The Role of Plate Tectonics in Earth Sciences
Dirk Spengler
Department of Geology and Mineralogy
Kyoto University
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The Role of Plate Tectonics in Earth Sciences
Outline
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What are Earth Sciences?
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What is Plate Tectonics?
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Ocean Floor and Continents Under Pressure
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’Listen’ to the Rocks
The Role of Plate Tectonics in Earth Sciences
What are Earth Sciences?
Earth Sciences ...
... are all type of science
related to the planet Earth
... investigate the Earth’s:
Structure
Composition
Deformation processes
Climate
Life
Environmental
preservation studies
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The Role of Plate Tectonics in Earth Sciences
What are Earth Sciences?
Earth Sciences ...
Disciplines
Geography
(surface processes,
human interactions)
Geology
(rocks)
Geophysics
(Earth’s interior)
Geodesy
(size, shape, position)
Usage of
Physics
Chemistry
Biology
Mathematics
Chronology (dating)
To understand the Earth’s
Past
Present
Future
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The Role of Plate Tectonics in Earth Sciences
What are Earth Sciences?
Earth’s Evolution
Sun
?
Solar nebular Proto planets
Future (0.5 Ga?)
4.54 Ga ago
(4.540.000.000)
Today
Solar nebular→planets
Early Earth = hot,
homogeneous
No tectonic plates
Earth = cooled
surface, chemically &
physically layered
Active plate tectonics
Takahashi, 1990
Earth = frozen
End of plate tectonics
Continuous heat loss
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Historical View of the Earth (Fixism)
Egyption idea
(∼5000 years ago)
Greek idea
(∼2200 years ago)
Modern proof
(1519-1522)
Plate shape
Sphere shape
Ferdinand Magellan
Continents are fixed
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Continental Drift Theory (Mobilism)
Continental shape
Antonio Snider-Pellegrini (1858)
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Continental Drift Theory (Mobilism)
Flora & Fauna distribution
Alfred Wegener (1915)
Pangaea (’Old Land’)
Distribution of fossils
across southern Pangaea
Driving force for continental drift?
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Ocean Floor Mapping
(1950’s and 1960’s)
World ocean floor
New findings
Mid ocean ridges
Magmatism
Magnetic anomalies
Sediment thickness
varies systematically
Deep trenches along
some continents
Renewal of the
improbable concept of
continental drift
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Seafloor Spreading
Formation of Ocean Floor and Passive Continental Margins
Hess and Dietz (1960/62)
Theory
Vine and Matthews (1963)
Proof of Theory
Pattern of
magnetic
stripes at midAtlantic ridge
Rising magma forms new ocean floor = Proof of continental drift
→ Global volume problem: where does the ocean floor go?
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Inclined Plane of Seismicity
Kiyoo Wadati (1935)
Wadati-Benioff-Zone
Plunging of Ocean Floor = Subduction Zone (Active Plate Margin)
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Plate Tectonic Model
= Continental Drift + Seafloor Spreading + Subduction Zones
Global seismic activity
Tectonic plates
The Earth outer shell is broken into large plates.
Size and position of these plates change over time.
Earthquakes and volcanoes help locating plate boundaries.
Divergent plate margins form plates, convergent plate
margins destroy plates (constant surface area of the
Earth).
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The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Plate Tectonic Model
Mantle convection
Synthesis (End of 1960’s)
Driving mechanisms:
3 major types of plate boundaries
(transform, divergent, convergent)
Widely accepted since 1970’s
Does not explain driving mechanism
→ Mantle convection model
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High density of
cooled ocean
floor (slab pull)
Low density of
hot soft mantle
(asthenosphere)
The Role of Plate Tectonics in Earth Sciences
What is Plate Tectonics?
