I. When plates move away from each other, new ocean crust is formed and the plate boundary is called a divergent boundary.
- A. Mid-ocean ridges mark the divergent plate boundary where the crust is formed.
- B. Magnetic reversals help to determine the age of the ocean floor.
- 1. If the magnetic reversals are correlated to certain ages, then by determining the magnetic reversal event during which ocean crust was formed, a corresponding age can be found.
- 2. Because the magnetic reversal events occurred worldwide, any piece of ocean floor basalt can be dated by the magnetic reversal technique.
- C. It is possible to determine the rates of spreading in different ocean basins.
- 1. The precise duration of each magnetic reversal event has been determined.
- 2. Different lateral thicknesses of ocean crust have been found to be formed during the same magnetic reversal thickness.
- 3. By dividing this thickness of ocean crust by the time period assoicated with each magnetic reversal, a formation rate for each ocean basin can be determined.
- a. In the North Atlantic, each side of the Mid-Atlantic Ridge spreads about 1 cm/year for a total thickness of 2 cm/yr.
- b. The South Atlantic has spread 2 cm/yr on each side of the Mid-Atlantic Ridge for a total of 4 cm/yr.
- c. The Pacific has been spreading at a rate of 4.5 cm/yr on each side of the East Pacific Rise for a total of 9 cm/yr.
- d. These rates correspond roughly to the rate at which your fingernail grows.
- D. Segments of the spreading center are bounded by transform faults.
- E. Divergent boundaries separating continental crust on either side of a spreading center are called rift zones.
- 1. After a continent has rifted apart, oceanic crust and a mid-ocean ridge begin to form.
- 2. Rifting activity can be found in East Africa (the Rift Valley running down Ethiopia, Tanzania, etc.) and in the Gulf of California (Baja separating from Mexico).
II. When two plates move toward each other, the plate boundary is said to be convergent. If one of those plates is an ocean plate, then subduction occurs.
- A. What evidence is there to support the existence of the subduction process?
- 1. Oceanic crust is being destroyed.
- a. It is known that new ocean crust is being made.
- b. We know the earth is not expanding, thus crust must be destroyed.
- 2. Deep focus earthquakes originate at the margins of oceans beneath continents.
- a. Many earthquakes that originate at spreading centers and transform faults (like the San Andreas Fault) are shallow focus earthquakes.
- b. Earthquakes at the margins of ocean crust show a pattern of progressively deeper foci (plural of focus) further toward the continent and away from the ocean plate. A deep focus earthquake can be as deep as 700 km.
- c. Such inclined zones are called Benioff zones.
- 3. Subduction zones have a characteristic geometry.
- a. A deep ocean trench lies at the margin of the ocean plate next to the continent. Examples of ocean trenches can be found off the Marianas Islands, the Aleutians, and Japan.
- b. On the other side of an ocean trench a line or arc of volcanoes lies parallel to the ocean trench.
- i. Often such a line of volcanoes produces a string of islands collectively called an island arc.
- ii. Examples of island arcs include Japan, the Aleutians, and the Tonga islands.
- iii. An instance of subduction volcanism that is not associated with island arcs occurs with the subduction of the Pacific plate beneath South American where the Andes are located.
- B. How does subduction work?
- 1. Ocean crust is denser than continental crust.
- 2. When ocean crust and continental crust converge, the denser ocean crust tends to sink and get pushed under the continental crust. Subduction also occurs at the convergence of two oceanic plates.
- 3. As the ocean crust continues to be subducted, it produces earthquakes and undergoes partial melting.
- a. The partial melting produces intermediate magmas which crystallize into andesite and diorite.
- b. The volcanism and compression produces mountain chains.
- 4. Eventually the crust is assimilated into the mantle.
- C. Subduction is present around much of the margin of the Pacific Ocean.
- 1. As a result, volcanic eruptions and earthquakes are quite common around the edge of the Pacific Ocean.
- 2. The active tectonic activity around the Pacific Ocean is labeled collectively as the "Ring of Fire.
- D. If two continents meet at a convergent boundary, a major mountain-building event is produced.
- 1. Because continental crust is relatively low in density compared to oceanic crust, the continental crust will not experience subduction.
- 2. The Himalayas are being produced by the convergence of the Indian subcontinent with the Asian plate.
- a. At one time India was situated further south next to Africa.
- b. For the past 180 million years India has been moving north toward the Asian continent.
- c. Evidence for India's northward movement can be seen on the floor of the Indian Ocean.
- d. The convergence of India with the Asian plate is responsible for the strong earthquakes that often occur in China.
III. When plates slide past each other, it is called a transform boundary.
- A. Transform boundaries are marked by transform faults (strike-slip).
- B. Compared to subduction zones, transform boundaries produce shallow focus earthquakes.
- C. An example of a transform boundary is the boundary between the North American and Pacific plates.
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