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Mid-ocean ridges - description (more)
Hydrothermal activity in rift valleys
Mid-ocean ridges - origin = Divergent plate boundaries
Spreading centers: creation of new oceanic lithosphere, volcanic
activity
Earthquakes? -- tensional forces as plates are rifted apart
Variation in width? -- variation in spreading rates:
fast spreading = wide ridges
slow spreading = narrow ridges
Continental rift systems (East African Rift) -- "developing"
divergent boundaries
Eventual formation of new ocean
Break-up of continents (like Pangea!)
Fracture zones - description
Deep, linear escarpments; thousands of km long.
_I_ to ridges; offset ridge axes
Earthquakes along offset segments only
Fracture zones - origin = Transform plate boundaries
Boundary: segment that offsets ridge axes
Earthquakes? plates moving in opposite direction
Long segment of fracture zones beyond the offset portion?
-- old, "inactive" boundaries
Marginal trenches - description
Western Pacific:
Volcanic island arcs (rock is "andesite," not basalt)
Zone of deep, strong earthquakes
Eastern Pacific:
Volcanic activity on continent (Andes, Mt. St. Helens)
Zone of deep, strong earthquakes
Marginal trenches - origin = Convergent plate boundaries
Ocean-ocean convergence -- Western Pacific
Subduction of the older, cooler plate
Volcanism? -- melting during descent of plate
Earthquakes? -- friction in the "cool" plate and between
plate and mantle
Ocean-continent convergence -- Eastern Pacific
Subduction of the oceanic plate
Volcanism? -- same as ocean-ocean.
Earthquakes? -- same as ocean-ocean.
Continent-continent convergence, or collision zone
Continental plates are not subducted -- too thick and bouyant
Thickening, uplift, deformation, earthquakes
Example: Himalayas (India-Asia collision beginning 45 m.y. ago)
Submarine hot-springs and geysers (hydrothermal activity). These extraordinary phenomena, first discovered in 1977 on the East Pacific Rise by the research submersible ALVIN, result from the circulation of sea water into very young oceanic crust. Sea water enters young basalt crust through cracks and fissures. This water is heated to 360 deg C when it encounters basalt magma chambers at depth. It then rapidly emerges at ridge crests as extremely hot geysers.
Mid-ocean ridges are sites where new ocean lithosphere is created. They are the "spreading centers" of Harry Hess's model of sea-floor spreading. They are the divergent boundaries of plate tectonics. As oceanic plates move apart (diverge), basalt magma is transported upward by convection currents and solidifies to form new lithosphere. This process accounts for the volcanic activity (including hydrothermal) so evident at mid-ocean ridges.
Why are there earthquakes (shallow, weak) along the axis of mid-ocean ridges? These result from tensional forces as plates are rifted apart. Earthquakes here are shallow and weak because young oceanic lithosphere is thin, hot, and relatively weak.
Why does the width of mid-ocean ridges vary from ocean to ocean? Because the rate of sea-floor spreading varies. In the Atlantic, spreading rates are 1-2 cm/yr -- the Mid-Atlantic Ridge is narrow. In the Pacific, spreading rates are 5-10 cm/yr -- The East Pacific Rise is more "spread out."
TASA Chapter 11 -- Divergent boundaries: Go through all six frames. Note rate of spreading -- divergence
A few divergent boundaries have been recognized within continents -- Continental rift systems. An example is the East African Rift. Continental rifts are regions where it appears that especially hot currents in the mantle ("mantle plumes") are rising, thining and fracturing the crust, and pulling it apart. As continents rift and separate, new ocean crust will eventually form. This is probably the mechanism that breaks-up continents both now (East Africa Rift) and in the past (the break-up of Pangea)
TASA Chapter 17 -- Hot Spots and Plate Tectonics:
Back in from Chpt. 18 -- 13 frames to ... (a) Pangaea; (b) Plume
and initial rifting;
(c) Further rifting and formation of new ocean basin. A total
of 4 frames.
Fracture zones are relatively deep, linear escarpments that can be thousands of kilometers long and extend across ocean basins (such as some in the Pacific). They are oriented about perpendicular to mid-ocean ridges and appear to offset the axis of mid-ocean ridges (you could also think of them as connecting offset portions of ridges). Earthquakes only occur on those portions of fracture zones that offset ridge axes.
