Geologic Time Scale
Life on earth started about 44 billion years ago and humans were one of the last
organisms to evolve. We wrack our brains just trying to imagine what would have
happened a few hundred years ago. To help us comprehend the full expense of
time, scientists have turned to the rocks.
Geologists are now able to describe crucial events in life’s history- from its origin to
disastrous extinction events. There was no idea what happened in what order. To
know when and how life originated, how it looked, Geological Time Scale was
made by geologists.
Geological Time Scale: Definition
Geological Time Scale is the tabulated form showing the sequence and duration
of the eras and the periods with their dominant form of life since the beginning of
life on the earth. (MODERN’S abc+abc+ of Biology)
History of Geological Time Scale
Let’s have a look at the history of geological time scale:
1. Scientist Nicolas Steno published the first law of stratigraphy in 1669.He
argued that the layers closer to the surface must be younger than the layers
below them. So, he thought, the farther we dig, the older the fossils that we
get.
2. Based on Steno’s ideas, Italian geologist Giovanni Arduino in
the 1760’s went a step further and began naming the layers of rock. But as
rock layers do not appear in this same order all over the world, there was no
way for geologists to compare rocks from one location to another.
3. Finally, in 1819, English geologist William Smith figured out the solution to
this problem- Fossils. Smith could match the ages of ancient organisms by
comparing their remains present in the rocks regardless of how far apart they
were. For example, Smith realized that fossils of many early species of
trilobites are found below ammonite fossils, which are in turn below certain
species of shellfish. So, anyplace in the world where we find these first
trilobites, we know that we are looking at a rock that is older than ammonites
lived.
4. Steno, Arduino and Smith, modern scientists have used these and other clues
to create what we call the Geological Time Scale or GTSGTS. The First
Geological Time Scale was published in 1913 by the British geologist Arthur
Holmes.
Geological Time Scale: Description
The Geological Time Scale has been reworked many times to reflect the latest
knowledge of Earth’s history. It is organized into 55 subgroups: – Eons, Eras,
Periods, Epochs and Ages.
Eons are the biggest which are divided into eras and eras further divided into
periods, epochs and ages.
A. Eons
I. These are the largest slices of time, ranging from a half-billion to
nearly 22 billion years long.
II. Earliest Eon is known as Hadean. It began with the very formation of
the Earth itself, around 4.64.6 BYA and ends 44 BYA.
III. This is the only Eon that does not have any fossils.
IV. During this time there was volcanic activity, cosmic bombardment,
temperatures were hot enough to melt rocks.
V. Small amounts of organic carbon have been discovered in Hadean
rocks which can be the evidence of earliest life.
VI. After the Hadean eon, the Archaeaneon came when cooling of
earth’s crust occurred and continents formed, which ran from 44 billion
to 2.52.5 billion years ago.
VII. First sign of life appeared.
VIII. Mostly CO2CO2 was there in the atmosphere until a prokaryote called
photosynthetic cyanobacteria came into existence.
IX. The Proterozoic eon came after the Archean eon, 2.52.5 billion
years ago.
X. Meaning of the Proterozoic eon meaning “earlier life”.
XI. Around this time, photosynthetic bacteria along with some multicellular
forms of life came which converted reducing atmosphere into
oxidizing one.
XII. Ancestral eukaryotes came during this eon whose cells had a nucleus
with organelles wrapped in a membrane.
XIII. Around 541541 MYA, the current Phanerozoic Eon came which
means ‘visible life’.
XIV. During this time, trees, dinosaurs, newts, aardvarks, and humans
came.
B. Eras
I. The Phanerozoic Eon is divided into three Eras, as, Paleozoic era, Mesozoic era
and Cenozoic era.
II. 541 million years ago the Paleozoic era came.
III. Here diversification of visible life occurred.
IV. This rapid growth and diversity in the ocean were so huge that all the eons
that came before it- Hadean, Archaean, Proterozoic are collectively known as the
Precambrian.
V. Fossil record of 25 MYA reveals the appearance of complex animals which had
shells, exoskeletons, etc.
VI. Of all these complex animals, Trilobites evolved first.
vii. The Mesozoic era is the age of reptiles, where dinosaurs, pterosaurs and a
variety of marine species evolved and many of them including dinosaurs became
extinct.
viii. Mammals, modern birds, and flowering plants also evolved during this era.
ix. 66 MYA, this Mesozoic era came to an end with another episode of
devastation and the Cenozoic era came.
x. The Cenozoic era is our era. It is the era that we are in today and marks the
rise of bmammals.
xi. By 40 million years ago, mammal’s groups like whales, bats, rodents and
primates came.
xii. But 34 MYA, the climate began to change again during which ice started to
grow at the poles taking up much of the planet’s water.
xiii. These drier conditions created a new habitat, the grassland, where ancestral
horses and antelope were hunted by the earliest cats and dogs.
xiv. 77 million years ago species of apes evolved.
xv. 2.62.6 MYA, ice expanded even more, and earth entered a glacial period
called Ice Age.
xvi. During this age, mammoth, ground sloths evolved.
xvii. Then 15000 years ago the climate started warming up.
xviii. By 11,700 years ago, modern humans inhabited nearly all the corners of
the globe.
xix. This is the era during which mammals, birds, and flowering plants are
dominant.
C. Periods- Periods in geological time scale refers to the unit which is smaller than
eras but bigger than epochs. All the eras are divided into smaller units called
periods. The Paleozoic era is divided into periods- Permian, Carboniferous,
Devonian, Silurian, Cambrian, Ordovician. All the other periods are given in the
table below.
D. Epochs- Periods are further divided into epochs like the Quaternary period of
Cenozoic era is divided into 33 epochs, i.e., Holocene, Pleistocene and Pliocene.
E. Ages- Epochs are divided into even smaller subunits called ages. It is the
smallest division of the Geological Time Scale
The
Geological
Time Scale:
Chart
Q.1. What are the 5 major divisions of geologic time?
Ans: 5 major divisions of geologic time are- eons, eras, periods, epochs, and ages.
Q.2. What are the 11 periods on the geologic time scale?
Ans: 11 periods on the geologic time scale are-
1. Quaternary
2. Tertiary
3. Cretaceous
4. Jurassic
5. Triassic
6. Permian
7. Carboniferous
8. Devonian
9. Silurian
10. Cambrian
11. Ordovician
Q.3. What is the largest time scale?
Ans: Eon is the largest geological time scale.
Q.4. Which era do we live in?
Ans: Cenozoic era is the era we live in.
Q.5. What period makes 90% of the earth’s history?
Ans: Precambrian time makes up 90% of the earth’s history which includes Hadean,
Archean, and Proterozoic eons.
