The History of the Earth in 1400 Words
by
Harold Richman
The Mississippi River starts in Minnesota and flows 2,000 miles down to the Gulf of Mexico. As the water flows south it carries with it an enormous amount of material. Some of this material is organic matter, but most is suspended small grains of clay. These small particles have a very special characteristic, they are positively charged. This means that each particle repels every other particle just as two similar poles of a magnet repel each other. Because of this repulsion, the particles are evenly distributed throughout the liquid.
If you took a glass of water from the Mississippi and placed it on your desk, the water would remain cloudy after one day, after a week, and after a year. In fact the particles would remain suspended indefinitely due to the positive charge. This is why the river looks muddy and is affectionately called "The Big Muddy".
When the river reaches the Gulf of Mexico something unusual occurs. The particles start to precipitate out and settle to the bottom. What has changed? The fresh water of the river has started to mix with the salt water of the Gulf. As you know, salt water can conduct electricity and so it neutralises the charged particles. Since they no longer repel each other they settle down to the bottom.
The river drops its load of material at the mouth of the river creating a delta. The river actually blocks its own channel, but water will always seek an alternate route which eventually will also be blocked. This is why rivers that empty into a body of salt water have many channels rather than one main channel. As more channels become blocked the delta becomes wider and wider. The delta at the mouth of the Mississippi extends from Florida across Alabama, across Mississippi, across Louisiana and half-way across Texas.
The river water can project out into the ocean for a considerable distance before all the particles are neutralised. Satellite pictures of the Amazon River show the brown colour of the river extending 400 miles out into the Atlantic. When the particles settle to the bottom, they form a layer of clay. Over millions of years the particles are cemented together with silica and form a rock called shale.
When the earth was formed about five billion years ago, it was a molten ball of glass. The outer layer is called magma; when it cooled sufficiently, about 4.5 billion years ago, it formed a crust. The rocks formed from molten magma are called igneous rocks.
Igneous rocks are still being produced by molten magma reaching the surface of the earth through a fissure or weakness in the crust. The most visible sign of this appears when a volcano erupts. The violent eruption of some volcanoes is caused by expansion of trapped gases. Strange as it may seem, the most violent eruptions are caused by water vapour. Volcanoes are usually located where two tectonic plates meet as this is where a fissure or weakness is most likely to occur.
Molten magma contains many different minerals. The type of rock produced will depend on which minerals are present and the rate at which the magma cools. If the magma cools slowly, the crystals formed by the minerals will have time to grow and become large, giving the rock a coarse appearance. if the magma cools quickly the crystals will be very small and the rock will have a smooth appearance.
The most common type of rock now found on earth is sedimentary rock.
Over millennia, igneous rock was eroded by wind, water, temperature changes, and the growth of plants to form sand, silt, clay and calcite. Some of the eroded material, such as sodium and calcium, was dissolved in water. Calcium forms calcium carbonate in the water and is used by crustaceans to build their shells.
The insoluble products of erosion are then used in the formation of sedimentary rock. For instance, sand produces sandstone, silt produces siltstone, clay produces shale, and calcite produces limestone.
In addition, there are other types of sedimentary rock produced by the sedimentation of a particular type of material. The white cliffs of Dover are chalk, a sedimentary rock formed by the shells of protozoans (a single-celled organism called forminifera). These shells are microscopic and yet they formed cliffs that are 400 feet thick and extend over an area of many square miles.
During the Precambrian period, there was a species of plant that used iron instead of carbon in its cell structure. These plants no longer exist, but when they died they formed sedimentary layers which eventually turned into rock called Banded Iron Formation. This rock is 30% to 50% iron oxide and is the most valuable type of iron ore.
Peat is the sedimentation of plant life that lived during the Carboniferous era. When peat is subjected to pressure, it is converted to lignite which will in turn be converted to bituminous coal.
All of these sedimentary rocks have been formed over a very long period of time which is called geological time. It is hard to comprehend such a long period, but let me give you an example of what can be accomplished in geological time. If the shells of the protozoans that formed the Dover cliffs were deposited at the rate of one-tenth of an inch per year, you would agree that this was possible. Over a period of 1,000,000 years, they would deposit 100,000 inches which is approximately 10,000 feet, or approximately two miles. So you can see how geological time can accomplish what at first looks like an impossible task.
Geological time can also work in reverse. For instance, there are mountain ranges that have been completely eliminated by erosion. If only one-tenth of an inch is eroded in one year, the same calculation will result in the erosion of a two-mile high mountain in 1,000,000 years.
There is one interesting exception. The Himalayan Mountains have remained at the same height for aeons. An Indian scientist performed an experiment near the Himalayas. He set up a plumb bob with a heavy weight and he expected that the large mass of the mountains would attract the weight. Much to his surprise, he found that the weight was repelled. It was then determined that the mass of rock on the opposite side was denser than the Himalayas. The mountains are actually floating, like an iceberg, in the molten magma located twenty miles below the surface of the earth. As the mountain is eroded, it is rising at the same rate and so it appears to be the same height.
One of the most spectacular sights on earth is the Grand Canyon. The Colorado River has cut a path through the rock for a depth of 7,000 feet. You would expect the canyon to extend all the way to the mouth of the river, but the canyon is only about two hundred miles long. This is another instance where the surrounding countryside is slowly rising and the river is cutting through the rock at the same rate. The river has remained at the same elevation but the rocks around it have been rising.
There is a third type of rock called metamorphic rock. This rock is produced when either igneous rock or sedimentary rock is subjected to very high temperature and pressure. For instance, limestone will be transformed into marble. Shale will be transformed into slate. Anthracite coal when subjected to extreme pressure will be converted into graphite.
Some crystals will only form under high pressure. Garnet and diamonds are two examples. The high pressure required is found at great depth. Later, due to movement caused by tectonic plates, some of the metamorphic rock will be brought near the surface of the earth where it can be found.
This brief description of the history of the earth in 1400 words will give you an appreciation of the many changes that have taken place to produce the earth we call home.