Geologic Time The History of the Earth Chapters 8/9

The Geologic Time Scale Because earth’s history is so immense, geologists have divided the time into units and organized it on a time scale. This is known as the Geologic Time Scale. The oldest division of time is at the bottom of the scale. Moving upward on the scale, each division is younger, just as the rocks usually are younger as you move upward.

The time scale is divided into units called eons, eras, periods and epochs. Eons – billions of years – Achaean, Proterozoic and Phanerozoic Era – hundreds of millions or billions, separated by different life forms, Precambrian, Paleozoic, Mesozoic and Cenozoic. Periods – millions of years, defined by life forms Epochs – tens of thousands of years.

The most important concept to remember when studying the eras is the idea of feedback systems. A feed back system is a cause/effect event. One event causes a specific response. It can be both positive and negative. Ex. Positive – when a pregnant animal begins having contractions- she produces oxytocin, which makes her have more contractions ( same ) Negative – the lower my body temperature- the more I shiver ( opposite)

Precambrian time (4.5 billion years ago to 570 million years ago) Life - no evidence of life on land. Simple marine plants and animals, stromatolites dominant. Earth – great volcanic activity and metamorphism of rocks The volcanic eruptions were releasing gases that would become necessary for life. Also, the same simple organisms in the water were photosynthetic, putting oxygen into the air. Metamorphism lead to the formation of metal ores like iron, copper and nickel.

Paleozoic ( 570 million years ago to 250 million years ago) Life – explosion of marine and land animals. ( oxygen in air) Trilobites, mollusks, amphibians, fish, reptiles exist. Mass extinction of 95% of marine invertebrates; seed ferns and scale trees die out Earth – Mountain building (orogeny) is expansive. Some periods of cold; most hot weather. Coal forming swamps in North America. Earth was changing, constantly. This contributed to the extinctions. Also, it was getting hotter toward the end, as much as 10%-15% of oceans evaporated.

Mesozoic ( 250 million years ago – 65 million years ago) Life – Reptiles dominated the earth; reign of the Dinosaurs. All types of plants thrive. Giant insects evolved with the flowering plants. Dinosaurs died out. Earth – Volcanism is still active; much of North America was under water. Temperatures decreased Pangaea is breaking up. Turmoil is everywhere. A Possible giant meteorite hit in the Mexican Peninsula.

Cenozoic ( 65 million years ago - today Life – Mammals dominate the earth. First human fossils around 5 million yrs. Domestication of animals.