MRT to magma chamber: field inquiry on plate tectonics and the rock cycle at Little Guilin, Singapore

Transcription

1 MRT to magma chamber: field inquiry on plate tectonics and the rock cycle at Little Guilin, Singapore Field session for SEAGA conference, 29-Nov-2012 Jamie McCaughey Earth Observatory of Singapore, Nanyang Technological University Singapore Abstract Little Guilin is probably Singapore's most dramatic and accessible exposure of bedrock, showing two stages of magma intrusion more than 230 million years ago to form plutonic igneous rocks. A short walk away, sandstone of the Jurong Formation is exposed. Through field inquiry, students will make direct observations of the composition, texture, and field relations of these rocks to interpret a geologic history of this part of Singapore. Keywords field inquiry; physical geography; inquiry learning INTRODUCTION Field inquiry approach This site and the suggested questions and activities below provide an excellent opportunity for guided inquiry learning in the field. However, being in the field does not, by itself, ensure that inquiry learning is taking place. In inquiry learning, students pose questions, seek out relevant information, then evaluate the information in an effort to answer the original questions (e.g. Margaret, 2003). In practice, inquiry is guided to varying degrees by the teacher, depending on student prior knowledge, available time, and the complexity of the topic or available information. To promote inquiry learning, answer student questions with guiding questions rather than direct answers (Table 1).

2 Table 1. Possible teacher responses to a student who asks, "What rock is this?" Direct answers (including confirmations of student guesses) inhibit inquiry learning, whereas guiding questions foster inquiry learning. Pedagogy Teacher role Student role Teacher responses to the student question, "What rock is this?" Didactic teaching in the field Teacher as knowledge dispenser Student as knowledge recipient It's a norite. It's a norite, which is high in iron and cooled slowly underground from a magma. See the coarse mineral grains and dark colour? Look right there. That's how you know it's a norite, and that it cooled slowly underground from an iron-rich magma. Inquiry learning in the field Teacher as guide and learning coach Student as investigator Tell me something about this rock. What colour is the rock? Can you see individual mineral grains in this rock? What is their size? What does the colour tell you about the composition of the rock? What does the grain size tell you about how this rock formed? If a rock had a lot of iron, would it be darker or lighter in colour? If liquid rock cooled slowly, would the mineral crystals grow large in size? Where would liquid rock cool more slowly - deep underground or on the Earth's surface? Field learning lends itself to an inquiry-learning approach (Mogk and Goodwin, 2012). When the two are combined, students can benefit greatly. Field inquiry helps students critically evaluate simplications and abstractions of natural systems that are presented in textbooks and in the classroom (Stillings, 2012). Field inquiry also helps students to form a more integrated, less compartmentalised understanding of natural systems through both sensory experiences and the process of integrating observations and concepts into a coherent whole (Mogk and Goodwin, 2012). In geography education in Singapore, both the Ministry of Education and National Institute of Education are advocating for field-based inquiry learning to become a standard component of students' geography learning. A key habit for inquiry learning is the practice of separating observations from interpretations. Students might be tempted to leap immediately to the interpretation, "It's granite!" Instead, guide them to start with describing what they see and using that information as evidence to support their interpretations.

3 Cultural history This site is a former quarry. The rocks were cut and blasted out of the ground for use as building stones and other purposes. Student preparation Students should already be familiar with: The rock cycle The processes that produce the three rock types (igneous, sedimentary, and metamorphic) General physical features that distinguish the three rock types Rocks are made of minerals Minerals are crystals that occur in a range of sizes Isostatic adjustment: if significant erosion occurs, the lithosphere rises up in response Logistics The field site is walking distance from Bukit Gombak MRT (Figure 1). As a well-manicured and easily accessible park, the risks are generally low. Students should not climb on steep rocks, enter closed areas, or swim.