Consequences of Plate Tectonics
Ocean floor age
Supercontinents
≤180 million years
Pangaea 300 – 180 Ma
Rodinia 1100 – 750 Ma
Columbia 1800 – 1500 Ma
Periodical opening and closing of ocean basins (Wilson Cycle)
Ocean floor is recycled, continents are stable
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The Role of Plate Tectonics in Earth Sciences
Ocean Floor and Continents Under Pressure
Convergent Plate Boundaries
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Ocean–Ocean
Ocean–Continent
Continent–Continent
(Japan)
(Andes)
(Himalaya)
Subduction of (heavy) oceanic plate
Subduction of (light)
continental plate
What happens with ocean floor
under pressure?
What’s about continents
under pressure?
The Role of Plate Tectonics in Earth Sciences
Ocean Floor and Continents Under Pressure
Subduction of Oceanic Crust
Seafloor composition
0.5 km
Sediments
1.5 km
Pillow lava (Basalt) ➀
5.0 km
Gabbro ➁
Seafloor magmatic rocks
Crystallization from a melt
Lava at seafloor
Lava at land surface
3 minerals:
feldspar (white)
pyroxene (black)
olivine (green)
Regular distributed
Gabbro
Every rock consists of minerals.
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The Role of Plate Tectonics in Earth Sciences
Ocean Floor and Continents Under Pressure
Subduction of Oceanic Crust
Metamorphic rocks
Metamorphic facies
Change in minerals and/or texture
➀➁
(Photo: B. Hacker)
Blueschist ➂
glaucophane (blue)
feldspar (white)
garnet (red)
garnet (red)
omphacite (green)
zoisite, quartz (white)
Subduction transforms magmatic
into metamorphic rocks
Stability of minerals depends on
P & T → Mineral assemblage
tells the history of a rock
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➂
➃ Eclogite
➃
Groups of minerals that occur together
➀➁
➂
➃
The Role of Plate Tectonics in Earth Sciences
Ocean Floor and Continents Under Pressure
Subduction of Continental Crust
Continent–Continent
Known
Sampling of mantle rocks
Continental and oceanic lithosphere
break apart
Subduction of light continental crust is
temporarily → exhumation
0
km
➅
➄
400
Less known
Max depth of continental subduction
Timing
3 possible scenarios for
subduction / exhumation
Suitable study objects
High grade metamorphic gneiss ➄
Enclosed mantle fragments ➅
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The Role of Plate Tectonics in Earth Sciences
Ocean Floor and Continents Under Pressure
Subduction of Continental Crust
High grade metamorphic rocks
Index minerals
20 µm
Mineral microstructures
Since 1984:
Since 1998:
Coesite
(>70 km)
Majoritic
garnet
(>180 km,
f (T))
Massonne (2001)
Since 1990:
Diamond
(>120 km)
10 µm
Korzakov & Hermannn (2006)
Today: subduction / exhumation probably max. 200 km depth
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The Role of Plate Tectonics in Earth Sciences
’Listen’ to the Rocks
How do we get to know all this?
As a Geologist
Motivation: without interest nothing gets discovered
Observation
In the field (excursions)
In the laboratory (analysis of natural / artificial rocks)
Comparing data of natural and experimental rocks
Discussions: different people have different ideas
Testing ideas
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The Role of Plate Tectonics in Earth Sciences
’Listen’ to the Rocks
How does it look in practise?
Geological Field Work
British GS mapping in Scotland (1902)
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MSc Student maps eclogite, Norway (2002)
The Role of Plate Tectonics in Earth Sciences
’Listen’ to the Rocks
How does it look in practise?
Geological Field Work
Taking samples
Taking directions
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Taking scale
Taking notes
The Role of Plate Tectonics in Earth Sciences
’Listen’ to the Rocks
How does it look in practise?
Analyses
Light microscopy
Electron microscopy
Laser ablation
ICP-MS
Relationship between
minerals
Crystal orientations
Mineral trace elements
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The Role of Plate Tectonics in Earth Sciences
’Listen’ to the Rocks
Thanks for listening!
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The Role of Plate Tectonics in Earth Sciences