Integrated Science Geologic Time Notes Section 1: Geologic Time To help in the analysis of Earth s rocks, geologists have divided the history of Earth into time units based upon the fossils contained in the rocks. 1. Geologic Time Scale a record of Earth s history from its origin 4.6 billion years ago to the present. Eon the longest time unit in the scale, and is measured in billions of years The eons are: Archean (4,600 to 2,500 millions of years before present (M.Y.B.P.), the Proterozoic (2,500 to 540 M.Y.B.P), and the Phanerozoic (540 M.Y.B.P. to present) Era the next longest time unit, measured in hundreds of millions of years Within the Phanerozoic Eon there are three eras: Paleozoic, Mesozoic, and Cenozoic Eras are defined by the differences in life-forms found in rocks The Paleozoic Era was dominated by a variety of marine plants and animals At the end of the Paleozoic Era (245 M.Y.B.P.) 90 % of all marine species became extinct The Mesozoic Era was dominated by the development of amphibians and reptiles Dinosaurs were the dominant land creature Mammals also began to evolve during this era By the end of the Mesozoic Era (66 M.Y.B.P.) the dinosaurs were extinct Periods time units measured in tens of millions of to hundreds of millions of years Periods are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited Some periods were named for the geographic region in which the rocks of that age were first observed, studied, and described There are seven periods in the Paleozoic Era, three in the Mesozoic Era, and three in the Cenozoic Era Epochs time units measured in millions of years to tens of millions of years The fossil record of the Cenozoic Era is relatively complete, and the rocks and fossils are easily accessed and studied The Paleogene Period is divided into three epochs, the Neogene into two, and the Quaternary into two epochs Different groups of fossils have been used to distinguish the various epochs Regardless of how a geologic time unit is defined, each unit contains specific characteristics that set it apart fro the rest of geologic history Section 2: Age Dating of Rocks Geologists believe that Earth is 4.6 billion years old. How did they determine the age of the Earth? How can geologists determine the age of fossils and the rocks in which they are found? Scientists use two methods to determine the age of the Earth and the rocks and fossils present in the Earth. 1. Relative Age Dating of Rocks Relative age dating places the ages of rocks and the events that formed them in order, but without exact dates. This is done by comparing one event or rock layer to another. Relative age dating is based the principle of uniformitarianism which states that the processes occurring today have been occurring since Earth formed. Only the rate, intensity, and scale with which they occur have changed There are geologic principles that guide the process of relative age dating The principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers
While the actual age of the rock layers is not known, geologists assume that the oldest layers are on the bottom and each layer going toward the top is younger The principle of superposition states that in an undisturbed rock sequence, the oldest rock is on the bottom and each successive layer is younger than the layer beneath The principle of cross-cutting relationships states the intrusion or fault is younger than the rock it cuts across The Black Hills is an igneous intrusive, however, most of the rock through which it intruded has been eroded away Relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it. These particles are called Using the principle of superposition, geologists inclusions have determined the relative ages of these rock layers. It is very difficult to find an undisturbed sequence of rock layers. The Earth is constantly changing as a result of processes such as weathering, erosion, earthquakes, and volcanism. Where these processes have removed a portion of the rock record, and a new layer of rock is then deposited there is a gap in the rock record. This gap is called an unconformity. Disconformity when horizontal layers of rock overlie horizontal layers of rock Nonconformity the contact point between nonsedimentary and sedimentary rock Angular unconformity the contact between uplifted and tilted rock layers and the horizontal rock layers that were deposited at a later time A discontinuity forms when a sedimentary rock layer is deposited on top of an eroded sedimentary rock layer. A nonconformity forms when a sedimentary rock layer is deposited on top of an eroded metamorphic rock layer.
