The age of the Earth
Until about 300 years ago, most people believed that the earth was formed qiute recently. Some considered that the earth was created in 4004 BC, a date that was calculated from the number of generations of people listed in the Bible. Fossils found in some sedimentary rocks were thought to be creatures that perished in the Flood that is described in the Bible.
In the 1880s, however, scientists began to realize that fossils were the remains of creatures which may have lived thousands or millions of years ago. They also came to understand that fossils represented the story of life on earth. But at the time no one could determine the true age of rocks and fossils. With the discovery of radio-activity and radioisotopes it became possible to fix the age of rocks by a process called radiocarbon dating. Now it is believed that the earth is about 4550 million years old and the oldest rocks about 3500 million years.
By studying rocks, scientists have been able to trace a broad history of the earth. They have identified periods of great volcanic activity, great changes of climate, periods when great mountains were worn down to flat plains.
Geology and paleontology
Geology is the study of the earth, in particular the nature and distribution of the materials that form the crust. It has many branches. The study of rocks is called petrology, the study of minerals is mineralogy. There is also geophysics which applies the laws of physics to the study of the earth, geochemistry which applies chemistry to the study of earth’s crust and structural geology, which studies the arrangement and structure of the rocks of the earth’s surface. All of these branches of geology come under the heading of ‘physical geology’. Historical geology covers the history of the earth, and includes paleontology, the study of rock srata and fossils.
When scientists began the study of sedimentary rocks they discovered that, in some places, they are hundreds of metres deep. It takes a long time for sedimentary material to accumulate on the bottom of seas and lakes today, so this was the first hint that the earth is very, very old. Taking into account that when rocks remain undisturbed the oldest will lie at the bottom, the scientists realized that fossils could be used to establish the relative ages of the rocks. One of the pioneers in this work was William Smith, a British engineer. He discovered that, although some fossils occur in many strata or layers of rock, others, called index fossils in different rocks, geologists could establish that the rocks were of the same age.
Using Smith’s methods, geologists began to classify rocks in order of age and were able to divide the history of the earth into six main eras. These eras were then divided into periods, and the periods were divided into epochs. These are named so that the when they are discussed everyone, wherever they come form, knows which part of the earth’s history is being talked about.
Geological eras
Rocks formed more than 570 million years ago are called Pre-Cambrian and they contain very few fossils. However, rocks formed in the seas of the Cambrian period, about 570 to 500 million years ago, are quite rich in fossils, providing that some forms of life were abundant at that time.
The word ‘Cambrian’ come from Cambria, the old Roman word for Wales, because it was there that these rocks were first studied. In the Cambrian period, shallow seas spread over large areas, including most of Britain and about one-third of North America. In some parts of North America, movements of the earth’s crust pushed and folded the rocks up into mountains such as the Appalachians, and the seas disappeared.
Life at this time was of the invertebrate kind, that is, animals which have no backbones. Typical animals were the sea-dwelling brachiopods, molluscs, trilobites and an early type of coral. As far as we know there was no plant or animal life on land at this time.
The Palaeozoic era was a long period, lasting about 345 million years, during which plant and animal life began to evolve slowly on earth. The first vertebrates or animals with backbones appeared. These included fish, amphibians and reptiles. The first land plants that we know of also seem to have occured during this period.
An important part of the Palaeozoic era is the Carboniferous period. It lasted about 65 million years during this time the coal that we use today was formed from rich plant life that grew in swamps. Coal-forming plants included trees that grew to 30 metres tall, and giant ferns that are now extinct. Some big deposits of limestone were also formed during this period.
Beginning about 225 million years ago, the Mesozoic era lasted for 160 million years. For convenience, scientists divide it into three periods, the Triassic, the Jurrasic and the Cretaceous. This was a very exciting evolutionary era, for it was then the dinosaurs made their appearance. The dinosaur is one of the largest creatures ever to walk the earth. Other spectacular creatures included the flying reptile called the pterodactyl, various marine reptiles and the first tru bird. Primitive mammals also evolved during this time and flowering plants spread rapidly. In the seas were the spiral-shaped animals called ammonites, and coral.
The word ‘cenozoic’ comes from Greek words meaning ‘new life’, and the Cenozoic era covers the last 65 million years of the earth’s history. Throughout this time most of the living things that exist today evolved. The great mountain ranges of the world, the Alps, the Himalayas and the Andes, were also formed during the Cenozoic era. It was towards the end of this era that man developed.
