Geologists have divided the Earth’s history into a series of time intervals using significant events in the history of the earth. Geological Time Scale categorizes geological events based on successive smaller time spans like eons, periods, eras, epochs and ages . Eon is a primary and the largest division of geological time lasting over 500 million years. Four eons have been identified by geologists including Hadean, Achaean, Proterozoic, and phanerozoic. Eons are divided into eras which last for several hundred million years. Eons are sub-divided into Epoch which last tens of millions of years. On the other hand an age usually lasts for several million years but duration shorter than epoch. These periods are divided on the basis of the changes in climate, physical characteristics and life forms .
Cenozoic Era is the last of the three identified geological eras of the Phanerozoic Eon covering the last 10% of the Earth’s whole geological history. It is also called the era of recent life or the Age of Mammals. During this period, mammals such as whales, Saber-tooth cats and elephants appeared in large numbers. The most significant life form was the human being. This period began about 65 million years ago when the dinosaurs became extinct to the present time. It is divided into tertiary and the Quaternary periods. This is the period during which the present continents have been formed from the original Pangaea super continent .
The period is also marked by tectonic movements which have resulted into the increase in size of the Atlantic Ocean and the formation of Himalaya Mountains from the coalition of India and Asia among other effects. During the same period, the global climate has been becoming colder resulting into the return of giant glaciers and ice caps in some parts of the world. The period is marked with quick spread and growth of mammals which was boosted by the extinction of dinosaurs. Mammals were therefore very rampant. More species of birds and fishes also emerged while at the same time some animals and plants faced extinction .
The appearance and the spread of humanity has caused a lot of alteration of the face of the earth. Man has cleared the natural environment in search for places to build towns and establish farms. This has led to alteration in the composition of plants and animals. Man destroyed of some plants and animals leading to their extinction. However, the number of the domesticated animals can be said to be rising. Humans have since become the dominant terrestrial life form. Humans themselves have greatly increased in number posing increasing threat to plants and animals today.
It is the second of the three geological eras of the Phanerozoic econ. It represents the Ancient Earth and it is also called the middle life. Mesozoic era began around 25 million years ago and remained for about 180 million years till around 65 million years ago. It is divided into the Triassic, the Jurassic, and the Cretaceous periods. The formation of the earth took place during the Jurassic period. The earth resulted from the coalition of the Gondwanaland and Laurasia which formed the Pangaea. The continued plate tectonics has led to the breakage of the super continent resulting into the formation of the formation of today’s present continents. An example of the movement was that of North America which broke away from the North West coast of Africa.
The reptiles, especially the dinosaurs, became the most dominant animals. This begun in the Triassic and quickly spread during the Jurassic becoming dominating the earth during the Cretaceous. The domination of the dinosaurs reptiles made this age be known as the age of reptiles. Several new bird’s and mammal’s species also emerged during this period as well as the deciduous trees and flowering plants . The period experienced warm climate and therefore the initial ice caps melted including those at the poles. During this period more than half of the existing life forms disappeared including all the dinosaurs.
This is the first of the three geological Eras of the Phanerozoic econ. This period is also known as the Ancient Life. It begun around 570 years ago and it lasted for about 325 years up to about 245 million years ago. The era is divided into seven geological time periods. It marked the period when the interior of the earth had cooled down and the volcanic processes were not so rampant. However, there were both the minor and major eruptions resulting from the effects of the plate tectonics. During this period the two supper continents the Gondwanaland and the Laurasia, were still intact. Because of the cold climate, there were huge glaciers covering the interior of Gondwanaland.
The percentage of oxygen in the atmosphere increased as more oxygen was being released to the atmosphere by the photosynthetic algae floating on the ocean. This condition became suitable for large animals leading to explosion of life forms in this period. There was also a shift from the dominant ocean based life forms like the shellfish and sponges. For the first time in history, the land plants appeared near the end of the Ordovician. Features such as forests and swap lands war formed especially during the Mississippian and Pennsylvanian periods that because of their warm climates.
This is the reason behind the existence of giant coal beds in the United States. Animal life such as the arthropods, amphibians and the reptiles became common on the land surface. The shellfish and the insects were the most common making the period be referred to as the age of invertebrates by many scholars. This period experienced massive extinctions especially at the end of the Ordovician, Devonian and Permian. It was at the beginning of Paleozoic era that animals developed hard parts like shells, teeth and bones while at the same time woody plants appeared.
The Precambrian Eon
The period is also known as the Cryptozoic or obscure life Eon because some of its rocks had fossil fuels. Precambrian Eon covered around 90% of the history of the Earth. The Precambrian era further divided into Hadean Era which begun about 4.6 billion years ago. During this time the Earth was formed from dust. Volcanic activity was at its peak and liquid rocks and craters became very common phenomena. The air majorly composed of carbon dioxide and water vapor. The moon also formed during this era majorly as part of the splash. There was no life form during this era.
The era began about 3.5 billion years after the formation of the Earth. The earth had cooled down leading to the formation of such features as the global ocean. The percentage of CO2 also dropped and the air was majorly nitrogen. During this era the normal clouds and rain begun to be formed in the sky. The ocean floor was formed from the cooling lava and many volcanoes were still erupting. The islands were the only earth surface the continents had not formed. The plate tectonic caused the collision between various small Islands forming larger Islands which eventually collided to form the cores of today’s continents. The only life form was the blue green algae. The oldest known fossils are from the bacteria of this age. Some of its rocks can still be found today.
This era occurred about 700 million years ago and begun about 2 billion years after the formation of the Earth. During this time there were two supper continents separated by the equator. The era had fewer volcanoes than the Achaean era. The continental cores had acquired stable and large centers. Life only existed in the ocean. Oxygen formed a greater percentage of the air. The oxygen released from the oceanic algae caused extinction of a larger percentage of life forms as it was poisonous. The earth’s climate was also very cold during this era.