Dating of rocks and geologic events exercise 13
Geologists get a wide range of information from fossils. Although the recognition of fossils goes back hundreds of years, the systematic cataloguing and assignment of relative ages to different organisms from the distant past — paleontology — only dates back to the earliest part of the 19th century. The oldest undisputed fossils are from rocks dated around 3. The oldest well-understood fossils are from rocks dating back to around Ma, and the sedimentary record from that time forward is rich in fossil remains that provide a detailed record of the history of life. However, as anyone who has gone hunting for fossils knows, that does not mean that all sedimentary rocks have visible fossils or that they are easy to find.
The major focus of this section is to help learners understand how scientists use deductive reasoning to understand the past. Scientists can never know exactly what happened in the past, but they are able to use various forms of evidence clues to piece together a coherent picture. When evidence from many different areas start to agree with each other it lends more credibility to the theory. Different forms of evidence, such as that provided by geological records and fossil evidence, will be introduced.
Scientists have divided the history of life into different time periods using the geological timescale. In this section we will present the key evidence scientists have used to construct this time scale. We will discuss the continental drift theory which explains how the continents were formed. We will also briefly touch on the theory of natural selection which explains how one life form can evolve into another over many years by adapting to its changing environment. We will then present the methods by which we are able to determine evolution of life forms over time through examining the fossil record.
The geological timescale is a 'calendar' of events in the Earth's history. It shows major geological and climactic events, and how these events affected the emergence and disappearance of species over time. In order to help us make sense of this vast amount of time, scientists divided it into smaller units of time. In descending order, these units are: An era: The Mesozoic era for example lasted from million years to 66 million years ago.
A period: Figure While you will not be expected to remember the names of specific periods, you will be expected to understand the eras and their particular characteristics. On this clock, human existence constitutes less than a minute of the evolutionary history of life on Earth. In this representation, the two million years' of human history constitute an effect too small to be visible on the timescale.
This website has an infographic showing geological time on a clock. Each hour on the clock corresponds to a period of the Earth's history, and shows the characteristics of each period and the key events within them. In this exercise you will learn to combine all the information given on the history of life and depict it on a simple geological timescale of your own. In this exercise learners are required to combine all the information given on the history of life and to depict this information onto a simple geological timescale of their own.
Learners can do this in poster format or as a class exercise. They may work individually or in small groups of 2 to 4. Pictures from the Internet to illustrate their time scale can be used. Any time line drawn by a learner may be acceptable, provided that it meets the following criteria:. Learners do not need to know the dates involved in continental drift. These dates are only approximate and are given as a guideline and to give a sense of the time-scale. Continental drift is the breakup and movement of the Earth's continents relative to each other by drifting across the Earth's surface.
Since the initial continental drift hypothesis was proposed, the study of plate tectonics has helped us understand why continents move. Plate tectonics is the study of the folding and faulting of the Earth's crust lithosphere. Biogeography is the branch of biology focusing on the geographical distribution of plants and animals. It has been instrumental in developing our understanding of the evidence for continental drift. There is considerable evidence for the theory of continental drift that draws upon fossil evidence, plate tectonics theory and studies of glacier sediments.
For example:. Using this evidence, scientists have inferred that in the past the Earth existed as a super-continent known as Pangea until the early Mesozoic era. There were three major phases in the break-up of Pangea:. Watch an animation of continental drift at the following link: Through a process known as natural selection , adaptations that allow one organism to survive better than another and produce more off-spring, results in future generations of organisms who have acquired those traits.
For Grade 10, you are required to understand the following key features of evolution by natural selection:. The fossil record is a key source of evidence that helps scientists understand life's history. Fossils are the remains, impressions or traces of animals and plants from the remote past. Fossils usually take the form of a mold or a cast in rock.
Examples of common fossils are skeletons or parts of skeletons, shells or teeth. Sometimes plants or animals can leave imprints that get preserved as fossils. The study of fossils across geological time, how they are formed, and how ancient organisms have evolved in relation to other phylogenetic groups, is called palaeontology. For fossilisation to occur, a plant or animal must first die.
Soft tissues decay quite quickly, therefore animals that have hard exoskeletons and woody plants tend to fossilise better than soft-bodied organisms. The organism plant or animal must be buried beneath mud and or soil shortly after death. Although decay still takes place, the lack of oxygen slows it down. As more and more layers of mud and soil are added, the sediments become compressed. Eventually, this compression turns the sediment into rock, which forms a mould around the shape of the original skeleton.
Sometimes the original bone or shell softens and dissolves completely, sometimes the bone or shell remains. Water that is rich in dissolved minerals trickles in through the layers of sediment into the mould. The mineral-rich water enters the hollow and crystallises to create a natural cast of the original organism. Otherwise, the minerals slowly seep into the skeleton, changing its chemical composition and making it capable of surviving for a long time.
