How Is Earth Time Measured ?

Just as we divide human history into periods according to events or the rulers of the time like Pre-Columbian, Ming, victoria and so on, so we divide the vast sweep of geological time into periods and eras based on the kinds of animals and plants that existed during those times.


For about seven-eights of the time that the Earth has been in existence, what life there was left little in the way of fossils. For this reason , this vast period called the Precambrian era, tends to be dismissed. It is divided into two eons: the Archaean, in which there was no life, and the Peroterozoic, in which life of some sort existed.

Then, 570 million years ago, the Palaeozoic Era began. This opened with the Cambrian in which there was a sudden flourishing of animals with hard shells. From then onwards, the rocks are full of fossils. The lower Palaeozoic, consisting of the Cambrian, Ordovician and Silurian periods, showed a great development of life in the sea and the evolution of the first backbone animals --- the fish. In the Upper Palaeozoic --- the Devonian, Carboniferous and Permian periods --- life colonized the land. Plants left the water first, followed by invertebrates that developed into insects and spiders, then came tha amphibians and these evolved into reptiles.


During the Mesozoic era --- consisting of the Triassic, Jurassic and Cretaceous periods --- the pattern of life changed. At the begining, the primitive reptiles evolved into dinosaurs, that dominated the land thorough out the era, and into the mammals, that were to play a less prominent role for a while. In the sea, too, changes were taking place, with modern-type fish evolving.


A sudden mass extinction that saw brought  the Mesozoic to an end --- an extinction that saw the loss of the dinosaurs and many of the sea  creatures. The Cenozoic era then began, and in this time the mammals radiated to fill all the niches left vacant by the extinction of the great reptiles. The bulk of the Cenozoic is taken up by the Tertiary period. The last silver is known as Quaternary period and this embraced the last Ice Age and modern times in which human beings developed.
       
The rocks that were laid down in these various periods can usually be dated by the types of fossils found in them. That is fine when dealing with sedimentary rocks in which fossils are abundant, but what about metamorphic rocks and igneous rocks, or even unfossiliferous sedimentary roks? Here the principles of cross- cutting relationships is used. If a metamorphic rock has been formed by the alteration of a sedimentary rock containing Silurian fossils, then the ignious rock must have been formed later still. If this whole sequence is then overlain by undisturbed sedimentary rock containing Carboniferous fossils, then we can deduce that the metamorphism and the igneous emplacement took lace between Silurian and Carboniferous times, in the Devonian period.


How Do We Put Dates To All This ?

We can glibly talk about the hundreds of millions of years involved in the geological time scale, but how is such a scale calibrated? The secret lies in the radioactive elements in the Earth's crust. A radioactive element will decay to a non-radioactive form in a particular length of time. For Example, after a mineral containing potassium-40 is formed it will take a a known length of time for half the mass of period of time for half the mass of potassium-40 to decay into argon-40, then the same period of time for half of the remainder to decay, and so on. This period is known as "half life". By finding the proportion of potassium-40 to argon-40 in a mineral we can work out how long ago that mineral formed. Many radioactive elements are used, each with a different half life.

What Was Earth's Early History ?

The oldest known rock lies in Greenland. It is part of a sequence of rocks that is 3,800 million years old.

 On the face of earth this may seem a long time ago but as the earth is some 4,500 million years old, this rock was formed quite a way into the story.The Ancient rocks in the Greenland are examples of gneissic rock and are metamorphic --- that is, they were formed as heat and pressure changed a rock that was already in existence. Previously this rock would have been a sedimentary rock, like sandstone, made up of tiny grains and fragments. These fragments would, in turn have been worn off some other rock that existed even before this. earth history is extremely complex!

Some of the most ancient rocks are sedimentary and have not been metamorphosed. These are known as greenstones and are interesting because they contain iron ore that has not been combined with oxygen, suggesting that there was no free oxygen in the atmosphere at that time. Iron ores that form nowadays tend to be reddish in color as they are combined with oxygen. We are familiar with this effect in the rusting of exposed iron. The red substance known as rust is an oxide of Iron.  

Life Evolves:

At around 3,800 million years ago life got going. The earliest form of life occurred in the sea and was probably no more than a molecule that had the power to reproduce itself by absorbing chemicals in the water round about. Any change to the molecule that made reproduction more efficient would have been passed on to the offspring molecules, and so evolution would have begun. The first living cell were probably cyanobacteria --- sometimes called "blue-green algae" --- similar to those that can form a poisonous scum on modern waterways. Microscopic organisms like these leave no fossils but they can leave signs of their passing. Where mats of cyanobacteria lie on the sea bed, the currents bring sand layer, which will in turn attract more sand. the result is a dome-like structure, called a stromatolite, consisting of alternating sand and bacteria layers. such structures can only form in waters where there are no other living things to disturb them. They have been found in rocks 3,500 million years old.

Some premitive  micro-organisms powered their reproductive cycle by absorbing energy from the sun. We can think of these as the first plants. Other organisms did not use the raw chemical materials but absorbed the ready-made foodstuffs and molecules produced by the early plants. These were the first animals. The chemical process that uses the sun's energy for reproduction produces oxygen as by product, and this was given off and dissolved in the ocean waters or put into the atmosphere. Free oxygen --- oxygen gas that is not combined with any other substance --- was present in the atmosphere by 2,000 million years ago.

The Growth Of Continents

      
While all this was happening in the sea, what was happening on the land? As wall as the metamorphic rocks which formed the cores of the continents, and the sedimentary rocks int the surrounding seas, there is a third type of rock which was produced by volcanic action. This is called igneous rock and was created when molten rock solidified. The continents grew building processes along the edges threw up  chains of sedimentary rock, shot through by igneous rock.
Nowadays, most continents consist of a core of ancient metamorphic rock surrounded by successively younger suites of sedimentary and igneous rocks in the form of mountains.