A Teaching Tool for Earth Sciences Education
John Etches, Environmental Educator
The focus of this teaching tool is to foster an understanding of the scientific approach to the interpretation of the Earth's history and, therefore, provides insight into the discipline of geology.
The exercise itself takes about one hour to conduct and requires a badminton court with regulation boundaries. Students perform mapping of an idealized geological terrain and utilize simple deduction to interpret observed data. Accurate data collection and application of geological knowledge leads to a reconstruction of a correct sequence of geological events. Coupled with pre-activity and follow-up discussion, this activity can assist in satisfying a great number of expectations within a number of Ontario Ministry of Education curriculum documents, specifically;
Science Grades 11 and 12; Earth and Space Science, Grade 12, University Preparation (SES4U)
Science and Technology Grades 1-8; Earth and Space Systems, Grade 7, The Earth's Crust
History and Geography Grades 7 and 8, Grade 7 Geography
The Themes of Geographic Inquiry
Patterns in Physical Geography
Through this exercise, students will directly witness how the application of knowledge can result in the valid interpretation and understanding of observed phenomena. The importance of geological mapping and the correct identification of rocks and minerals in the interpretation of the Earth's history is specifically brought to light.
The importance of identification & interpretation of rocks & minerals
The correct assignment of names to rocks is much more than just a matter of interest. The ramifications of an incorrectly named rock are quite serious indeed. The names of rocks tell of their mineral content, texture and how the rocks formed. An incorrect name represents false conclusions drawn about all of the above. It may lead to a misinterpretation of a piece of the earth's history!
Also, once a rock is named, the name may be used in reports, journals and maps. People referring to these geological documents trust the information to be as correct as possible. Geologists and prospectors can infer a good deal of information about a rock just by learning what it has been called. But if a rock type is wrongly named, a false impression of the rock is set forth. The usefulness of the map or report will be questionable.
The purpose of the following is to illustrate the importance of identifying and interpreting the rocks correctly. Closely linked with the identification of rocks is a procedure known as geological mapping.
Geological Mapping and Economic Mineral/Rock Type Associations
The most important phase of any mineral exploration endeavour is geological mapping. It represents the foundation upon which all other exploration work is conducted. The basic function of geological mapping is to identify the geology or rock types present in a given area. This includes showing as accurately as possible the trace of the boundaries between the different types of rocks. But why is it so important to know what rock types are where?
Through years of observation and study it has been found that there is often a definite association between concentrations of economic minerals and the rock type in which they are situated. Occurrences of certain valuable minerals have repeatedly been found in the same type of rock regardless of geographic location. Some economic minerals, such as diamond, occur in one specific type of rock. Diamonds have been found in a rock type called kimberlite. This is a rare type of igneous rock which has crystallized at very high temperatures and pressures. Being able to establish this economic mineral-rock type association for a valuable mineral is an extremely powerful observation. Here lies one of the reasons why geological mapping and the correct identification of rock types is so important.
The search for new mineral resources is truly a "needle in a haystack" undertaking where a concentration of valuable minerals may or may not be evident at the earth's surface. Also, surface expressions are often of little assistance as they are usually of limited spatial extent. The establishment of an economic mineral-rock type association aids in reducing an area of search by identifying the most probable rock type and excluding all others. Geological mapping will locate the favourable rock type, if it is present within the area being explored. Once found and isolated, the search can concentrate just within the smaller area occupied by that rock type only. The rocks occupying the surrounding ground can be ignored being recognized as improbable hosts of the minerals being sought after.
As an example, an imaginary exploration company, Minorex Resources, has decided to explore for diamonds. Attention has been drawn to a particular area since a single, loose diamond crystal was found by a little girl playing in the gravel of an abandoned pit. A 5 kilometre by 5 kilometre area is chosen to be explored approximately centred around the discovery site. The goal of the project is to find the bedrock source of the diamond. Initially, all the company has are maps showing the terrain, lakes and rivers. Within this area of 25 square kilometres, where do they begin to look?
The company's geologist is wise and decides to first map the geology of the area by examining the rock types in outcrops. After days of field work, all or most of the outcrops have been located and evaluated by the geologist. The position of each outcrop is plotted on a base map. Using lakes, rivers and topographic features as reference points, the outcrops can be plotted with a good degree of certainty.
The geologist has identified three types of rocks outcropping within the map area, sandstone (ss), gneiss (gn) and kimberlite (k). With the locations and rock types of the outcrops plotted on the map, decisions can now be made regarding where the boundaries or contacts between the different rock types are situated. Remember, outcrop is just where underlying bedrock pokes through soil and other overburden cover. If two outcrops of the same type of rock are separated by an area covered by soil, it is safe to assume that the bedrock under the soil is also of that rock type. If there is no evidence to suggest otherwise, this is an acceptable assumption. But if the two outcrops are of different rock types, then there must be a geological contact between the outcrops where the two rock types meet.
