Chapter 4: Earth's Structure and Plate Tectonics Flashcards
Stress is responsible for these three deformation processes. What are they?
- Compression
- Tension
- Shear
Definition of Deformation
a process of physically compressing rock sequences to cause them to slide up and over adjacent cratons (overthrusting) or to exert a compressing force that reduces the width of a mobile belt, producing steeply dipping, folded sequences by flattening and elongating the rocks sequence perpendicular to the stress.
What is compression?
pushes on rocks from opposite directions which
causes rocks to be shortened parallel to the stress applied.
What is Tension?
pulls rocks from opposite directions, resulting it to
become stretched/lengthened.
What is Shear?
occurs when rocks are being pushed in an uneven
manner, causing the rocks to be skewed such that different sides of a rock body slide or move in opposite directions.
True or False. Rocks on the surface of the Earth are elastic.
True
What is Elasticity?
The elastic force developed in an elastic object is directly proportion. As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force.
is the ability of a material to return to its original shape after being stretched or compressed.
When all elastic materials reach this point, deformation becomes permanent and they no longer behave elastically.
Elastic limit
This type of deformation is permanent by flowing plastically.
In physics, a plastic flow (sometimes written as plastic flows) refers to any fluid flow in which the rate of motion is proportional to the force that is being applied (above the yield value). (geology) Any deformation that is induced by a force that acts continuously.
Ductile Material
This type of deformation is permanent by fracture.
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane.
Brittle Materials
What happens to rocks near surface when buried?
Since they are typically brittle and will fracture, high temperatures and pressures will cause them to become ductile and deform plastically.
When rocks deform, they often slide past one another along a fracture plane, this point of fracture is called ______.
All faults involve some type of slippage movement, whereas fractures do not.
Fault
A fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements.
How do waves travel when seismic waves generated by earthquakes and human-made explosions encounter a low density layer?
Reflective
How do waves travel when seismic waves generated by earthquakes and human-made explosions encounter a high density layer?
Refracted
It is the transition zone between SIAL and SIMA. Found between upper and lower crust.
Conrad Discontinuity
A boundary within the Earth’s continental crust that can be detected seismically at about 10–12 km depth, although exploratory deep drilling has failed to locate it. The boundary separates the crust into a lower, basic layer and an upper, granitic layer.
Found between lower crust and upper mantle. This boundary is located approximately 24 miles below the earth’s surface and 6 miles below the oceanic floor, a distance which varies from place to place.
Mohorovicic Discontinuity
Divides mantle in upper and lower, it is approximately between 660 and 700 km deep. Passing through this discontinuity, seismic waves increase it’s speed. As explained in post about tectonic plates, lower mantle is hotter and liquefied while upper is cooler and pasty.
Repetti Discontinuity
This is the transition zone between the lower mantle and the outer core.
Gutenberg Discontinuity
This is the layer separating the outer core from the inner core.
Lehmann Discontinuity
In geology, this term is used for a surface at which seismic waves change velocity.
Discontinuity
Thickness of the Crust (low density rock)
7-70 km thick
Physical property of the outer core
liquid
Physical property of the inner core
solid
Chemical composition of the core
Iron and nickel
Composed largely of dark-colored, mafic rocks enriched in oxides of magnesium, iron and calcium (MgO, FeO, and Cao) relative to average crust.
Oceanic Crust
Depth of oceanic crust to the Moho averages 5-7km. However, under some oceanic islands, its thickness reaches 18km
The Oceanic Crust consists principally of which rocks?
Basalt and Gabbro
May be slices of ocean crust thrust onto continental margins.
Ophiolites
A section of Earth’s oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks.
One of the most studied upper mantle-crust sequence in the Philippines, had most of the studies concentrated on the northern portion (Masinloc Massif) of the ophiolite.
The Zambales Ophiolite Complex (ZOC)
The ZOC is made up of three massifs, Masinloc, Cabangan and San Antonio as one goes from north to south.
Oceanic Crust Ophiolite Model
- Sediment
- Pillow Lava
- Sheeted Dykes
- Gabbro
- Layered Gabbro
- Layered peridotite
- Upper Mantle
Generalized as “granitic” in composition, enriched in K2O, Na2O, and SiO2 relative to average crust. Thicker than oceanic crust.
Continental Crust
This is the oldest well-documented continental crust (Stern & Bleeker, 1998).
4.03 Ga rocks from the Northwest Territories of Canada
The reason why some continent crust result into great age
Relative Buoyancy
Thickness of mantle relative to the radius of the Earth (~6370km)
~2900 km
The mantle constitutes how many percentage of Earth’s total volume?
~83%
Composition of Mantle
Ultrabasic composition
contains average of approximately 40-45% SiO2
It is the uppermost part of the mantle and the crust together constitute this relative rigid which is strong enough to rupture in response to stress.
Lithosphere
Occurs within the upper mantle at depths of ~100-250 km below the surface. The top of this zone marks the contract between the strong lithosphere and the weak asthenosphere.
discrete low velocity zone (LVZ)
Is more plastic and flows slowly, rather than rupturing, when subjected to stress.
Asthenosphere
Mineral found at 410 km depth
Wadleysite/Beta Spinel (Mg2SiO4)
Minerals at the Transition Zone
- Ringwoodite (Mg2SiO4)
- Garnet (Mg2SiO4)
Minerals at 60 km depth
- Perovskite [(Mg,Fe,Al)SiO3]
- Periclase (MgO)
Proposed by Williams and Garnero (1996) that may be related tot he formation of deep mantle plumes within the lower mantle.
ultra-low velocity zones (ULVZ)
This discontinuity ranges from ~130 to 340 km above the core -mantle bounary.
