Interpreting Rock Layers

lnterpreting Rock Layers Getting the ldea KeyWords geology absolute age radiometric dating relative age law of superposition intrusion unconformity index fossil The science of geology is the study of Earth, including its history. One goal of geology is to develop а timeline of Earth's past-ordering and dating the events throughout history. This timeline includes events such as the formation of rock layers or chains of volcanoes. Geologists study rock layers and fossils to learn about Earth's history. Determining the Age of Rocks Scientists use rocks and fossils to try to date geologic events, or determine when the events took place оп Earth. When studying rocks and fossils, scientists use techniques to determine their absolute or their relative ages. Absolute age is the actual age of а rock or fossil, or how long ago it formed, such as 2,500,000 years ago. Scientists determine absolute age Ьу using а technique called radiometric dating. Radiometric dating measures the age of а material Ьу comparing the amount of а radioactive form of an element with the amount of its decay product. An absolute age gives the most precise measurement of how long ago а rock or fossil formed. However, this information cannot always Ье determined. When absolute dating is not possiЫe, scientists make use of relative-age data. Relative age describes the age of а rock or а fossil in comparison to that of another rock or fossil. Scientists determine which ones formed earlier or later than others without giving definite dates. Geologists often apply the law of superposition to determine relative age. The law of superposition states that in undisturbed sedimentary rock layers, older layers of rock lie beneath younger rock layers. This idea makes sense because the older sediments must Ье deposited before the younger ones сап pile up оп top. Effects of Plate Movements, Magma, and Erosion Sometimes rock layers become disturbed, and even overturned, so that older layers of rock lie above or beside younger rock layers. Plate movements сап disturb rock layers through the processes of folding, uplifting, and faulting. When tectonic plates collide, rock layers get squeezed together and form folds, or ripples, in the crust. This process is called folding. Folds in rock layers сап arch upward, downward, or sideways. When folding pushes rock layers upward, the process is called uplifting. Pressure builds up as plates press together, slide Ьу each other, or pull away from each other. When the rocks cannot tolerate the pressure, they break and slip, forming а fault. The process Ьу which such а break in Earth's surface forms is called faulting. Molten rock сап also affect rock layers. Magma сап move upward and push into existing rock layers. When the magma cools and hardens, it forms а body of igneous rock called an intrusion. An intrusion is always younger than any of the rock layers through which it cuts. Oldest Oldest --------------- Youngest Overturned bed-rocks оп the bottom are now the youngest Undisturbed bed-no erosion, uplift or disturbance � Oldest Angular bed-horizontal rocks are younger than the tilted rocks lntrusion-igneous rocks are younger than апу sedimentary rocks they cut Rock layers сап also become eroded, exposing older rocks S below. lf new layers are later deposited, а gap in the rock record (Did You Know occurs between the older and new layers. This gap in the rock The familiar record is called an unconformity. ln some cases, an unconformity plated herblvore may represent thousands or millions of years of missing time. stegosaurus is Unconformities сап make it more difficult to understand how the state fossil of Colorado. Earth changed during а specific period of _time. As explained lts bones in the captions оп the previous page, understanding how rock аге plentiful beds have been disturbed and how the disturbance affects the in Morrison sequence of rock layers is the key to determining the relative ages Formation of the rocks. sediments. Using Fossils to Find the Age of Rocks Fossils сап Ье used to determine the geologic history, or age, of the rocks that contain them. Suppose two similar rocks in different locations contain the same kinds of fossils. The fossils may suggest that the rocks were laid down in а similar place and time and later became separated. When the absolute age of а fossil is known, the fossil сап Ье used to identify the approximate age of the rock layer in which it was found. lndex fossils are helpful for determining the age of а rock layer. An index fossil is the fossil of an organism that existed for only а short period of time. For example, triloЬites lived from 500 million years ago to 200 million years ago, and they evolved rapidly. Thus, а rock containing а particular kind of triloЬite must Ье from the short period when that species lived.