This exploration project has already met with good fortune because the geological mapping has revealed the presence of two kimberlite bodies in bedrock. They are surrounded by a metamorphic rock called gneiss. The remainder of the map area is occupied by a sedimentary rock called sandstone. The data collected by the geologist presented in the geological map above shows the location and boundaries of each rock type.
Diamonds have never been found in sandstone or gneiss. They are considered unfavourable for the occ-urrence of diamonds. For this reason, the area underlain by these two rock types can be ignored. All of a sudden, the area of search is reduced to a small fraction of the original 25 square kilometres. An exploration target has been defined.
The success of this phase of the project has hinged on an ability to correctly identify the different rock types present within the area. Without proper identification skills, the kimberlite may have been missed or, even worse, disregarded.
A tremendous variety of information over and above just identification can be derived from each and every outcrop. The map accompanying this text will serve as a simple example showing the kind of information that can be read from the rocks. The example again highlights the economic incentive for interpreting the rocks correctly.
The geologists of an exploration company are confronted with an interesting problem. Minor gold mineralization has been found in a pegmatite dyke exposed in Outcrops 10 and 14 (Dyke A). In the hope of finding a more extensively mineralized zone another pegmatite dyke identified in Outcrops 17 and 21 has also been sampled. This dyke (Dyke B1) which is apparently mineralogically and texturally identical to the one containing the gold mineralization, contains not even trace amounts of gold. How can two dykes so close together that appear to be the same rock type be different? The question of whether or not to continue spending time and money analyzing samples from Dyke B1 becomes an important one. The geologists decided to spend a day mapping the ground adjacent to their study area. It is hoped that other differences (or similarities) between the two dykes will become evident.
After a thorough day's worth of mapping, many interesting features have been found. A third rock type has been identified. A third dyke has been found. A fault has been recognized. With all the outcrops plotted on the map, some geological assumptions can be made.
First of all, the boundary between the newly found sandstone and the granite can be drawn. Looking at the rock types in each outcrop, it can be deduced where the two types of rocks meet. Outcrops 3 and 16 actually show the contact. Outcrop 16 holds another interesting piece of information. Dyke B2 extends up to the contact between the sandstone and the granite but does not cross into the sandstone. This simple fact makes the age relationships between the sandstone, granite and the dyke very clear.
First of all, between the granite and the pegmatite of Dyke B2, which must be younger? Because the dyke is igneous forming from hot fluids invading a crack or fissure in the granite, the pegmatite must be the younger of the two. This is called a cross-cutting relationship.
Dyke B2 is observed to not cross into the sandstone. This shows that the sandstone was emplaced after the dyke was introduced. If the opposite were true the dyke would cross-cut the sandstone as well as the granite. Using these observations, the relative ages of these three rock types is known. The order from oldest to youngest is granite, pegmatite (Dyke B2) and sandstone.
The geological mapping has also revealed the presence of another pegmatite dyke that cross-cuts the sandstone. Compilation of the geologist's data on the map shows that this is a continuation of Dyke A connecting Outcrops 7 and 4. With this knowledge it is apparent that a major difference exists between Dyke A and Dyke B2. The cross-cutting relationship between Dyke A and the sandstone says that the pegmatite of Dyke A is younger than the sandstone. But it is also known that the sandstone is younger than the other pegmatite and the granite. The order from oldest to youngest for all four identified rocks must then be granite, pegmatite (Dyke B2), sandstone, and pegmatite (Dyke A). By just looking at the positions of the rocks with respect to each other, it has been found that the two pegmatite dykes are not of the same age at all.
From the mapping of the entire area, Dyke B1 and Dyke B2 can also be interpreted to have been off-set or faulted. These two dykes were originally the same dyke. The shifting of Dyke B1 and Dyke B2 is mimicked by the sandstone/granite boundary showing the same shift direction and amount of off-set.
Dyke A on the other hand appears to cross the fault without being dislocated or faulted. This means that the faulting had to have occurred after Dyke B was introduced but before Dyke A was emplaced. The two dykes, again, cannot be of the same age.
It is possible that millions of years passed between the emplacement of the two dykes. The "parent" igneous fluids which formed the two dykes may have come from two completely different sources within the earth. With this new evidence, it is not surprising that the dykes differ in their gold content. Chemically, they are probably different in many other respects.
With definite evidence that the dykes do not share the same history, the exploration company decides to concentrate on the younger of the two dykes (Dyke A) where the gold mineralization is known to occur. Geological mapping and correct interpretation of the rocks provided the information needed to make this decision.
