Plate Tectonics Laboratory Key

Exercise 18

  1. Draw the Plate Boundaries.

Oceanic trenches, island arcs, volcanic arcs (continental) and active mountain belts indicate Convergent Boundaries.Mid-ocean ridges and continental rift valleys indicate Divergent Plate Boundaries.Offset mid-ocean ridges and fracture zones indicate transform plate boundaries (donít forget the San Andreas Fault in California).Passive continental margins are indicated by continental shelfs.

 

  1. List the feartures shown on the map that indicate plate motion.

Oceanic trenches, island arcs, volcanic arcs (continental), active mountain belts Mid-ocean ridges and continental rift valleys, hotspot volcanoes

 

  1. Show the direction of plate motion with a series of arrows.

In general, plates move away from divergent boundaries and toward convergent boundaries

 

  1. Draw an idealized cross section through the plate margins that occur near (a) South America, (b) Central America, (c) Japan, (d) Himalaya Mountains, and (e) the Indonesian Islands.
    1. Western South America is an example of and Ocean-Continent convergent plate boundary.An Oceanic plate (Nazca Plate) is subducted beneath the South American continental crust forming an oceanic trench, and a mountain belt with active composite.Donít forget to show the asthenosphere, lithosphere, oceanic crust, continental crust, mountain range, volcanoes
    2. Central America is an example of an Ocean-Ocean convergent plate boundary. An oceanic plate (Cocos Plate) is subducted beneath another oceanic plate (Caribbean Plate) forming an oceanic trench to the west and active composite volcanoes that have coalesced into the isthmus of Central America.
    3. Japan is another example of an Ocean-Ocean convergent plate boundary with an oceanic trench and an island arc.
    4. Himalayan Mountains are an example of a Continent-Continent convergent plate boundary where two slabs of continental crust have collided and compressing the crust and the acretionary wedge material and lifting it to form the Himalayan Mountains.
    5. Indonesia is another example of an Ocean-Ocean convergent plate boundary. An oceanic plate (Cocos Plate) is subducted beneath another oceanic plate (Caribbean Plate) forming an oceanic trench to the west and active composite volcanoes that have coalesced into the isthmus of Central America.
  2. Explain the origin of (a) the Red Sea, (b) the Black Sea, (c) the Mediterranean Sea, (d) the Gulf of California.
    1. A divergent plate boundary is forming a narrow linear sea (basaltic oceanic crust) separating two slabs of continental crust, northwest Africa and the Arabian Peninsula.
    2. The Black Sea is forming in the forearc region of the convergent plate boundary between the African Plate and the Eurasian Plate.
    3. The Mediterranean Sea is forming over the subduction zone and trench of the convergent plate boundary between the African Plate and the Eurasian Plate.
    4. The Gulf of California is a divergent plate boundary forming a narrow linear sea (basaltic oceanic crust) pulling the Baja Peninsula away from the North American Plate.
  3. The Himalayan Mountains are higher because two slabs of continental crust and all the material of the acretionary wedge is being compressed and lifted up at the Continent-Continent convergent plate boundary.Subduction has been stopped by the collision and larger stresses are accumulating.The Andes are forming at an Ocean-Continent plate boundary with less material being compressed and lifted during mountain building and less stress building because subduction continues.
  4. The Indonesian Islands, Himalaya Mountains and the Alps are all convergent plate boundaries long the southern edge of the Eurasian plate.
  5. The Kuril trench since old crust is being consumed at a convergent boundary farthest from the new crust being generated at the East Pacific Rise.
  6. The distance from the Mid-Atlantic Ridge to South America and to Africa is generally 1,200km.Because new crust is generated at the mid-Atlantic ridge (divergent plate boundary) and the oldest crust is at the continental rise 1,200km away. age = (120,000,000cm)/(3.8cm/yr) = 32 million years old.
  7. Compression of the crust at a convergent boundary folds, faults and lifts the crust.Also, magma is generated at the subduction zone resulting in explosive, silicic volcanism and forms composite volcanoes.

 

17: North American Continent.

    1. Schists and gneisses of sedimentary and granitic origin within the Precambrian foldbelts of the Canadian Shield were formed in the roots of Precambrian mountains in the region.
    2. 4 within the shield (plus the Appalachians and the Rockies wich are not technically in the shield)
    3. Accretion states that a continent grows at the margins and thus will be older in the center becoming younger toward the edges.This is what is found in the Canadian shield.

f.†† The mountain chains of North America are found parallel to the continental margins with folds and faults running parallel to the margins.Plate tectonics states that compression at convergent plate boundaries will fold and fault rocks perpendicular to the compression and parallel to the continental margin.

g.†† Plate tectonics states that silicic magma is generated at subduction zones and intruded into the folded and faulted continental crust above.

h.†† The rocks of the Baja Peninsula were originally formed at a convergent plate boundary and are now being pulled away from the North American plate by a divergent plate boundary forming the Gulf of California.

i.††† Silicic magma is generated at the subduction zone forming due to the convergence of the Juan De Fuca plate and the North American plate.Silicic magma is viscous and is rich in dissolved gasses so it will erupt explosively and form composite volcanoes

j.††† The 6000 ft depth between 4000 and 1000 indicate a trench forming at a convergent boundary (the subducted plate is to the southwest).

k.†† Because the subducted plate is to the southwest of Central America the subduction zone and volcanism will be along the southwest coast.

 

20: Appalachian Mountains.

a.       Your version of this image is not clear enough to distinguish the anticlines from the synclines but the resistant sedimentary beds form ridges and the less resistant beds form the linear valleys.

b.      The anticlines are regions that have been warped up and eroded.The synclines have been warped down exposing younger rocks in their cores.

c.       The forces forming folds are compressive ŗ ~~~ Ŗ

Exercise 19.

Problem 8.

  1. The types of volcanic eruptions are non-explosive fissure eruptions extruding non-viscous basaltic lava floods.
  2. Basaltic lava floods occur where the crust is in tension.This tension depressurizes the mantle causing it to melt and form basaltic magma.Thus the linear features are probably normal faults and rifts.
  3. There are no volcanic cones because the lava is non-viscous and hot causing it to flow out over large areas.
  4. The youngest basalt flows are the darkest at the right of the image.The older flows are generally darker.

 

Problem 13.

  1. Length=1000km, Width: 50-100km.The East African rift valleys are about the same width but about twice as long.It appears that the Rio Grande Rift is shorter and narrower.
  2. The rocks seem to have the same density, width and orientation (northeast trend) as the mid-continent gravity anomaly.
  3. Most of the continental crust is granitic, metamorphic and sedimentary so the regions shown in red (higher gravity) are underlain by denser rocks, probably basalt.
  4. The cross section should show a rift valley filled with basalt and conglomerates and overlain by horizontal sedimentary layers.