Geologic
History
Geologic Time Scale
Geologic Time Scale
Chart of Earth’s history showing events, time
units, & ages
Era: the four largest subdivisions of the geologic
time scale – Precambrian, Paleozoic,
Mesozoic, Cenozoic
Period: subdivisions of eras on the geologic
time scale
Epoch: subdivisions of periods on the geologic
time scale – ONLY in the Cenozoic
Traces from our past
• Index Fossils: a fossil of a species that
existed briefly and was widespread
geographically, used in determining the
relative age of rock layers (i.e. trilobites)
• Fossils and Ancient Environments:
fossils can be used to determine what the
environment of an area was like long ago.
Traces from our past
Fossils: the remains or traces of a onceliving organism preserved in rock.
How fossils form:
– organisms have a better chance of being
preserved if they have hard parts (bones,
shells, teeth, or wood)
– usually found in sedimentary rocks
Petrified Remains
Petrified Remains – plant or animal
remains that have been turned to rock;
this happens when minerals carried in
groundwater replace the original
material.
Petrified Wood
Carbonaceous Film
•
Carbonaceous Film – a fossil impression in a
rock, consisting only of a thin carbon deposit
Mold and Cast
Mold – a cavity in a
rock that has the
shape of a fossil
that was trapped
there; water
dissolved the
fossil away,
leaving its imprint
Mold and Cast
Cast – type of fossil
formed when an earlier
fossil in rock is
dissolved away, leaving
behind the impression
of that fossil, and new
sediments or minerals
enter the mold
Original Remains
•
Original Remains –
sometimes the actual
organism or parts of
organisms are found
(mosquitoes in
amber, wooly
mammoth frozen in
ice)
Trace Fossil
•
Trace Fossil –
footprints, worm
holes, burrows &
other traces of
animal activity
preserved in rock
Trace Fossil
Geologic Time Scale
Age Dating Techniques
Relative Dating: determining the order of events
and the relative ages of rocks by examining the
positions of rocks in layers
Age Dating Techniques
Relative Dating – another good diagram
Uniformitarianism
• In 1795, James Hutton described a ‘new
concept’ stating that:
– The geologic processes now at work were
also active in the past.
– The present physical features of Earth were
formed by these same process, at work over
very long periods of time
• “the present is the key to the past”
Relative Dating
• Law of Original Horizontality: sediments
are deposited in a horizontal fashion
Relative Dating
• Law of
Superposition: in
layers of undisturbed
rock, the oldest are
on the bottom, and
rocks become
younger toward the
top
Law of Superposition
Relative dating
• Law of CrossCutting
Relationships:
igneous rock
intrusions or faults
are younger than
the rocks that
have been
intruded or faulted
Relative Dating
• Law of Included Fragments: pieces of
one rock found in another rock must be
older than the rock in which they are
found
Relative Dating
• Correlation:
the matching of
rock layers
from one area
to another to
establish a
relative date
Relative Dating
Unconformity: one or more missing layers in a sequence
of rocks, this is the result of gaps in the time/rock record
• angular unconformity – tilted rock layers meet horizontal
rock layers, this indicates layers are missing and there is
a gap in the time record
• disconformity – the top rock layer is eroded before the
next layer can be deposited causing a gap in the time
record
Angular Unconformity
Angular Unconformity
Disconformity
Disconformity
Age Dating Techniques
• Absolute Dating: determining the age of
rocks using the radioactive decay of
atoms.
• Radiometric Dating: an Absolute Dating
method that uses the rate of decay of
radioactive isotopes in rocks.
Age Dating Techniques
• Radioactive Decay: the decay of an atom of one
element to form another element, occurring when
an alpha particle or beta particle is expelled from
the original atom (Ex. parent atom (K) decays to
daughter product (Ar))
• Half-Life: the time it takes for half of the parent
atoms of an isotope in an object to decay into the
daughter product (K-Ar = 1.3 billion years)
Age Dating Techniques
Tree rings – counting the number of rings on a
cross section of a tree
Radio-Carbon dating – the radioactive carbon
isotope used for dating organic material:
Carbon 14  decays to  Carbon 12
The ‘half life’ of Carbon 14 is 5700 years…
Half life – how long it takes for half of the material
to decay
Precambrian Time
•
•
•
•
from 4.6bya to 545mya
makes up ~90% of Earth’s history
very little is known about fossils from this period
cyanobacteria appeared around 3.5 bya
– responsible for oxygen and
indirectly for ozone in
the atmosphere
• invertebrates developed
at end of era
545mya to 245mya
Paleozoic
• began when animals
developed hard parts that
could be fossilized
• life moved from ocean to
land
• end signified by mass
extinction probably
caused by formation of
Pangaea
• Early - “Age of the
Invertebrates”
• Late – “Age of the Plants”
Era:
Mesozoic Era
• 245 mya to 66 mya
• Laurasia &
Gondwanaland
developed from the
break-up of Pangaea
• Reptilian eggs developed
a hard shell
• Dinosaurs evolved during
the Triassic and
dominated during the
Jurassic and Cretaceous
periods
Mesozoic Era (cont.)
• Mammals appeared
during the Triassic
• Birds appeared during
the Jurassic
• Angiosperms
appeared during the
Cretaceous
• End signified by mass
extinction
• “Age of the Reptiles”
(Dinosaurs)
Cenozoic Era
• 66mya to present
• Increased tectonics created the Alps, Himalayas,
and the Appalachians
• Global temperature drop
• Mammals evolved and
dominated
• Homo sapiens emerged
500,000yrs. ago
• “Age of the Mammals”
Evolution
• Organic Evolution: gradual change in lifeforms through time
• Species: group of organisms similar to
each other and that typically reproduce
only with each other.
• Natural Selection: natural process by
which some organisms survive and
reproduce because they have traits
favorable to survival in an environment,
while others die out because they lack
those traits
Evolution
• Endangered: describes a species that has
a small number living and thus in danger
of dying out
• Habitat: any place where organisms live,
grow, and interact
General Related Facts –
(the amazing impact humans have had)
• Four years ago 19,836sq. km. of Amazon
rainforest was destroyed
• More than 80% of all deforestation of the
Amazon has occurred since 1980
• More than ¼ of all pharmaceuticals come
from rainforest plants
General Related Facts –
(the amazing impact humans have had)
only 22% of Earth original forest coverage
remains…..
– Western Europe lost 98% of its primary
forests;
– Asia 94%;
– Africa 92%;
– N. America 66%;
– S. America 54%