plate tectonics theory: basic concepts & some common misconceptions chan lung sang department of...
TRANSCRIPT
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Plate Tectonics Theory:
Basic Concepts & Some Common Misconceptions
Chan Lung SangDepartment of Earth Sciences
Faculty of Science, HKU
Refresher Programme for Geography Teachers
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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
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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]
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1. From Continental Drift to Plate Tectonics
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Alfred Wegener
• Paleoclimate• Fossil evidence• Geological evidence• Coastline geometry
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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
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Plate Boundaries, Earthquakes and Major Volcanoes of the World
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* Note new designation of plate boundary at Japan.
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Plate Boundaries
* Note associated landforms: axial rift at mid-ocean ridge, sea trench at subduction zone, sea mounts and island arc.
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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
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2. Earth’s Interior Structure
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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
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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
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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.
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Earlier idea on crustal structure
* The concepts of sial and sima are old and imprecisely describe current understanding of crustal structure.
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*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
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3. Crustal Provinces & Structures
Crustal provinces and structures at different scales are results of tectonic evolution and associated stress system
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Growth of North America Continent
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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
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Most folded mountain belts are continental suture zones
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Structures on smaller scale
Overturned syncline
Graben structure
Tension gashes
Plunging S-fold
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4. Rock Formations
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Rock Cycle in Plate Tectonics Context
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Igneous & Metamorphic Rocks
* Most magmas form from melting of mantle and crustal material in the overriding plate and not the descending plate.
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Ophiolite: suite of oceanic crust and mantle rocks
Pillow basalt intruded by mafic dikes, Cyprus
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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.
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5. The Energy Source
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Earth’s Source of Energy
• Primordial heat
• Radioactivity
• Gravitational heat
• Phase changes
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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.
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Mantle Convectionheat energy from core
地幔對流熱源來自地核
* Not all spreading ridges are located above mantle divergence.
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6. Mineral and oil Formations
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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
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7. Tectonic Evolution of Hong Kong Region
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Geological Map of Hong Kong (Geological Survey, GEO)
Department of Earth Sciences, HKU http://www.hku.hk/earthsci
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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