Tutorial 3 crib sheet
Question 1.
North Pacific
 2500km of new crust in 65Myr
 average rate ~38.5km/Myr
Note Spreading rate is this amount in each direction
Question 2
2a) #2 t=0 & t0 =10Ma
#2 linear velocity of plate B = 100km/Myr, but since plate A is fixed, ½ of this is due to
addition of crust to plate A west of ridge and ½ to addition of new crust to plate B west of
ridge. Spreading rate of each plate is thus 50km/Myr.
 ridge has moved 500km E due to new plate A
 but 1000km of plate B subducted
 however, also 500km of new plate B so net loss of B is 500km
 anchor subducted and ammonite moves E 1000km from start position of 500 to
1500km
 E of ridge active transforms, W inactive
#3 ridge E 20My x 50km/Myr --- 1000km (new plate A)
plate B also has 1000km new crust, plus has moved east 1000km due to new plate A.
Thus over 20Myr 2000km of plate B subducted, anchor and ammonite have gone; net
loss of B is 1000km.
2b) linear velocity is 50km/Myr; spreading rate = 25km/Myr Note A is subducting
beneath B
At t0 = 10Ma  ridge is 10x25km 250km E of start
subduction zone is 10x(25+25km)=500km E
At t0 = 20Ma  ridge is 20x25km 500km E of start
subduction zone is 20x(25+25km)=1000km E
2c) B subducted under A and A is fixed therefore subduction is static
linear velocity of B is 40km/Myr; there is no ridge
B therefore moves 400km NW by t0 = 10Ma and 800 km NW by t0 = 20Ma
2d) A is subducting under B and therefore trenches will migrate NE; again linear
velocity is NW 40km/Myr.
45
x = 40 cos 45° (or sin 45°)
= 28
40km
Therefore horizontal velocity component i.e. westwards 
28km/Myr
45
x
Therefore in 10Myr subduction zones migrate 280 km W, and in 20Myr moves 560 km
W. Relative distance between them is maintained.
 the eastern spreading ridge propagates northwards as the transform is gradually
subducted, however transform itself propagates NW maintaining same relative length of
transform
2e) Remember A is fixed so N subduction zone remains static whilst southern
subduction zone moves west at 28km/Myr. Transform is consumed.
After t0 = 20Ma transform has reverse sense of motion.
2f) Linear velocity 80km/Myr, spreading rate 40km/Myr (in 4 Myr - 10km)
After 10 Myr spreading ridge is 400km to west
400km of new oceanic crust on plate A
400km of new oceanic crust on plate B
800 km of plate B subducted
After 20 Myr x2
2g) B linear velocity NW 40 km/Myr
W velocity component 28km/Myr
A is fixed so lower subduction zone doesn’t move
Upper zone migrates NW. Transform lengthens
After 10Myr northernmost subduction zone moves 280km westwards, 560km after
20Myr.
2h) A is fixed so subduction zones in N and S remain fixed.
Central subduction zone subducts A beneath B and thus with westward movement of B
this trench migrates westward
600km after 10Myr and 1200km after 20Myr.
2j) A is fixed so central subduction zone is static whilst N and S trenches migrate
westwards. Transforms lengthen
2k) A is fixed, ridge in west with transform trench is east.
Linear velocity of B is 100km/Myr, therefore spreading rate is 50km/Myr.
After 10Myr  ridge moves 500km E
 1000km of B is subducted
 500km of oceanic crust is added to each of plates a and B
After 20Myr x 2