4 BUKIT GOMBAK SMRT SITE1 SITE 3 SITE 2 SITE 4 North 0 100m Figure 1. Location of field sites at Little Guilin. (Basemap from streetdirectory.com) SITE 1: OBSERVING AND INTERPRETING ROCKS IN THE FIELD Learning objectives: students will be able to... 1a) Distinguish bedrock from loose soil/regolith. 1b) Distinguish fresh and weathered rock surfaces. 1c) Interpret whether rocks are igneous, sedimentary, or metamorphic

5 Access From Bukit Gombak MRT, walk southeast a few hundred metres on Bukit Batok Ave 5. Pass the "meeting point" sign and enter the park along the main path (at grade, not climbing the staircase.) Continue along the lakefront path about 50 m (Figure 2.) Figure 2. Site 1, looking south. Objective 1a) Distinguish bedrock from loose soil/regolith. Here there are dramatic exposures of bedrock, which in geological terms means solid rock that is physically connected to the rest of the crust. Note that "bedrock" does not mean a particular rock type, only that the rock is connected to the rest of the crust. In contrast, loose soil or regolith sits on top of bedrock but is not physically connected to it. Student questions: What is bedrock? What is soil? Find, sketch, and describe examples of each. o Guiding questions: Which rock looks like it's connected to rock below the surface? Which rock looks like it is resting loosely upon the surface? o Answer: Bedrock can be seen on the large cliffs across the pond, on the small hill on the near side of the pond, and where the park path crosses over a narrow opening in the rock (Site 2, Figure 1). Soil can be seen as the loose dirt along the top of the cliffs, and also the dirt throughout the grassy areas of the park. The boulders in the grassy areas are not bedrock. If we break a piece of rock off of the cliff, is it still bedrock? Why or why not? o Guiding questions: How did we define bedrock? o Answer: It would no longer be bedrock, since it is no longer connected to the rest of the crust. However, its composition

6 would not change; if you break a piece of norite off the cliff, it is still norite, just no longer bedrock. Why is there a cliff here? Why is there a pond here? o Guiding questions: Do you normally find large rock cliffs in Singapore? Do you normally find large boulders laying around in Singapore? o Answer: People dug up and removed this rock in the past to make buildings and other things. This is a former quarry. Objective 1b) Distinguish fresh and weathered rock surfaces. Direct students to look at boulders with varying surface appearances (Figure 3). Figure 3: Weathered vs. fresh rock. The boulders near the bottom have fresher rock surfaces, which are quite close to the actual colour of the rock. The same rock has a different appearance once weathered, as seen in the rocks above and behind.

7 Figure 4: Closeup of a weathered boulder along the park path. Different lichens are seen as orange, a little black, and two shades of green. The blue is spray paint! Student questions: Some of these boulders have heavily weathered surfaces, which means that chemical reactions and biological activity have modified the surface appearance. Other boulders have fresh surfaces, which are much closer to the original colour of the rock. Which are which? o Guiding questions: If you leave metal outside in the rain over the long term, what happens to it? o Answer: The medium-dark gray boulders are slightly weathered, whereas the boulders with reddish and greenish colours are heavily weathered (Figures 3 and 4). What are some processes that may have weathered these boulders? o Guiding questions: What does the dark gray colour of the rocks tell you about the amount of iron in the rocks? What might happen to the iron in our tropical climate? Do living things grow on rock or concrete surfaces? o Answer: Oxidation (rust) of iron in the rocks results in red-orange colours. The blackish, green, and some orange colours are lichen. Lichen are a symbiotic relationship of algae and a fungus; the algae photosynthesise sugars, providing food to the fungus, while the fungus secretes acid that breaks down the rock to provide necessary mineral nutrients for life. All of these processes break down the rock and change its surface appearance (Figures 3, 4, and 5).

