In-Class Activity 1
12 September 2006
Plate Tectonics

Grading:  This activity was graded based on 4 points (1 pt for participation, 2 pts for question1 and 1 pt for question 2).  Partial credit was awarded for some answers.

Note that this activity included two figures.  Figure 1 showed a divergent plate boundary similar to what is observed in the Atlantic Ocean today (two oceanic plates diverged - each oceanic plate was attached to a continental plate.  Figure 2 showed a convergent plate boundary, where a slab of seafloor was subducting under continent 'Y'.  The slab of seafloor was attached to continent 'X'.

1. Fig. 1 shows two lithospheric plates and the boundary between them.  Use this figure to answer the following.  Note that “B” and “C” refer to specific points on the seafloor.  “E”, “F” and “G” refer to points below the seafloor. 

a) (0.25 pt) The hottest solid rocks are located at:  point C (a point near the top of the mid-ocean ridge)

b) (0.25 pt) The oldest seafloor rocks are located at: point B (a point on the seafloor far from the ridge).

c) (0.25 pt) The two lithospheric plates in the sketch are (check one):

___  moving together
_x_  moving apart
___  not moving

d) (0.25 pt) At point “F”, material is moving toward point: point E (material from within the asthenosphere is moving upward toward the seafloor.

e) (0.25 pt) Consider the rocks at points “A”, “B” and “C”.  (note that A=continental rock, B=seafloor rocks far from the top of the mid-ocean ridge and C=seafloor rocks near the top of the mid-ocean ridge).
    Which are most dense?   __B_
    Which are least dense?   __A__

 f) (0.25 pt) The feature at point C is: a mid-ocean ridge 

g) (0.25 pt) Why does the feature at point C sit up higher than the seafloor around it?
The mid-ocean ridge consists of rocks that have recently solidified from molten magma.  These rocks are warmer and less dense than seafloor rocks on either side of the mid-ocean ridge, thus they "sit up" higher than the surrounding seafloor.

2a. (0.25 pt) If the processes depicted in Fig. 1 on the previous page continue in the future, the ocean will most likely:
    __x__ get broader
    ____ get narrower
    ____ remain the same width 

b. In Fig. 2, if plate motion continues in the direction indicated by the dashed arrows, what will eventually happen to the ocean?  What will eventually happen to continents X and Y?

(0.25 pt) The ocean will: eventually close up, as more of the slab of seafloor is subducted.

(0.25 pt) Continent X will: collide with continent Y.  Both continents are about the same density, so neither will subduct and a mountain range will form.

(0.25 pt) Continent Y will: collide with continent X.  Both continents are about the same density, so neither will subduct and a mountain range will form.