Download Chapter 9 Notes III. Continental Tectonics I. Great ocean basins

Chapter 9
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Chapter 9 Notes III. Continental Tectonics
I. Great ocean basins, continents, mountains and plains appear to us to be everlasting.
A. Human life span is tar too short to permit a view of lands heaving and moving about
1. immense perspectives of the history of the Earth can he provided by historical geology
B. Evidence of the occurrence of upheavals
1. Layers of fossiliferous marine sediments that are now poised in mountain peaks
2. exposures of originally deep-seated rocks (Metamorphic)
3. incised deep canyons (changing base level - Colorado River)
C. Uplifts along with the many other movements of the Earth s crust. break and deform rocks
and produce geologic structures such as faults, folds, and unconformities
II. Tectonics - Plate Boundaries
A. Rifting - Divergent Boundaries
1. Mid ocean ridges are rifted or divergent boundaries where new oceanic crust is
produced
a. High heat flow makes ridges occupy a shallower position below sea level. Hot material
is more buoyant than cold material.
b. as plates move laterally, they cool and become more dense. Eventually, they subduct at
the trenches and sink into the mantle. (see below)
2. Passive Continental Margin (see figure 9-4)
a. East Coast of US - Atlantic Ocean is best example
1. basalts
2. lake beds and volcanism
3. evaporites
4. Normal (gravity) Faulting - A break in the Earth's crustal rocks along which there
has been movement
a. the mass of rock that lies above the shear plane called the hanging wall appears to
move downward relative to the opposite side or foot wall
b. rocks are subjected to tensional forces and tend to stretch the crust
3.Three -Armed rifts
a. Afar Triangle: Red Sea-Gulf of Aden-East African Rift (Fig. 9-2,3)
b. Aulocogen - Failed arm - Important because it provides evidence for rifting
1. basalts
2. lake beds and volcanism
3. evaporites
4. normal faulting (see below)
5. but no ocean crust
6. you are sitting within an aulocogen - Newark Basin tried to open with the rifting of
No. America for NW Africa. we observed 1-4 her in central Jersy
B. Convergent Boundaries
1. Deep see trenches are convergent boundaries where old oceanic crust is subducted
under either continental or oceanic crust
a. Characteristics of trench environments
1. Reverse (also thrust) faults. Small thrusts occur just above the trench in the
accretionary prism.
a. exhibit a hanging wall that has moved up relative to the foot wall
b. rocks are subjected to compressional forces most prevalent in the world's great
mountain ranges
2. Earthquakes -most powerful and numerous are at convergent boundaries in general.
a. occur along plane of the descending crustal slab.
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b.Earthquake pattern shows that shallow earthquakes are proximal to the trench and the
deepest earthquakes occur further away from tranch - They form an angle that
approximates 45° - This is the Benioff Zone - evidence for plate movement
3. Volcanism - Volcanic arcs - Ring of Fire around the Pacific
a. Volcanoes Erupt to release the pressure built up in the magma chamber below the
volcano. The eruption point is the Vent, where the magma breaks the surface.
b. the siliceous and carbonate sediment are volatile and melt as the basaltic slab
subducts. This molten, buoyant material will make its way to the surface.
c. Of all volcanoes:
80% - at Convergent boundaries
15% - at Divergent bounddaries
5% - Intra Plate - Hot Spots, Yellowstone, Long Valley, Hawaii to name some US
d. Quiet vs. Explosive
i. This is controlled by the composition and temperature of the magma. The
temperature, Silica content, and water content determine the viscosity of the magma
and hence the explosivity of the eruption. A synonym for Viscosity is stiffness.
a. Higher temperature = lower viscosity. Think of syrup.
b. Higer SiO2 (silica) = higher viscosity; stiffer
c. Higher water = water lowers the melting point meaning that the magma boils at a
lower temperature and is cooler and has a higher viscosity.
ii. Increased viscosity increases the explosiveness of the eruption. Think of
boiling down syrup. As it gets thicker and thicker, what happens to the bubbles
coming out of the syrup, they burst/explode more violently.