8 Figure 5: Weathering of rock surfaces. The black colours are lichen; lichen preferentially grow where water runs more frequently down the rock face, hence the streaky pattern on the bottom part of the cliff. Objective 1c) Interpret whether rocks are igneous, sedimentary, or metamorphic Students should already know the rock cycle and general characteristics of the rock types (Table 2). Table 2. The main rock types. Rock type Igneous Sedimentary Metamorphic Common mode of formation Common distinguishing characteristics Liquid rock (magma or lava) cools and crystallises Mineral crystals are interlocking Rock particles (gravel, sand, or clay) are deposited and later compacted and cemented together Grains of sediment do not interlock; layers might be seen A pre-existing rock is changed by heat and pressure Mineral crystals align and sometimes separate into bands of different compositions "It's not the shape, it's what's inside." People tend to focus on the outward shape of rocks. However, the outward shape does not tell us very much about the processes that formed the rocks. Instead, focus students' attention on the materials and texture within the rocks. As an analogy, if you wanted to know the taste of a new ice cream, you shouldn't focus on whether the ice cream is in a bowl or in a cone - you should take a taste to see what's inside.

9 Student questions: Are these rocks igneous, sedimentary, or metamorphic? How do you know? Sketch the defining characteristics of the rock. o Answer: Igneous, because the mineral crystals interlock but are not aligned or banded. SITE 2: DESCRIBE AND INTERPRET ROCKS Learning objectives: students will be able to... 2a) Describe and classify the rocks. 2b) Interpret how the rocks formed. Access From site 1, continue south along the pond shore until the path passes through a narrow cut in the bedrock (Figure 6).

10 Figure 6: Closeup of the intrusion, where the park path passes through a narrow cleft cut into the rock. The lighter-coloured rock on the left is the Bukit Timah granite; this is a narrow dike that intruded into a larger, already existing body of the Gombak norite (darker colour on the right). Where the path passes, the intrusion of granite is a few metres wide. Objective 2a) Describe and classify the rocks. Students have already interpreted that these rocks are igneous, because the mineral crystals in this rock are interlocking and there is no banding or layering. Student questions How many distinct rock units are present? o Guiding questions: Scientists group rocks by common characteristics. Are all the rocks here the same? If not, which rocks are similar to one another? o Answer: There are two distinct igneous rock units (Figure 6); one lighter-gray and one dark-gray.

11 For each rock unit, describe the grain size, grain relationships (interlocking or not?), colour, and any characteristics of the minerals that you can observe. o Answer: Both rock units have coarse, interlocking mineral crystals. Crystal sizes range from barely visible to a few mm across. Which rock unit has more iron and magnesium? How do you know? o Guiding questions: What's something that has iron? What colour is it? o Answer: (reminder from objective 1b above) The dark-gray colour indicates that the rock has more iron and magnesium. The darker rock also tends to show more oxidation on its surface. After they have made descriptions, provide students with the rock names: the dark-gray rock is the Gombak norite, and the lighter-gray rock is the Bukit Timah granite (Lee & Zhou, 2009). Both norite and granite are general rock terms for igneous rocks that cooled slowly underground; the key difference is that norite is richer in iron and magnesium. Rock units are named for the locations where they were first described, which is also often the place where they are most visible. Objective 2b) Interpret how the rocks formed. Student questions If liquid rock cools slowly, the mineral crystals grow large. If liquid rock cools quickly, however, the resulting mineral crystals are too small to see without a microscope. Which process would happen underground, and which process would happen in a volcanic eruption onto the Earth's surface? Why? o Answer: Magma cools slowly underground because it is "insulated" by the surrounding rock, and because temperatures are hotter deeper inside the Earth than at the surface. Lava erupted onto the Earth's surface cools quickly because it loses heat rapidly to its surroundings, and and because temperatures on Earth's surface are cooler than deep underground. Did this igneous rock cool slowly underground, or did it cool quickly on the surface after being erupted from a volcano? o Answer: This rock cooled slowly underground from a magma. We know this because the grains are all coarse (visible without a microscope).

