Transcript Geologic Ages_Fall09_1

Geologic Time
Geologic Time
• Radioactive isotope dating tells use that the age of the
earth is 4.6 billion years old!
– Oldest rocks found on Earth are 4.28 billion years old
• Found in northern Quebec on the coast of the Hudson Bay
• Moon rocks (4.5 billion years) and meteorites (4.6
billion years) have been recorded
• Life has existed on earth for 3.6-3.8 billion years
• Multi-cellular life has existed since 550 million years
ago
• Homo sapiens have only been around for the past
100,000 years (0.002% of total earth history!!)
The Geologic Time Scale
• Early geologists divided time based on
what kinds of rocks/fossils were present.
•
The Geologic Time Scale is broken up into
Eons , Eras, Periods and Epochs
– Eons
• There are two Eons
– Precambrian and Phanerozoic
– Eras
• The Phanerozoic has been divided into 3
Eras based on the timing of mass
extinctions
– Periods
• the 3 Eras are divided into Periods based
on the dominant life-forms that were
present
– Epochs
• The Periods in the Cenozoic Era are also
subdivided into Epochs
Determining Age
Relative age: stratigraphic relationships
between rock formations to determine
the order in which they were created
Absolute ages: Uses radioactive decay
to determine a numerical age of a
formation
Relative Geologic Dating 2
Stratigraphic Laws
• Principle of original horizontality
– Sedimentary rock layers are deposited in
horizontal layers
Stratigraphic Laws
• Principle of stratigraphic superposition
– The oldest rocks are at the bottom of a
sequence and the youngest rocks are on top
• Rocks can become deformed and overturned - recognized by “up
indicators”
If they were overturned…
Up
Down
Down
Up
Stratigraphic Laws
• Cross-cutting relationships
– A geologic feature may cut across another feature
and indicate a relative age
• Which is younger, Intrusion W or Intrusion P?
Inclusions
• Law of Inclusions
– When fragments of one rock unit have been enclosed
within another than the fragments are older than the
rock in which they are enclosed
Principle of fossil succession
• Index fossils - fossils or organisms that only existed
for a short period of time but were geographically
widespread
Quaternary
Mesozoic
Tertiary
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Unconformities
• A time gap in the rock record
– boundary between rocks of distinctly different
ages
– Indicates that rocks were:
• deposited and then eroded
• simply not deposited
• intruded by an igneous body
• 3 types:
– Angular Unconformity
– Disconformity
– Nonconformity
Angular Unconformity
• An unconformity
between non-parallel
strata
• Older strata lie at a
different angle than
younger strata
Disconformity
Limestone
Dolostone (very similar to limestone)
Nonconformity
Triassic Dolostone
Tertiary Igneous
Intrusion
• Igneous or metamorphic rocks in
contact with sedimentary rocks
Atom
• Proton: a nuclear particle
having a positive charge
• Neutron: a nuclear
particle having a mass
almost identical to the
proton, but has a neutral
charge
• Electron: a negative
charged particle with a
mass of 0
• Atomic number: the
number of protons in the
nucleus of an atom (Z)
• Mass number: the total
number of protons and
neutrons in a nucleus (A)
Isotopes
• Atoms whose nuclei have the same
atomic number but different mass
numbers
• The nuclei have the same number of
protons but different number of
neutrons
Radioactive Decay
• Many nuclei are radioactive – unstable
• Decays by emitting a particle (alpha,
beta, or gamma), changes the energy
state of the nucleus
• A chain of decays takes place until a
stable nucleus is reached
Absolute age dating
• Absolute age reflects the amount of time
since that mineral crystallized and locked in
the parent atoms
• A numerical age is determined by measuring
the ratio of parent atoms to daughter atoms
and applying the half life.
Half-life
The length of time it takes for half of the
radioactive parent atoms of one element
to decay to their daughter atoms
Radioactive isotope systems
• An isotope system with a short half life
is good for dating processes that
happened recently
– 14C half life is ~5730 years
• An isotope systems with a long half life
is good for dating processes that
happened long ago.
– 87Rb half life is ~48,800,000 years
Questions