Download What They Say in Hong Kong Geography Books and Exam

What They Say in Hong Kong Geography Books and Exam Papers vs. What Earth
Scientists Actually Know
LS Chan
There are many inaccuracies and errors in the information on the earth sciences given in
text books and public examination marking schemes in Hong Kong. In general, these
inaccuracies can be classified into three categories.
Category I.
Inaccuracies due to simplification and generalization
An example is the plate boundary for the 2004 Indonesian Tsunami earthquake. In
simplified term, this is a boundary between the Indian-Australian Plate and Eurasian
Plate. However, since that part of Eurasia Plate may comprise a number of small plates,
the boundary could be referred to as one between Australian Plate and Burma Plate.
This kind of simplification may be acceptable, and sometimes even necessary to avoid
presenting overly tedious details.
Category II
Old or obsolete knowledge
Since advances in the Earth Sciences are made constantly and often at very fast rates,
many old terms and concepts have become obsolete or been superceded by refined
information. For example, Japan is often regarded as part of Eurasian Plate. New data
have revealed that Japan is probably located instead within North American Plate. Upto-date descriptions therefore would regard earthquakes off the eastern coast of Japan to
be generated by the subduction of Pacific Plate under the North American Plate. Also,
it is customary for textbook authors to duplicate information from older textbooks, and
many old and rarely used terms somehow find their ways to pass on over decades. It is
sometimes amazing to see such terms as ‘hypabyssal rock’, ‘geo’, and ‘sial’ used
commonly by geography students in Hong Kong. The terms are indeed rarely used by
earth scientists worldwide and textbook authors should make a better effort to learn
updated knowledge in Earth Sciences.
Category III
Errors and misconceptions.
These are simply wrong concepts and misconceptions. They reflect the deficiency in
the subject knowledge on the part of the authors. This kind of inaccuracies is
unacceptable and should be corrected as soon a possible.
The intention of the current list is not to patronize the authors or persons behind the
published misconceptions, but to help teachers and students acquire a clearer understanding
of the concepts.
What they say (in textbooks): In the molten mantle, it is so hot that the rocks remain in
molten form, called magma…
What we actually know: Lower mantle is solid. We know this for fact because it transmits
shear waves. It is not composed entirely of magma. Only at a depth of about 100-200 km,
the temperature of the mantle approaches that of the melting point (called the solidus) of
the mantle rock, and partial melting of the mantle may occur. This idea is supported by the
presence of a decrease in the seismic wave velocity profile at the same depth. But shear
waves, a kind of earthquake wave which cannot propagate in fluids, can still be recorded in
the layer, suggesting the layer is not entirely molten.
What they say (in textbooks): Continental crust … is found above the oceanic crust….
What we actually know: Continental and oceanic crusts are simply very different.
Continental crust is less dense, but it sits directly on the mantle, not oceanic crust. Many
geography textbooks used by secondary schools in Hong Kong show the presence of
oceanic crust beneath continental crust. This is wrong and unacceptable.
What they say (in textbooks): The sima layer is thinner and more uniform in
thickness.
What we actually know: Oceanic crust varies in thickness from 0 km to about 15 km. We
also know the layering structure of oceanic crust very well. On land, geologists have
identified what we call ophiolite suites which represent exposed sections of oceanic crust.
Sima is an obsolete term to describe the composition of the crustal material. We should
avoid using this term.
What they say (in textbooks): The earth’s inner core and outer core has the same
composition, except that one is a solid and the other is a liquid…
What we actually know: Not exactly. There is likely a compositional difference between
the outer and the inner core as well. The outer core is probably more FeS+FeO while the
inner core FeO+Fe.
What they say (in textbooks): …(for conservative plate boundaries) Plates slide
against each other when the currents flow past each other (like a shear motion)…
What we actually know: No. It can’t happen this way. Transform faults don’t occur where
the mantle currents slide past each other. In fact, there may not be a strong association
between the plate motion and the mantle convection pattern.
What they say (in textbooks): …(mountain building)…In the beginning stage, the
mountains grow faster than denudation can wear them down. These mountains are
young fold mountains. In time, the growth of the mountains slow down…These
mountains are old fold mountains.
What we actually know: No. Most mountains formed when continents collided with each
other. Old fold mountains are those that formed long ago, and the collision has long ended;
young fold mountains are those still forming and at where plate collisions are still active.
What they say (in textbooks): The power driving the cells probably comes from the
breakdown of radioactive elements in the mantle which releases large amounts of
heat.
What we actually know: The elemental composition of the rocks making up the mantle
contains no or very little radiogenic elements. The heat that drives the mantle convection
likely comes from the core. Chemical phase changes occurring at the inner core-outer core
boundary releases a great amount of heat, which is transferred to the mantle to cause
convection. Mantle convection, and for that, plate tectonics, is actually a process for heat
transfer and dissipation.