2. Absolute Age Dating of Rocks Absolute age dating allows scientists to determine the actual age of a rock, fossil, or other object. Scientists have developed a method for dating very old objects using the decay rate of radioactive isotopes. A radioactive isotope is an unstable form of an atom that, over time, decays into a stable form of a different atom Example: Uranium 238, which is radioactive, decays into Lead 206, which is very stable The original radioactive isotope is called the parent, and the new, stable element is called the daughter The radioactive isotopes decay by emitting (giving up) protons and neutrons at a constant rate As the number of parent atoms decreases the number of daughter atoms increase by the same amount As the number of protons and neutrons change with each nuclear emission, the element is converted to a different element Radioactive decay the process in which a radioactive isotope emits protons and neutrons at a constant rate until a stable nuclear structure is achieved Because it often takes a very long time for an entire amount of a radioactive isotope to decay, geologists use the length of time it takes for one-half of the original amount to decay. This amount of time is known as the half-live of the isotope Half-life period of time it takes for a radioactive isotope to decay to one-half of its original amount Different radioactive isotopes have different half-lives Half-lives of Selected Radioactive Isotopes Radioactive Isotope (parent) Approximate Half-live Decay Product (daughter) Rubidium-87 48.6 billion years Strontium-87 Thorium-232 14.0 billion years Lead-208 Potassium-40 1.3 billion years Argon-40 Uranium-238 4.5 billion years Lead-206 Uranium-235 0.7 billion years Lead-207 Carbon-14 5,730 years Nitrogen-14 Some igneous and metamorphic rocks contain radioactive isotopes that allow geologists to determine the absolute age of the rocks Radiometric dating the process whereby scientists attempt to determine the ratio of parent nuclei to daughter nuclei for the purpose of determining the absolute age of the object By finding this ratio the scientists know how many half-lives have occurred since the radioactive decay began Example: The radioactive decay of Carbon-14 to Nitrogen-14 Percent Parent Percent Daughter Elapsed Number of Element Element Years Half-lives Time 1 100.0 0.0 0 0 Time 2 50.0 50.0 5,730 1 Time 3 25.0 75.0 11,560 2 Time 4 12.5 87.5 17,290 3
The isotope used for dating depends on the estimated age of the object to be dated Example: If Uranium-238 (half-life = 4.5 billion years) is used to date an object estimated to be only a few thousand years old, the ratio of parent atoms to daughter atoms will be too large to be useful. Likewise if Carbon-14 (half-life = 5,730 years) is used to date a rock estimated to be over a billion years old, the parent/daughter ratio will to low to be useful 3. Other Ways to Determine Age Determining the relative of absolute age of an object or event is not limited to the use of rocks or chemical elements. 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 events, such as a forest fire, a drought, a flood, or a volcanic eruption Tree Rings The age of trees can be determined by counting the number of annual tree rings in a cross section of the tree During the spring months a tree experiences its greatest growth, during the winter months its growth is less. A pair of spring and winter growth rings represents an annual tree ring Seasonal Climate Changes About 11,000 years ago, glaciers covered the northern part of the United States During the summer the glaciers would partially melt, and the water would carry sediment downstream and deposit the sediment in large lakes Summer sediment deposits are generally light-colored and relatively thick compared to the thinner, dark-colored sediments of winter Deposits from different glacial lakes can be compared to determine ages from about 15,000 to 20,000 years ago Distinctive Sediment Layers When a rock layer is formed by an instantaneous or short lived event, geologists can determine the date of the event by radiometric dating The layer then becomes a time marker called a key bed, which can be used to correlate rock layers across wide areas Key bed sediment layer that serves as a time marker in the rock record and results from volcanic ash or meteorite-impact debris that spread out and covered large areas of the Earth Example: at the end of the Cretaceous Period a large meteor impact threw large amounts of dirt and stuff into the air and resulted in an overall lowering of the planet s temperature and caused the mass extinction of the dinosaurs. This dust later settled and formed a sediment layer that can be found in many parts of the world today. Section 3: Fossils 1. Fossils the evidence of once living plants or animals Provide evidence of the past existence of a wide variety of life-forms, most of which are extinct Also provide evidence that populations have undergone change through time in response to changes in their environment Evolution adaptations of life-forms due to changing environmental conditions Fossils can also provide information about past environmental conditions and can be used to correlate rock layers from are to another 2. Three concepts are important in the study and use of fossils: Fossils represent the remains of once-living organisms Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time