iii. Eruptions release:
a) Lava - heat becomes sufficent to break the bonds of the minerals. Rocks are
composed of different minerals which have different melting points. Therefore,
the magma is a slurry of molten rock, dissolved gases, and mineral crystals.
b) Tephra - Explosive ejection of pyroclastic material - Pyro - fire + clastic - broken
a) airborn ash that reaches the stratosphere
c) pyroclastic flows - very dangerous mixture of hot gases and tephra. These are
heavier than air and flow along the ground at speeds up to 150 mph.
d) Volcanic Gases - Dissolved gases are an important component because they help
to determine the explosiveness. Higher the gas content the more explosive.
Common gases include:
i) water vapor - most abundant
ii) carbon dioxide
iii) Sulfur dioxide
iv) carbon monoxide
vi) hydrogen sulfide (rotten eggs)
e. The intermediate composition of the magma (Dioritic) leads to very Explosive
eruption at Convergent margion (Andesite)
2. Ocean-Continent collisions - The continental environments.
Convergent boundaries where old oceanic crust is subducted under continental - trench forms
at boundary - Landward of the Trench there are:
a. Forearc Basin - accumulation of marine sediment in fromnt of the trench
1.Accretionary Wedge - where marine sediments are scraped up (Thrust) - it becomes
very jumbled - Melange
b. Volcanic Arc - ediface built up from successive volcanism - Andesitic composition
c. Backarc or Foreland Basin 1. marine sedimentation called Flysch
2. eventually Foreland Basin fills and is uplifted - sedimentation is now non-marinecalled Molasse
C. Transform Boundaries
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1.boundaries movement past each other in the horizontal plane. There is little to no vertical
movement.
2 Fault type is Strike Slip or Later
a. Lateral (right and left) 1. lateral: can be further designated right lateral or left lateral if one looks across the fault
zone to see if the opposite block has moved to the right or left
2 San Andreas Fault is the most famous of these
a. Important geologic changes in Northern California - Volcanism in the
Cascades, but ends in CA, puts back up in Central America.
b. Reason is that there is no subduction along the San Andreas Fault to fuel the
volcanism
D. Folds: Bends in rock strata- direct evidence of crustal movement
1. Anticlines and synclines
a. anticlines: uparched rocks in which the oldest rocks are in the center and the youngest
rocks are on the flanks
b. synclines: downwardly folded rocks that have youngest beds in the center and oldest
rocks on the flanks
2. Domes and basins
a. domes: similar to anticlines except that they have elliptical to roughly circular outcrop
patterns
b. basins: similar to synclines except that they have elliptical to roughly circular outcrop
patterns
3. Monoclines
D. Formation of folds
1. compression
2. differential compaction
3. slumping of mounds of sediment at the time of its accumulation
4. upward protuberance of rock masses t'rom below
5. draping over uplitted fault blocks
6. crumpling as great blankets of strata slide over an older rock surface
III. Mountain Building - Two examples
A. Orogenesis - process of mountain building
1. long known that mountains tend to parallel coast line - zone of deformation
a. only the edges of the continents are deformed during collisions
2. Ural mountains an apparent exception - important in plate tectonic models
a. suturing - the joining of continents - Ural indicate the suturing of two continents
b. Sutures are found in mountain ranges
B. Andes - Central and South America range
1. Subduction of the Nazca Plate under So. America and Coccos Plate under Central
Amercia
2. Volcanism
3. Thick continental crust - 70 km
4. Amazon Basin was Foreland Basin 10 million years ago. (Flysch)
a. filled in and uplifted to be Amazon River (Molasse)
C. Himalayas - Collision of India and Asia
1. most impressive recent (last 100 myr) mountain building events
2. India separated from Antarctica during the Cretaceous.
3. Initial collision was ~40 Ma.
4. Some of the fore Arc material (marine sediments and coral reefs) and Basalts (Ophiolites)
were Obducted up onto the Asian Plate.
5. Up near the summit of Everest, one can find Palecene and Eocene limestones.
6. Still going up.