Plate Tectonics Theory:

Basic Concepts & Some Common Misconceptions

Chan Lung SangDepartment of Earth Sciences

Faculty of Science, HKU

Refresher Programme for Geography Teachers

地殼(固態，大陸地殼為花崗質，海洋地殼為玄武質 )

地幔(固態，超基質 )

地核(外核液態，硫化鐵質。內核固態，純鐵質。 )

Landform and Endogenetic Processes

Part 1: Plate Tectonics• 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]

1. From Continental Drift to Plate Tectonics

Alfred Wegener

• Paleoclimate• Fossil evidence• Geological evidence• Coastline geometry

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

Plate Boundaries, Earthquakes and Major Volcanoes of the World

* Note new designation of plate boundary at Japan.

Plate Boundaries

* Note associated landforms: axial rift at mid-ocean ridge, sea trench at subduction zone, sea mounts and island arc.

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

2. Earth’s Interior Structure

Studying Earth’s Interior

• Earthquake waves• Whole-earth free oscillation • Rock records• Meteorite compositions• Geomagnetism• Heat flow & geothermal gradient• Lab experimentation• Numerical modeling

Earth scientists use the following techniques to determine Earth’s interior structure

Earth’s Abundance

• 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

Shell Name Depth (km) Composition State

A Crust 0-30 Al-rich silicate solid

BNoncrustal

Lithosphere Upper

Mantle

30-100Mg-rich silicate

(olivine)

solid

C Asthenosphere 100-640 near melting

D’ Lower Mantle 640-2800 solid

D” Mantle-Core Transition 2800-2890 soild (softer)

E Outer Core 2890-4600 FeS+Fe liquid

F OC-IC Transition 4600-5160Fe+FeO

liquid+solid

G Inner Core 5160-6370 solid

* Mantle is not liquid; convection occurs due to material rheology.

Earlier idea on crustal structure

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

3. Crustal Provinces & Structures

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

Growth of North America Continent

板塊俯衝形成火山弧

弧後地區形成摺曲山脈

大陸板塊碰撞形成巨型造山帶

Most folded mountain belts are continental suture zones

Structures on smaller scale

Overturned syncline

Graben structure

Tension gashes

Plunging S-fold

4. Rock Formations

Rock Cycle in Plate Tectonics Context

Igneous & Metamorphic Rocks

* Most magmas form from melting of mantle and crustal material in the overriding plate and not the descending plate.

Ophiolite: suite of oceanic crust and mantle rocks

Pillow basalt intruded by mafic dikes, Cyprus

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.

5. The Energy Source

Earth’s Source of Energy

• Primordial heat

• Radioactivity

• Gravitational heat

• Phase changes

Outer Core-Inner Core Relation

Outer core condenses gradually to become part of inner core. The process probably involves also a compositional change.

* Phase change, not radioactivity, is the primary energy source for plate motion.

Mantle Convectionheat 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

ACTIVE VS PASSIVE MARGINACTIVEMARGIN

• magmatic activity

• mountain belt with thick continental crust

• narrow continental shelf

• active seismicity

• mineral deposit

PASSIVEMARGIN

• Stable

• Wide continental shelf

• Continental crust thins out gradually

• Oil and gas resources

7. Tectonic Evolution of Hong Kong Region

Geological Map of Hong Kong (Geological Survey, GEO)

Department of Earth Sciences, HKU http://www.hku.hk/earthsci

End of Part 1chanls@hku.hkDepartment of Earth Sciences,

University of Hong Kong

Useful Web Links

pubs.usgs.gov/publications/text/dynamic.html

geology.er.usgs.gov/eastern/tectonic.html

www.school.discovery.com

www.geolsoc.com.hk