The amount of time that is involved in the carving of the landscape, the formation of rocks, or the movement of the continents is an important scientific question. Different hypotheses about the age of the earth can essentially change our perspective of the workings of geologic events that molded the Earth. Scientists studying rocks were able to piece together a progression of rocks through time to construct the Geologic Time Scale Figure 1. This time scale was constructed by lining up in order rocks that had particular features such as rock types, environmental indicators, or fossils. Scientists looked at clues within the rocks and determined the age of these rocks in a comparative sense. This process is called Relative Dating , which is the process of determining the comparative age of two objects or events. For example, you are younger than your parents. As time progressed, scientists discovered and developed techniques to date certain rocks as well as the Earth itself.
Geologic Time. From the beginning of this course, we have stated that the Earth is about 4. How do we know this and how do we know the ages of other events in Earth history? Prior to the late 17th century, geologic time was thought to be the same as historical time.
One of its goals is to answer the question “How can geologists claim to know the age is in Railsback’s GEOL lecture on “correlative and relative dating“.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava. Lava properly called magma before it erupts fills large underground chambers called magma chambers. Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios.
Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger. This calls the whole radiometric dating scheme into serious question. Geologists assert that older dates are found deeper down in the geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older.
But even if it is true that older radiometric dates are found lower down in the geologic column, which is open to question, this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later. Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down. A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger.
Teaching about Earth’s history is a challenge for all teachers. Time factors of millions and billions of years is difficult even for adults to comprehend. However, “relative” dating or time can be an easy concept for students to learn. Once they are able to manipulate the cards into the correct sequence, they are asked to do a similar sequencing activity using fossil pictures printed on “rock layer” cards. Sequencing the rock layers will show students how paleontologists use fossils to give relative dates to rock strata.
Plate tectonics laboratory using plate motions (3 exercises) by Richard Harwood of Interpreting geologic sections through Athro, Limited – students can test their Relative dating laboratory by Richard Harwood of Black Hawk College – a.
September 30, by Beth Geiger. Dinosaurs disappeared about 65 million years ago. That corn cob found in an ancient Native American fire pit is 1, years old. How do scientists actually know these ages? Geologic age dating—assigning an age to materials—is an entire discipline of its own. In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating. Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you. Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old. To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones.
This is called the Rule of Superposition. This rule is common sense, but it serves as a powerful reference point. Relative age dating also means paying attention to crosscutting relationships. Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
1.2: Geologic Time
Most sedimentary rocks are laid down in flat, horizontal layers. These can later tilt and fold due to tectonic activity, and river cuttings can cause gaps among the layers. Relative dating arranges geological events — and the rocks they leave behind — in a sequence. The method of reading the order is called stratigraphy layers of rock are called strata.
dating geological materials. – Relative ages – Based upon order of formation. • Qualitative method developed s of years ago. • Permit determination of.
Surfing Sites: You must visit all of the surfing sites and write a description of the content of each one. Please note that a web site may consist of many web pages and you should visit enough of these to be able to describe what the entire site is about, not just the home page. Your descriptions should be brief – perhaps a paragraph for each site – but must include 1 a general description of the web site and 2 at least one example of something you learned from the Web site. Web Exercise: You must 1 describe in detail what you did in the Web Exercise and 2 report to me your results.
Be sure to include in your report the answers to any questions which are asked on the web site, copies of any forms which you filled out and any certificates of completion which you receive. Activity 1. Web Exercise 1. Go to Geologic Time Chart. Learn the Geologic Time Scale Here’s a web site that might help you to remember all those strange new words: Dr. Bob’s Geologic Time Page.
Go to Geologic Time Chart for testing.
This is an example of a relative age diagram. It is a cross-section through the Earth. Relative age diagrams can include rock layers, intrusions, unconformities, and geologic structures folds and faults.
Dating Principles. Why is the Geologic Time Scale important? • Rocks record geologic and Applying Principles. Sequence of Events exercise.
Use the theory of radioactive isotope decay to calculate parent and daughter isotope abundances and determine the absolute age of geological materials. Determine relative geologic age sequences using the basic principles of relative age determination. Distinguish different types of unconformities in the stratigraphy of the Grand Canyon. There are two parts to this lab exercise A. Part 1 is on absolute age determinations using radioactive isotopes and their daughter products in geological materials.
In Part 2 you will use the principles of relative dating to determine sequences of geological events as indicated by the layers and structures in the rocks. Part 2 ends with an application of the principles of relative dating to the rocks of the Grand Canyon.
Geologic dating exercise
The simplest and most intuitive way of dating geological features is to look at the relationships between them. For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top.
The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it Figure 8. Figure 8. The lava flow took place some time after the diorite cooled, was uplifted, and then eroded.
The simplest and most intuitive way of dating geological features is to look at the Exercises. Exercise Cross-Cutting Relationships. outcrop The outcrop.
We usually think in terms of a few tens of years, since that measures a lifetime. Hundreds or thousands of years are considered ancient history. But in geology, we must think in time spans of millions of years, hundreds of millions of years, even of billions of years. Geologists measure such tremendous spans of geologic time in a relative sense, that is, one event occurred after another and prior to a third event. In that sense, a geologic time scale was developed in the ‘s to make handy “boxes” of relative time periods in order to think of events in an understandable manner.
The geologic time scale is shown in fig. Figure 3 –Geologic time scale showing geological eras and periods recognized in North America from oldest to youngest in ascending order. Length of time of each period and million years before present are rough estimates based on radiometric dating of rocks ad relative rock relationships. Earliest humans did not appear until about the beginning of the Pleistocene Epoch some two million years ago.
There are many reasons to believe that the Earth formed about 4. However, it was not until the beginning of Cambrian time, about million years ago, that sedimentary rocks containing fossils became abundant. The first four billion years of Earth history, lumped by geologists into the Precambrian Era, can only be dated where radioactive minerals are available.