Warm Up Atoms with the same atomic number but different mass numbers are called ____. a. ions c. neutrons b. protons d. isotopes 2) Radiocarbon dating could be used to date which of the following? a. 60,000-year-old metamorphic rock b. 60,000-year-old mammoth bone c. 15-million-year-old lava flow d. 65-million-year-old meteorite 3) Radioactivity is produced when unstable nuclei ____. a. break apart c. bond together b. become cooler d. expand Answers: 1) d. 2) b. 3) a. 1) The Geologic Time Scale Chapter 12, Section 4 Geologic Time Scale Based on their interpretation of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time Geologic Time Scale – the division of Earth history into blocks of time—eons, eras, periods, and epochs; the time scale was created using relative dating principles Since the time scale was created in the 19th Century, when radiometric dating was unavailable, it wasn’t until the 20th Century that radiometric dates were incorporated Structure of the Time Scale Eons represent the greatest expanse of time Eons are divided into eras Each era is subdivided into periods Periods are divided into still smaller units called epochs The eon that began about 540 million years ago is the Phanerozoic, Greek for “visible life” There are three eras within the Phanerozoic: the Paleozoic (“ancient life”), the Mesozoic (“middle life”), and the Cenozoic (“recent life”) Each era is subdivided into periods, each of which is characterized by a somewhat less profound change in life forms as compared with the eras The periods of the Cenozoic are divided into still smaller units called epochs The epochs of other periods are not usually referred to by specific names Concept Check What is the geologic time scale divided into? Eons, eras, periods, and epochs Precambrian Time The detail of the geologic time scale does not begin until the start of the Cambrian Period (540 million years ago) The 4 billion years prior to the Cambrian are divided into eons, and generally called the Precambrian The Precambrian represents about 88% of Earth’s history, but little is known about it During Precambrian time, there were fewer life forms; these life forms are more difficult to identify and the rocks have been disturbed often Precambrian Fossils Difficulties With the Geologic Time Scale For a radiometric date to be useful, all minerals in the rock must have formed at about the same time Radioactive isotopes can be used to determine when minerals in an igneous rock crystallized and when pressure and heat made new minerals in metamorphic rock A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not the same age as the rock in which they occur The sediments that are eventually cemented together into a sedimentary rock have been weathered from older rocks The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock first formed; the date may indicate when the rock was metamorphosed Absolute Dating of Sedimentary Rocks Concept Check How can geologists overcome the problem of sedimentary rocks and dating the time units of the geologic time scale? Geologists can date igneous rocks, then determine how the igneous rocks relate to nearby sedimentary rocks. Dendrochronology Assignment Read Chapter 12, Section 4 (pg. 352-355) Do Chapter 12 Assessment #1-33 (pg. 259-260) For Section 4: Do #’s 5 & 11 Study for Chapter 12 Quiz!