Geological Structures

Is Presented to you by:

Sheryl Shirley

For the Spring Semester 2002
ES 111 Introduction To Earth Science Lab
Emporia State University, Emporia, Kansas USA

This page is being created for my Introduction to Earth Science Lab at  Emporia State University for the Spring semester of 2002.  I have chosen the Web Page Option , covering Geological Structures, Chapter 7, in the Introduction to Earth Science Lab Manual.


Susie Aber
(http://www.emporia.edu/
earthsci/abersusa.htm) 		My instructor, Susie Ward Aber (shown at the left), is a Gemologist and lecturer at Emporia State University. She also likes amber. Visit her The World of Amber, that you will find really interesting. Her assistant, Elizabeth Wilson (image to the right), is a graduate assistant and helped tremendously in the class. Elizabeth Wilson
(http://www.emporia.edu/
earthsci/images/wilson1.jpg)

Table of Contents

Rules

Images of Geologic Structures

Anticline and Syncline

Faults

Glossary of Terms

References and Links

Rules of Geologic Structure

Since I am a novice on geological structures this page will be written to that audience but I hope that everyone will find something of interest here.  The primary focus of this web page will be Geological Structures, although I have include an interactive time machine which is very interesting! First, I will start with some basic rules that are needed for understanding. These rules are quoted directly from a handout given in this course.
  • Older rocks will be in the center of an anticline. (An easy way to remember this is "antique" = old = anti cline = old rock will be in center.) 
  • Younger rocks will be in the center of a syncline.  (An easy way to remember this is "sin" = young = syn cline = younger rock will be in the center.)  (No offense to any of you youngsters out there! I was young once myself!)
  • Rock layers will dip down perpendicular in direction of the youngest rock at the surface.
  • The steeper a layer of rock dips the more narrow the width of it's exposure belt and a wider dip represent a gentle dip.
  • Exposure belts formed on the limbs of an eroded non plunging fold are
    • Straight
    • Parallel
    • Non converging
    • Outcrop belts do not converge to form a "U" shape
  • Outcrop belts formed on the limbs of plunging folds tend to converge and form a "U" shaped pattern.  This "U" points or converges in the direction of plunge in anticlines, bur opposite to direction of plunge in syncline.
    • (An easy way to see this is to take a piece of paper and fold it and dip one end to see which way the dip will go.)
  • Compressional faults (reverse) the hanging wall moves up relative to the foot wall.  The older rocks will be exposed on the hanging wall at the surface.
  • Tensional faults (normal) the hanging wall moves down relative to the foot wall.  The older rocks will be exposed on the foot wall at the surface.  These press up and are compressional.  
LAW OF SUPERPOSITION:
  • In a sequence of rock layers, each layer is older than the next layer above.
PRINCIPLE OF HORIZONTALITY:
  • Layers of rock dip downward in the same direction as that which the youngest rock layers are exposed at the surface.
  • The older rocks are exposed in the center of eroded anticlines and domes.
  • The younger rocks are exposed in the center of eroded synclines and basins.
  • Plunging anticline form "U" shaped outcrop belts that point in the same direction that the fold plunges.
  • Plunging synclines form "U" shaped outcrop belts that point in the opposite direction that the fold plunges.
  • The steeper the dip of the layer, the more narrow the width of its outcrop belt.
  • Compressional faults (the two walls are pushed together) the hanging wall tends to move up relative to the foot wall.
  • Tensional faults (the two walls are pulled apart) the hanging wall tends to move down relative to the foot wall.

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Images of Geologic Structures

Mother Nature creates some wonderful geological structures with Her fascinating techniques.  To see some of these magnificent structures you will definitely want to take time to view the following images up close.  After looking at the beauty of these pictures I appreciate my lab class more than I did in the beginning because now I know how they were formed and can pass on the information to others.

Please select any photograph to view an enlarged version. These images are taken from the Project IN-DEPTH album from http://www.geo.cornell.edu/geology/indepth/MainMenu/Photo/Structure.html






















Corky the Hornet Return to Top.

Anticlines and Synclines

Anticlines and synclines are folded rock layers. Anticlines are folds in which the oldest rock lies in the center or core and synclines have the youngest rock in its center or core.  Most often anticlines are arch shaped and synclines are trough shaped. (Encarta) Visit this external site to see an image of an overturned fold and anticlines and synclines (from Winona State University Geoscience, Winona, MN).


Image taken from:
 www.educ.uvic.ca/Faculty/jtinney/
earth%20science/Images/antsyn.gif)

Anticlines and Synclines form when rock layers are compressed, (pushed together) similar to carpet folds when its sides are pushed together. When rocks are compressed they can either bend, creating folds, or they can fracture, creating faults. Anticlines are sought out by geologists who explore for oil and gas because the arches form natural traps these hydrocarbons. (Encarta)


Image taken from:
http://www.priweb.org/ed/pgws/systems/
traps/structural/anticlinetrap.gif .

