Geologic Time Scale
Geologic Time Scale
Table of Contents
- Introduction
- The Geologic Time Scale
- Precambrian Era
- Paleozoic Erai
- Mesozoic EraCenozoic Era
- Conclusion.
* Objectives
After attending this lesson, the user would be
able to highlight the various periods of the
earth's history.
The origin and evolution of life that have
happened during different periods will also be
known.
✰ Introduction
- While studying the physical geology and history of the planet earth, it is necessary to know about the Geological Time Scale, in detail.
- The study of the geological time scale is necessary to every student of earth, atmospheric and all life sciences.
- The origin and evolution of the Earth took place over a period of billions of years.
- The evolution of life on earth is also a part of that prolonged Earth's history. It is a fact that the planet earth has evolved as one of the planets from the common nebulae.
- Due to this reason, the age of the earth is also related to the age of the Solar System. From the Big-Bang theory, till today, the time span or time-frame of the Earth's evolution is very extensive.
- The periods of remarkable development in the evolution of life, the stages in the evolution of continents, the stages in the evolution of ocean basins, the origin and evolution of life on earth, and their proliferations or mass-extinctions, are represented in the form of geological records on the surface rocks and sediments all over the world.
- They clearly show the fossilized evidences of the past life and environment.
- The whole historical past of the earth from the origin of life till date are classified into eras, periods, epochs and ages.
- The time scale which spreads the earth's geological history into such divisions and sub-divisions is known as the Geological Time Scale.
- This time scale is very wide and cannot be mathematically shown in a small graph.
- This lesson highlights the importance and details of the geological time scale.
The Geological Time Scale was constructed using the evidences collected from various sources, like
- Field observations
- Fossil records Stratigraphic correlations Radioactive dating Paleomagnetic orientations
- Orbital revolution pattern of the Earth and
- The duration of the daily rotation of the earth around its axis.
These evidences have indicated that there were several major incidences that happened in the history of the earth, during various stages and ages.
Some of the major events like
• Mass extinctions,
• Appearance of new species or genera of life,
• Mountain-building movements
• Drifting of continents
• Spreading of ocean floors
• Widespread glaciations
• Dominance of certain species
• Massive migration of life between land and water are all considered while accounting geologic time scale.
These have shown very interesting aspects and features.
>The understanding of the geologic time scale is necessary while studying the geology of the earth. In this lesson, the following aspects of the Geologic Time Scale are highlighted:
- HISTORY OF THE TIME SCALE
- THE GEOLOGICAL TIME SCALE
- PALEOZOIC ERA
- MESOZOIC ERA
- CENOZOIC ERA
Each one of these major sections, is classified into several chronological sequences and divisions.
✰ History of The Time Scale
• Everything we do has got a time-frame. The life span is limited to some years.
• Time is not just a unit or feature or phenomenon of the present.
• Was the earth originated along with the Universe or at a later stage? How long is the history of the earth?
• There are several such questions which are to be answered while studying the history of the earth.
• To answer all these questions, scientists have constantly tried and searched the clues and sequences on the Planet from its origin.
• They studied the Solar System and its evolution, very meticulously.
• It is the change in the components of the sedimentary sequences that made people to know about the duration, and that has provided the basis for the measurement of the past with time units.
The change that was considered were,
•The Emergence or coming into existence of something new,
•The alteration of something that was already existing (or)
•The disappearance (or) loss of something, widespread, in a
region.
✰ The Passage of Time
•The passage of time can be related to a series of events.
•In order to construct the history of past events, one must determine how much time elapsed between the events and how long it took for the events to occur.
•Time is also marked by certain characteristic or unique set of events.
•When we list the events in an order in which they took place, we establish a chronological sequence of all the events.
✰ Two kinds of Clocks
The measurement of time is accomplished with two kinds of clocks known as
A.Mechanical clocks and
B.natural clocks.
Our wall clock (or) our wrist watch is a mechanical
clock.
There are several kinds of natural clocks and calendars used to refer the events of the past to the present.
For understanding the past geological events, geologic clocks were used.
They are:
1. Radioactive clocks (or the atomic clocks) and
2. Stratigraphic units correlated with Fossil population.
✰ Radioactive Disintegration
•Radioactive disintegration is an accurate phenomena to find out the date of earth's naturally occurring materials.
