Transcript Plate Tectonics

Plate Tectonics
The grand unifying theory of Geology

Plate tectonics controles



Distributions of geologic materials and
resources (e.g., Minerals, Energy, Water…)
Geologic Hazards (e.g., earthquakes,
volcanoes, landslides, tsunamis…)
Landscape features (e.g., mountain ranges,
oceanic trenches, continents, rift valleys…)
Formation of Earth
Birth of the Solar System
Nebular Theory
(pg. 24, Kehew)




Rotating nebula contracts
Begins to flatten and collapse
into center to form the sun.
Clusters of asteroids
coalesced to form
planetesimals and moons
(planetary accretion) around
4.6 billion years ago (bya)
(Meteorites are iron-rich or
rocky fragments left over from
planetary accretion)
http://www.psi.edu/projects/planets/planets.html
Orion Nebula
www.hubblesite.org
www.geol.umd.edu/~kaufman/
ppt/chapter4/sld002.htm
www.psi.edu/projects/
planets/planets.html
Formation of the Planets



The mass of the center
of the solar system
began nuclear fusion to
form the sun
The inner planets were
hotter and gas was
driven away leaving the
terrestrial planets
(Fe, O, Si, Mg…)
The outer planets were
cooler and more
massive so they
collected and retained
the gasses forming the
“Gas Giants”
Terrestrial Planets
Gas Giants
www.amnh.org/rose/backgrounds.html
Differentiation of the Planets


The relatively uniform
iron-rich proto planets
began to separate into
zones of different
composition: 4.5 bya
Heat from impacts,
pressure and
radioactive elements
cause iron (and other
heavier elements) to
melt and sink to the
center of the terrestrial
planets (Kehew Fig. 2-4)
Lab. Man., Fig. 1.7a: Zones of the earth’s interior
Further Differentiation
of Earth


Lighter elements such as
Oxygen, Silicon, and
Aluminum rose to form
a thin, rigid crust
The crust, which was
originally thin and
basaltic (iron rich
silicate), further
differentiated to form
continental crust which
is thicker, iron poor, silica
rich and lighter
Deepest
Mine
Deepest
Well
Continental
Crust
(Silicic)
Mid-Ocean
Ridge
(New Crust)
Oceanic
Crust
(Basalt)
Kehew Fig. 2.5
Composition of Earth and Crust
Chemical
Symbol
% of
% of Crust
Earth
(by Weight)
Change in
Crust Due to
Differentiation
Oxygen (8)
O
30
46.6
Increase
Silicon (14)
Si
15
27.7
Increase 
Aluminum (13)
Al
<1
8.1
Increase 
Iron (26)
Fe
35
5.0
Decrease 
Calcium (20)
Ca
<1
3.6
Increase 
Sodium (11)
Na
<1
2.8
Increase 
Potassium (19)
K
<1
2.6
Increase 
Magnesium (12)
Mg
10
2.1
Decrease 
~8
1.5
Element
(Atomic #)
All Others
Crust and Mantle
Lithosphere and Asthenosphere




The uppermost mantle and
crust are rigid, solid rock
(Lithosphere)
The rest of the mantle is soft
and solid (Asthenosphere)
The Continental Crust
“floats” on the uppermost
mantle
The denser, thinner
Oceanic Crust comprises
the ocean basins
Rocks and Sediment
Products of an Active Planet
Earth’s structure leads to
intense geologic activity




Inner core: Solid iron
Outer core: Liquid iron,
convecting (magnetic field)
Mantle (Asthenosphere) :
Solid iron-magnesium
silicate, plastic, convecting
Crust (Lithosphere): Rigid, thin

O, Si, Al, Fe, Ca, Na, K, Mg…
47%, 28, 8,
5,
4,
3,
3, 2
Crust:
Rigid,
Thin
Mantle:
Plastic,
Convecting
Pangea
225 million years ago
135 mya
65 mya
Today
Evidence of Continental Drift

Glacial striations
match across
oceans
Kehew, Fig. 2.27
Evidence of Continental Drift

Matching rock
types and
mountain ranges
Kehew, Fig. 2.27
Evidence of Continental Drift

Fossils of land plants and animals
Evidence of Continental Drift

Magnetic Evidence

Reversals in Earth’s magnetic field
are recorded in newly formed rocks
Kehew, Fig. 2.7
Evidence of Continental Drift

Age of Earth’s Oceanic Crust
Kehew, Fig. 2.32
Lithospheric Plates
See Kehew, Figure 1.19



The Lithosphere is broken into “plates” (7 maj., 6 or 7 min.)
Plates that “ride around” on the flowing Asthenosphere
Carrying the continents and causing continental drift
Lithospheric Plates
Kehew, Fig 2.24
Three Types
of Plate
Boundaries