Since oil and gas are less dense than water they tend to migrate upward through permeable rock When rock is folded into an anticline and capped by an overlying impermeable rock, then oil and gas will migrate up the slope of the fold to the crest and accumulate there. Just like geologists search for anticlines, hydrologists search for synclines because they can form natural traps for water.  This can provide an underground water supply, or aquifer. (Encarta)

Image taken from:
http://www.suburbanwatersystems.com/graphics/aquifer.jpg,
an image from Suburban Water Systems.

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Types of Faults

A normal fault is one of two faults that fall into the dip-slip category, or movement is along the inclination or dip. A normal fault will move vertically and the hanging wall will move in a downward movement relative to the foot wall.

This is a Normal Fault.
Image taken from:
http://www.educ.uvic.ca/Faculty/jtinney/
earth%20science/ESMain.html#fault
 Click on animation to see how it works.

A reverse fault is much of what the name implies. It moves in a reverse fashion to the normal fault. By this I mean that the hanging wall moves upward relative to the footwall.

This is a Reverse Fault.
Image taken from:
http://www.educ.uvic.ca/Faculty/jtinney/
earth%20science/RFault.html 		Click on animation to see how this works.

A thrust fault is a reverse fault with the dips less than 45o and have displaced rocks due to compressional stresses.

This is a Thrust Fault.
Image taken from:
http://www.educ.uvic.ca/Faculty/jtinney/
earth%20science/TFault.html

Click on animation to see this in action.

A strike-slip fault has the dominant displacement along the strike or trend of the fault rather than along the inclination or dip. Many of these faults are associated with plate boundaries. They have nearly vertical dips and connect large structures, such as segments of an ocean ridge. (Earth Science, pg. 252-253). The most famous of these faults is the San Andreas Fault (from a USGS external site).

The last of the faults is the Strike-Slip Fault.
Image taken from:
http://www.educ.uvic.ca/Faculty/jtinney/
earth%20science/ESMain.html#lateralfault.

Click on animation to see this action.

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Glossary of Terms:

Anticline
 A fold in sedimentary strata resembling an arch.
Aquifer
 Rock or soil in which water moves easily.
Syncline
 A linear downfold in sedimentary strata; the opposite of anticline.
Fault
 A break in a rock mass along which movement has occurred.
Fold
 A bent rock layer or series of layer or series of layers that were originally horizontal and subsequently deformed.
Geology
 The science that examines Earth, its form and composition, and the changes it has undergone and is undergoing.
Geologic Time Scale
 The division of Earth history into blocks of time - eons, eras, periods, and epochs.  The time scale was created using relative dating principles.
Permeable Rock
 Rock that allows movement of fluids.
Reverse Fault
 A fault in which the material above the fault plane moves up in relation to the material below.
Normal Fault
 A fault in which the rock above the fault plane has moved down relative to the rock below.
Water Table
 The upper level of the saturated zone of groundwater.
Strike-Slip Fault
 A fault along which the movement is horizontal.

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References and Links

Aber, S. W., Johnston, P., Morales, M., Sleezer, R., 2000. Introduction to Earth Science Lab Fifth Edition.  USA.  Emporia State University Earth Science Department, Department of Physical Sciences.

Tarbuck, Edward.  2000/1997.  Earth Science.  New Jersey.  Prentice-Hall.  

Microsoft Encarta Encyclopedia 2000 CD.

Web URL: http://www.winona.msus.edu/geology/MRW/mrwimages/faults.jpg

Web URL: http://www.winona.msus.edu/geology/MRW/text/text3-13.html

Web URL: http://www.educ.uvic.ca/Faculty/jtinney/earth%20science/ESMain.html

Web URL: http://geoweb.tamu.edu/courses/geol101/lab/structural/sld001.htm

Web URL: http://pubs.usgs.gov/publications/text/San_Andreas.html

Web URL: http://www.natureandco.co.nz/land_and_wildlife/landforms/mountains/rock.php3

Web URL: http://www.geo.cornell.edu/geology/indepth/MainMenu/Photo/Structure.html

Other related links to visit:

Visit the Time Machine and take an interactive approach to learning the Time Scale.

Dr. Bob's Geologic Time Page full of valuable information.  This page included information of the geologic time scale with learning and mnemonic devices to help you learn and remember the order in which the periods and epochs occurred.

Structural Geology, http://geoweb.tamu.edu/courses/geol101/lab/structural/sld001.htm

Student Projects from Emporia State University

Corky the Hornet Return to Top.

This web page was designed for ES 111 Introduction to Earth Science Lab, taught by Susie Ward Aber, Emporia State University. For questions or comments contact Sheryl Shirley. Created on May 12, 2002.

Return to ES 111 Student Webpages.