EARTH AND SPACE SCIENCE Chapter 9 A View of Earth’s Past 9.1 Geologic Time
9.1 Geologic Time Objectives • Summarize how scientists worked together to develop the geologic column. • List the major divisions of geologic time.
Introduction • The Earth’s surface is constantly changing – mountain chains are formed and eroded, oceans rise and fall, and continents move about. • The changes in the Earth’s surface affect life living on that surface. • Evidence of change in life on Earth over time is recorded in the layers of sedimentary rock. • Scientists have developed a geologic time scale to describe the sequence of changes in life on Earth.
The Geologic Column • The law of superposition states that in a column of sedimentary rock, the lower the rock unit, the older it is in relation to the others. • As one ascends through the rock units, the rock units get progressively younger. • In the 19th century, scientists were able to determine relative ages of sedimentary rock layers around the world by applying the law of superposition. • No single area on Earth contains the entire segment of geologic time, so observations must be combined to create a composite idealized arrangement of rock units.
The Geologic Column • A geologic column is an ordered arrangement of rock layers that is based on the relative ages of rocks and in which the oldest rocks are at the bottom. • Rock layers in a geologic column may be distinguished by the type of rock from which they are made and the types of fossils that they contain.
The Geologic Column • A geologic column could be viewed as a timeline of Earth’s history. • Fossils found in the sedimentary rock layers of the geologic column follow the law of faunal succession. • The law of faunal succession states that as fossil fauna become progressively younger, it becomes more familiar. • The older the fossil fauna, the more dissimilar it is when compared to modern fauna.
The Geologic Column • When first developing the geologic rock column, some scientists would attempt to apply average depositional rates to estimate ages. • Later, radiometric dating methods allowed scientists to determine much more accurate ages for rock units. • Rock layers that cannot be dated using radioisotopes are assigned ages based on their position in the geologic column. • So when a fossil is found in a particular sedimentary layer, a relatively close absolute date can be assigned to the fossil based on the rock unit from which it was collected.
Divisions of Geologic Time • The geologic time scale is divided into eons, eras, periods, and epochs. • The units of time are not divided evenly. • The Earth’s history is marked by changes in Earth’s surface, climate, and organisms. • The divisions of geologic time are marked primarily by changes in life on Earth.
Divisions of Geologic Time • The Eon is the largest geologic unit of time. • There are four Eons. • Hadean • Archean • Proterozoic • Phanerozoic • The first three eons are commonly lumped into the designation Pre-Cambrian time – the first 4 billion years of Earth’s history. • Very few fossils exist in Pre-Cambrian rocks.
Divisions of Geologic Time • Eons are divided into eras. • The Phanerozoic Eon is divided into three eras. • Paleozoic (292 million years) • Mesozoic (183 million years) • Cenozoic (65 million years) • Paleozoic rocks contain an abundance of marine (and later terrestrial) fossils. • Eras are divided into periods which are usually named after the location where typical fossils were first discovered for this age.
Divisions of Geologic Time • Each period can be further subdivided into smaller time units called epochs. • Epochs may be divided into even smaller units called ages. • Ages are defined by the occurrence of distinctive fossils found in rocks of that particular age.
References • Bardstown Reef – Ashley Allen photo • Grand Canyon Superposition - http://www.calstatela.edu/faculty/acolvil/geo_time.html • Geologic Correlation - http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.html • Eocrinoids – Ashley Allen photo
References • Elrathiella alabamensis – Ashley Allen photo • Hardouinia bassleri - Ashley Allen photo http://www.squali.com/fossili/squalicorax.php • Carcharodon carcharias – Ashley Allen photo • Geologic Time Scale - http://gator.gasd.k12.pa.us/~dpompa/Mini%20Lecture.html
References • Archimedes – Ashley Allen photo • Geologic Time Scale (Ages) - http://geowords.com/histbooknetscape/k09.htm • Dating Fossils Using Relative Data and Absolute Dating Techniques - http://www.talkorigins.org/faqs/dating.html