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A Trip through Geologic Time ______________________________________________________________________________ Fossils: 1. Help scientists infer how Earth’s surface has changed 2. ________________________________________________________________________ How a Fossil Forms ______________________________________________________________________________ ______________________________________________________________________________ Fossils are normally found in sedimentary rock. Sedimentary rock is the type of rock that is made of hardened sediment. Made of remains of dead organisms and rock particles Steps to form a fossil 1. ______________________________________________________________________________ 2. Sediment covers the animal 3. ______________________________________________________________________________ 4. Weathering and erosion eventually expose the fossil at the surface. Types of Fossils Fossil Types _____________________________________ Petrified fossils ____________________________________ Trace fossils _____________________________________ Molds and Casts The most common fossils are molds and casts ______________________________________________________________________________ A cast is a solid copy of the shape of an organism. ______________________________________________________________________________ Petrified Fossils The term petrified means “turned to stone” Petrified fossils are fossils in which minerals replace all or part of an organism. _____________________________________________________________________________ Some of the original substance remains but the minerals have hardened and preserved it. Carbon Film ______________________________________________________________________________ How they form: 1. Sediment buries the organism 2. ________________________________________________________________________ 3. Carbon is left behind leaving behind only the carbon from the organism Trace Fossils ______________________________________________________________________________ Example: Footprints There are many things that trace fossil tells us: _________________________________________________________ Behavior of an organism Two legs or four _________________________________________________________ Where the organism lived _________________________________________________________ Preserved Remains Some processes preserve and organism with little or no change. ______________________________________________________________________________ Change Over Time Scientists who study fossils are called Paleontologists. Collect information from fossils _____________________________________________________ Arrange organisms in order of which they lived ______________________________________________________ All the information that paleontologists have gathered about past life is called the Fossil Record. ______________________________________________________________________________ The fossil record also shows that different groups of organisms have changed over time. Fossils and Past Environments ______________________________________________________________________________ Example coal is found in Antarctica. Since coal is formed from the remains of dead plants and animals, this means that at one time in Earth’s history, Antarctica had to be warm enough to support that type of climate. Scientists also use fossils to learn about changes in Earth’s surface. Example: If you find a fossil of an aquatic animal in the middle of the desert, you can infer that at one time, there was a region of water where the fossil was found. The Relative Age of Rocks ______________________________________________________________________________ The absolute age of a rock is the number of years since rocked formed The Position of Rocks ______________________________________________________________________________ ______________________________________________________________________________ Geologists use the law of superposition to determine the relative ages of sedimentary rock layers ______________________________________________________________________________ ______________________________________________________________________________ Determining Relative Age To determine relative age, geologists also study extrusions and intrusions of igneous rock faults and gaps in the geologic record. Clues from Igneous Rocks ______________________________________________________________________________ An extrusion is always younger than the rocks below it Below the surface where magma pushes through bodies of rocks and cools is called an intrusion ________________________________________________________________________ Clues from Faults A fault is a break in Earth’s crust ______________________________________________________________________________ The fault is always younger then the earth it cuts through Gaps in the Geologic Record The geologic record is not always complete ______________________________________________________________________________ ______________________________________________________________________________ An unconformity shows where some rock layers have been lost because of erosion Using Fossils to Date Rocks To date rock layers, geologists first give a relative age to a layer of rock at one location. ______________________________________________________________________________ To be useful as an index fossil it must: _________________________________________________________ Occurs in many areas Index Fossils are useful because they tell the relative ages of the rock layers in which they occur ______________________________________________________________________________ ______________________________________________________________________________ Radioactive Decay All matter, including those in rocks, is made of tiny particles is called atoms ______________________________________________________________________________ ______________________________________________________________________________ The majority of elements are stable. But some elements exists in forms that are unstable. Over time these unstable elements break down or decay by releasing particles and energy in a process called radioactive decay ______________________________________________________________________________ During radioactive decay, the atoms of one element break down to form atoms of another element Radioactive elements occur naturally in igneous rocks. Scientists use the rate at which these elements decay to calculate the rock’s age ______________________________________________________________________________ The amount of radioactive element goes down, but the amount of the new element goes up. ______________________________________________________________________________ This rate of decay is the element’s half-life This rate of decay is the element’s half-life. The half-life of a radioactive element is the time it takes for half of the radioactive element to decay Determining Absolute Age ______________________________________________________________________________ Steps to determine absolute age using radioactive dating: 1. ______________________________________________________________________________ 2. Compare the amount with the amount of the stable element into which the radioactive element decay Potassium-Argon Dating ______________________________________________________________________________ Potassium-40 is useful in dating the most ancient rocks because of its long half-life Half-life of 1.3 billion years Carbon-14 Dating A radioactive form of Carbon is Carbon-14 ______________________________________________________________________________ Carbon-14 is very useful in dating materials from plants and animals that lived up to about 50,000 years ago. C_____________________________________________________________________________ For this reason, it cant be used to date very ancient fossil or rocks The amount of carbon-14 would be to small Radioactive Dating of Rock Layers Radioactive dating works well of igneous rocks, but not for sedimentary rocks ________________________________________________________________________ Scientists use intrusion and extrusions to date the age of sedimentary rocks The Geologic Time Scale ______________________________________________________________________________ Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history. ______________________________________________________________________________ Divisions of Geologic Time Geologists have found that over time, major changes in life form at certain times. ________________________________________________________________________ The divisions of the geologic timescale depends on the history of life on Earth Precambrian Geologic time begins with a long span of time called Precambrian time. Precambrian Time: ________________________________________________________________________ Was the time at the beginning of Earth ________________________________________________________________________ Divisions of Geologic Time After Precambrian Time, the basic units of the geologic time scale are eras and periods. The time after Precambrian are divided into three units of time called eras ______________________ Mesozoic Era ______________________ Paleozoic Era The Paleozoic ________________________________________________________________________ Lasted approximately 300 million years ________________________________________________________________________ Mesozoic Era Mesozoic Age of the Reptiles ______________________________________________________________________ Lasted 180 million years _______________________________________________________________________ Cenozoic Era Cenozoic ________________________________________________________________________ Ceno – means “recent” ________________________________________________________________________ Divisions of Geologic Time ______________________________________________________________________________ The names of many geologic periods come from places around the world where geologists first described the rocks and fossils of that period.