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تاريخ التسجيل : 11/05/2015
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The Geologic Time Scale

في الأحد مايو 17, 2015 5:49 pm
GEOLOGIC TIME
The Geologic Time Scale


The time during which
Earth has existed is known
as geologic time. The
geologic time scale is a
system that scientists use to
organize their findings
about Earth’s history. In the
Investigate, you made your
own geologic time scale.
You divided the time into
different parts. This helped
you to understand what
scientists do when they
divide geologic time into
different parts.You likely noticed that some parts had more unique events
than others. Some events occurred only once in Earth’s past. For example,
the formation of Earth and the development of the core, the mantle, and
the crust occurred only once. The development of the atmosphere was
also a one-time event. Other events are
cyclical. Mountain formation, the eruption
of volcanoes, and earthquakes occur often.
There have also been many changes in
climate and in sea level.
It is not easy to define the parts of
Earth’s history using processes that repeat
themselves. For this reason, scientists
divide geologic time into smaller parts.
They use the history of the biosphere.
Fossils are evidence of once-living
things. These organisms are preserved in
sediments and rocks. Different species of
organisms have appeared and disappeared
over time. Fossils indicate when this
happened. Extinct species do not reappear
later. Scientists use the arrival and
extinction of different fossils as markers.
They are used to identify when certain
parts begin and end.
The longest units of time in Earth’s
history are called eons. So far, there have
been four eons. Each has its own unique
story. The Hadean Eon is the first part
of Earth’s history. This part of Earth’s
history lasted 800 million years. During this time, the Moon formed,
and meteorites bombarded the geosphere. Later, the core, mantle, and
solid crust developed. Toward the end of the Hadean, simple life had
gained a foothold on Earth. The Archean Eon lasted from 3.8 to 2.5 Ga. It
contains evidence of early cellular life and photosynthesis. Next came the
Proterozoic Eon (2.5 Ga–542 Ma). It is marked by the appearance of early
life forms. However, it was the time before abundant life. The first major
glaciation took place 2.3 Ga. Presently, Earth is in the Phanerozoic Eon.
It has lasted about 542 million years so far. It is a time of abundant life.
This is about an eighth of geologic time. During the Phanerozoic Eon,
evolution has led to large-scale changes in the biosphere. Scientists have
used these to further define the geologic time scale.
Evidence comes from many different places in time and space.
Eons are divided into shorter units of time called eras. The Phanerozoic
Eon is divided into three eras. The Paleozoic Era is the era of ancient life.
It lasted about 290 million years. The Mesozoic Era is the era of middle
life. It lasted about 185 million years. The Cenozoic Era is the current

era of modern life. It has lasted about 65 million years so far. Changes in
the mix of animals and plants in the biosphere define the beginning and
end of each era. These changes can be very dramatic. For example, the
Mesozoic Era is called the Age of Dinosaurs. It ended when most of life on
Earth became extinct. The next era is called the Cenozoic. It included the
evolution of mammals. Many of these mammals live on Earth today.
Eras are divided into shorter units of time called periods. Each period is
named by the scientists who discovered it. Sometimes the names are from
a particular region. For example, the Cambrian, Ordovician, and Silurian are
names of three periods. They are named after Welsh tribes. Each period
lasts tens of millions of years. They subdivide geologic time in a way similar
to how hours divide a day. Each one also tells a unique part of Earth’s
history. For example, the Cretaceous Period was when large dinosaurs were
abundant. It lasted about 80 million years. After the Cretaceous Period
ended, the number of dinosaurs decreased. A new period began. This was
called the Paleogene Period. Scientists have defined this period according to
the appearance of a large number of mammal fossils.
Even smaller units of time, known as epochs, are part of the geologic time
scale. They help define time even more precisely. This is much like minutes
within an hour. Epochs are often determined by special events in other
parts of the Earth system. They can include times when Earth’s climate
changed. They might be determined by when Earth’s magnetism reversed
or when distinctive layers of rock were deposited.
Compared to a human lifetime, geologic time is extremely long. This
makes it difficult to imagine its length, or the relative scales of its eons,
eras, and periods. For example, think about the Phanerozoic Eon. It lasted
about 540 million years. Imagine it as one year. In this year, animals with
backbones would have emerged on the continents in mid-April. Dinosaurs
would have appeared in July. They would have died out suddenly in late
October. Humans would have arrived about two hours before midnight on
New Year’s Eve.
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