Common Misconceptions in Physical Geography

Chan Lung SangDepartment 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.

perihelion aphelion

focuscenter

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 AbundanceWhat 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

Fe+FeO

FeS+Fe

Mg-rich silicate

(olivine)

Al-rich silicate

Composition

liquid+solid4600-5160OC-IC TransitionF

solid5160-6370Inner CoreG

liquid2890-4600Outer CoreE

soild (softer)2800-2890Mantle-Core TransitionD”

solid640-2800Lower MantleD’

near melting100-640AsthenosphereC

solid30-100UpperMantle

NoncrustalLithosphere

B

solid0-30CrustA

StateDepth (km)NameShell

Moho

• 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.

Earlier idea on crustal structure

* The concepts of sial and sima are old and imprecisely describe current understanding of 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

The Obsolete Concept of Geosyncline

*What formerly called geosynclines are now known to be mountains that formed in active continental margins

*The term should not be used anymore.

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

What geologists know:

Any kind of rock can be converted into any kind in the rock cycle

Misconception: Rrocks follow a specific path of evolution in the rock cycle, transforming from igneous into sedimentary, and fromsedimentary into metamorphic rocks.

River system:

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: 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.

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

Thank You!chanls@hku.hk

Department of Earth SciencesUniversity of Hong Kong

www.hku.hk/science/temp/LSChan

speleothems found in Hong Kong

stalactite helictite drip cutain