 2002
Fossils (from Latin fossus, literally "having been dug up")
are the mineralized or otherwise preserved remains or traces
(such as footprints) of animals, plants, and other organisms.
Petrified wood (left), Eocene fish Knightia (center), Ammonites (right)
(photos from en.wikipedia.org/wiki/Fossil)
Law of Fossil Succession: The kinds of animals and plants found
as fossils change through time because of evolution of life.
pubs.usgs.gov/gip/fossils/
Correlation:
matching of rock units
in different locations
Rock units with distinct
characteristics such as
tephra layers or coal beds
called key beds are
especially useful in
correlation
Turbuck & Lutgens, 2002
Fossils and correlation
Overlapping ranges
of fossils are used to
correlate rock units
at different locations
Index fossils are
especially useful.
They must
- have wide geographic range
- have lived during short
geologic time
- be easily identified
Turbuck & Lutgens, 2002
Discovery of radioactivity
Antoine Henri Becquerel
(1850-1908)
Pierre Curie
(1859-1906)
Maria Skłodowska-Curie
(1867-1934)
A.H. Becquerel, Pierre and Marie Curie shared Nobel Price in Physics in 1903 for their
discovery of radioactivity. Marie Curie was awarded the second Nobel Price in Chemistry
in 1911 for discovery of radium and polonium. She was the first female
professor in Sorbonne.
Radioactivity is the set of various processes by which unstable
atomic nuclei emit subatomic particles (radiation).
Decay is said to occur in the parent nucleus and produces a
daughter nucleus.
Examples of decay:
9 α-particle (2 protons and 2 neutrons)
emission
9 β-particle (electron) emission
9 γ radiation emission: nucleus releases
photon
Decay chains
intermediate decay products
U238->x->x->…Pb206
Parent
(unstable)
Daughter
(stable)
At the time of rock formation,
ratio of parent/daughter=100/0
With increasing rock age, the
amount of parent isotope decreases
while the amount of daughter
isotope increases
%
Half-life: the time required for a half of the nuclei in a sample to
decay. After one half-life, the ratio parent/daughter=50/50 or 1/1, after
two half-lives the ratio becomes 25/75 or 1/3, after three
half-lives- 12.5/87.5 or 1/7 and so on.
Radiometric age calculation
Some radioactive isotopes used in absolute dating
Not all rocks can be dated by radiometric methods. The best rocks
for absolute dating are igneous. Some sedimentary rocks can also
be used, e.g. organic-rich rocks younger than 50,000 y. Metamorphic
rocks are generally unsuitable.
Dating of rock sequences using absolute dates, e.g. from igneous rocks
Wasatch Formation is younger than 66 my
Rock formations
between Volcanic ash
and Wasatch Formation
are between 160-66 my
Morrison and Summerville formations are older than 160 my
Radiocarbon dating
Used for organic-rich materials
younger than ~75,000 y. Previous
age limit of the method was ~50,000 y.
Dendrochronology
Simply put, dendrochronology is the dating of past events (climatic changes)
through study of tree ring growth. Botanists, foresters and archaeologists
began using this technique during the early part of the 20th century.
Discovered by A.E. Douglass from the University of Arizona.
In some regions, sequences dating back more than
10,000 y are available
Geologic time scale
Units: eons, eras, periods, epochs
Eons:
- Precambrian (Hadean, Archean,
Proterozoic) lasted
4.55 ba-540 ma
- Phanerozoic began 540 ma,
consists of three eras:
Paleozoic (540- 248 ma),
Mesozoic (248-65 ma)
Cenozoic (65 ma-present)
Eras consist of periods (see
Table to the right)
540
modified from USGS
Turbuck & Lutgens, 2002
Geologic Time and Engineering Risk
Risk assessment is a step in the risk management process.
Risk assessment is measuring two quantities of the risk R,
the magnitude of the potential loss L, and the probability p
that the loss will occur. Probability is related to recurrence
interval T of the event p=1/T
The larger the magnitude (energy) of the event
(and potential loss) the longer its recurrence interval
Magnitude versus frequency for asteroid impacts
(m)
1908 Siberia
65 ma
Tunguska impact location, Siberia
K/T (Cretaceous/
Tertiary) impact
location-Yucatan
Peninsula
All man-made structures are designed to last certain amount of time
(design life).
Egyptian pyramids (left) and ziggurats (temples) of Iraq (right) are among the oldest
man-made structures (more than 4000 years old)
www.crystalinks.com
Some of the modern structures must have very long design lives,
e.g. high-level radioactive waste disposal sites