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The Geologic Time Scale OBJECTIVES Describe the geologic time scale. Distinguish among the following geologic time scale divisions: eon, era, period, and epoch. Vocabulary Original preservation Eon Absolute Altered hard part Era Half-life Radiometric Dating Period Varve geologic time scale Epoch Mold Uniformitarianism Key bed Cast Unconformityperposition Paleontology Law of Crosscutting Index fossil Law of Superposition Trace fossil Dendrochronology Evolution Permineralization THE GEOLOGIC TIME SCALE *By studying the characteristics of rocks and the fossils within them, geologists can interpret the environments in which the rocks were deposited, reconstruct Earth’s history, and possibly predict events or conditions in the future. HOW OLD IS EARTH? We know that Earth must be at least as old as the oldest rocks in the crust. – The age of the oldest rocks on Earth is between 3.96 to 3.8 billion years. – Evidence of 4.1- to 4.2-billion-year-old crust exists in the mineral zircon that is contained in metamorphosed sedimentary rocks in Australia. EARTH’S “BIRTH” For about the first 4 billion years of Earth’s 4.6-billion-year existence, most of the life-forms that inhabited Earth were unicellular organisms. It wasn’t until 2.1 BYA eukaryotic algae was 1st appeared. THE ROCK RECORD Geologists have divided the history of Earth into time units based upon the fossils contained within the rocks. • The geologic time scale is a record of Earth’s history from its origin 4.6 billion years ago to the present. This scale - • geologic events • environmental changes • development of life-forms preserved in the rock record. The Geologic Time Scale THE ROCK RECORD GEOLOGIC TIME The oldest division of time is at the bottom of the geologic time scale. The time scale is divided into units called eons, eras, periods, and epochs. An eon, measured in billions of years, is the longest time unit of the geologic time scale. An era, defined by the differences in life-forms found in rock and measured in hundreds of millions to billions of years, is the secondlongest span of time on the geologic time scale. A period is the shortest time span. GEOLOGIC TIME Precambrian Time - 90% of geologic time, is divided into the Archean and Proterozoic Eons. Proterozoic (end), the more recent of the two, is marked by the first appearance of organisms with hard parts. GEOLOGIC TIME Plants and Animals Evolve – During the Paleozoic Era, the oceans became full of a wide diversity of plants and animals; land plants appeared, followed by land animals. – The Mesozoic Era, the emergence and evolution of dinosaurs, reef-building corals, large predatory reptiles, and flowering plants and trees. – During the Cenozoic Era, mammals increased both in number and diversity, human ancestors developed, and grasses and flowering plants expanded on land. GEOLOGIC TIME Periods of Geologic Time Periods (measured in terms of tens of millions of years to hundreds of millions of years) are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited. The Cenozoic is divided into three periods: Paleogene Neogene Quaternary. GEOLOGIC TIME Epochs of Geologic Time – Epochs are smaller divisions(usually measured in millions of years to tens of millions of years). – The Cenozoic Periods - further divided into epochs - Paleocene & Oligocene. – Different groups of organisms are in the various epochs. – Each unit has characteristics that set it apart from the rest of geologic history. SECTION ASSESSMENT 1. Match the following terms with their definitions. C eon ___ A period ___ D ___ era B ___ epoch A. time periods defined by the life forms that were present; usually measured in terms of tens of millions to hundreds of millions of years B. smaller divisions of time; usually measured in millions to tens of millions of years C. the longest period of time; measured in billions of years D. second longest period of time; measured in hundreds of millions to billions of years SECTION ASSESSMENT 2. How does the geologic time scale correspond with the rock record? Moving upward on the scale, each division is younger, just as the rock layers in the rock record grow younger as you move upward. SECTION ASSESSMENT 3. Identify whether the following statements are true or false. ______ true Precambrian Time represents 90 percent of geologic time. ______ false Human ancestors developed during the Paleozoic Era. ______ false The boundaries between the periods of the Cenozoic are marked by mass extinctions. ______ true Organisms in Precambrian Time had soft bodies with no shells or skeletons. RELATIVE-AGE DATING OF ROCKS Uniformitarianism the forces that continually change the surface features of Earth today have been occurring since Earth formed. Only the rate, intensity, and scale have changed. The resulting sediments and rocks record & preserve: • the environment and fossils within the rocks • preserve evidence of the life-forms that lived PRINCIPLES FOR DETERMINING RELATIVE AGE Relative-age dating places the ages of rocks and the events that formed them in order, but without exact dates. One event or rock layer is compared to another. PRINCIPLES FOR DETERMINING RELATIVE AGE Original horizontality – Sedimentary rocks are deposited in horizontal or