Physical Geography - Version 1 Unit 3 Introduction to Geology & Geologic Time

. at . Figure Crater in Valley , California . Image by Jeremy is used under a license . UNIT INTRODUCTION TO GEOLOGY GEOLOGIC TIME Goals Objectives of this unit Explain what geology is , how it incorporates the other sciences , and how it is different from the other sciences . Discuss why we study Earth and what type of work geologists do . Use the notation for geological time , gain an appreciation for the vastness of geological time , and describe how very slow geological processes can have enormous impacts over time . Apply basic geological principles to the determination of the relative ages of rocks . Explain the difference between relative and absolute techniques . Summarize the history of the geological time scale and the relationships between eons , eras , periods , and . GEOGRAPHY

WHAT IS GEOLOGY ?

In its broadest sense , geology is the study of Earth , it interior and its exterior surface , the rocks and other materials that are around us , the processes that have resulted in the formation of those materials , the water that flows over the surface , and lies underground , the changes that have taken place over the vastness of geologic time , and the changes that we can anticipate will take place in the near future . Geology is a science , meaning that we use deductive reasoning and scientific methods to understand geological problems . It is , arguably , the most integrated of all of the sciences because it involves the understanding and application of all of the other sciences physics , chemistry , biology , mathematics , astronomy , and others . But unlike most of the other sciences , geology has an extra dimension , that of time , deep time , billions of years of it . Geologists study the evidence that they see around them , but in most cases , they are observing the results of processes that happened thousands , millions , and even billions of years in the past . Those were processes that took place at incredibly slow rates , millimeters per year to centimeters per year , but because of the amount of time available , they produced massive results . The Sierra Nevada is a mountain range in the Western United States , between the Central Valley of California and the Great Basin . The vast majority of the range lies in the state of California , although the Carson Range spur lies primarily in Nevada . The Sierra Nevada is part of the American Cordillera , a chain of mountain ranges that consists of an almost continuous sequence of such ranges that form the western backbone of North America , Central America , South America , and Antarctica . The Sierra runs 400 miles ( 640 ) and is approximately 70 miles ( 110 ) across . Notable Sierra features include The General Sherman , the largest tree in the world by volume Lake Tahoe , the largest alpine lake in North America Mount Whitney at ( the highest point in the contiguous United States and Yosemite Valley sculpted by glaciers from granite , containing high waterfalls . The Sierra is home to three national parks , twenty wilderness areas , and two national monuments . These areas include Yosemite , Sequoia , and Kings Canyon National Parks and Devils National Monument . Geology is also about understanding the evolution of life on Earth about discovering resources such as metals and energy about recognizing and minimizing the environmental implications of our use of those resources and about learning how to mitigate the hazards related to earthquakes , volcanic eruptions , and slope failures . GEOGRAPHY

What Do Geologists Do ?

Geologists are involved in a range of widely varying occupations with one thing in common the privilege of studying this fascinating planet . Many geologists work in the resource industries , including mineral exploration and mining and energy exploration and extraction . Other major areas where geologists work include hazard assessment and mitigation ( assessment of risks from slope failures , earthquakes , and volcanic eruptions ) water supply planning , development , and management waste management and assessment of geological issues on construction projects such as highways , tunnels , and bridges . Most geologists are employed in the private sector , but many work for geological organizations , such as the United States Geological Survey , Figure Faculty Explaining Physical Weathering of Rock Material . Image by is used under a license . Many people are attracted to geology because they like to be outdoors , and many geological opportunities involve fieldwork in places that are as amazing to see as they are interesting to study . But a lot of geological work is also completed back in offices or laboratories . GEOLOGICAL TIME The Geologic Time Scale and the basic outline of Earth history were worked out long before we had any scientific means of assigning numerical units of age like years to events of Earth history . Working out Earth history depended on realizing some key principles of relative time . Nicholas Steno introduced a basic understanding of stratigraphy ( the study of layered rocks ) in 1669 with the basic principles of stratigraphy . William Smith ( working with the strata of the GEOGRAPHY