12 SITE 3: INTERPRET GEOLOGIC HISTORY Learning objectives: students will be able to... 3a) Interpret the relative ages of the Gombak norite and Bukit Timah granite 3b) Interpret the geologic history of Little Guilin. Access Return to near site 1, where you can get a clear view of the large cliff on the far side of the pond (Figure 7). Figure 7: Large cliff at Little Guilin. Most is made of Gombak norite, except for a thin intrusion of the lighter-coloured Bukit Timah granite shown by the arrows. Objective 3a) Interpret the relative ages of the Gombak norite and Bukit Timah granite Going back to Hutton and Lyell, the principle of cross-cutting relations holds that a geological feature that cuts another is the younger of the two features. This is actually quite a simple idea: by analogy, you have to bake a cake before you can cut it.

13 Student questions Which is younger, the Bukit Timah granite or the Gombak norite? o Guiding questions: How did these rocks form? If new magma intruded, would it cut across the pre-existing rock or be cut by it? o Answer: The Bukit Timah granite forms a thin dike in a larger body of Gombak norite, as seen on the cliff (Figure 7). Therefore, the Gombak norite was there first; the Bukit Timah granite later intruded into it (Lee & Zhou, 2009). Objective 3b) Interpret the geologic history of Little Guilin. Student questions Develop a geologic history of Little Guilin by listing events from the oldest to the youngest. Note where there is a gap in time in your history. o Guiding questions: Did these rocks form on the surface or deep underground? How could rocks that formed deep underground now be exposed on the surface? What is happening to these rocks today? Has this process been going on for a long time? If rocks are eroded, how does the lithosphere respond? o Answer (oldest at top): Iron-rich magma intruded deep underground, cooled to form Gombak norite Some time passed Iron-poor magma intruded deep underground into fractures in the Gombak norite, then cooled to form Bukit Timah granite Some time passed A very long period of erosion removed rocks that were overlying the Gombak norite and Bukit Timah granite. This erosion unloaded the lithosphere, which rose up in response (isostatic adjustment); this process eventually brought up to the surface rocks that had formed deep underground. In the past century, humans quarried these rocks for use as building stones. More recently, NParks landscaped the site for use as a public park. Radiometric dating allows for estimations of absolute rock ages based on the decay of radioactive isotopes since the time of crystallisation of the rock. Radiometric dating studies of Singapore rocks provide estimated ages of million years for the Gombak norite and million years for the Bukit Timah granite (Oliver et al., 2011; and pers. comm.)

14 Student questions Are these dates consistent with your geologic history? What does it mean about the period of erosion? o Answer: Yes, the dates are consistent with the relative ages indicated by cross-cutting relations. The dates suggest that the period of erosion could have been as long as ~200 million years. These rocks are how much older than you? o Answer: The Gombak norite is about 17 million times older than a 15-year-old student (255 million divided by 15). These rocks crystallised from a magma more than 200 million years ago. What does that mean about the plate tectonic setting of Singapore at that time? o Guiding questions: Does magma intrude everywhere around the Earth? (No, mainly at subduction zones.) What kinds of plate boundary are associated with volcanism? (Subduction zones and divergent boundaries.) Which of these could produce a granite? o Answer: Singapore was at a subduction zone at the time of these rocks, about million years ago. Based on your tectonic interpretation, what kinds of natural hazards would have been common in the area that is now Singapore at about million years ago? o Guiding questions: What kinds of natural hazards are common at subduction zones today? What happened in Japan in 2011 and Aceh in 2004? What happened at Krakatoa? o Answer: The area that is now Singapore would have faced frequent large earthquakes, tsuamis, and volcanic eruptions, just as in modern subduction zones including Japan, Sumatra, much of the Philippines, and others. SITE 4: CONTRAST JURONG FORMATION AND INTERPRET AGE RELATIONS Learning objectives: students will be able to... 4a) Describe rock at Block 503 4b) Interpret how the rock formed 4c) Hypothesise age relations between Little Guilin and Block 503 Access From Little Guilin, follow Bukit Batok East Ave 5 to the south, then turn right on Bukit Batok East Ave 2. Site 4 is immediately behind bus stop 43421, along a pathway toward Block 503 (Figures 1 and 8).

