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.