English coal mines , noticed that strata and their sequence were consistent throughout the region and eventually produced the first national geologic map of Britain becoming known as the Father of English Using Steno principles , a relative time scale was developed in the nineteenth century with names derived from areas studied and characteristics of the rocks in those areas . The figure below shows the names applied to units and of the Geologic Time Scale . Using this time scale as a calendar , all events of Earth history can be placed in order without ever knowing the numerical age A useful mechanism for understanding geological time is to scale it all down into one year . The origin of the solar system and Earth at Ga ( billion years ago ) would be represented by January , and the present year would be represented by the last tiny fraction of a second on New Year Eve . At this scale , each day of the year represents million years each hour represents about years each minute represents years , and each second represents 145 years . Some significant events in Earth history , as expressed on this time scale , are summarized in the table below . Table Geological Events Geologic Event Approximate Date Calendar Equivalent Formation of oceans and continents Ga January 15 Evolution of the first primitive life forms Ga Early March Stromatolites the origin of Earth Ga April 15 Oxygen Fauna 600 Ma November 11 Cambrian Explosion 545 Ma November 16 Animals first crawled onto land 360 Ma December 15 Extinction of the dinosaurs 65 Ma December 26 Beginning of the ice age Ma or 200 ka on December 315 Native Americans made the Channel 10 ka on December 315 Islands their home . The arrival of the first Europeans on the 250 years ago Two seconds before midnight northern west coast of North America on December 315 GEOGRAPHY

The Geological Time Scale William Strata Smith worked as a surveyor in the and industries in southwestern England in the late and early . While doing his work , he had many opportunities to look at the and sedimentary rocks of the region , and he did so in a way that few had done before . Smith noticed the textural similarities and differences between rocks in different locations , and more importantly , he discovered that fossils could be used to correlate rocks of the same age . Smith is credited with formulating the principle of faunal succession ( the concept that specific types of organisms lived during different time intervals ) and he used it to great effect in his monumental project to create a geological map of England and Wales , published in , rocks A . I , is , rocks rocks ' Figure William Smith Sketch of the Succession and Their Relative Altitudes . Image by NASA is in the public domain . Inset into Smith great geological map is a small diagram showing a schematic geological section extending from the Thames estuary of eastern England the west coast of Wales . Smith shows the sequence of rocks , from the rocks of Wales and western England , through the rocks of central England , to the rocks of the area around London . Although Smith did not put any dates on these , because he did know them , he was aware of the principle of superposition ( the idea , developed much earlier by the Danish theologian and scientist Nicholas Steno , that young sedimentary rocks form on top of older ones ) and so he knew that this diagram represented a stratigraphic column . And because almost every period of the Phanerozoic is represented along that section through Wales and England , it is a primitive geological time scale . Smith work set the stage for the naming and ordering ofthe geological periods , which was initiated around 1820 , first by British geologists , and later by other European geologists . Many GEOGRAPHY

of the periods are named for places where rocks of that age are found in Europe , such as Cambrian for ( Wales ) for Devon in England , Jurassic for the Mountains in France and Switzerland , and Permian for the Perm region of Russia . Some are named for the type of rock that is common during that age , such as Carboniferous for the and bearing rocks of England , and Cretaceous for the chalks of England and France . The early time scales were only relative because geologists did not know the ages of the rocks . That information was not available until the development of isotopic dating techniques early in the 20 century . The geological time scale is currently maintained by the International Commission on Stratigraphy ( which is part of the International Union of Geological Sciences . The time scale is continuously being updated as we learn more about the timing and nature of past geological events . Geological time has been divided into four eons , and Phanerozoic , and as shown below , the first three ofthese represent almost 90 of history . The last one , the Phanerozoic ( meaning visible life ) is the time that we are most familiar with because Phanerozoic rocks are the most common on Earth , and they contain evidence of life forms that we are all somewhat familiar with . Eons 4570 Ma 3850 Ma 2500 Ma 540 Ma Eras Periods Figure The Eons of Earth History . Image is used under a license . The Phanerozoic , the past 540 Ma of Earth history , is divided into three eras the ( early life ) the ( middle life ) and the ( new life ) and each ofthese is divided into several periods . Most ofthe organisms that we share Earth with evolved at various times during the Phanerozoic . GEOGRAPHY

