Plate Tectonics Theory:
Basic Concepts
Dr LS Chan
Department of Earth Sciences
Faculty of Science, HKU

Crust 地殼
( 固態，大陸地殼
為花崗質，海洋
地殼為玄武質 )
Mantle 地幔
( 固態，超基質 )
Core 地核
( 外核液態，硫化
鐵質。內核固態
，純鐵質。 )

• History of Development
• Basic Concepts
– Earth’s interior model
– Crustal provinces & structures
– Rock formation
– Energy sources
– Mineral and oil formation
• Tectonics and Geology of Hong Kong Region
[MANY OF THE FIGURES USED IN THIS PRESENTATION WERE
DOWNLOADED FROM THE INTERNET. THEIR CREATORS , OFTEN
ANONYMOUS, ARE GRATEFULLY ACKNOWLEDGED]

• Paleoclimate
• Fossil evidence
• Geological evidence
• Coastline geometry
Alfred Wegener

Birth of the Plate Tectonics Theory
Plate tectonics theory was founded upon the following new findings in the late 1960s:
• Ocean floor topography
• Marine magnetic anomalies
• Paleomagnetism & geomagnetic field reversals
• Sea floor spreading

Topography of
Earth’s Surface
Deep Sea Drilling Project Ocean Drilling Program

NORMAL
地球磁場源於
外核
REVERSED

Geomagnetic Field
Polarity Reversal
Apparent Polar
Wander Paths

Marine magnetic anomalies and sea floor spreading
Variation in total magnetic field intensity
Marine magnetic anomaly map of East Pacific

Plate Boundaries, Earthquakes and Major
Volcanoes of the World

Plate Boundaries

A paradigm shift - A unifying theory accounting for
• Earth’s interior structure
• Origin of atmosphere & hydosphere
• Crustal structures and provinces
• Volcanism & earthquakes
• Rock record
• Fossil record
• Sedimentary basins
• Petroleum and mineral formation
• Geophysical fields

Earth scientists use the following techniques to determine
Earth’s interior structure
• Earthquake waves
• Whole-earth free oscillation
• Rock records
• Meteorite compositions
• Geomagnetism
• Heat flow & geothermal gradient
• Lab experimentation
• Numerical modeling

• Four major elements: O, Fe, Si, Mg 85%
• Only 3% S
• Earth’s crust depleted in siderophile (Fe, Ni,
Cu etc.) but enriched in K and Al
• Mantle - Mg silicate
• Core – FeS, FeO and Fe

Earth’s Interior Model
B
C
D’
D”
E
F
G
Shell
A
Name Depth (km) Composition
Al-rich silicate Crust
Noncrustal
Lithosphere
Asthenosphere
Lower Mantle
Moho
Upper
Mantle
Mantle-Core Transition
Outer Core
OC-IC Transition
Inner Core
0-30
30-100
100-640
640-2800
2800-2890
2890-4600
4600-5160
5160-6370
Mg-rich silicate
(olivine)
FeS+Fe
Fe+FeO
State solid solid near melting solid soild (softer) liquid liquid+solid solid

Earlier idea on crustal structure

*Lithosphere includes the crust and part of the upper mantle.
Asthenosphere is not a molten layer.
*Note deepening of Moho beneath mountains due to isostasy.
* The distinction between lithosphere and asthenosphere is by means of physical strength, not composition

Crustal provinces and structures at different scales are results of tectonic evolution and associated stress system

Growth of North
America Continent

Three types of convergent boundary

板塊俯衝
形成火山弧
Subduction of oceanic plate and formation of magmatic arc
弧後地區
形成摺曲山脈
Folding in back-arc region
大陸板塊碰撞形成巨型
造山帶
Collision of continental plate to form folded mountain belt

Most folded mountain belts are continental suture zones

Continental Rifts
& Oceanic Crust

Jingxi Region,
Guangxi
50 km

Overturned syncline Tension gashes
Graben structure
Plunging S-fold

Rock Cycle in Plate Tectonics Context

Igneous & Metamorphic Rocks

Ophiolite: suite of oceanic crust and mantle rocks crust mantle
Pillow basalt intruded by mafic dikes, Cyprus

Earth’s Source of Energy
• Primordial heat
• Radioactivity
• Gravitational heat
• Phase changes

* Phase change, not radioactivity, is the primary energy source for plate motion.
Outer Core-Inner Core
Relation
Outer core condenses gradually to become part of inner core. The process probably involves also a compositional change.

Mantle
Convection heat energy from core
地幔對流
熱源來自地核
* Not all spreading ridges are located above mantle divergence.

6. Mineral and
Oil Formations

VS
ACTIVE
MARGIN
• magmatic activity
• mountain belt with thick continental crust
• narrow continental shelf
• active seismicity
• mineral deposit
PASSIVE
MARGIN
• Stable
• Wide continental shelf
• Continental crust thins out gradually
• Oil and gas resources

7. Tectonic Evolution of Hong Kong Region

Tibet
South China
India-
Australian
Plate
Sunda Block
Philippines
Sea Plate
Pacific
Plate

Ma gm ati c a rc
Tectonic Evolution of S China Coastal Province
Jurassic-Cretaceous northward plate subduction

Rift basins
Tectonic Evolution of S China Coastal Province
Tertiary N-S opening of South China Sea

Tectonic Evolution of S China Coastal Province
Recent collision of Philippine Sea Plate with Taiwan

Landsat imagery of Mirs Bay

Late Mesozoic Volcanic Calderas in Hong Kong

Geological Map of Hong Kong (Geological Survey, GEO)
Department of Earth Sciences, HKU http://www.hku.hk/earthsci

helictite drip cutain stalactite speleothems found in Hong Kong
Department of Earth Sciences,
University of Hong Kong

pubs.usgs.gov/publications/text/dynamic.html
geology.er.usgs.gov/eastern/tectonic.html
www.school.discovery.com
www.geolsoc.com.hk