15 Figure 8: Site 4, behind bus stop and facing Block 503. Objective 4a) Describe rock at Block 503 The rock is slightly exposed on either side of the path (Figure 8). Better exposures can be found on the grassy slope above the path and just into the edge of the forest; however, mind the uneven footing. Student questions Describe the rock, including grain size, grain relationships, colour, and any other features that you can observe. Note that the rock here is much less well exposed than at Little Guilin; be sure that the features that you are describing are part of the rock itself. o Guiding questions: Are the grains interlocking? Are the grains rounded, angular, or in between? Do you see layering or banding? o Answer: Grains are fine to coarse sand, moderately rounded. Grains do not interlock. No banding is present. Layering is present but not obvious.

16 Figure 9: Closeup of Jurong Formation exposed at site 4. Objective 4b) Interpret how the rock formed Student questions Based on your description, what rock type is this? How do you know? o Guiding questions: If the grains don't interlock, what rock type can we rule out? What process could round the grains? o Answer: Sedimentary rock, sandstone in particular. This is indicated by the lack of interlocking grains, the lack of banding, and the partial rounding of the grains. How did this rock form? o Sediments were deposited by water, then buried, compacted, and cemented to form this sandstone. Objective 4c) Hypothesise age relations between Little Guilin and Block 503 This rock is part of the Jurong Formation, which includes a range of sedimentary and volcanic (igneous) rocks (Lee & Zhou, 2009). In places, the Jurong Formation has been deformed by folding and faulting and affected by low-grade (slight) metamorphism, though these features are not visible at site 4. Student questions From our observations, is it possible to determine whether the Jurong Formation is older or younger than the Bukit Timah granite and Gombak norite? o Answer: No. The contact between the two rocks is not exposed.

17 If you could find a place where the contact between these rock units is exposed, what evidence would you look for to know the correct age sequence? o Answer: If the Jurong Formation is younger, it would be deposited on top of the granite and norite. The Jurong Formation would not show metamorphism along the contact (contact metamorphism). If the Jurong Formation is older, the norite and granite might intrude (cross-cut) as dikes into the Jurong Formation. Along the contact, the Jurong formation would show signs of metamorphism, such as recrystallisation and alignment of mineral grains. The absolute age of the Jurong Formation is not well known. Researchers have not found intrusions of granite or norite in the Jurong Formation, nor contact metamorphism of the Jurong Formation. There is granitic material within the sediments of the Jurong Formation that may have been weathered out of nearby granites. Volcanic rocks included in parts of the Jurong Formation indicate that subduction was still active at the time. Researchers thus conclude that the Jurong Formation is slightly younger than the Bukit Timah granite (Lee & Zhou, 2009; Oliver et al., 2011). REFERENCES Lee, K. W., and Zhou, Y. (2009) Geology of Singapore, 2nd Ed., Defense Science and Technology Agency, Singapore, 90 p. Mogk, D. W., and Goodwin, C. (2012) Learning in the field: Synthesis of research on thinking and learning in the geosciences, in Kastens, K. A., and Manduca, C. A., eds., Earth and mind II: A synthesis of research on thinking and learning in the geosciences: Geological Society of America Special Paper 486, p Oliver, G., Zaw, K., and Hotson, M. (2011) Dating rocks in Singapore, Innovation Magazine, v. 10, no. 2, p Roberts, M. (2003) Learning through enquiry: Making sense of geography in the key stage 3 classroom: Geographical Association, 212 p. Stillings, N. (2012) Complex systems in the geosciences and in geoscience learning, in Kastens, K. A., and Manduca, C. A., eds., Earth and mind II: A synthesis of research on thinking and learning in the geosciences: Geological Society of America Special Paper 486, p