•The life of the radioactive element is expressed in terms of its half-life, which is the time required for half of any original amount of the material to be transformed.
•The actual rate of decay has been found to be entirely independent of environmental factors.
•This is a major advantage for adopting and accepting the measurements.
✰ Relative Ages of The Rocks
•Using this method, the relative ages of most of the rocks of the earth's surface were dated during the early 1800's and late 1700's.
•By following the principles of superposition, fossil correlation and relative dating of atomic clocks in rocks, the Geologic Time Scale was prepared in 1833 and 1905.
•The original idea has come from the works of Johann Lehmann (1719-1767)
• Continuous Annual deposition of sediments with life remains in water bodies can create sequential beds of sediments containing the fossils of animals and plants.
•These are called as stratigraphic sequences.
•These evaluate the evolutionary trend of ancient life and the geological formations. stratigraphic sequences were correlated to evaluate the evolutionary trend of ancient life and the geological formations.
✰ Scale for The Entire Earth's History
>The geologic time scale is a reference scale for the entire Earth's history.
> It helps to understand the entire history of the earth into workable units.
>Based on all the available evidence, the earth is found to be around 4500 million years old.
>In other words, the earth is almost 4.5 billion years old, since the starting point of the hot universe, which gave birth to the galaxies.
✰ The Geologic Time Scale
•The Hot original matter of the Universe is the starting point of the time scale. About 15.0 billion years ago, the hot Universe matter, with unimaginable temperature, was believed to have been in existence.
•We know that the Universe cannot be younger than its products.
•It must be older than the individual galaxies, older than the earth and older than all the chemical elements.
•Evidences indicate that an important event occurred about 10 to 12 billion years ago, that this mass got expanded and exploded.
•Due to this, the galaxies got separated from this mass and in a similar process, the stars and their planets was separated.
•If the Universe just keeps on expanding, then the galaxies will get farther and farther apart until each one is 'alone' in space.
✰ Age of The Galaxy
•A rough calculation shows that this would have been about 10 to 12 billion years ago.
•The Big-Bang theory also states this globe of energy and its sequential release of galaxies, very clearly.
•Formation of Galaxies, Stars and Planets happened from this hot universe.
•About 10 billion years ago the first generation galaxies started to spiral around and got released one after the other.
•The Milky Way Galaxy got released at about 6 billion years ago.
•From this mass, the Sun and the other members of the Solar System got originated at about 5 billion years ago.
•In an accurate estimate, it is found, that the Sun and its Planets were born around 4.56 billion years ago.
✰ Dating The Periods
Millennium, century and decade are the references made to denote the periods used in these time units.
In addition, the divisions in the time scale were made into the following units:
a) EON is the mega unit comprising a duration of half a billion years (or) more time
b) ERA is used to refer the duration of several hundred millions of years.
C) PERIOD is used to cover hundreds of millions of
years.
D)EPOCH is used to denote the duration of Tens of Millions of years and
E) AGE is used to cover a duration of Millions of years in the earth's history.
✰ Dividing The Geologic Time
The Geologic Time Scale of the earth is divided into two major eons as:
>Phanerozoic eon
>Cryptozoic eon.
The term phanerozoic was derived from the Greek word "Phoneros" meaning " Visible", and Zoo means life.
•The term Phanerozoic was used to refer the eon of visible life on earth.
•The term azoic was used to denote the eon of "no life" on earth.
•The term CRYPTOZOIC was derived from the Greek word Crypto which means Hidden and Zoo means life.
• Cryptozoic eon refers to the eon of hidden life.
✰ Eons and Eras
These two Eons are divided into various ERAS. The geologic time of the earth is divided into five major eras as:
A)CENOZOIC ERA
(Greek: Kaines - Recent, Zoo-life) - 65 Mya to the present
B)MESOZOIC ERA
- (Greek: Mesos - Middle, Zoo-life) Age of Reptiles and Ammonoids - 245 - 66.4 Mya
C)PALEOZOIC ERA - (Greek: Paleos - ancient, Zoo-life) - 540 - 245 Mya
D)PRECAMBRIAN ERA (OR PROTEROZOIC ERA)-Greek: Protero
- Primitive, Zoo-life) 4500-540 Mya.
E)ARCHAEAN ERA
Precambrian Era
The short-form of millions of years is Mya. The duration of PRECAMBRIAN ERA refers to the Span of time between 4500 Mya - and 540 Mya.