Until about 300 years ago, most people believed that the earth was formed qiute recently. Some considered that the earth was created in 4004 BC, a date that was calculated from the number of generations of people listed in the Bible. Fossils found in some sedimentary rocks were thought to be creatures that perished in the Flood that is described in the Bible.
In the 1880s, however, scientists began to realize that fossils were the remains of creatures which may have lived thousands or millions of years ago. They also came to understand that fossils represented the story of life on earth. But at the time no one could determine the true age of rocks and fossils. With the discovery of radio-activity and radioisotopes it became possible to fix the age of rocks by a process called radiocarbon dating. Now it is believed that the earth is about 4550 million years old and the oldest rocks about 3500 million years.
By studying rocks, scientists have been able to trace a broad history of the earth. They have identified periods of great volcanic activity, great changes of climate, periods when great mountains were worn down to flat plains.
Geology and paleontology
Geology is the study of the earth, in particular the nature and distribution of the materials that form the crust. It has many branches. The study of rocks is called petrology, the study of minerals is mineralogy. There is also geophysics which applies the laws of physics to the study of the earth, geochemistry which applies chemistry to the study of earth’s crust and structural geology, which studies the arrangement and structure of the rocks of the earth’s surface. All of these branches of geology come under the heading of ‘physical geology’. Historical geology covers the history of the earth, and includes paleontology, the study of rock srata and fossils.
When scientists began the study of sedimentary rocks they discovered that, in some places, they are hundreds of metres deep. It takes a long time for sedimentary material to accumulate on the bottom of seas and lakes today, so this was the first hint that the earth is very, very old. Taking into account that when rocks remain undisturbed the oldest will lie at the bottom, the scientists realized that fossils could be used to establish the relative ages of the rocks. One of the pioneers in this work was William Smith, a British engineer. He discovered that, although some fossils occur in many strata or layers of rock, others, called index fossils in different rocks, geologists could establish that the rocks were of the same age.
Using Smith’s methods, geologists began to classify rocks in order of age and were able to divide the history of the earth into six main eras. These eras were then divided into periods, and the periods were divided into epochs. These are named so that the when they are discussed everyone, wherever they come form, knows which part of the earth’s history is being talked about.
Geological eras
Rocks formed more than 570 million years ago are called Pre-Cambrian and they contain very few fossils. However, rocks formed in the seas of the Cambrian period, about 570 to 500 million years ago, are quite rich in fossils, providing that some forms of life were abundant at that time.
The word ‘Cambrian’ come from Cambria, the old Roman word for Wales, because it was there that these rocks were first studied. In the Cambrian period, shallow seas spread over large areas, including most of Britain and about one-third of North America. In some parts of North America, movements of the earth’s crust pushed and folded the rocks up into mountains such as the Appalachians, and the seas disappeared.
Life at this time was of the invertebrate kind, that is, animals which have no backbones. Typical animals were the sea-dwelling brachiopods, molluscs, trilobites and an early type of coral. As far as we know there was no plant or animal life on land at this time.
The Palaeozoic era was a long period, lasting about 345 million years, during which plant and animal life began to evolve slowly on earth. The first vertebrates or animals with backbones appeared. These included fish, amphibians and reptiles. The first land plants that we know of also seem to have occured during this period.
An important part of the Palaeozoic era is the Carboniferous period. It lasted about 65 million years during this time the coal that we use today was formed from rich plant life that grew in swamps. Coal-forming plants included trees that grew to 30 metres tall, and giant ferns that are now extinct. Some big deposits of limestone were also formed during this period.
Beginning about 225 million years ago, the Mesozoic era lasted for 160 million years. For convenience, scientists divide it into three periods, the Triassic, the Jurrasic and the Cretaceous. This was a very exciting evolutionary era, for it was then the dinosaurs made their appearance. The dinosaur is one of the largest creatures ever to walk the earth. Other spectacular creatures included the flying reptile called the pterodactyl, various marine reptiles and the first tru bird. Primitive mammals also evolved during this time and flowering plants spread rapidly. In the seas were the spiral-shaped animals called ammonites, and coral.
The word ‘cenozoic’ comes from Greek words meaning ‘new life’, and the Cenozoic era covers the last 65 million years of the earth’s history. Throughout this time most of the living things that exist today evolved. The great mountain ranges of the world, the Alps, the Himalayas and the Andes, were also formed during the Cenozoic era. It was towards the end of this era that man developed.
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