AP BIOLOGY

Detailed explanation-1: -The geologic time scale is the “calendar” for events in Earth history. It subdivides all time into named units of abstract time called-in descending order of duration-eons, eras, periods, epochs, and ages.

Detailed explanation-2: -What does the time scale represent? The geologic time scale divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet. These divisions are called geochronologic units (geo: rock, chronology: time).

Detailed explanation-3: -For example, the entire age of the earth is divided into four eons: the Hadean Eon, the Archean Eon, the Proterozoic Eon, and the Phanerozoic Eon. These four eons are further subdivided into eras (Table 7.3). Fig.

Detailed explanation-4: -eon = The largest unit of time. era = A unit of time shorter than an eon but longer than a period. period = A unit of time shorter than an era but longer than epoch. epoch = A unit of time shorter than a period but longer than an age.

Detailed explanation-5: -Some examples of events listed on the geologic time scale include the first appearance of plant life on Earth, the first appearance of animals on Earth, the formation of Earth’s mountains, and the extinction of the dinosaurs.