That segment of a fracture zone that connects ridge axes is the boundary between between two oceanic plates that are moving in opposite directions. Thus, those segments are transform plate boundaries. Earthquakes occur because of the opposite motion of the two plates. The part of a fracture zone that extends beyond that segment (the longest part of fracture zones in the oceans) is no longer "active." Those parts used to be transform boundaries, but they are now part of a single plate.
A few transform boundaries occur occur on continents. One example is the San Andreas fault in California. On the west side of the fault, the Pacific plate and a small sliver of California is sliding north past North America.
Most of the deep trenches occur around the margins of the Pacific.
In the western Pacific, volcanic islands (called island arcs) occur on the continental-side of trenches. The volcanoes often erupt explosively, with great outpourings of volcanic ash. The type of volcanic rock erupted here is "andesite," different in composition from basalt of mid-ocean ridges and elsewhere in the oceans. In addition, strong, deep earthquakes (to a depth of 700 km) occur beneath trenches and island arcs.
In the eastern Pacific along Central and South America, trenches occur just at the edge of (narrow) continental margins. Volcanic activity occurs on the continents. There are also strong, deep earthquakes.
Marginal trenches mark convergent plate boundaries, where two plates are coming together. An oceanic plate involved in the convergence is subducted back into the mantle. Volcanic activity and earthquakes result from this subduction.
In the western Pacific, two oceanic plates are converging.
This is called an "ocean-ocean" convergence.
The older and cooler plate is subducted. As the plate descends,
a small part of it and overlying mantle melts to produce rising
blobs of magma that erupts explosively. Earthquakes occur as friction
develops within the "cool" plate and between the descending
plate and mantle. In fact, earthquake zones trace the descent
of the plate.
In the eastern Pacific, an oceanic and a continental plate are
converging. This is called an "ocean-continent" convergence.
The oceanic plate is subducted. Volcanic activity and earthquakes
are generated in the same way as they are in the ocean-ocean convergence
in the western Pacific.
There is a third type of convergent boundary -- a "continent-continent" convergence, or collision zone. When two continental plates collide, neither can be subducted because continental crust (and lithosphere) is to bouyant and thick to be forced back into the mantle. What happens instead is that one plate penetrates into and below the other, resulting in much crustal thickening and elevation as well as deformation and earthquakes -- it's like a continental-scale train wreck! The best example is the Himalayas, the world's highest mountain range, that is the result of the collision of India with Asia that began about 45 m.y. ago.
First frame shows world's trenches.
Skip through the chapter to show animation of the 3 types of boundares.
(The entire chapter is quite good and illustrative.)
Study/Review Questions:
7-1. According to the theories of sea-floor spreading and plate
tectonics, what is the origin of mid-ocean ridges?
7-2. Why do earthquakes occur at the axis of mid-ocean ridges?
7-3. Why does hydrothermal activity (hot-springs and geysers,
black and white "smokers") occur along mid-ocean ridges?
Give two reasons why this activity is important?
7-4. Why does the width of mid-ocean ridges vary from ocean to
ocean?
7-5. Give an example of a divergent plate boundary on a continent.
What is the significance of continental rifting to the theory
of continental drift and the creation of new oceans?
7-6. What are fracture zones? How are they oriented with respect
to mid-ocean ridges?
7-7. According to the theory of plate tectonics, what type of
plate boundary are fracture zones?
7-8. Sketch a simple map view of a mid-ocean ridge spreading center
with several transform faults and fracture zones. Indicate where
earthquakes occur in that system. Explain why.
7-9. Name a transform plate boundary that is located on a continent.
7-10. Describe the important geologic processes associated with
marginal trenches in the western Pacific and the eastern Pacific.
7-11. According to the theories of sea-floor spreading and plate
tectonics, what is the origin of marginal trenches and associated
features (e.g., volcanic arcs)?
7-12. Why do deep earthquakes occur in well-defined zones beneath
marginal trenches? Why don't earthquakes occur at depths greater
than 700 km?
7-13. What happens when two plates with continents at their leading
edges collide? Contrast that to ocean-ocean and ocean-continent
convergence boundaries? Give an example of a continent-continent
collision?
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