The three concepts are summarized in the Law of Fossil Succession: The kinds of animals and plants found as fossils change through time If we begin at the present and examine older and older layers of rock, we will come to a level where no fossils of humans are present. If we continue backwards in time, we will successively come to levels where no fossils of flowering plants are present, no birds, no mammals, no reptiles, no four-footed vertebrates, no land plants, no fishes, no shells, and no animals. The Law of Fossil Succession: The kinds of animals and plants found as fossils change through time 3. Evolution How do scientists explain the changes in life forms, which are obvious in the record of fossils in rocks? Early explanations were built around the idea of successive natural disasters or catastrophes that periodically destroyed life After each catastrophe, life began anew In the mid-nineteenth century, both Charles Darwin and Alfred Wallace proposed that older species of life give rise to younger ones According to Darwin, this change or evolution is caused by four processes: variation, over-reproduction, competition, and survival of those best adapted to the environment in which they live Darwin's theory accounts for all of the diversity of life, both living and fossil. His explanation gave scientific meaning to the observed succession of once-living species seen as fossils in the record of Earth's history preserved in the rocks. Darwin's theory of evolution has been refined and modified continuously as new information has accumulated All of the new information has supported Darwin's basic concept--that living beings have changed through time and older species are ancestors of younger ones 4. Types of fossils Original preservation describes a fossil with soft and hard parts that have not undergone any change since the organism s death These fossils are very uncommon because their preservation requires extraordinary circumstances such as freezing, drying out, or oxygen-free environments In Alaska the woody parts of plants are imbedded in the permafrost from 10,000 year old bogs Soft parts of mammoths and saber-toothed tigers are preserved in the La Brea Tar Pits in California
Soft parts are also preserved when plants or animals have been dried out and their remains mummified. Most mummies are found in dry caves or deserts Altered hard parts fossils whose organic material has been removed and whose hard parts have been changed by recrystallization or mineral replacement Permineralization process in which pore spaces in a fossil are filled with mineral substances Example: When volcanic ash spreads over a large area, entire forests may become buried. Over a long period of time, quarts and other minerals in the ash combine with groundwater and slowly fill in the spaces between the cell walls of the trees and silicify them. This fossilized tree is called petrified wood. The process of permineralization Changes in temperature and pressure may also result in changes in shell or bone material Shells or exoskeletons (from coral) can be affected by a process called recrystallization. The exterior of the shell remains the same, but shell microstructure is destroyed in the process Index fossil remains of plants or animals that can be used by geologists to correlate rock layers over large geographical areas or to date a particular rock layer An index fossil is easily recognized, abundant, and distributed over a large geographic area An index fossil must also have lived during a short period Some fossils do not contain any shell or bone material. They may be molds or casts. Mold fossil that is formed when a shelled organism decays in sedimentary rock and is weathered away leaving a hollowed impression Cast fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment The process of cast formation Indirect evidence of plant and animal life are called trace fossils Examples of trace fossils include worm trails, dinosaur tracks, gastroliths, and corprolites
Gastroliths smooth, rounded rocks dinosaurs had in their stomachs to help them digest and grind their food. Coprolites the remains of solid waste materials of animals 5. Why study fossils The study of fossils allows scientists to interpret and describe Earth s history Fossils from different geologic time periods describe how organisms have changed throughout time Fossils also show evidence of ancient environmental conditions They also may help scientists find patterns and cycles that can be used to predict future phenomena, such as climate changes The study of fossils allows geologists to locate energy resources 6. How fossils are studied The study of fossils and the rocks that contain them occurs both out of doors and in the laboratory The field work can take place anywhere in the world In the laboratory, rock saws, dental drills, pneumatic chisels, inorganic and organic acids, and other mechanical and chemical procedures may be used to prepare samples for study Preparation may take days, weeks, or months--large dinosaurs may take years to prepare. Once the fossils are freed from the rock, they can be studied and interpreted In addition, the rock itself provides much useful information about the environment in which it and the fossils were formed