Over many millions of years, rock that was once buried rises again to the surface and is eroded away, exposing the fossils. If the rocks surrounding a fossil are distorted and squeezed by geological forces, it will result in distortion of the fossils within them. Fossilisation is very rare and only happens when a plant or animal dies in exactly the right circumstances. Usually animal corpses are eaten by other animals or decomposed by bacteria before fossilisation can occur.
Even hard parts, like bones and shells, are eventually destroyed through erosion and corrosion. This website has a video detailing the fossilisation of Lucy: It takes a rare set of circumstances to turn a living creature into fossilised bone. In the case of Lucy, the famous hominid fossil discovered in Ethiopia's Great Rift Valley in , there is no evidence that she met a violent death. No predator or scavenger found her body before it began rotting in the lake's soft sediments.
Her bones, which settled in the mud, may have been cracked or shattered by animals roaming around the shore. Heavy rains gradually washed in enough sand and gravel to bury her bones. These deposits built up over thousands of years, burying her remains hundreds of metres deep. The calcium in her bones, molecule by molecule, was replaced by minerals from these deposits, turning the bones to stone. She remained buried over millions of years, while the Earth's crust moved constantly, forcing the remains of her body closer to the surface.
Heavy storms beating down on the earth eroded the sediment and most likely brought her once again to the Earth's surface. Her exposure made it possible for anthropologists to later discover her remains some three million years after her death. We have learnt how fossils are formed over geologic timescales. In this section we will learn how we determine the age of a fossil.
There are two methods of dating fossils:. In order to understand radiometric dating, it is necessary to revise our understanding of the atom. The nucleus of an atom is made up of protons and neutrons. The number of protons in the nucleus define what type of element it is. However, the number of neutrons of an element may vary. Atoms with the same number of protons, but different numbers of neutrons are called isotopes. Some isotopes are stable, while others are unstable. Unstable isotopes undergo a process called radioactive decay , whereby they spontaneously change to elements of a different type.
We can never predict when a specific atom will undergo radioactive decay. However, when considering many atoms, we observe that the decay occurs at an exponential decay rate. Exponential decay means that over a certain period of time, called a half life , half of the unstable isotopes in a sample will undergo radioactive decay. Thus, using radiocarbon dating, scientists can determine how much carbon remains within a particular fossil, and thereby infer the age of the fossil.
Radiometric dating: Graph showing the half-life of C The amount of carbon halves every years. The isotopes of different elements undergo decay at different rates; some decay much more rapidly than others. This makes radiometric dating a useful tool as scientists can use different elements to date longer and shorter time-scales. Carbon has a relatively short half-life, and therefore it is not of much use when dating fossils that are millions of years old.
In order to date older fossils, scientists use uranium-lead dating and potassium-argon dating. The half-life of uranium is million years, and the half-life of potassium is 1. In order to estimate the age of incredibly old fossils, scientists date the age of the igneous volcanic rock in which the fossils are buried. This provides them with an estimate of the age of the fossils contained within them. In relative dating, fossils are dated relative to layers of igneous volcanic rocks that they are near.
Chapter 13 Streams and Floods The oldest undisputed fossils are from rocks dated around Ga, and although fossils this old are Most well-known types of organisms were decimated by this event, but only a few became Exercises. Exercise Dating Rocks Using Index Fossils. Cretaceous inoceramid clams. Study 20 Lab 1 - Dating of Rocks, Fossils, & Geologic Events flashcards from James D. on StudyBlue. Day, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, Year, -Named intervals that can be expressed in both absolute & relative dates Practice Problems.
Skip to main content. Log In Sign Up. Abdullah, PhD, There are two ways by which this can be accomplished. Absolute dating involves being able to date how many years old a rock is.
It is not about the theory behind radiometric dating methods, it is about their application , and it therefore assumes the reader has some familiarity with the technique already refer to "Other Sources" for more information.
The major focus of this section is to help learners understand how scientists use deductive reasoning to understand the past. Scientists can never know exactly what happened in the past, but they are able to use various forms of evidence clues to piece together a coherent picture. When evidence from many different areas start to agree with each other it lends more credibility to the theory.
Radiometric Dating and the Geological Time Scale
Adapted by Joyce M. McBeth, Michael B. First Edition. View Source. Relative time is an important tool for geologist to quickly construct a model for a series of geologic events, especially in the field. In the following section, apply what you have learned regarding relative time to the questions below.
8.3 Dating Rocks Using Fossils
Working out Earth history depended on realizing some key principles of relative time.
.Relative Dating - Example 1