What they say (in textbooks): …Convection currents (occur) in the magma of the
upper mantle…
What we actually know: A common misconception is that the mantle is made of magmas,
and that’s why it can flow and cause convection currents. The mantle is not a magma layer.
The reason mantle rocks can ‘flow’ is because mantle rocks are not a perfectly rigid body.
Under an applied load (force) for a long duration of time, rocks can behave like a very
viscous fluid. The flow rate is extremely slow, on the order of a few cm per year.
What they say (in a Geography exam marking scheme): The main cause of the
transform faults is the results of the earth’s rotation.
What we actually know: Earth’s rotation has nothing to do with transform faults.
Transform faults are simply plate boundaries along which the plates on both sides of the
fault slide past each other. Those transform faults along ocean ridges are needed to
accommodate the varying spreading rate along the spreading axis and to maintain the
spreading ridge at a medial position of the expanding ocean.
What they say (in a Geography exam marking scheme): Since the Earth is a sphere,
places at lower latitudes are subject to a higher linear speed of rotation while the
speed of rotation at high latitudes is lower….Places at lower latitudes may have
greater lateral force, so the displacement of the fault at low latitudes is greater.
What we actually know: The relative plate motion between each pair of plates can be
defined by a rotation about a pole which is known as an Euler pole. This Euler pole has
nothing to do with the earth’s rotation poles which are the North and South Poles. Since
there are numerous plate boundaries on earth, there are numerous Euler poles. These Euler
poles are distributed all over the earth. The notion that the Earth is a sphere and places at
lower latitudes are moving at a faster rate is flatly wrong. This misconception probably
arises since the particular Euler pole for the Mid-Atlantic Ridge happens to be located very
close to the North Pole of the Earth, giving the author of the examination question the
impression that plate motions are dictated by the earth’s rotation.
For each plate boundary, the linear plate motion rate increases with increasing distance
from the Euler pole, and the relationship is given by
R=A*Θ
In which,
R: linear spreading rate at a site
A: angular distance of the site from the Euler pole (in radians)
Θ: angular velocity of plate motion
What they say (in textbooks): …compression also causes some of the sedimentary
rocks to change their form, and they turn into metamorphic rocks….
What we actually know: One of the misconceptions about Rock Cycle is that rocks evolve
in specific path, igneous-sedimentary-metamorphic. In reality, any kind of rocks can be
turned into any other kind, or even its own kind. Metamorphic rocks can be formed from
igneous rock, sedimentary rocks, or other kind of metamorphic rocks.
What they say (in textbooks): …reverse fault… is also called a thrust fault.
What we actually know: Reverse fault and thrust fault are not equal. Thrust faults are a
class of reverse faults with a very shallow dip angle.
What they say (in textbooks): Granite has three sets of well-developed …joints –
vertical horizontal and oblique.
What we actually know: The number of joint sets
depends on the history of the rock, not the rock type.
Some rocks may have few sets of joints, and some may
have more. It is not an intrinsic property of granite to
have three sets of joints. Any kind of rock can have any
number of joint sets. (Figure: Multiple joint sets in
granite at Lamma Island)
What they say (in textbooks): tear fault…caused by lateral forces…also called a
lateral fault.
What we actually know: It is more common to use the term strike-slip fault or
transcurrent fault. It is not clear what is meant by ‘lateral forces’. Reverse faulting is also
caused by a lateral compression.
What they say (in textbooks): normal faults are caused by tension, reverse fault by
compression and tear faults are caused by ‘lateral forces’….
What we actually know: The authors obviously never took a course in soil or rock
mechanics. Inside the crust, rocks are subjected to pressure (called stress) from different
directions. We can resolve the applied stress into three ‘principal stresses’, which are at
right angle, like the axes of a coordinate system, to each other. The greatest principal stress,
normally a compression, is called the (compression) P-axis. The smallest principal stress,
often but not necessarily always, is the (tensional) T-axis. Whether you get normal or
reverse faults depends on the orientation of the P- and T-axes. Reverse faults occur when
P-axis is horizontal, T-axis vertical; normal faults occur when P-axis is vertical, and T-axis
horizontal. If both P- and T-axes are horizontal, and the rocks break at an angle to the axes,
strike-slip faults are produced. (Figure: Relationship between principal stress and fault
type)
Normal
Reversed
Strike-slip
What they say (in textbooks): Hypabyssal rocks: Example--porphyry
What we actually know: Porphyry is a term describing the presence of large phenocrysts
in an igneous rock. Both volcanic and pkutonic rock can be porphyritic. The term
hypabyssal is rarely used. In fact, you won’t even find that term in the 2003 edition of
Dictionary of Earth Science by McGraw-Hill.