nearly horizontal layers. Superposition - In an undisturbed rock sequence, the oldest rocks are at the bottom and each successive layer is younger than the layer beneath. Cross-cutting relationshipsAn intrusion or a fault is younger than the rock it cuts across. PRINCIPLES FOR DETERMINING RELATIVE AGE Inclusions Relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it. These particles, called inclusions, indicate that the rocks in the lower layer are older than those on top. OTHER MEANS OF DETERMINING RELATIVE AGE The fact that Earth is constantly changing makes it difficult to find an undisturbed sequence of rock layers. An unconformity is a gap in the rock record caused by an erosional surface becoming buried by younger rocks. http://www.classzone.com/books/earth_scienc e/terc/content/visualizations/es2902/es2902pa ge01.cfm?chapter_no=visualization (Unconformity Visualization) OTHER MEANS OF DETERMINING RELATIVE AGE The unconformity is called a disconformity when horizontal sedimentary rocks overlie horizontal sedimentary rocks. A different type of unconformity exists when sedimentary rocks overlie nonsedimentary rocks. The contact point between rock is called a nonconformity. Angular unconformity is created when horizontal sedimentary rocks are uplifted & tilted, then exposed to erosional processes, and deposition resumes. OTHER MEANS OF DETERMINING RELATIVE AGE Correlation of Rock Strata – Correlation is the matching of outcrops of one geographic region to another. – Geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers. – Correlation allows geologists to accurately locate that same rock layer in another location. SECTION ASSESSMENT 1. Match the following terms with their definitions. A. principle which states that A ___ original horizontality C superposition ___ B unconformity ___ D correlation ___ sedimentary rocks are deposited in horizontal layers B. a gap in the rock record C. principle which states that oldest rocks are at the bottom and that each successive layer is younger D. matching of outcrops from one geographic region to another SECTION ASSESSMENT 2. What is the principle of uniformitarianism? The principle of uniformitarianism states that the forces that change the surface of Earth that are occurring today have been occurring since Earth formed. SECTION ASSESSMENT 3. Identify whether the following statements are true or false. ______ false Relative-age dating allows geologists to determine the age of rock formations. ______ true A limestone layer is older than a shale layer that is above it. ______ false Rock layers are often found undisturbed if you dig deep enough. ______ false The grains in a rock layer can be from a younger layer of rock. ABSOLUTE-AGE DATING OF ROCKS Absolute-age dating enables scientists to determine the actual age of a rock, fossil, or other object using the decay rate of radioactive isotopes. – Radioactive substances emit nuclear particles at a constant rate regardless of any physical or environmental changes. – The original radioactive element is referred to as the “parent,” and the new element is referred to as the “daughter.” – As the numbers of protons and neutrons change with each nuclear emission, the element is converted to a different element. ABSOLUTE-AGE DATING OF ROCKS Radioactive decay is the emission of radioactive particles and the resulting change into other elements over time. USE OF RADIOACTIVE ISOTOPES In a process called radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil to determine its absolute age. http://science.discovery.com/videos/100-greatestdiscoveries-shorts-radiometric-dating.html Radiometric Dating Videos http://www.pbs.org/wgbh/evolution/library/03/3/quickt ime/l_033_01_56.html USE OF RADIOACTIVE ISOTOPES Half-life is the length of time it takes for one-half of the original amount of an isotope to decay. Geologists use the half-life of an isotope to date samples. USE OF RADIOACTIVE ISOTOPES – Carbon-14 (C-14) is a radioactive isotope that is commonly used to determine the absolute age of an object, especially of organic origin. – C-14 is accurate for dating objects up to 75 000 years old. – For the dating of a particularly old rock sample, a radioactive isotope with a longer halflife must be used. OTHER WAYS TO DETERMINE AGE Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also be used to help geologists determine the age of an object or event. Tree Rings – The age of a tree - determined by counting the number of annual tree rings in a cross section of the tree. – The widths of tree rings are directly related to the climatic conditions during growth periods. – Dendrochronology is the science of comparing annual growth rings in trees to date events and changes in past environments. OTHER WAYS TO DETERMINE AGE Seasonal Climatic Changes – About 11,000 years ago, continental glaciers covered the northern part of the United States. (Indiana) – Varves are bands of alternating light- and dark-colored sediments of sand, clay, and silt found in lakes that resulted from summer and winter runoff from glaciers. – Varves from different lakes can be compared to determine the ages of glacial lake sediments from about 15,000 to 12,000 years ago. OTHER WAYS TO DETERMINE AGE Distinctive Sediment Layers – When a widespread sediment