Divergent
|

Convergent
|

Transform
e.g., Pacific NW
See Kehew, Fig. 2.38
Divergent
Plate Boundaries



Where plates move away from each other the
iron-rich, silica-poor mantle partially melts and
Extrudes on to
the ocean floor
or continental
Lithosphere
Lithosphere
crust
Simplified
Asthenosphere
Cool and
solidify to form
Basalt: Iron-Rich, Silica-Poor, Dense Dark,
Fine-grained, Igneous Rock
Block
Diagram
Characteristics of
Divergent Plate Boundaries

Divergent Plate Boundary





Stress
Earthquakes
Volcanism
Rocks
Features
Lithosphere
New Oceanic Crust
Forming at Mid-Ocean Ridge
Shallow
Earthquakes
Fissure
Eruptions
Crust
Magma
Generation
Asthenosphere
See Kehew, Fig. 2.29
Characteristics of
Divergent Plate Boundaries

Divergent Plate Boundary





Stress: Tensional  extensional strain
Volcanism: non-explosive, fissure eruptions,
basalt floods
Earthquakes: Shallow, weak
Rocks: Basalt
Features: Ridge, rift, fissures
Magma
Generation
Crust
Locations of Divergent Plate Boundaries
Mid-Ocean Ridges
(Mid-Arctic Ridge)
East Pacific Rise
 Mid Atlantic Ridge
 Mid Indian Ridge
 Mid Arctic Ridge
Fig. 1.10

See Kehew, Figure 2.24
Divergent Plate Boundaries
Rifting and generation of shallow earthquakes (<33km)
0
30
70
0
33
70
150
150
300
300
500
500
800
Depth
(km)
E.g., Red Sea and
East African Rift Valleys


Thinning crust, basalt
floods, long lakes
Fig. 19.21
Fig. 19.22
Rift
Valley
Shallow
Earthquakes
Linear sea, uplifted
and faulted margins
Oceanic Crust
Rift
Valley
Passive continental
shelf and rise
See Kehew, Figure 2.33
Convergent
Plate Boundaries


Where plates move toward each other, oceanic
crust and the underlying lithosphere is subducted
beneath the other plate (with either oceanic crust or
continental crust)
Wet crust is partially melted to form silicic (Silicarich, iron-poor, i.e., granitic) magma





Stress: Compression
Oceanic Trench
Earthquakes
Volcanic Arc
Plate Movement
Volcanism
Lithosphere
Lithosphere
Rocks
Magma
Subducted
Generation
Simplified
Features
Plate
Asthenosphere
Shallow and Deep Block
Diagram
Earthquakes
Convergent Plate Boundary
e.g., Pacific Northwest

Volcanic Activity



Strong Earthquakes



Explosive, Composite
Volcanoes (e.g., Mt. St. Helens)
Arc-shaped mountain ranges
Shallow near trench
Shallow and Deep over
subduction zone
Rocks Formed
 Granite (or Silicic)
Iron-poor, Silica-rich
 Less dense, light colored
Usually intrusive: Cooled slowly, deep down, to form large crystals and
course grained rock


The “Ring of Fire” (e.g., current volcanic activity)
A ring of convergent plate boundaries on the Pacific Rim









New Zealand
Tonga/Samoa
Philippines
Japanese Isls.
Aleutian Island arc
and Trench
Cascade Range
Sierra Madre
Andes Mtns.
Fujiyama
Pinatubo
Also: Himalayans
to the Alps
Composite Volcanic Arcs (Granitic, Explosive)
Basaltic Volcanism (Non-Explosive)
Depth of Earthquakes
at convergent plate boundaries
Seismicity of the Pacific Rim 1975-1995
0
33
70
150


Shallow quakes at
the oceanic trench
(<33km)
Deep quakes over
the subduction zone
(>70 km)
300
500
800
Depth
(km)
Major Plates and Boundaries


See Kehew, Figure 2.24
Each major plate caries a continent except the Pacific Plate.
Each ocean has a mid-ocean ridge including the Arctic Ocean.


Divergent bounds beneath E. Africa, gulf of California
The Pacific Ocean is surrounded by convergent boundaries.

