(This Is Transcribed from a Prentice Hall

Geologic Time Chapter 17 section 1 (This is transcribed from a Prentice Hall read only slide show)

Geologic Time Scale:

Slide Number Information 761 To understand Earth, it is important to realize that our planet is very old. 762 Modern methods of radiometric dating put Earth’s age at 4.5 billion years. 763 This immense time span is hard to comprehend. Guess about how long it would take just to count to 4.5 billion if you could count one number per second without stopping. 150 years 764 An appreciation for the magnitude of geologic time is important because many of Earth’s processes are so gradual that vast spans of time are needed for significant changes to occur. 765 It took millions of years to create these mountains and it will take millions more to wear them away. Geologists have divided all of Earth history into units of varying magnitude. Together they comprise the geologic time scale.

766 767 The time scale provides a framework for arranging events of the geologic past. 768 Eon represents the greatest expanses of time. The eon that began about 540 million years ago is called the Phanerozoic, Greek for “visible life”.

769 770 The sedimentary rocks and deposits of Phanerozoic eon contain an abundance of fossils. 771 The Phanerozoic eon is divided into units called eras. 772 The era names refer to import differences in dominant life-forms.  Paleozoic refers to ancient life. Example Trilobite fossils.  Mesozoic refers to middle life.  Cenozoic refers to recent life. 773 Each era is divided into smaller units called periods.

774 The changing fossil record also played an important part in establishing each of the periods. 775 Periods may be divided into epochs. Seven epochs are names for the periods of the Cenozoic era. 776 The epochs of other periods usually are just termed early, middle, and late. 777 The Pleistocene epoch, which ended just 10,000 years ago, coincides with a time span commonly known as the ice age. 778 The detail on the time scale does not begin until about 540 million years ago, the date for the start of the Cambrian Period. 779 The vast expanse of time prior to the Cambrian Period is divided into three eons- the Hadean, the Archean, and the Proterozoic. This greater than 4 billion-year span is also commonly called the Precambrian.

780 781 About what percent of Earth’s 4.5 billion-year history is represented by Precambrian time? 88% 782 Why are the eons of Precambrian time not divided into numerous eras and periods? 783 One reason is that Precambrian life was soft- bodied and therefore the fossil record is meager compared to the Phanerozoic eon. 784 In addition, much of the Precambrian rock record is buried beneath younger rocks and/or altered by metamorphism. The Grand Canyon’s ancient Vishnu schist is buried beneath a huge pile of Paleozoic rocks. 785 Which is the greatest expanse of time? eon 786 Which is a subdivision of an epoch? None of these 787 Mesozoic is an example of era. 788 Eons are divided into these era. 789 Which is the smallest time span on the geologic time scale? epoch 790 Cambrian and Jurassic are examples of period. 791 The age of the Earth is about: 4.5 billion years 792 The Phanerozoic eon began about years ago.

793 We are presently living in the Cenozoic era. 794 The Hadean, Archean, and Proterozoic are: eons that collectively comprise the Precambrian. 795 Which era of the Phanerozoic eon has the most periods? Paleozoic

796 Eocene, Pleistocene, and Holocene are examples of: epochs