Transcript Physical Geology

Learning Plate Tectonic Geography
Brushing up on basic geography will help you learn Plate Tectonics
Once you know your basic geography (continents and major
mountain ranges) and ocean basin features (Mid Ocean Ridges,
Oceanic Trenches) you can
- Learn the 7 major plates
- Learn the types of plate boundaries
- Learn why those features are where they are
I. Introduction (cont.)
• E. Forces shaping the
Earth at the surface
and from within
– 1. Surficial Processes
Solar energy and gravity
shaping the landscape
– 2. Internal Processes
Internal energy and
forces that buckle and
break Earth’s crust
If External Processes Only?
Mississippi River Delta
• Question:
– If 550 million tons of rock
are broken down and
transported to the sea from
the United States each
year,
– Why has our continent not
been worn flat after the
billions of years of its
existence and
– Why haven’t the oceans
been filled in?
Mississippi River
Drainage Basin
Erosion, transport,
deposition
Mississippi
River Delta
1. Surficial Processes
Results of the “External Heat Engine”
• Weathering
– Chemical and
Mechanical
Breakdown of solid
rock into sediments
• Erosion
– Removal of rock and
sediment from source
by Gravity, wind,
water, ice
Surficial Processes (cont.)
Results of the “External Heat Engine”
• Transport over large
distances by water,
wind and ice.
Surficial Processes (cont.)
Results of the “External Heat Engine”
• Deposition of large
amounts of sediment
in seas and oceans
2. Internal Earth Processes
Results of the “Internal Heat Engine”
• Evidence of internal
energy and forces
working on our
earth.
–
–
–
–
Intricate landscapes
Volcanoes
Earthquakes
Geothermal
Gradients (deeper is
hotter)
Another Question: What is the source of all this energy?
3. Formation of Earth
Birth of the Solar System
Nebular Theory
– nebula compresses
– Flattening of spinning nebula
and collapse into center to form
sun
– Condensation to form planets,
planetesimal, moons and
asteroids during planetary
accretion around 4½ billion
years ago
– (Meteorites are iron-rich and
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 ignite the sun
• The inner planets were
hotter and gas was
driven away leaving the
terrestrial planets
• The outer planets were
cooler and more
massive so they
collected and retained
the gasses hence the
“Gas Giants”
Terrestrial Planets
Gas Giants
www.amnh.org/rose/backgrounds.html
Differentiation of the Planets
• The relatively uniform ironrich proto planets began to
separate into zones of
different composition:
4.6mya
• Heat from impacts,
pressure and radioactive
elements cause iron (and
other heavier elements) to
melt and sink to the center
of the terrestrial planets
The zones of the earth’s interior
Further Differentiation of Earth
• Lighter elements such as
Oxygen, Silicon, and
Aluminum rose to form a
crust
• The crust, which was
originally thin and heavy
(iron rich silicate) Like
today’s Oceanic crust,
• Further differentiated to
form continental crust
which was thicker, iron
poor and lighter
Figure 1.7, the zones of the earth’s interior
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
but solid (Asthenosphere)
• The Continental Crust
“floats” on the uppermost
mantle
• The denser, thinner
Oceanic Crust comprises
the ocean basins
Figure 1.7, Detail of crust and Mantle
Lithospheric
Plates
• The Lithosphere is broken into “plates”
(7 major, 6 or 7 minor, many tiny)
• Plates that “ride around” on the flowing
Asthenosphere
• Carrying the continents and causing
continental drift
Plates Shown by Physiography
Types of Plate Boundaries
• -Convergent
-Divergent
-Transform
Lithospheric Plates and Boundary types
Three Types
of Plate
Boundaries
• Divergent
|
• Convergent
|
• Transform
e.g., Pacific NW
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: 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
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)
Fig. 2-16
Pg. 41
E.g., Red Sea and
East African Rift Valleys
Fig. 2-15
Pg. 40
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
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
–
–
–
–
–
Fig. 2-17
Pg. 42
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
– Explosive, Composite
Volcanoes (e.g., Mt. St. Helens)
– Arc-shaped mountain ranges
• Strong Earthquakes
– Shallow near trench
– Shallow and Deep over
subduction zone
• Rocks Formed
– Granite (or Silicic)
Fig. 2-18
Pg. 42
• 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
• Shallow quakes at
the oceanic trench
(<33km)
• Deep quakes over
the subduction zone
(>70 km)
0
33
70
150
300
500
800
Depth
(km)
Major Plates and Boundaries
•
•
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
Transform
Plate Boundaries
• Offset Midocean ridges
• May cut
continents
– e.g. San
Andreas Fault
Fig. 2-21
Pg. 44
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
The
Rock
Cycle
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