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Notes On Geologic Periods Of The Earth

1938 words - 8 pages

The Precambrian era is about 90% of the earth’s geologic life. It refers to all geologic time before 600 million years ago. During the Precambrian era, the earth formed along with the oceans and atmosphere. Originally, the earth was in a molten state, but as it cooled down, it developed a hard crust and oceans that developed water vapor to form an atmosphere. About five hundred million years after the earth was formed, small continents started to form. The plate tectonics also started to build up. As the earth grew older within this period, the layers of the earth started to form. The continents were thought to be joined together in one super continent. It is thought that the end of the Precambrian era was the start of a global ice age.
The earth was more than six hundred million years old when the first signs of life started to show. The planet had cooled down from its molten state enough to support life. Scientists believe that the water was the reason that the earth formed its first life. The earliest living organisms were microscopic bacteria. There is fossil evidence of them from as early as 3.4 billion years ago. The first mulitcelled organisms have fossil evidence that dates back to about six hundred million years ago. The main forms of life consisted of sponges, cnidarians, and annelids.

The Cambrian time period was the first in the Paleozoic era. It lasted about fifty-three million years. As the period started, the continents started to pull apart. Land masses were scattered. During this time period the oceans started to oxygenate. The Cambrian was thought to be in the middle of two ice ages; however, there were no significant ice formations during this time period. No continents were located near the poles, so the temperatures were relatively mild. The sea levels rose and lowlands were flooded and much of the world was covered completely in water.
During this time period, life in the ocean started becoming more abundant. There is fossil evidence of sponges and worms. The Cambrian echinoderms were not the same as the ones we might find today. However, invertebrates were still abundant in the Cambrian time period. Many early Cambrian invertebrates are only known due to the shell like fossils. A wiwaxia was found during the Cambrian period; they were found at the bottom of the ocean floor. They might have been a relative of the mollusks or the annelids.
During this time period, the continents were mainly together in a super continent. As time in this period went on, part of the continent started to move towards the South Pole. North America was mainly centered at the equator and almost the entire continent lay underwater. Towards the middle of the period, most of the continent was no longer underwater. There was an increase in sea floor spreading, causing more volcanic activity. Ocean currents also started to change. The climate started to...

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