Geologic Time - School of Earth Sciences

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Geologic Time

A: By correlating
stratigraphic records
worldwide, the Geologic
Column or Time Scale was
developed in the 19th century

(From: Murck and Skinner, 1999)

Q: How was this Earth
geologic history created?

Problems facing the Earth scientist:



Only partial information is available



Processes can only be observed as they are
acting now



Time scale for many processes much too long
to observe

Uniformity or Uniformitarianism:



natural laws are permanent: under the same
conditions a given cause will always produce the
same results



the present is the key to the past

Catastrophism:



opposing earlier view



Earth originated through supernatural means and
had been affected by a series of catastrophic
events such as the biblical Flood

Mudcracks in tidal flat

(From: Marshak, 2004)

Mudcracks preserved in Paleozoic siltstone bed

(From: Marshak, 2004)

Stratigraphy:

the study of rock layers

and layering



water
-
borne sediments are deposited in horizontal
layers or strata



in any undisturbed sequence of layers, each layer
is younger than the layer below and older than the
layer above



a rock unit must always be older than any feature
that cuts or disrupts it

(From: Marshak, 2004)

Relative ages can be determined based on

three principles:



original horizontality



stratigraphic superposition



cross
-
cutting relationships

Horizontal strata (~1 km thick) deposited on older strata that tilted and
deformed tectonically prior to deposition of the overlying strata

Absolute ages cannot be determined from

one stratigraphic record alone:



rates of sedimentation vary greatly



sediments are often compressed



gaps in the stratigraphic records



unconformity: boundary representing a gap



gaps may occur because sedimentation stopped
or because a layer was eroded

We

ll return to absolute ages a little later in the lecture.

(From: Marshak, 2004)

Siccar Point unconformity in Scotland

Movie 1 Geologic History

Q: What is the sequence of events that leads to these stratigraphic structures?

A: Watch the movie!

A: Correlating strata: the
fossil record

Rare fossilized fish

Species: Phareodus testis

Eocene Age

Kemmener, Wyoming


http://www.stonesbones.com/

Complete fossilized articulated shrimp

Species: Carpopenaeus callirostris

Age: Late Cretaceous

Location: Hajoula, Lebanon

What is the role of fossils in determining
the Earth

s geologic history?

Dinosaur
-
like swimming
reptile

Name: Keichousaurus
hui

Age: Triassic

Formation: Huixia Beds

Location: Guanglin,
Guizhou Province, China

Beautiful Fossilized Dragonfly

Name: Aeschnidum cancellosa

Age: Late Jurassic to Early Cretaceous

Location: Beipiao, Liaoning
-

China

http://www.stonesbones.com/

Sliced pyritized ammonite

Kosmoceras in Matrix

Upper Jurassic

Location: Ulyanovsk, Russia


Triple Kosmoceras in Matrix

Upper Jurassic

Location: Ulyanovsk, Russia


http://www.stonesbones.com/

Correlating strata at three different locations; at location 3, stratum D is
missing

Principle of fossil succession

(From: Marshak, 2004)

Fig. 10.12a

W. W. Norton

Fig. 10.12b

W. W. Norton


Fossil record restricted to the youngest eon, the
Phanerozoic



Precambrian: animals were soft
-
bodied and
fossils are imprints



Cambrian: appearance of animals with hard shells
(trilobites)

Complete Trilobite in Matrix

Kanops raymondi
-

Devonian
Age
-

Oklahoma


Q: Why is the stratigraphic
column mostly detailed for the
last one
-
tenth of Earth

s
geologic history?


A: See above 3 points.

Absolute age: the radioactive rock clock


Radioactivity: the process by which an element
transforms spontaneously into another isotope
of the same element or another element



Rate of decay is unaffected by changes in the
chemical or physical environment



Radiometric dating: use of naturally occurring
radioactive isotopes to determine the absolute
age of minerals or rocks

Absolute age: radiometric dating



radioactivity: transformation (decay) of an element
(parent) into another isotope of the same element
or another element (daughter)



the proportion of atoms that decays during each
time unit is constant



half
-
life: time needed for the number of parent
atoms to be reduced by one
-
half



once a mineral grain has formed, atoms are locked
in



by measuring the number of remaining parents
and the number of daughter atoms, the age when
the mineral was formed can be determined

Radioactive decay

Fig. 10.19

W. W. Norton

Using the rate of radioactive decay, the
absolute ages can be associated with the
stratigraphic column.

Earth

s
Biography:

The movement of
plates throughout
time.


Q: How were the Himalayan
Mountains and Tibetan Plateau
formed?


A: By the successive
accretions of various terrains to
Asia.

Fig. 11.02 a, b

W. W. Norton