Also Himalayans to the Apls
Divergent Plate Boundaries
Rifting and Formation of new Basiltic Oceanic Crust
Oceanic Crust*






Thin (<10 km)
Young (<200my)
Iron Rich (>5%) /
Silica Poor (~50%)
Dense (~ 3 g/cm3)
Low lying (5-11 km
Iceland
Etna
Visuvius
Kilimanjaro
deep)
Formed at Divergent Plate
Boundaries
*Make a “Comparison
Table” on a separate page
Composite Volcanic Arcs (explosive)
Basaltic Volcanism (non-explosive)
Convergent Plate Boundaries
Formation of Granitic Continental Crust
Oceanic Crust






Continental Crust
Thin (<10 km)
Young (<200 my)
Iron Rich (~5%) /
Silica Poor (~50%)
Dense (s.g. ~3 x H2O)
Low lying (5-11 km deep)
Formed at Divergent Plate
Boundaries






Thick (10-50 km)
Old (>200 m.y. and up to 3.5 b.y.)
Iron Poor (<1%) /
Silica Rich (>70%)
Less Dense (~ 2.5 g/cm3)
High Rising
(mostly above see level)
Formed at Convergent Plate
Boundaries
Isostatic Adjustment

Why do we see,
at the earths surface,







Intrusive igneous rocks and
Metamorphic rocks
Formed many km deep?
Thick, light continental crust
buoys up even while it erodes
Eventually, deep rocks are
exposed at the earth’s surface
Minerals not in equilibrium
weathered (transformed) to clay
Sediments are formed
The Hydrologic Cycle
Works with
Plate-Tectonics to

Shape the land

Weathering
clay, silt, sand…




Erosion
Transport
Sedimentation
Geologic
Materials


Sediments
Sedimentary
Rocks
The 3 rock types form at
convergent plate boundaries



Igneous Rocks: When rocks
melt, Magma is formed, rises,
cools and crystallizes.
Sedimentary Rocks: All rocks
weather and erode to form
sediments (e.g., gravel, sand,
silt, and clay). When these
sediments accumulate they are
compressed and cemented
(lithified)
Metamorphic Rocks: When
rocks are compressed and
heated but not melted their
minerals re-equilibrate
(metamorphose) to minerals
stable at higher temperatures and
pressures
See Kehew, Figure 2.34
The
Rock
Cycle
See Kehew, Fig. 2.53
Igneous and Sedimentary Rocks
at Divergent Boundaries and
Passive Margins



Igneous Rocks (basalt)
are formed at divergent
plate boundaries and
Mantle Hot Spots. New
basaltic, oceanic crust is
generated at divergent
plate boundaries.
Sedimentary Rocks are
formed along active and
passive continental
margins from sediments
shed from continents
Sedimentary Rocks are formed on continents where a basin forms
and sediments accumulate to great thicknesses. E.g., adjacent to
mountain ranges and within rift valleys.
See Kehew, Figure 2.30
“Continental Accretion”
How continents are built

The Ancestral Atlantic
Ocean looked like
today’s Pacific




Island Arcs
Oceanic Trenches
Bounding Continents
~500 mya
Convergent
Boundaries


Cause new terrains to
collide and
be accreted to the old
continental Cratons
~400 mya
“Continental Accretion”
How continents are built

Mountains are built during
accretion




Rocks are folded (bent) and
faulted (broken and shifted)
Volcanoes continue to form
Rocks are metamorphosed in the
Cores Mountains
Weathering and Erosion of
Mountains



Sediments are shed and
Lithified to produce
A venire of Sedimentary rocks
~350 mya
~300 mya
~250 mya
Rock Types of Continents
Rock Types of Continents

Metamorphic




Formed by intense
pressure and heat
Deep within
mountain cores
Exposed by isostacy
and erosion
Igneous


Magma intruded
into cores of
mountains
Lava extruded at
volcanoes

Sedimentary


Weathered and
eroded mountains
shed sediments
Covering the
continental interior
with a venire of
sedimentary rocks
Rock Types of Continents
B
A
A
B
Virginia / Penn.
Ohio
Michigan
Canada
Deciphering the Geology of Ohio
Using Steno’s Principles
Sandstone
Shale
Limestone

By characterizing the
sequence of sedimentary
rocks found in Ohio, we can
decipher the geologic
history preserved in the
rocks using the basic
principles of geology
Deciphering the Geology of Ohio
Using Steno’s Principles (~1650s)




Uniformitarianism
Original Horizontality
Original Continuity
Superposition
Sandstone
Shale
Limestone
Sedimentary Rocks of Ohio
Demonstrate the Use of Steno’s principles

Generalized sequence of rocks and ages in
millions of years
Sandstone
Shale
Limestone
350
380
450




Principle of Uniformitarianism
Principle of Original Horizontality
Principle of Original Continuity
Principle of Superposition
Sedimentary Rocks of Ohio

Uplift during the Tertiary period (26 mya)
Erosion
Sandstone
Shale
Limestone
350
380
450
Regional Uplift
Sedimentary Rocks of Ohio

Exposed older rocks in central and western Ohio
Erosion
Sandstone
Shale
Limestone
350
380
450
Regional Uplift
Sedimentary Rocks of Ohio

