Download field inquiry on plate tectonics and the rock cycle at Little Guilin

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
[email protected]
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).
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
Didactic teaching in the
field
Inquiry learning in the field
Teacher role
Teacher as knowledge
dispenser
Teacher as guide and learning coach
Student role
Student as knowledge
recipient
Student as investigator
Teacher
responses to
the student
question,
"What rock
is this?"
• 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.
• 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.
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.
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
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
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.
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).
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
Common mode of
Liquid rock (magma
formation
or lava) cools and
crystallises
Common
distinguishing
characteristics
Mineral crystals are
interlocking
Sedimentary
Rock particles
(gravel, sand, or
clay) are deposited
and later compacted
and cemented
together
Grains of sediment
do not interlock;
layers might be seen
Metamorphic
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.
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).
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.
• 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).
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.
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 250260 million years for the Gombak norite and 230-245 million years for the
Bukit Timah granite (Oliver et al., 2011; and pers. comm.)
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 230-260 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 230-260
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).
Figure 8: Site 4, behind bus stop 43421 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.
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.
• 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. 131-163.
Oliver, G., Zaw, K., and Hotson, M. (2011) Dating rocks in Singapore,
Innovation Magazine, v. 10, no. 2, p. 22-25.
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.
97-111.