#28: Earth History
• Scientists believe that the Earth is 4.6
billion years old
• History of the Earth can be investigated
by studying fossils & rocks
Uniformitarianism: the principle that states that
Earth processes occurring today are similar to
those that occurred in the past
• Weathering
• Erosion
• Plate Tectonics
Law of Superposition: In a layer of undisturbed rock
the oldest rocks and fossils are found on the bottom
and the youngest on the top
Relative Dating
• Using the age of a rock or fossil by
comparing it to other things
• Age based on position
Unconformity: a gap in the rock record caused
by erosion or a pause in deposition
• Shown as a curvy line in a rock formation
Absolute Dating
• A method used by geologists to determine the
age, in years, of a rock or fossil.
• Uses the chemical properties of atoms
• Radioactive decay: when the number of
protons in an atom is changed and a new
element is formed
• Radiometric dating: method that uses the
rate of decay to determine the age of a rock
or fossil
Half-life: the time it takes for half of an
element (Parent material) to decay into another
element (Daughter material)
Parent
Uranium
Carbon
Daughter
Lead
0.7 billion years
Nitrogen 5730 years
# of
HalfLife
0
1
2
3
Amount of Amount of
Parent
Daughter
Material
Material
80 g
# of
HalfLive
0
1
2
3
Amount of Amount of
Parent
Daughter
Material
Material
80 g
0g
40 g
40 g
20 g
60 g
10 g
70 g
Earth History is divided into units based on:
1 Life forms
2 Geologic events
Divisions of time (Listed largest to smallest)
Eon
Era
Period
Epoch
Eons: Based upon the abundance of fossils
Era: major, striking, and worldwide
changes in types of fossils and life forms
Period: smaller division of an era,
based
on types of life living worldwide
and
geologic events
Epoch: smaller division of a
period
based on differences in life
forms
by smaller regions
Precambrian time
• Longest part of Earth’s history (4.5 billion
years ago to 542 mya)
• A super eon which is divided into 3 eons
• Very few fossils
• Examples of fossils
• Stromatolites
• Cyanobacteria
(blue green algae)
Flora and fauna of the Precambrian
Phanerozoic Eon
• Current geologic eon: 541 mya to present
• Abundant animal and plant life
• Divided into 3 Eras based on life forms
• Paleozoic: paleo = ancient
• Mesozoic: meso = middle
• Cenozoic: ceno = present
Paleozoic Era
• Trilobite and other invertebrates
• Small vertebrates evolved in the late
Paleozoic (fish and reptiles)
Mesozoic Era
• Reptiles (dinosaurs), fish, birds, flowering
plants, first mammals
• Ended with the major extinction of
dinosaurs
Cenozoic Era
• Era we live in
• Large mammals
• First humans (Homo sapiens)
Fossils
• Remains or traces of once living organisms
that lived over 10,000 years ago
• Human remains are called artifacts
• Most fossils are found in sedimentary rocks
• Some remains may be found in
metamorphic rocks such as slate or
volcanic tuff
• Paleontologist: a scientists who studies
ancient life (fossils)
• Fossil evidence indicates that life forms
have changed and become more
advanced over time
5 types of fossils
1. Permineralized remains (petrified remains)
• Bone replaced by minerals
2. Trace fossils
• Footprints, burrows, coprolites, gastroliths
3. Molds & casts
4. Original remains
• Amber, tar pits, glacier
5. Carbonaceous film
Mold & cast
Petrified remains
Carbonaceous film
Trace fossil
Original remains
Index fossils
• Fossil that gives clues as to the age of a
rock layer
• Graptolite, trilobite
Fossils in Virginia
• Most are marine fossils
• Tells us that Virginia has been covered by
a shallow sea several times in its geologic
history
• State Fossil
• Chesapecten jeffersonius
https://www.youtube.com/watch?v=hOX8_wqqRPs