2802835|dlese.org|1007936
This document describes how geologic time is approached in discussions of geologic topics. The uses of relative time and absolute time are compared, and a geologic time scale is provided to represent both concepts. References are provided.
Summary
| Subject keyword(s) | Astronomy, Chronometric, Chronostatic, Earth and space science, Earth science, Evolution, Geologic time, Geology, Geoscience, Instructional materials, Physical sciences, Space Science, Space sciences, Teacher content knowledge |
|---|
| Grade level | Elementary School, Middle School, High School, Higher Education, Informal Education, Vocational/Professional Development Education |
|---|
| Intended audience | Educator, Learner |
|---|
| Resource type | Audio/Visual, Reference Material, Text |
|---|
| Resource format | text, text/html |
|---|
| Rights | Copyright 1996. This file may be freely used for non-commercial purposes provided its original source is indicated. Please contact the author for other arrangements.
Copyright 2003, 2004, The University of Calgary. |
|---|
Using this resource
Found in collection(s)
Click on the logo to get more information about the collection.
| Title | DLESE Community Collection |
|---|
| Link | http://www.dlese.org/ |
|---|
| Description | The Digital Library for Earth System Education (DLESE) mission is to improve the quality, quantity, and efficiency of teaching and learning about the Earth System, by developing, managing, and providing access to high-quality educational resources and supporting services through a community-based, distributed digital library. Intended users include students, educators and citizens of all ages, in both formal and informal learning environments. The collection favors resources that convey linkages and connections; the general with the specific, theory with evidence, global with local, and that demonstrate the application of science to solving real world problems. This includes providing access to Earth data sets and imagery, and the tools and interfaces that enable their effective use. DLESE resources are community-contributed, by individuals, organizations, and formal collection efforts. |
|---|
| Full description and distribution of resources |
|---|
| Title | Middle School Portal: Math and Science Pathways (MSP2) |
|---|
| Link | http://msteacher2.org/ |
|---|
| Description | The Middle School Portal 2: Math and Science Pathways (MSP2) supports middle grades educators with high-quality, standards-based resources and promotes collaboration and knowledge-sharing among its users. Educators use MSP2 to increase content knowledge in science, mathematics, and appropriate pedagogy for youth ages 10 to 15. MSP2 employs social networking and digital tools to foster dynamic experiences that promote creation, modification, and sharing of resources, facilitate professional development, and support the integration of technology into practice. MSP2 is a project of the Ohio State University College of Education and Human Ecology, National Middle School Association, and Education Development Center, Inc., and is funded by the National Science Foundation. The partners integrate resources, tools, and services across projects, and support multiple methods of resource discovery to meet the needs of this audience. |
|---|
| Full description and distribution of resources |
|---|
Content contained within the resource
Geological Time ScaleFew discussions in geology can occur without referenceto geologic time. Geologic time is often dicussed in two forms: Relative time ("chronostratic") -- subdivisions of the Earth'sgeology in a specific order based upon relative age relationships (mostcommonly, vertical/stratigraphic position). These subdivisions are givennames, most of which can be recognized globally, usually on the basisof fossils. Absolute time ("chronometric") -- numerical ages in"millions of years" or some other measurement. These are most commonlyobtained via radiometric dating methods performed on appropriate rock types. Think of relative time as physical subdivisions of the rock foundin the Earth's stratigraphy, and absolute time as the measurements taken uponthose to determine the actual time which has expired. Absolute timemeasurements can be used to calibrate the relative time scale,producing an integrated geologic or "geochronologic" time scale. It is important to realize that with new information aboutsubdivision or correlation of relative time, or new measurements ofabsolute time, the dates applied to the time scale can and do change. Revisions to the relative time scale have occurred since the late 1700s.The numerically calibrated geologic time scale has been continuouslyrefined since approximately the 1930s (e.g., Holmes, 1937), although theamount of change with each revision has become smaller over the decades (seefig. 1.5 and 1.6 of Harland et al.) and a few numericalestimates were available previously (but often for the duration of theentire scale rather than its individual subdivisions). In addition, likeany good scientific measurement, every dated boundary has an uncertaintyassociated with it, expressed as "+- X millions of years". These can notbe included in the diagram for practical reasons, but can be found inHarland et al., 1990, along with a detailed description of thehistory of earlier-proposed time scales and the terminology,methodology and data involved in constructing this geological time scale. Because of continual refinement, none of the valuesdepicted in this diagram should be considered definitive, eventhough some have not changed significantly in along time and are very well constrained (e.g., the Cretaceous/Tertiaryboundary has been at 65+-1 Ma for decades, and has been testedinnumerable times, with almost all dates somewhere between 64 and 66million years). The overall duration and relative lengthof these large geologic intervals is unlikely to change much, butthe precise numbers may "wiggle" a bit as a result of new data. This gelogical time scale is based upon Harland et al.,1990, but with the Precambrian/Cambrian boundary modified according to themost recently-published radiometric dates on that interval, revisingthe boundary from 570+-15 million years to 543+-1 million years ago(Grotzinger et al., 1995). Other changes have been proposed since1990 (e.g., revision of the Cretaceous by Obradovich, 1993), but arenot incorporated because they are relatively small. The time scale is depicted in its traditional form with oldestat the bottom and youngest at the top -- the present day is at the zero mark.Geologic time is finely subdivided through most of the Phanerozoic (seeHarland et al., 1990 for details), but mostof the finer subdivisions (e.g., epochs) are commonly referred to bynon-specialists only in the Tertiary. Because of the vast difference inscale, the younger intervals have been successively expanded to the rightto show some of these finer subdivisions. geological time scale ReferencesBlatt, H.; Berry, W.B.N.; and Brande, S., 1991. Principles of StratigraphicAnalysis. Blackwell Scientific Publications: Boston, p.1-512.ISBN 0-86542-069-6 [Chapter 4 provides an introduction to geologic time. This is a good starting point to get the basic principles.] Grotzinger, J.P.; Bowring, S.A.; Saylor, B.Z.; and Kaufman, A.J., 1995(Oct.27). Biostratigraphic and geochronologic constraints on earlyanimal evolution. Science, v.270, p.598-604. [The most recentrevision of the age of the Precambrian/Cambrian boundary.] Harland, W.B.; Armstrong, R.L.; Cox, A.V.; Craig, L.E.; Smith, A.G.; and Smith, D.G., 1990. A geologic time scale, 1989 edition. Cambridge University Press: Cambridge, p.1-263. ISBN 0-521-38765-5[One of the more recent compilations of the entire geologic timescale.] Holmes, A., 1937. The Age of the Earth (new edition, revised).Nelson:London, p.1-263. [One of the earlier attempts at anintegrated geochronologic time scale.] Obradovich, J.D., 1993. A Cretaceous time scale. IN: Caldwell, W.G.E.and Kauffman, E.G. (eds.), Evolution of the Western Interior Basin.Geological Association of Canada, Special Paper 39, p.379-396.[Proposes revisions to the Cretaceous time scale at the resolutionof stages (finer divisions than shown on diagram above) andsub-stages.] Authorship and distribution(c) 1996 macrae@geo.ucalgary.ca This file may be freely used for non-commercial purposes provided itsoriginal source is indicated. Please contact the author for otherarrangements. Back to the Geology and Geophysicshome page.