It is further divided into two eras as:
1-PROTEROZOIC ERA - from 2500 Mya to 540 Mya
2-ARCHAEAN ERA - from 4500 Mya to 2500 Mya.
These are further divided into various periods.
Precambrian Period
The notable observations of Precambrian period are:
• First appearance of life on earth
• Tectonic Plates were present and began moving
• Atmosphere became enriched in oxygen
• Dry and cold climate to warm and moist conditions prevailed on the earth.
• This is the earliest of all the geologic ages.
• It is further divided into Proterozoic Era 2500540 Mya) and Archean Era 39602500 Mya).
• The age of the earth is probably about 4500 Mya.
The Archaean Era
The Archean Era is the oldest era of the earth between 3960 Mya and 2500 Mya. The significant observations of this period are:
>Extensive mountain-building,
>Formation of banded iron ores and Greenstone belts,
>Existence of shallow seas,
>Accumulation of free oxygen, and Origin of life, especially the prokaryotes, bacteria and blue-green algae.
The Proterozoic Era
The Proterozoic Era 2500540 Mya) is the second era after Archean era. The significant observations of this period are:
>Dry and Cold climate,
>Warm and moist conditions,
>Origin of Eukaryotic cells and multicellular life Occurrence of earliest known fossils including of soft-bodied marine invertebrates,
>Origin of Sponges,
>Cnidarians and Annelids.
Paleozoic Era
• The duration of PALEOZOIC ERA is the time of Ancient life.
• This is the second major era of earth's history.
• It is the age of Invertebrates.
• This era has the span over a period between 540 Mya and 245 Mya.
>It is further divided into six periods as:
• Permian - 286-245 Mya
• Carboniferous 360-286 Mya
• Devonian-408-360 Mya this is the Age of fish
• Silurian - 438-408
• Ordovicia - 505 – 438 Mya this is the Age of Graptolites
• Cambrian - 540 - 505 Mya this is the Age of trilobites
These are very important periods in the earth's history. Each period has shown a very significant change in the planets constituents.
The Cambrian Period
The Cambrian Period is spread between 540 and 505 Mya. This the period of abundant life on Earth, after the Proterozoic. The significant observations of Cambrian period are:
>Prevalence of Mild climate,
>Extensive seas,
>Spilling over continents,
>Shelled Marine invertebrates
>Explosive growth of Eukaryotic organisms,
>Swimming,
>Floating,
>Crawling,
>Clinging and burrowing sea animals,
> Appearance of Trilobites,
>Brachiopods,
>Gastropods,
> Radiolarians,
> Sponges,
>Echinoderms,
>Starfish,
>Sea cucumbers,
>Jelly fish,
> Worms and water scorpions,
>Existence of plants was observed only as algae.
•This period is marked by the absence of land plants.
•There was a significant prosperity of life in the oceans than lands, during this period.
•During this period the continents had a soil crust and were resembling deserts.
✰ The Ordovician Period
The Cambrian Period is followed by the Ordovician period.
It has the duration between 505 and 438 Mya.
The significant observations of the Ordovician period are:
Mild climate-Adaptive Radiation (Ordovician radiation) Shallow seas retreating from land and spreading back all plants and animals still restricted to water,
the first vertebrates originated as jawless fishes Invertebrates dominated,
• the Crustaceans,
• trilobites,
• graptolites,
• brachiopods,
• bryozoans,
• Echinoderms,
• corals,
• mollusks and
• cephalopods all dominated in the water masses.
The First fungi originated during this period.
>Invasions of land by plants started.
>The Sea levels were high during Ordovician, the First known marine transgressions occurred during this period.
>The Southern Continents were collected into a single land
mass.
>It was named as Gondwana Land.
>The world's major mountain-building activity happened during this period.
✰ The Silurian Period
>The Ordovician period is followed by the Silurian Period.
>It is spread between 438 Mya and 408 Mya. The significant observations of the Silurian period are:
>The world has seen a mild climate, and experienced a stable and warm temperature during this period.
>The continents were generally flat and flooded.
>The notable mountain building occurred in Europe.
>The rise of fishes and reef building corals were originated in this period.
>There was marked abundance of shell-forming sea animals, dominance of Sea lilies, eurypterids and land scorpions, and invasion of land by arthropods.