layer is formed by an instantaneous or short-lived event, geologists may be able to determine the time of the event through radiometric dating. – A key bed is a layer which has been dated and acts as a time marker, which can be used to correlate rock layers across large areas. – http://vsx.onstreammedia.com/vsx/AFG/search/PB SPlayer?assetId=67655&ccstart=914479&pt=1 (Indiana Fossils – Falls of the Ohio) SECTION ASSESSMENT 1. Match the following terms with their definitions. A. alternating light- and dark___B half-life ___ D dendrochronology ___ A varve ___ C key bed colored sedimentary deposits in glacial lakes B. the time period until the ratio of parent-to-daughter atoms is equal C. a widespread layer that has been accurately dated D. the science of comparing annual growth rings in trees to date events and environmental changes SECTION ASSESSMENT 2. How old is an object of organic origin if it has 25 percent of carbon-14 remaining? Why? The object is 11 460 years old. To reach the 25 percent level, the carbon-14 has experienced 2 half-life cycles of 5730 years each. SECTION ASSESSMENT 3. Why would rubidium-87 with a half-life of 48.6 billion years probably not be useful in dating an object that is 100 000 years old? With such a long half-life, the ratio of parent-to-daughter atoms would be too small to measure. REMAINS OF ORGANISMS IN THE ROCK RECORD Fossils are the evidence or remains of once-living plants or animals. The fossil record provides evidence of evolution. Evolution is an adaptive change in the DNA of populations as a result of mutation and/or environmental change. Fossils preserved in the rock record also provide information about past environmental conditions and can be used to correlate rock layers from one area to another. TYPES OF FOSSILS Fossils with original preservation are the soft and hard parts of plant and animal remains that have not undergone any kind of change since the organisms’ deaths. Such fossils are uncommon - preservation requires extraordinary circumstances such as freezing, drying out, or oxygen-free environments. How Fossils are Formed http://www.classzone.com/books/earth_science/terc/c ontent/visualizations/es2901/es2901page01.cfm?chap ter_no=visualization TYPES OF FOSSILS – Altered hard parts are fossils (all the organic material has been removed) and the hard parts of a plant or animal have been changed either by mineral replacement or by recrystallization. – Permineralization is the process by which pore spaces in a fossil are filled in with mineral substances. – During the process of recrystallization, the exterior of the shell or a bone remains the same, but the shell microstructures are destroyed. TYPES OF FOSSILS Form by mummification, amber, tar seeps, freezing, & petrification. – Index fossils are remains of plants or animals that can be used by geologists to correlate rock layers over large geographic areas or to date a particular rock layer. – An index fossil is easily recognized, abundant, and widely distributed geographically and must also have lived during a short period of time. TRACE FOSSILS Carbon films, molds, casts, coprolites, & gastroliths. – A mold is formed when the original shell parts of an organism within a sedimentary rock are weathered and eroded. A hollowed-out impression, or mold, of the shells is left in their place. – A cast of an organism is created if the cavity later becomes filled with minerals or sediment. – Carbon films -surface features on carbon film. – Coprolites -solid wastes from the animal. – Gastroliths -stones dinosaurs swallowed. TYPES OF FOSSILS Indirect Evidence of Past Life – Trace fossils are indirect evidence of plant and animal life. – Trace fossils can provide information about how an organism lived, how it moved, or how it obtained food. WHY STUDY FOSSILS? The study of fossils allows scientists to interpret and describe Earth’s history. Fossils may help scientists find patterns and cycles that can be used to predict future phenomena (events), such as climatic changes. The study of fossils allows geologists to locate energy resources (oil, gas, & coal companies hire geologist to search for new wells or mines). http://fossils.valdosta.edu/ Fossil Home Page SECTION ASSESSMENT 1. Match the following terms with their A. fossils in which all organic definitions. C preservation ___ A altered hard part ___ B permineralization B. __ D cast ___ material has been removed and bones or shells have been structurally changed process by which pore spaces are filled in with mineral substances C. fossils in which soft and hard parts of an organism have not undergone any kind of change D. formed when a mold becomes filled with minerals or sediments SECTION ASSESSMENT 2. What conditions are necessary to allow original preservation? Original preservation requires extraordinary circumstances such as freezing, drying out, or oxygen-free environments. SECTION ASSESSMENT 3. How might petroleum geologists use fossils? Petroleum geologists use certain index microfossils to determine whether oil might be present at a particular site. These fossils provide information about the ages of rocks and, in some cases, information that indicates whether the temperature and pressure conditions needed to form oil or gas were present in those layers.