This example of geological mapping is the basis of the indoor mapping exercise which can be performed by students at any school with a regulation badminton court. Students plot on graph paper the position of mock outcrops laid out on the auditorium floor as shown by the outcrop pattern diagram. The position of geological contacts, dykes, the fault and the age relationships of the rock types can all be interpreted by the student by plotting the outcrops accurately. The exercise serves as a concise and illustrative example of the value of geological mapping.
Geological mapping exercise badminton court lay-out
Set-up Time: 10-15 minutes
Requirements: badminton court with regulation boundaries, set of outcrop cut-outs, exercise worksheets, clipboards or notebooks, pencils with erasers
Activity Duration: 1 hour
NOTE: For ease of lay-out, all cut-out positions are at boundary intersections or mid points between intersections as per Sheet #1.
1) According to the pattern given on Sheet #1, lay out all granite only and all sandstone only outcrops first. They are generic; any sandstone cut-out can be placed at any location requiring a sandstone outcrop, and the same for the granite. This will allow you to place 17 of the 27 cut-outs in less than 5 minutes.
2) Lay out the outcrop cut-outs that are all pegmatite only (all black). There are just two of these.
3) There should now only be eight cut-outs left to place. These should all contain two or three rock type patterns in each cut-out.
Important; ensure that the angles of all rock type contacts on the cut-outs are in the correct rotation according to the pattern shown on Sheet #1. If these angles are not correct, the exercise will not work.
Conducting The Exercise
- A notebook or, preferably, a clipboard should be provided. A pencil with an eraser is also strongly recommended.
- Explain to students that the cut-outs represent outcrops and, as such, are visible glimpses of the Earth's crust. In other words, pretend that most of the court is actually covered by soil and the cut-outs are bedrock poking up through the soil creating outcrops.
- Hand out Sheet #2 to students. (the blank outcrop template sheet) This sheet shows the position of the outcrops relative to each other.
- Notice the legend and what rock type that each pattern represents.
- Each cut-out has an index number for reference.
- Ask students to find the outcrops on the floor that corresponds to the outcrops on their blank outcrop sheet, Sheet #2, using the index numbers. As examples, point to a couple of cut-outs and ask students to find them on their blank sheet and give the correct index number.
- Task #1 (give students a 15-20 minute time limit) For all the outcrops;
- Locate each outcrop on the map
- Apply the correct shading according to legend for the rock type indicated by each outcrop
- For outcrops with two or more rock types indicated, draw in the boundaries between the different rock types VERY ACCURATELY including the correct angles as they exist in the cut-outs!!
(teaching note; demonstrate an example of this with at least one multi-rock type outcrop)
- Do not move any of the outcrops. If any outcrop is accidentally kicked, tell your teacher and the outcrop will be repositioned.
- Task #2 Deduce and draw in the geological contacts.
- Note where geological contacts are actually revealed in the outcrops. These are special places where there needs to be no guessing about where the geological contacts are located. You can see them.
- There are three types of contacts in this exercise; sandstone against granite (#3, #16), sandstone against pegmatite (#4, #16), and granite against pegmatite (#14, #16, #17, #19, #21, #23)
- The next step is to connect the geological contacts you can see across areas of the map where the contacts are hidden. This is called "inferring" where the contacts are.
- Remember that if two outcrops of different rock types are observed, a contact where the two rock types meet must be present somewhere between the two outcrops. Even though this geological contact can't be seen, it has to exist between outcrops of differing rock types.
- First draw the contact between the granite and the sandstone. Ignore the pegmatite for now. Start with an outcrop that actually reveals the contact between the granite and the sandstone (#3).
- Scan the map for granite and sandstone outcrops. Place a dot between the granite and the sandstone outcrops.
- Starting at Outcrop #3, draw a dashed boundary line connecting your in-between dots. There should be a dot between Outcrops #1 and #5, between Outcrops #5 and #6, between Outcrops #10 and #11, between Outcrops #11 and #15.
- The contact between the granite and the sandstone is also exposed in Outcrop # 16. Connect your new geological contact to this exposed contact.
- You may safely extend this new geological contact to the edge of the badminton court boundaries beyond Outcrops #3 and #16. The new geological contact should look a bit like a zig-zag line across the page.
- Again, ignore the pegmatite. If the new granite and sandstone geological contact has been drawn correctly, all the outcrops on one side of the boundary should be granite. All the outcrops on the other side of the boundary should be sandstone.
- Now, draw in the pegmatite boundaries. Notice in Outcrop #23 that the pegmatite is occurring in narrow, linear structures. These are called dykes. Assume that the dykes are around the same thickness as shown in Outcrop #23.
