Transcript Science 7: Unit E: Planet Earth

Science 7: Unit E: Planet Earth
Topic 9 – Geologic Time
How Old is the Earth?
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The Earth is roughly 4.5 billion years old, the
age of the solar system.
The history of the Earth from start to finish is
divided into separate periods called eons. These
eons in turn can be divided into eras. These eras
are then divided into periods.
The eras can be millions, even billions of years
old and they are not all the same length of time.
What are the Eras of Earth?
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One era ends and another begins usually when
there is a mass extinction of life-forms such as
when the dinosaurs died out after the meteor
hit. Usually when this happens, a new class of
organisms (living things) become dominant on
Earth.
The order of eras is Precambrian, Paleozoic
(ancient life), Mesozoic (middle life), and
Cenozoic (recent life).
Because one era can be hundreds of millions
years long each era is divided into periods. See
p. 430.
Where are We at Now?
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We are in the Cenozoic era and the Quaternary
Period of that era.
The Cenozoic era started after the Mass
Extinction of the Dinosaurs and is characterized
by the dominance of mammals and ultimately
humanity.
The Cenozoic era is roughly 65 million years old,
and the Quarternary Period occupies the last 2
million years of that period.
How to Determine the Ages of
Rocks and Fossils
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I. Relative Dating – is the dating of rocks by
comparing them to other rocks or fossils of well
known organisms. There are two methods of
relative dating.
II. Absolute Dating – is the dating of rocks and
fossils using the radioactive properties of certain
elements (uranium and the carbon-14 isotope).
Radioactivity basically means that over time,
certain materials such as uranium will change into
another material. By studying the amount of the
original material that is left, we can calculate the
age of the rock. There are two methods of absolute
dating: radiometric dating and radiocarbon dating:
Relative Dating
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1. Principle of Superposition – In an undisturbed
sedimentary rock formation we know that the
lower down the strata/layer of rock is, the older
it is.
2. Index Fossils – If geologists find a fossil of a
well known organism such as a dinosaur we can
assume that the rock layer is as old the fossil
that’s in it. For example we know that trilobites
lived in the early Mesozoic Era and therefore if
we find a rock containing trilobite fossils, then
the rock itself is from that era as well.
Absolute Dating
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1. Radiometric Dating – The study of the
amount of uranium (parent material)
compared to amount of lead (daughter
material) contained in a sample of rock. The
lower the percentage of uranium, the older
the rock is. The half-life of uranium is 4.5
billions old, the same age as the Earth. No
rock should therefore have a percentage of
uranium of less than 50% compared to lead
and few if any rocks have a percentage near
50% (because rocks that old have been
recycled by the Rock Cycle).
Absolute Dating Continued
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2. Radiocarbon Dating – The study of the
amount of Carbon-14 (parent material)
compared to the amount of nitrogen (daughter
material) contained in a fossil sample. The lower
the percentage of Carbon-14, the older the
fossil. This technique only works with fossils of
less than 50’000 years. The half life of Carbon14 is 5730 a meaning that half of the carbon-14
will turn into nitrogen over that time.