D” layer (discontinuity)
Density of outer core
10-12 g/cm^3
Density of inner core
~13g/cm^3
Seismic studies have shown that the inner core is seismically _________.
Anisotropic
How did the magnetic field formed?
results of the planet’s rotation causing metallic ions inthe outer core to circulate.
How can magnetic field help the Earth from hazards?
It helps shield the Earth from harmful radiation streaming from the Sun, hence it is important to the biosphere.
Heat sources in the Earth
- Heat from when the planet formed and accretted which has not yet been lost.
- Frictional heating, caused by denser core material sinking to the center of the planet.
- Heat from the decay of Naturally Occuring Radioactive Elements/Nuclides
It is a mantle convection model which suggests that cold oceanic lithosphere sinks to great depths and stirs the entire mantle; the ultimate burial ground for subducting slabs is the core-mantle boundary. This downward flow is balanced by bouyantly rising mantle plumes that transport hot material toward the surface.
Whole-Mantle Convection Model
Has two largely disconnected convective layers. A dynamic upper layer driven by descending slabs of a cold oceanic lithosphere and a sluggish lower layer that carries heat upward without appreciably mixing with the layer above.
Layer Cake Model
The lithosphere is broken along major fault systems into large pieces called “plates”, moving on top of a plastic asthenosphere, that move relative to one another.
Fundamental tenet of plate tectonics (Isacks et al., 1968; Le Pichon 1968)
Three major types of plate boundary segments
- Divergent
- Convergent
- Transform
Describes one of the earliest ways geologists thought continents moved over time. This map displays an early “supercontinent,” Gondwana, which eventually moved to form the continents we know today. Fossils of similar organisms across widely disparate continents encouraged this revolutionary theory
Continental Drift
Matching of coastlines on different continents i.e., the South American and African continents seem to fit together.
Continental Fit
This evidence shows organisms that were restricted in geographical distribution but appeared in 2 or more areas that are presently separated by major barriers.
Fosil Evidence
Aquatic reptile with fossils limited to eastern South America and Southern Africa. It lived during the early Permian period (286 to 258 million years ago).
Mesosaurus
A mammal-like reptile that existed during the Triassic Period (250-240 mya). Its fossils are found only in South Africa and South America.
Cynognathus
Thought to have been a herbivore with a stout build like a pig. Relatively dominant on land during the early Triassic period (250 mya). Lystrosaurus fossils are only found in Antarctica, India, and South Africa.
Lystrosaurus
known as woody, sead bearing tree, the Glossopteris is named after the Greek description for tongue due to its tongue shaped leaves and is the largest genus of the extinct descendant of seed ferns. The Glossopteris fossil is found in Australia, Antarctica, India, South Africa, and South America- all the southern continents.
Glossopteris
Evidence of mountain belts such as the Appalachians in eastern US and British Isles and Scandinavia have comparable age & structure
Rock Type and Structural Similarities
Glacial deposit evidence. At the end of the Paleozoic era (220-300 mya), ice shets covered extensive areas of the Southern Hemisphere; at present these areas are in the subtropics to tropics.
Paleoclimatic Evidence
Records the orientation of the magnetic field at the time the rocks formed.
Magnetite-rich rocks
How to determine the paleolatitude?
- steep inclination of “fossil magnets” indicates formation of the rock at high latitude.
- Very low angle = near the equator
What is magnetite’s Curie Temperature?
580 degrees Celsius (?)
Are volcanic locales thought to be fed by underlying mantle that is anomalously hot compared with the surrounding mantle.
Hotspot
It is a Hawaiian Island northwest of Molokai with an age rank of 2.2 to 3.3 MA
Oahu
-dating of the ocean floor using fossils
-age of sediment increases with distance from the ridge
Evidence from Ocean Drilling
Depth of the seafloor ________ away from the ridge
decreases
the outer rigid lithosphere consist of rigid segments
plate
Oceanic crust is ____ than continental crust
thinner
hotter and weaker zone; facilitates motion of teh earth’s rigid outer shell
asthenosphere
two places on the same plate do not move relativee to each other
rigid plates
Where do all major interaction between plates take place?
along plate boundaries
This drives mantle convection which results into plate motions, plumes, volcanoes, earthquakes, mountain ranges, mineral deposits, etc.
Earth’s internal heat
Areas where divergent plate boundaries occur (where to plates are moving apart relative to their boundary).
Areas characterized by horizontal extension and vertical thinning of lithosphere
marked by continental rift systems and in oceanic lithosphere by the oceanic ridge system.
horizontal extension in continental lithosphere
form where horizontal extension occurs in continental lithosphere. In such regions, the lithosphere is progressively stretched and thinned, like a candy bar being stretched in two. This stretching occurs by brittle, normal faulting near the cooler surface and by ductile flow at deeper, warmer levels.
Continental Rift Systems
Modern examples of continental rift valleys
East african rift, the Rio Grande Rift in the United States, and Dead Sea rift in the Middle East
the process by which supercontinents such as Pangea and Rodinia were broken into smaller continents such as those we see on Earth’s surface at present.
Complete continental rifting
When continental rifting happens, a new and growing ocean basin begins to form between the two continents by this process
Sea Floor Spreading