✰ The Devonian Period
>The Silurian period is followed by the Devonian Period. It is spread between 408 Mya and 360 Mya. The significant observations of the Devonian period are:
>Violent volcanic eruptions and crustal movements, prevalent folding, mountain-building activities and drier Climate.
>The seas were covering most of the land.
>This is the age of fishes, sharks and rays.
>The fishes move into the open seas. Lunged fishes and amphibians appeared in this Devonian period.
>I The Mollusks were abundant.
>It has noticed the extinction of primitive vascular plants.
>Origin of modern Vascular plants with true leaves, roots and stems were seen from this period.
>I The Earth appeared to look green. Some plants started to produce seeds rather than spores.
✰ The Carboniferous Period
The Devonian Period is followed by the carboniferous period.
It is spread between 360 Mya and 286 Mya.
The significant observations of the Carboniferous period are:
•It is known for its coal deposits, lime stones and Grit stones.
•The Hercynian orogenic movements were stared.
•There was abundant of life on land and water.
•The earth has seen Large Primitive Trees, First land vertebrates, and Sea Invertebrates.
• There was a prevalence of Foraminifers, Bryozoans, Brachiopods, Cephalopods, Blastoids, Crinoids and Corals.
• The Brachiopods are the zone fossils of Carboniferous period.
•The Lamellibranchs and winged insects were important fauna.
•The Swamp forests with ferns were existing.
✰ The Permian Period
* The Primitive reptiles dominated in places.
* Extinction of all Trilobites, Graptolites and Blastoids, were seen in this period.
* The Beetles and flies got evolved.
* Much marine life flourished in warm shallow reefs.
* There was an abundance of Spiriferid brachiopods, bivalves, foraminifers and ammonoids.
*The Cone-bearing Gymnosperms (the first true seed plants) and the first true mosses appeared.
* The landmasses united into the super continent Pangaea, creating the Appalachians, in this period.
* This period records the end of Permo-Carboniferous glaciations.
✰ Mesozoic Era
The duration of MESOZOIC ERA is the time of Middle life in the history of the earth. This is the Age of reptiles. This era has the span of time between 245 Mya and 66.4 Mya.
It is further divided into three periods as:
a)Cretaceous - 144 - 66.4 Mya
b)Jurassic - 208 - 144 Mya
c) Triassic - 245 - 208 Mya.
This is the world's fascinating era, with very unique life and features.
✰ The Triassic Period
> Formation of Salt, Gypsum and red beds have been observed.
> The dominance of Archosaurs on land as dinosaurs, ichthyosaurs, nothosaurus in oceans and pterosaurs in the air.
> The appearance of Primitive Mammals was found in the Triassic.
> The domination of Reptiles and Cycads, reduction of Marine Invertebrates, first appearance of mammals and crocodilian were the unique things.
> Dicroidium flora were common on land.
> Extreme abundance of ceratitic ammonoids were seen.
> The modern Corals appeared.
> Orogenic Movements prevailed in some parts.
> The Pangaea was still in existence.
> Altering global climate and ocean circulation happened.
> The appearance of modern conifers, cycadeoids, first turtles, lizards, mammals and dinosaurs, have been seen in this period.
✰ The Jurassic Period
> The world's most famous period is the Jurassic period.
> The Dominance of Dinosaurs in Jurassic got registered in every mind of the human life due to global media coverage.
> It is spread between 208 Mya and 44 Mya.
> The significant observations of the Jurassic period are:
> Atmospheric CO, levels 4-5 times more than the present day levels 12001500 ppmv).
> Many types of Dinosaurs-Sauropods, Carnotaurus and steepsaurs dominated.
> Great plant eating dinosaurs were in existance.
> Lush growth of ferns and palm-like cycads, Gymnosperms also have grown much.
✰ The Cretaceous Period
The cretaceous period is a notable period in geologic history. It is spread between 144 and 66.4 Mya.
The significant observations of the Cretaceous period are:
* This is the last period of the age of Dinosaurs.
* First primates, angiosperms appeared.
* World continent Pangaea begins.
* Atmospheric Co, close to present day levels.
* Modern flowering plants proliferated along with new types of insects.
* Ammonites, belemnites, bivalves, echinoids and sponges were all common.
* Many new types of dinosaurs and crocodilians appeared on land.
* Modern sharks appeared in the sea.
* Primitive birds appeared (toothed birds and flying reptiles).
