To get a better understanding of this whole process, I would like you to peruse this web site. It is
tutorial of sort that goes through the entire process of plate tectonics. Begin your assignment by going
to the Developing the Theory web site and answering the following questions:
1. What four major scientific developments spurred the formulation of the plate-tectonics theory:
Four major scientific developments spurred the formulation of the plate-tectonics theory: (1)
demonstration of the ruggedness and youth of the ocean floor; (2) confirmation of repeated
reversals of the Earth magnetic field in the geologic past; (3) emergence of the seafloorspreading hypothesis and associated recycling of oceanic crust; and (4) precise documentation
that the world's earthquake and volcanic activity is concentrated along oceanic trenches and
submarine mountain ranges.
2. Explain the significance of paleomagnetic banding.
Paleomagnetic banding shows that at one point, older rock further from the ridge had been
formed at the divergent boundary- only way to explain the zebra like banding of alternating
magnetic orientation of the sea floor.
3. Explain the process of seafloor spreading and recycling of ocean floor.
Where there is a divergent boundary- new ocean floor is created when magma seeps up from the
asthenosphere at a divergent boundary between two plates and solidifies into new ocean floor.
This then causes plates to collide and conduct- when an more dense ocean plate is involved, it
sinks back into the asthenosphere where it originally came from.
4. Explain the significance of the concentration of earthquakes.
All of the earthquakes occur at the plate boundaries- therefore a map of the earthquake zones
outlines and shows us where all of the plates are.
Next I would like you to go back to the first web site and choose the site titled “Understanding Plate
Motion.” I would like you to study this section very carefully. I am especially interested in each
student having a thorough diagram and description of each boundary type.
1. Describe the four types of plate boundaries
There are four types of plate boundaries:
a. Divergent boundaries -- where new crust is generated as the plates pull away from each
other.
b. Convergent boundaries -- where crust is destroyed as one plate dives under another.
c. Transform boundaries -- where crust is neither produced nor destroyed as the plates slide
horizontally past each other.
d. Plate boundary zones -- broad belts in which boundaries are not well defined and the
effects of plate interaction are unclear.
Divergent Boundaries
2. Give a thorough description of what a divergent boundary is.
Divergent boundaries occur along spreading centers where plates are moving apart and new crust
is created by magma pushing up from the mantle. Picture two giant conveyor belts, facing each
other but slowly moving in opposite directions as they transport newly formed oceanic crust
away from the ridge crest.
3. How fast is the mid ocean ridge moving?
Most mid ocean ridges are spreading at about 2.5 cm or 1 inch a year.
4. Where will earth’s next major ocean occur and why?
The next major ocean will occur in eastern Africa. Geologists believe that, if spreading
continues, the three plates that meet at the edge of the present-day African continent will separate
completely, allowing the Indian Ocean to flood the area and making the easternmost corner of
Africa (the Horn of Africa) a large island.
Convergent Boundaries:
5. Describe the three types of convergent boundaries. There are three types of convergent
boudaries dependent upon the types of plates that are colliding.
a. Ocean plate with an Ocean plate
b. Continental plate with a continental plate
c. Continental plate with an ocean plate.
6. Create a thorough diagram for each type of convergent boundary. These diagrams need to be
neat and well labeled and will be the most important aspect of this assignments grade. Make
sure you provide a description of what is occurring at that boundary and why! Be thorough and
neat!
7. Descriptions are provided with the diagrams!
Ocean-Ocean Convergent Boundary
When an oceanic crust collides with an oceanic crust, the ocean crust that is more dense will subduct
forming a deep sea trench. The subducted oceanic
crust will then become part of the convection
currents in the asthenosphere. Portions of the
subducted oceanic crust can be melted by the
increase in temperature in the asthenosphere to
magma that rises and forms volcanic Islands (called
an Island Arc) on the surface of the oceanic crust
that does not subduct.
Continental-Continental Convergent Boundary:
When a continental plate collides with a continental
plate- neither plate subducts because they are very
similar in their density. However, a portion of an
ancient oceanic plate can be found hundreds of
miles below the surface of the mountains and
remains of the oceanic floor can also be found at the
top of the mountains that form as well. The
mountains form when the two plates crumple
against one another. Behind the mountains, the
plate can be pushed to a higher elevation – which
forms a plateau.
Continental –Oceanic Convergent Boundary.
This boundary is very similar to the ocean-ocean boundary- except the volcanic islands are
volcanic mountains. The oceanic plate subducts
because it is denser than the continental plate.
Where the oceanic plate subducts, it forms a deep
sea trench. The oceanic plate then will sink down
into the asthenosphere and become part of the
convection currents in the asthenosphere. Because
the temperature is very high in the asthenosphere.
Portions of the oceanic plate can melt and form
magma. This magma rises to the surface of the
continental plate to form volcanoes. The
Continental plate will crumple where it meets the
oceanic plate forming a mountain chain. Behind the mountains, the pate can be pushed up by the
crumpling forming a Plateau.
Transform Boundaries
8. Describe the SanAndreas fault- what two plates does it separate and how fast is it moving?
San Andreas fault is a transform fault between the Pacific Plate and the N.American Plate
“Some Unanswered Questions”
9. Explain what is causing plates to move.
Convection currents in the asthenosphere are causing the plates to move. The plates diverge
where the convection currents rise beneath the crust and the plates subduct where the convection
currents sink. The crustal plates ride the convection currents as they move beneath the crust.
“Hot Spots.”
10. Explain what a hot spot is.
A Hot Spot is a volcanic area that is not caused by a plate boundary. Hot spots are caused by an
unusual upwelling of thermal heat – that is rising through the asthenosphere from the core.
11. Describe where the magma comes from.
Heat from this hotspot produced a persistent source of magma by partly melting the overriding
Plate. The magma is a part of the crust that is melted by the heat rising from the core. The
magma, which is lighter than the surrounding solid rock, then rises through the mantle and crust
to erupt onto the seafloor, forming a volcano or volcanic island