Because the portions of the earth's interior and differences between continental and oceanic crust are an essential part of plate tectonics, it is worth our while to review these concepts briefly: Earth's crust (lithosphere) is composed of several elements crucial to our existence. In order of their abundance, these eight (8) elements are:
1.Oxygen
2.Silicon
3.Aluminum
4.Iron
5.Calcium
6.Sodium
7.Potassium
8.Magnesium
The Upper Mantle and Crust
•Crust (5 mi for ocean, 25 mi for continents)
•Dense iron-rich basalts (mafic) make up ocean floor Fe, Silica, magnesium
•Silica-rich rock makes up the continents (felsic) Silica, aluminum2011/
The Earth’s Interior
•General trends: temperature, density
•Horizon composition, behavior
What is ‘tectonics’?
•From Greek ‘tektonikus’ meaning building or construction
•Plate tectonics refers to the process of plate formation, movement, and destruction.
What is a ‘Plate?’
•Lithospheric plate: crust + upper mantle
•Asthenosphere: plastic mantle
Lithosphere and the solid earth: the solid earth lies beneath the atmosphere and the oceans and composes 29% of the earth's surface. It is divided into several distinct units or layers:
a.Lithosphere or crust: two (2) types of crust: oceanic and continentalwith basic differences
*Oceaniccrust is thinner and denser and usually darker in color
*Continentalcrust is lighter in weight, less dense, light in color, and tends to float over oceanic crust
b.Mantle: beneath the crust; houses molten rock material called magma
c.Outer core: composed of liquid iron and nickel; very dense material
d.Inner core: composed of solid iron and nickel; extremely dense material
The upper mantle and lower crust (lithosphere) are referred to as theasthenosphere.
*There is a distinct seismic discontinuity where seismic waves slow down considerably due to the composition of molten rock. This is located in the asthenosphere and is referred to as the Mohorovicic Discontinuity, after the Russian scientist who discovered it. We refer to it as the "Moho".
History of Events Leading up to the Formulation of the Theory of Plate Tectonics
*Note that plate tectonics is a theory. It is not something that we can directly sample or touch, or for that matter prove. That is why we will refer to it as a theory.
*In 1915, a Bavarian scientist named Alfred Wegener (later referred to as the "Father of Plate Tectonics") noticed, while working near the North Pole, that his compass needle did not point to where north "should" have been. In other words, truenorth and magneticnorth were in two separate localities. Wegener theorized that the poles (both North and South) were "wandering" with time. He called this "Polar Wandering".
*Subsequent to his theory, he began to also notice how continents fit together like a jigsaw puzzle…most notably the western coast of Africa and the eastern coast of South America. In addition, rocks from these localities were the same rock type, same age, and contained the same age and type of fossils. His revised theory became known as "Continental Drift", because he realized that it was not the poles that shifted, but the continents themselves.
*Wegener died of a heart attack on a voyage studying glaciers near the North Pole in early 1930 and his work was virtually forgotten for several decades.
•1915 Alfred Wegener proposes theory of continental drift.
•Supercontinent Pangaea (‘all-earth’) [225mya].
•Fragmentation and drift to current positions.
*It was not until the advent of World War II that a technology (Echo Sounding) developed to a degree that a stunning discovery was made by a geologist and seaboat commander, Harry Hess. He noticed that rocks on either side of a prominent geologic feature in the middle of the Atlantic Ocean (called the mid-oceanic rift zone) were a perfect mirror image of each other on either side of the rift zone. He theorized that the rift zone was oozing out magma material from submarine volcanoes and that the material spread laterally across either sides of the rift. As time progressed, Hess took more and more samples to back up his findings, as part of a series of drill voyages aboard the research vessel, Glomar Challenger.
*Later in the 1960’s, Frederick Vine and Drummond Matthews, a Cambridge University professor/student team, discovered the principles of magnetic patterns on the ocean floor and went on the road to present their discoveries. They published first in a rather obscure journal and then on the lecture circuit at Scripps Oceanographic Institute. Further findings revealed that not only did the stripes have the same age of rocks, but a magnetic polarity image resulted as well, showing that during earth history there have been several "magnetic reversals" (A time when the compass needle would have pointed south instead of north).
