Plate Tectonics Laboratory Key
Exercise 18
- 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 (don’t forget the San Andreas Fault in California). Passive continental margins are indicated by
continental shelfs.
- 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
- Show
the direction of plate motion with a series of arrows.
In general, plates move away from divergent
boundaries and toward convergent boundaries
- 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.
- 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. Don’t forget to show the
asthenosphere, lithosphere, oceanic crust, continental crust, mountain
range, volcanoes
- 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.
- Japan
is another example of an Ocean-Ocean convergent plate boundary with an
oceanic trench and an island arc.
- 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.
- 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.
- Explain
the origin of (a) the Red Sea, (b) the Black Sea, (c) the Mediterranean
Sea, (d) the Gulf of California.
- 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.
- The
Black Sea is forming in the forearc region of the convergent plate
boundary between the African Plate and the Eurasian Plate.
- The Mediterranean
Sea is forming over the subduction zone and trench of the convergent
plate boundary between the African Plate and the Eurasian Plate.
- 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.
- 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.
- The
Indonesian Islands, Himalaya Mountains and the Alps are all convergent
plate boundaries long the southern edge of the Eurasian plate.
- 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.
- 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.
- 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.
- 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.
- 4 within
the shield (plus the Appalachians and the Rockies wich are not
technically in the shield)
- 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 à
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Exercise 19.
Problem 8.
- The
types of volcanic eruptions are non-explosive fissure eruptions extruding
non-viscous basaltic lava floods.
- 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.
- There
are no volcanic cones because the lava is non-viscous and hot causing it
to flow out over large areas.
- The
youngest basalt flows are the darkest at the right of the image. The older flows are generally darker.
Problem 13.
- 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.
- The
rocks seem to have the same density, width and orientation (northeast
trend) as the mid-continent gravity anomaly.
- 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.
- The
cross section should show a rift valley filled with basalt and
conglomerates and overlain by horizontal sedimentary layers.