* Breakup of Gondwana and beginning of Rocky Mountains.
* Widespread epicontinental Seas.
* Extensive chalk deposits.
* Marine Pelecypods were very abundant.
✰ Cenozoic Era
• The duration of CENOZOIC ERA is the time of recent life. This is the age of mammals.
• This era has the Span of time from 65 Mya ago to the present day.
• It is further divided into two major eras as:
• Quaternary Era (Age of man) and
• Tertiary Era (Age of mammals).
✰ The Tertiary Era
The Tertiary era includes the periods:
> Pliocene 05-02 Mya
> Miocene 25-05 Mya
> Oligocene 38-25 Mya
> Eocene 55- 38 Mya
> Paleocene 65-55 Mya
✰ The Quaternary Era
The Quaternary era includes the periods:
>Holocene 10000 years to the present
>Pleistocene 1.6-0.01 Mya
✰ Paleocene Period
Paleocene Period (65-55 Mya):
The significant observations of this period are:
> Climate was Tropical.
> The modern plants appeared.
> The mammals diversity into a number of primitive lineages following the extinction of the dinosaurs.
> The world has seen the first large mammals.
> There was the Alpine orogeny.
> The Himalayan orogeny between 55 Mya. and 48 Mya.
✰ Eocene Period
Eocene Period (55-38 Mya):
The significant observations of this period are:
> Sea was Marginal.
> There was extensive terrestrial sedimentation.
> Reglaciation in South Pole happened.
> The Dawn of mammalian dominance was seen.
> The Subordinate position for reptiles happened.
> Moderate, cooling climate was noticed.
> The Archaic mammals flourished.
> The primitive whales diversity was seen.
> The First grasses appeared.
✰ Oligocene Period
Oligocene Period (38-25 Mya):
The significant observations of this period are:
> Warm but cooling climate prevailed on the earth.
> There was a rapid evolution and diversification of fauna especially the mammals and modern flowering plants.
> The Early ancestral elephants appeared.
> The Carnivores and ungulates become well-established.
✰ Miocene Period
Miocene Period (25-5 Mya):
The significant observations of this period are:
> Moderate icehouse climate.
> Extensive glaciation in Southern Hemisphere.
> Orogeny in Northern Hemisphere.
> Modern mammal and bird families become recognizable.
> Horses and Mastodons diverse.
> The first Apes appeared.
> Whales, Apes and grazing mammals dominated.
> Widespread volcanism and basalt flows were seen.
> Notable advances in the horses and elephant families.
> Spread of grasslands as forests contracted.
✰ Pliocene Period
Pliocene Period (05-02 Mya):
The significant observations of this period are:
> Worldwide elevation continues.
> Continental uplift and mountain building.
> The Ice Age began.
> The Seas were restricted.
> The world has experienced Cool and dry climate.
> Many of the existing generation of mammals and recent mollusks appear.
> Homo habilis appears. The Horses and Elephants became almost modern in appearance.
> First known appearance of hominids (human like primates).
> The large carnivores were dominated.
✰ Pleistocene Period
Pleistocene Period (1.6-0.01 Mya):
The significant observations of this period are:
> The Era of ice ages. Glacial climate.
> Four great ice advances and retreats.
> Flourishing and then extinction of many large mammals.
> Evolution of modern humans was seen in this period.
> The Dawn of human stone-age cultures.
> Volcanic eruptions destroy human populations.
> CroMagnon man - First appearance of present species.
> The Neanderthal man - Paleolithic culture started.
✰ Holocene Period
Holocene Period (10000 years to the present)
The significant observations of this period are:
> Rise of human civilization.
> Major habitat change and deforestations caused by Introduction of Pests and habitat destruction.
> The beginning of Agriculture happened in this period only.
> Humans built cities.
> Paleolithic and Neolithic cultures began around 10000 BC.
The world has seen the following ages:
> Copper Age 3500 BC.
> Bronze Age 2500 BC.
> Iron Age (1200 BC).
> Man used iron implements in 1350 BC.
> Youthful landforms and high order relief features were
seen.
✰ Conclusion
* Time has been flowing since the beginning of the earth. Time will continue to flow long after the present generation also.
* To understand the past, we should know the present.
* To understand the present, we should know the past.
* The age of the earth is one important aspect in Earth Science studies. Every period has to be studied in detail.
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