Transcript Présentation PowerPoint

Refresher Programme for Geography Teachers
Plate Tectonics Theory:
Basic Concepts & Some
Common Misconceptions
Chan Lung Sang
Department of Earth Sciences
Faculty of Science, HKU
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
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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
Earth scientists use the following techniques to determine
Earth’s interior structure
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Earthquake waves
Whole-earth free oscillation
Rock records
Meteorite compositions
Geomagnetism
Heat flow & geothermal gradient
Lab experimentation
Numerical modeling
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
* Mantle is not liquid; convection occurs due to material rheology.
Earth’s Interior Model
Shell
Name
Depth (km)
Composition
State
A
Crust
0-30
Al-rich silicate
solid
B
C
Noncrustal
Lithosphere
Asthenosphere
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
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
*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
5. The Energy Source
Earth’s Source of Energy
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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
ACTIVE VS PASSIVE MARGIN
ACTIVE
MARGIN
PASSIVE
MARGIN
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Stable
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mountain belt
with thick
continental
crust
Wide continental
shelf
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Continental
crust thins out
gradually
narrow
continental shelf
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Oil and gas
resources
active seismicity
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mineral deposit
•
magmatic
activity
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 1
[email protected]
Department 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