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 19 th 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 - o_time.html o_time.html Geologic Correlation - ml ml Eocrinoids – Ashley Allen photo

References Elrathiella alabamensis – Ashley Allen photo Hardouinia bassleri - Ashley Allen photo php php Carcharodon carcharias – Ashley Allen photo Geologic Time Scale - %20Lecture.html %20Lecture.html

References Archimedes – Ashley Allen photo Geologic Time Scale (Ages) - pe/k09.htm pe/k09.htm Dating Fossils Using Relative Data and Absolute Dating Techniques - g.html g.html