Download Common Misconceptions in Physical Geography

Common Misconceptions in
Physical Geography
Chan Lung Sang
Department of Earth Sciences
Faculty of Science, HKU
Causes of Inaccuracies
• Inaccuracies caused by technical
details
• Obsolete concepts
• Erroneous concepts
About Earth’s motion in space
• Misconception: the Earth is revolving about the Sun in an
elliptical orbit, with the Sun located at the center of the
ellipse.
• What scientists know: the Earth is revolving about the
Sun in an elliptical orbit, with the Sun located at one of
the foci of the ellipse.
focus
perihelion
center
aphelion
About insolation and radiation budget
Source: NASA
• Misconception: the Earth is closest to the Sun at summer
solstice, and farthest at winter solstice.
• What scientists know: The solstices occur when Earth’s
rotation axis is tilted at the maximum angle relative to the
axis of the ecliptic plane. As the Earth’s orbit around the
sun is elliptical, the distance between the Earth and the
Sun varies. The two points on the elliptical orbit where the
Earth is closest and farthest from the Sun are known as,
respectively, perihelion and aphelion. The position along
the Earth’s orbit around the Sun where the summer solstice
happens varies gradually over time, with a period of about
23,000 years. This is known as the precession of the
equinoxes.
Earth’s Abundance
What scientists know:
• Four major elements: O, Fe, Si, Mg make up
85% of earth material
• Earth contains about 3% Sulphur
• Earth’s crust is depleted in siderophile (Fe, Ni,
Cu etc.) but enriched in K and Al
• Mantle is mainly a Mg silicate
• Core – FeS, FeO and Fe
Earth’s Interior Model
Shell
Name
Depth (km)
Composition
State
A
Crust
0-30
Al-rich silicate
solid
B
C
Noncrustal
Lithosphere
Asthenosphere
Moho
Upper
Mantle
30-100
100-640
D’
Lower Mantle
640-2800
D”
Mantle-Core Transition
2800-2890
E
Outer Core
2890-4600
F
OC-IC Transition
4600-5160
solid
Mg-rich
silicate
(olivine)
near melting
solid
soild (softer)
FeS+Fe
liquid
liquid+solid
Fe+FeO
G
Inner Core
5160-6370
solid
• Misconception: the Earth’s mantle is in molten form,
consisting of molten rock material called
• What scientists know: the Earth’s mantle is in general a solid,
rocky layer. Only locally at a depth around 50-150 km, the
temperature of the mantle is quite close to the melting point
of the rock. A small change in the pressure or introduction of
water into the rock may lower the melting point enough to
cause partial melting of the rock. The misconception that the
mantle is liquid also arises from the concept of mantle
convection. The truth is material can behave very differently
on different time scales. When a force is applied suddenly to
a rock, the rock may break and behave like a brittle solid. If
the rock is subjected to a prolonged force, the rock may
actually ‘flow’ and behaves like a viscous fluid. This dual
behavior of material is known as rheology.
* The concepts of sial and sima are old and imprecisely
describe current understanding of crustal structure.
Earlier idea on crustal structure
Source of figures: Internet, original author unknown
* Lithosphere includes
the crust and part of the
upper mantle.
Asthenosphere is not a
molten layer.
* The distinction between
lithosphere and
asthenosphere is by
means of physical
strength, not composition
* No oceanic crust
beneath continental crust
Source of figures: Internet, precise author unknown
*What formerly called geosynclines are now known to be
mountains that formed in active continental margins
*The term should not be used anymore.
The Obsolete Concept of Geosyncline
Source: http://www.uwgb.edu/DutchS/platetec/geosync.htm
Earth’s Internal Sources of Energy
•
•
•
•
Primordial heat
Radioactivity
Gravitational heat
Phase changes
Source of figures: Internet, original author unknown
Outer Core-Inner Core
Relation
Outer core condenses
gradually to become
part of inner core. The
process probably
involves also a
compositional change.
Heat is released in the
process.
Source of figures: Internet, original author unknown
Misconception: The earth’s internal heat is due to
radioactive decay in the earth’s core.
What earth scientists know: There may not be much
radiogenic elements in the Earth’s core. We know
most radiogenic elements are concentrated in the
earth’s crust. Phase change at the inner-outer core
boundary, not radioactivity, is probably the primary
internal energy source for the Earth.
Rocks and rock cycle
Misconception: Rrocks follow a specific path of evolution in the
rock cycle, transforming from igneous into sedimentary, and from
sedimentary into metamorphic rocks.
What geologists know:
Any kind of rock can be converted
into any kind in the rock cycle
River system:
Misconception: Rivers have greatest
velocity in the upper course and lowest
in the lower course. A river’s function is
erosion in the upper course, transportation in
the middle course, and deposition in the
lower course.
The facts are:
• Flow velocity of river continues to increase downstream
• River erosion, transportation and deposition occur in all
courses of a river, although the relative importance differs
Misconception: Landslide occurs after rainstorm because
water acts as a lubricant
What geologists could tell you
• The key role of water is NOT
lubrication; water increases
pore pressure and reduces
effective stress
• Human activities played an
important major role in many
of the major landslides in Hong
Kong
stalactite
helictite
speleothems found in Hong Kong
Thank You!
[email protected]
Department of Earth Sciences
University of Hong Kong
www.hku.hk/science/temp/LSChan
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