Forming the Findley Arch (with east flank in eastern Ohio)
Erosion
Sandstone
Shale
Limestone
350
380
450
Regional Uplift
Sedimentary Rocks of Ohio

And the pattern of rocks found across Ohio
Erosion
Sandstone
Shale
Limestone
350
380
450
Regional Uplift
Sedimentary Rocks of Ohio

The oldest rocks are found in southwestern Ohio
(along the axis of the Findley Arch)
Erosion
Sandstone
Shale
Limestone
350
380
450
Regional Uplift
Sedimentary Rocks of Ohio
Sandstone
Shale
Limestone
350
380
450
Sedimentary Rocks of Ohio
Sandstone
Shale
350
380
Limestone
450
Sedimentary Rocks of Ohio
Sandstone
Shale
Limestone
350
380
450
Sedimentary Rocks of Ohio
Sandstone
350
380
Limestone
450
Shale
Sedimentary Rocks of Ohio
Sandstone
Sandstone
350
380
Limestone
450
Shale
Shale
Limestone
Sedimentary Rocks of Ohio
Sandstone
Sandstone
350
380
Limestone
450
Shale
Shale
Limestone
Sedimentary Rocks of Ohio
Sandstone Shale Limestone
(325 my)
Thus rocks are
younger and change
lithology (rock type)
as you go west or
east from Ottawa
County
(400 my)
The Geologic Record in the Rocks
Sandstone
Shale
Limestone
Gneiss
Granite
Relative Age and the “Principles”



Uniformitarianism
Superposition
Original horizontality



Lateral continuity
Cross cutting
relationships
Inclusions
Sandstone
Shale
Limestone
Gneiss
Granite Gabbro
See Figure 8.1 – 8.12
1. Regional Uplift,
Tilting, or folding)
causes Erosion
2. Erosion surface
indicates gap in
geologic record
Formation of
Unconformities
Sandstone 350
Shale 380
Limestone
450
Gneiss (1,500)
Granite (280)
Gabbro (790)
240
million years ago
Formation of an
Angular Unconformity
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
Sedimentation (e.g., clay)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
220
million years ago
Formation of an
Angular Unconformity
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
Sedimentation (e.g., lime mud)
Shale (220)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
210
million years ago
Formation of an
Angular Unconformity
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
Sedimentation (e.g., quartz sand)
Limestone
(210)
Shale (220)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
200
million years ago
Formation of an
Angular Unconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
Sedimentation (e.g., immature sand)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
190
million years ago
Formation of an
Angular Unconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
Arkose
(190)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
180
million years ago
Formation of an
Angular Unconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
Arkose
(190)
1. Regional Uplift,
Tilting (or folding),
Erosion
2. Erosion surface,
gap in geologic
record
3. Continuous
Sedimentation
4. Sedimentation
ceases
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
170
million years ago
1. Erosion of horizontal beds
Formation of a
Disconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
Arkose
(190)
Erosion
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
160
million years ago
Formation of an
Disconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
Arkose
(190)
1. Erosion of horizontal beds
2. Loss of geologic record
(i.e., Arkose)
3. Formation of a horizontal
erosion surface
Erosion
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
150
million years ago
Formation of an
Disconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
1. Erosion of horizontal beds
2. Loss of geologic record
(i.e., Arkose)
3. Formation of a horizontal
erosion surface
4. Renewed Sedimentation
Arkose
Sedimentation (e.g., reef)
(190)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
140
million years ago
Formation of an
Disconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
1. Erosion of horizontal beds
2. Loss of geologic record
(i.e., Arkose)
3. Formation of a horizontal
erosion surface
4. Renewed Sedimentation
Arkose
(190)
Limestone (140)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (280)
Gabbro (790)
130
million years ago
Formation of an
Disconformity
Quartz Sandstone
(200)
Limestone
(210)
Shale (220)
1. Erosion of horizontal beds
2. Loss of geologic record
(i.e., Arkose)
3. Formation of a horizontal
erosion surface
4. Renewed Sedimentation
Arkose
(190)
Limestone (140)
Sandstone 350
Shale 380
Limestone 450
Gneiss (1,500)
Granite (290)
Gabbro (790)
120
million years ago
Summary: Types of
Unconformities

Deciphering Relative
Ages


Limestone
Quartz
Sandstone
Limestone
Shale
Sandstone
Shale
Limestone
Principles give
sequences of geologic
events
Unconformities
indicate gaps in the
geologic record
Disconformity
Angular
Unconformity
Gneiss
Granite
Gabbro
Nonconformities
The Grand Staircase

Correlation



Physical Continuity
Similar Rock Types
Fossils (index and assemblage)