. I . Cambrian Permian Jurassic Cretaceous 00 ED 01 01 IN in ED co in an In , Figure The Eras and Periods of the Phanerozoic . Image is used under a license . The , which represents the past Ma , is divided into three periods , and Quaternary , and seven . Dinosaurs became extinct at the start of the , after which birds and mammals radiated to fill the available habitats . Earth was very warm during the early and has steadily cooled ever since . Glaciers first appeared on Antarctica in the and then on in the and covered much of North America and Europe by the . The most recent ofthe ended around years ago . The current epoch is known as the . are further divided into ages or stages . Era Periods Ma Ma Ma Ma Ma Figure The Periods and of the . Image is used under a license . Most of the boundaries between the periods and of the geological time scale have been fixed based on significant changes in the fossil record . For example , as already noted , the boundary between the Cretaceous and the coincides exactly with the extinction of the dinosaurs . That not a coincidence . Many other types of organisms went extinct at this time , and the boundary between the two periods marks the division between sedimentary rocks with Cretaceous organisms below and organisms above . GEOGRAPHY

GEOLOGIC DATING Geological Dating is a technique used in Geology to date a certain type of rock that contains radiometric elements and those radiometric elements decay at a constant rate . This unit will discuss several different types of dating , both relative and absolute . Figure Geological Time Spiral . Image is in the public domain . Relative Dating Relative dating is the process of determining if one rock or geologic event is older than or younger than another , without knowing the specific age ( number of years ago the object was formed ) The principles of relative time are simple , even obvious now , but were not generally accepted by scholars until the Scientific Revolution of the 17 and 18 centuries . James realized that geologic processes are slow and his ideas on uniformitarianism ( the present is the key to the past ) provided a basis for interpreting the rocks of the Earth in terms of scientific principles . GEOGRAPHY

Stratigraphy is the study of layered sedimentary rocks . Below are a few principles of relative time that are used in all of geology , but especially useful in stratigraphy . Principle of Superposition In an otherwise undisturbed sequence of sedimentary strata ( rock layers ) the layers on the bottom are the oldest and the layers above are younger . Principle of Original Horizontality Layers of rocks deposited from above in a gravity field , such as sediments and lava flows , originally were laid down horizontally . This holds true except for the margins of basins , where the strata can slope slightly downward into the basin . Principle of Lateral Continuity Within the depositional basin in which they form , strata are continuous in all directions until they thin out at the edge of that basin . Of course , all strata eventually end , either by hitting a geographic barrier or by a depositional process being too far from its source , either a sediment source or a volcano . Strata that are subsequently cut by a canyon remain continuous on either side of the canyon . Principle of Fossil Succession of fossils contained in strata are unique to the time they lived and can be used to correlate rocks of the same age across a wide geographic distribution . Evolution has produced a succession of life whose fossils are unique to the units of the Geologic time Scale . A . Figure Example of Superposition , Red Rock Canyon , California . Image by Jeremy is used under a license . GEOGRAPHY

Figure Fossil Succession Showing Correlation Among Strata . Image is used under a license . Absolute Dating Relative time allows science to tell the story of the Earth , but does not provide specific numeric ages of events , and thus , the rate at which geologic processes operate . Based on Uniformity Principle ( Uniformitarianism ) early geologists surmised that geological processes work slowly and that the Earth is very old . Because science advances as the technology of its tool advances , the discovery of radioactivity in the late provided a new scientific tool by which actual ages in years can be assigned to mineral grains within a rock . Later we will identify how Earth history is understood using relative dating principles without actually knowing the numerical age of events . This was how scientists of that time interpreted Earth history , until the end of the 19 when radioactivity was discovered . This discovery introduced a new dating technology that allows scientists to determine specific numeric ages of some rocks , called absolute dating . The next sections discuss this absolute dating system called isotopic dating . Dating Given a sample of rock , how is the dating procedure carried out ?

