Transcript The Geologic Time Scale

Geologic Time
The Rock Record
• Rocks record geological events and
changing life forms
• Uniformitarianism: (James Hutton) the
forces and processes that we observe
today have been at work for all of earth’s
history
• Earth is a lot older than originally thought!
Relative Dating
• Tells us the sequence of events, not the
actual time the events occurred
• Law of Superposition: in an undeformed
sequence of sedimentary rocks, each bed
is older than the one above it and younger
than the one below it
• Original Horizontality: layers of sediment
are deposited in a horizontal position
Relative Dating
• Cross-Cutting: when faults cut through
rock layers, or magma intrudes, the
intrusion or fault is younger than the rock
affected
• Inclusions: Pieces of one rock are
contained within another; the rock
containing inclusions is younger
Relative Dating
• Unconformities: a long period during
which deposition stops, erosion removes
previously formed rocks, then deposition
resumes
• 3 Types: angular unconformity,
discomformity, and nonconformity
• Correlation: matching up rocks of similar
age in different parts of the earth
Relative Dating
• Angular unconformity: an unconformity where
horizontally parallel strata of sedimentary rock
are deposited on tilted and eroded layers
• Nonconformity exist when the sedimentary
rock lies above and was deposited on the preexisting and eroded metamorphic or igneous
rock
• Disconformity: unconformity between
parallel layers of sedimentary rocks which
represents a period of erosion or non-deposition.
A = Angular unconformity
B= Nonconformity
C=Disconformity
Correlation
Fossil Record
• Fossils are the remains or traces of
prehistoric life
• Fossil type depends on conditions at death
and how it was buried
• Fossils are typically found in sedimentary
rock
• Unaltered Remains: bones, teeth or
shells of an animal remain unchanged
Fossil Record
• Altered Remains: most likely event
• Petrified: “turned into stone”
• Molds: the structure is buried and
dissolved. Reflects only shape and surface
markings
• Casts: If the hollow spaces are filled with
mineral matter
• Trace fossils: indirect evidence; tracks
Fossils and Correlation
• Fossils succeed one another in a definite
order; any time period can be recognized
by the fossil content of the rock
• Index Fossils: fossils used as time
indicators
• Sometimes groups of fossils are used as
the index
• Also used to interpret the environment
Fossils and Correlation
• Conditions favoring preservation:
– Rapid burial
– Possession of hard body parts
(skeleton)
Types of Fossilization
Overlapping Ranges of Fossils
Dating with Radioactivity
Basic Atomic Structures
• Atomic number is the number of protons in
the atom’s nucleus
• Mass number is the number of protons
plus the number of neutrons in an atom’s
nucleus.
• Isotopes are atoms of the same element
containing a different number of neutrons
Dating with Radioactivity
Radioactivity
Radioactivity is the spontaneous decay
of certain unstable atomic nuclei.
Particles and energy are released from the
nucleus
A more stable daughter is the result
Common Types of Radioactive Decay
Dating with Radioactivity
Half-Life
A half-life is the amount of time necessary
for one-half of the nuclei in a sample to
decay to a stable isotope.
The Half-Life Decay Curve
Dating with Radioactivity
Radiometric Dating
• Each radioactive isotope has been
decaying at a constant rate since the
formation of the rocks in which it occurs
• Radiometric dating is the procedure of
calculating the absolute ages of rocks and
minerals that contain radioactive isotopes
Dating with Radioactivity
• As a radioactive isotope decays, atoms of
the daughter product are formed and
accumulate
• An accurate radiometric date can be
obtained only if the mineral remained in a
closed system since its formation.
Radioactive Isotopes Frequently
Used in Radiometric Dating
Dating with Radioactivity
Dating with Carbon-14
• Radiocarbon dating is the method for
determining age by comparing the amount of
carbon-14 to the amount of carbon-12 in a
sample
• When an organism dies, the amount of carbon14 it contains gradually decreases as it decays.
By comparing the ratio of carbon-14 to carbon12 in a sample, radiocarbon dates can be
determined
The Geologic Time Scale
Structure of the Time Scale
• Based on their interpretations of the rock
record, geologists have divided Earth’s
4.56-billion-year history into units that
represent specific amounts of time.
• Taken together, these time spans make up
the geologic time scale
The Geologic Time Scale
Structure of the Time Scale
• Eons represent the greatest expanses of
time.
• Eons are divided into eras.
• Each era is subdivided into periods.
• Finally, periods are divided into smaller
units called epochs
The Geologic Time Scale
Structure of the Time Scale
There are three eras within the Phanerozoic
eon:
• Paleozoic, which means “ancient life,”
• Mesozoic, which means “middle life,”
• Cenozoic, which means “recent life.”
The Geologic Time Scale
• Each period within an era is characterized
by somewhat less profound changes in life
forms as compared with the changes that
occur during an era
• The periods of the Cenozoic era are
divided into still smaller units called
epochs, during which even less profound
changes in life forms occur.
The Geologic Time Scale
Precambrian Time
• During Precambrian time, there were
fewer life forms. These life forms are more
difficult to identify and the rocks have been
disturbed often
The Geologic Time Scale