- Start with Outcrop #4. Notice that the dyke boundary in the outcrops trends toward Outcrop #7 which is all pegmatite. Continuing along the same trend, notice that Outcrop #10, which is also partly pegmatite, lines up with Outcrops #4 and #7. Again, notice that Outcrop #14 is also along the same trend.
- All four of the above outcrops can be connected including the visible boundaries in Outcrops #4 and #10, by drawing a narrow band across the page. This indicates a pegmatite dyke cross-cutting the sandstone and the granite.
- Scan the outcrop layout. Notice that this mapped dyke (Dyke A) does not seem to connect at all with the other pegmatite in Outcrops #17 #19, #21 or #23. There must be more than one dyke.
- Outcrop #21 and #17 show pegmatite boundaries that indicates a common dyke direction. Draw in a narrow dyke structure that connects Outcrops #21 and #17. This dyke has to "dead end" because no pegmatite is present in Outcrop #15. End this new dyke between Outcrop #15 and #17. Call this dyke, "Dyke B1".
- Outcrops #23, #19 and #16 show other pegmatite boundaries that indicates a common dyke direction. Draw in a narrow dyke structure that connects Outcrops#23, #19 and #16. This dyke has to "dead end" because no pegmatite is present in Outcrop #26. End this new dyke between Outcrop #26 and #23. Call this dyke, "Dyke B2".
- This third dyke shows something else interesting in Outcrop #16. The pegmatite does not cut into the sandstone.
- All rock type boundaries have now been defined. You are well on your way to creating a geological map. Refer to Solution Sheet #3.
- Another geological feature can now be inferred from your interpretation of the data. Notice two things;
- Dyke B1 and "Dyke B2 are parallel. The dead ends of these two dykes in the middle of the map could at one time have lined up and been connected to form one dyke. It appears that one dyke was off-set by what is called faulting. The faulting created Dyke A and Dyke B which were originally the same dyke.
- Also notice the zig-zag boundary between the granite and the sandstone. This zig-zag boundary indicates that the granite and the sandstone have also been off-set by by faulting. The shifting in the granite and the sandstone is the same distance and direction as the shifting of Dyke B.
- Draw a line to indicate a fault. Suggest how the fault cuts across the map area to explain the off-set of the granite/sandstone boundary and Dyke B.
- Task #3 Interpret the relative order of geological events. Put the sandstone, granite, the fault, Dyke A and Dyke B in order of occurrence.
- The oldest item or event in the map area will be what is cross-cut by everything else. Remember that the cross-cutting element has to be younger than what is being cross-cut.
- What is being cross-cut by both dykes and the fault? (the granite) The sandstone is NOT being cross-cut by Dyke B (outcrop #16), so, the sandstone cannot be the oldest. Therefore the granite is the oldest.
- Next... Outcrop #16 holds another interesting piece of information. Dyke B2 extends up to the contact between the sandstone and the granite but does not cross into the sandstone. This simple fact makes the age relationships between the sandstone, granite and the Dyke B2 very clear. First of all, between the granite and the pegmatite of Dyke B2, which must be younger? Because the dyke is igneous forming from hot fluids invading a crack or fissure in the granite, the pegmatite (Dyke B2) must be the younger of the two.
- Dyke B2 is observed to not cross into the sandstone. This shows that the sandstone was emplaced after the dyke was introduced. If the opposite were true the dyke would cross-cut the sandstone as well as the granite. Using these observations, the relative ages of these three rock types is known. The order from oldest to youngest is granite, pegmatite (Dyke B2) and sandstone.
- The cross-cutting relationship between Dyke A and the sandstone says that the pegmatite of Dyke A is younger than the sandstone. But it is also known that the sandstone is younger than the other pegmatite and the granite. The order from oldest to youngest for all four identified rocks must then be granite, pegmatite (Dyke B2), sandstone, and pegmatite (Dyke A).
- From the mapping of the entire area, Dyke B1 and Dyke B2 can also be interpreted to have been off-set or faulted. These two dykes were originally the same dyke. The shifting of Dyke B1 and Dyke B2 is mimicked by the sandstone/granite boundary showing the same shift direction and amount of off-set.
- The fault has off-set the granite, the sandstone and Dyke B, but not Dyke A. Therefore, the fault must be older than Dyke A, but younger than everything else.
- Therefore, the order of geological events is:
- pegmatite (Dyke A) Youngest
- pegmatite (Dyke B)
- granite Oldest
- By just looking at the positions of the rocks with respect to each other, the relative order of geological events can be figured out. Accurate geological mapping and interpretation can help piece together the Earth's history, even though events happened millions if not billions of years ago.