Using chemical analysis , the parent elements and daughter products can be separated from the mineral . Remember that elements behave chemically due to their atomic number . In the case of uranium , both the and isotopes are chemically separated out together , as are the and . An instrument called a mass spectrometer then separates the uranium isotopes from each other as well as the lead isotopes from each other by passing beams ofthe isotopes through a magnetic field . As these isotopic beams pass through the instrument , the path of the heavier isotope is deflected less so the two beams strike a sensor at different places . From the intensity of each GEOGRAPHY

beam , the amount of parent and daughter products is determined , and from this ratio , the age can be calculated . Figure Graph of the Number of Image is used under a license Here is a simple example of age calculation using the ratio of daughter product to parent isotope . When the mineral initially forms , there is a 100 parent isotope and daughter and the ratio of daughter to parent ( is . After one , half the parent has decayed so there is 50 parent and 50 daughter . The ratio is then . After two , there is 25 parent and 75 daughter , and the ratio is . This can be further calculated for a series of lives as shown in the table below . Note that after about ten , the amount of parent remaining is so small that accurate chemical analysis ofthe parent is difficult , and the accuracy of the method is diminished . Ten are generally considered the upper limit for use of an isotope for dating . Modern applications of this method have achieved remarkable accuracy of plus or minus two million years in billion years ( that ) Considering the technique , in any given sample analysis , there are two separate clocks running at the same time , and . The existence ofthese two clocks in the same sample gives a on each other . Many geological samples contain multiple pairs so clocks show dating to be highly reliable . Carbon Dating Another dating method involves carbon and is useful for dating archaeologically important samples containing organic substances like wood or bone . Carbon dating uses the unstable isotope ( and the stable isotope ( is constantly being created in the atmosphere by the interaction of cosmic particles with atmospheric ( The cosmic particles include neutrons that strike the nitrogen GEOGRAPHY

nucleus kicking out a proton but leaving the neutron in the nucleus . The atomic number is reduced by one from to forming carbon and the mass number remains the same at 14 . The quickly bonds with oxygen in the atmosphere to form carbon dioxide which mixes with the other atmospheric carbon dioxide and is incorporated into living matter . Thus , while an organism is alive the ratio of in its body does change since it is constantly exchanging with the atmosphere . However , when it dies , the radiocarbon clock starts ticking as the decays back to by beta decay with a of years . The radiocarbon dating technique is thus useful for about ten back years or so . Since dating relies on parent and daughter ratios and the amount of parent needs to be known , early applications of dating assumed the production and concentration of in the atmosphere for the last years or so was the same as today . But the production of since the Industrial Revolution by combustion of fossil fuels ( in which long ago decayed ) has diluted in the atmosphere leading to potential errors in this assumption . Other factors affecting the estimates of the composition of parent carbon in the atmosphere have also been studied . Comparisons of carbon ages with tree ring data and other data for known events have allowed calibration for the reliability ofthe radiocarbon method which is primarily used in archaeology and very recent geologic events . Taking into account these factors , dating is a reliable dating method in this range . SUMMARY All the events of earth history can be placed in sequence using the principles of relative time called the five Principles of Stratigraphy . The Geologic Time Scale was completely worked out in the 19 Century using these principles and used as a calendar for telling the story of the earth . Uniformity Principle , knowing how natural processes work in the present is key to understanding the past , provided a means to study and understand the processes involved in the events that have shaped the earth to its present form . The discovery of radioactivity in the late provided a tool to measure the actual ages of the events of earth history . However , certain types of rocks and minerals are better suited for dating and certain assumptions about those rocks and minerals require care and precaution in interpreting ages . Geologic time is now known to be vast and to have provided plenty of time for the evolution of the planet and life upon it to have taken place to produce the earth as we see it and all the life forms on it . GEOGRAPHY