*In the late 1960's and early 1970's, two scientists, revisited Wegener's findings and combined them with Hess' discoveries to formulate a new package called "Plate Tectonics". Robert Palmer and Donald Mackenzieare credited with naming and synthesizing the theory of “plate tectonics”.
•Problem with continental drift?
–No sound mechanism for the ‘drift’!
–Wegner hypothesizes spin of earth or tides…..
•New theory for motion: Arthur Holmes (1930s)
–thermal convective cells in the upper mantle (aesthenosphere)
–theory is largely ignored2011
•In the 1960s, Harry Hess and Robert Deitz (geophysicists) propose sea floor spreadingalong mid-oceanic ridgesfor plate motion.
The theory of plate tectonics is that rigid lithospheric plates move across the surface of the earth. There are approximately 12 major and 8 minor plates that move in concert with each other. Some pull apart, some push together and some move horizontally against each other.
The plate motion is driven by one or more of the following mechanisms:
1. Convection--heat transferred by movement of a fluid (magma)
2. Conduction--heat transfer by touching plates
3. Push-Pull Slab--heavy slabs pull plates downward and magma forced upward pushes plates to the surface (upwelling)
*There are several geological processes that occur where plates meet (called plateboundariesor margins):
1.Volcanoes tend to erupt at plate margins as a result of a process called subduction
2.Earthquakesoccur where plates grind against or over one other
3.Mountain buildingoccurs as one plate is pushed over another
4.Seafloor spreadingoccurs where two oceanic plates pull apart
There are three (3) major types of plate boundaries (margins):
1.Convergent--plates move towardseach other (compression)
2.Divergent--plates move awayfrom each other (tension)
3.Transform--plates horizontally grind against one another (strike-slip motion)
*Plate boundaries can occur on landmasses (continents) or in marine settings (oceans) or both at the same time.
Convergentplate movement is associated with the following:
a.Compression
b.Reverse faulting
c.Creation of a subduction zone
d.Mountain building processes
e.Collisions of plates:
–1.Continent vs. continent
–2.Continent vs. oceanic
–3.Oceanic vs. oceanic
Divergentplate boundaries are associated with the following:
•a. Tension or extension (pulling apart)
•b. Normal faulting
•c. Rifting (as in the mid-oceanic rift zone)
•d. Creation of magma material inside the rift zone
Transformboundaries are associated with the following:
•a.Horizontal grinding motion
•b.Strike-slip faulting
•c.Lateral offset of rock units
Volcanic Zones(both continental and oceanic in origin) associated with Plate Tectonics are located:
1.Subduction zones
a.Continent vs. ocean collision (ex.: Andes Mts., Pacific NW of U.S
Ocean vs. ocean collision (ex.: Japan, Philippines); Basaltic rocks
2.Rift Zones(Spreading centers) are located:
a. Ocean -ocean divergent zones (ex.: mid-oceanic rift); Basaltic rocks
b.Continental rift zone (ex.: East African Rift Zone); Granitic rocks
3."Hot Spot" Volcanismis located:
a. Oceanic; (ex.: Hawaiian Islands chain); Basaltic rocks
b.Continental; (ex.: Yellowstone Nat. Park); Granitic/Andesitic rocks
Seismic (Earthquake) Zonesassociated with Plate Tectonics:
1. Subducting oceanic plate; shallow focus as plate subducts
2.Intermediate focus earthquakes; partial melting and rising of magma; in the "Benioff Zone"
3.Deep focus as slab of crust is pulled by sheer gravity2011
Features/Landforms Associated with Plate Tectonics
1.1. Continent vs. Oceanic collision zone:subduction zone, deep sea trench associated, volcanic arc, andesitic volcanic rock.
2.2. Continent vs. Continent collision zone: Granitic rocks, mountain building processes, no volcanism, no magmatic activities.
3. Oceanic vs. Oceanic collision zone: deep sea trench associated, volcanic islandarc, basaltic volcanic rock.
4. Divergent zone: Oceanic, basaltic magma, spreading center (Mid- Ocean Ridge). Also, Continental: granitic, (East African Rift Zone)
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