Geologic Time
Essential Questions
What is the geologic time
scale?
What is absolute dating?
What is the difference between
relative and absolute age?
What is the geologic time
scale?
Geologic
Time
Scale
Geologic Time: Earth
Revealed Series
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30mins
Phanerozoic Eon
The last 10% of Earth's history is
called the eon of "evident life."
This means that rocks from the
Phanerozoic contain fossils, lots of
fossils!
The Phanerozic Eon is divided into 3
eras
Cenozoic
Mesozoic
Paleozoic
Cenozoic ("Recent Life")
Era
This is the last of three geologic eras
squeezed into the Phanerozoic ("Evident
Life") Eon that covers the last 10% of
Earth's whole geologic history.
This is the "Age of Mammals" in which
whales took over the oceans, saber-tooth
cats shared the land with elephants and
giant sloths, and humans finally appeared.
The Cenozoic period began about 65
million years ago with the extinction of the
dinosaurs and continues through the
present.
The Cenozoic is divided into two time
periods:
the Tertiary, and the Quaternary.
Mesozoic ("Middle Life") Era
This is the second of three geologic eras
squeezed into the Phanerozoic ("Evident
Life") Eon that covers the last 10% of
Earth's whole geologic history.
This is the era we all think of when we
imagine the Ancient Earth! Rampaging
dinosaurs! Dive-bombing pterodactyls!
Endless forests of giant ferns! Erupting
Volcanoes!
The Mesozoic Era lasted about 180 million
years, from about 245 million years ago to
about 65 million years ago.
The Mesozoic is divided into three time
periods:
the Triassic, the Jurassic, and the Cretaceous.
Paleozoic ("Ancient Life")
Era
This is the first of three geologic eras
squeezed into the last 10% of Earth's
whole geologic history.
The Paleozoic period lasted about 325
million years, from about 570 million years
ago to about 245 million years ago.
So much happened during the Paleozoic
that it is divided into six periods:
Permian, Carboniferous (Pennsylvanian &
Mississippian), Devonian, Silurian, Ordovician,
Cambrian
The Precambrian Eon
The name means: "before the Cambrian period.“
This old, but still common term was originally used
to refer to the whole period of Earth's history before
the formation of the oldest rocks with recognizable
fossils in them.
In the last few decades, however, geologists have
found that there are some hard-to-discern fossils in
some Precambrian rocks, so this period also is now
known as the Cryptozoic or "obscure life" Eon (from
the words "crypt" = "hidden," and "zoon" = "life").
This eon covers almost 90% of the entire history of
Earth.
It has been divided into three eras:
the Hadean, the Archean and the Proterozoic.
Proterozoic ("Early Life")
Era
Well, here we are about 700
million years ago, near the end
of the longest time period in
geologic history.
It began about two billion years
after the formation of Earth and
lasted about another two billion
years!
Archaean ("Ancient" or
"Primitive") Era
This era begins about a billion years after
the formation of Earth, and things have
changed a lot!
Mostly everything has cooled down.
Most water vapor in the air has cooled and
condensed to form a global ocean.
Even most of the carbon dioxide is gone,
having been chemically changed into
limestone and deposited at the bottom of
the ocean.
The air is now mostly nitrogen, and the sky
is filled with normal clouds and rain.
Hadean ("Hades-like") Era
This era begins about 4.6 billion years ago
with the formation of Earth from dust and
gas orbiting the Sun.
During this era the surface of Earth is like
popular visions of Hades: oceans of liquid
rock, boiling sulfur, and impact craters
everywhere!
Volcanoes blast off all over the place, and
the rain of rocks and asteroids from space
never ends.
It is hard to take a step without falling into a
pool of lava or getting hit by a meteor!
What is absolute dating?
Radioactive Decay
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30mins
Absolute Dating
Geologists find absolute ages by
measuring the amount of certain
radioactive elements in the rock.
When rocks are formed, small
amounts of radioactive elements
usually get included.
Radioactive Decay
Radioactive decay
continued…
As time passes, the "parent"
radioactive elements change at
a regular rate into nonradioactive "daughter" elements.
Thus, the older a rock is, the
larger the number of daughter
elements and the smaller the
number of parent elements are
found in the rock.
The age of the rock in years can
be found by measuring the rate
at which a parent element
decays and then measuring the
ratio of parent element to
daughter element in the rock.
Half-Life
Half-life refers to the amount of
time in which only half the
amount of the original substance
is left.
Uranium to Lead
A common "parent-daughter"
combination that geologists use
is radioactive uranium and nonradioactive lead.
As the rock ages, more and
more of the uranium changes
into lead.
The half-life of U-235 decaying
to Pb-207 is 713 million years
Rubidium to Strontium
Rubidium occurs in nature as two isotopes:
radioactive Rb-87 and stable Rb-85.
Rb-87 decays with a half-life of 48.8 billion
years to Sr-87.
This half-life is so long that the Rb-Sr
method is normally only used to date
rocks that are older than about 100 million
years.
Examples include the mica family (biotite
and muscovite) and the feldspar family
(plagioclase and orthoclase). These
minerals are abundant in granite and
gneiss.
What is carbon dating?
For measuring shorter time intervals, in the
range of historical interest, radiocarbon
dating has proved invaluable.
The radionuclide 14C (with half-life of 5730
years) is produced at a constant rate in the
upper atmosphere.
By measuring the amount of radiocarbon
per gram of organic matter, it is possible to
measure the time that has elapsed since
the organism died.
Carbon dating is limited to organic material.
What is the difference between
relative and absolute age?
An absolute age is a specific
number of years, like 150 million
years ago.
A relative age simply states
whether one rock formation is
older or younger than another
formation.