UCL Earth Sciences
GEOL1013 The Earth
Tutorial Practical Exercise 3: Plate Tectonics …….. for Fun!
Objective: to encourage thinking about the geometry of relative movements of
tectonic plates in different situations at spreading ridges and at subduction zones.
Here are a series of exercises about plate tectonics which, hopefully, are fun but
should also test your geometrical thinking in the realm of plate tectonics. You
might find it useful sometimes to cut up the diagrams. Please photocopy copies as
necessary.
1). Sea-floor Spreading.
Diagram 1 shows magnetic anomalies determined by cruises which towed a
magnetometer across the North Pacific Ridge. Also shown are ages for dredged
samples measured by radiometric 40Ar-39Ar dating. Clearly rates of spreading can
be computed from dx/dt where x is the distance from the ridge and t the age of
the rock. On the diagram:
• mark the position of the spreading centre
• calculate the mean spreading rate since the beginning of the Tertiary
2). Plate Geometry
Figure 2 shows a series of sketches of plate boundaries. Note
• in each case Plate A is stationary and the linear velocity of Plate B is given in
km/Ma with the direction indicated by the arrow.
• unless otherwise labelled, Plate B is being subducted under Plate A.
• plates labelled U = undestroyed (upper plate) and D = destroyed (lower plate)
• the scale is the same in each sketch.
You should:
• label the boundaries of the plate with symbols for either ridge, trench
(subduction) or transform fault (see box below)
• sketch in second column what the plates would look like today (t=0) had motion
started 10 Ma ago (i.e. to = 10 Ma)
• sketch in third column what the plates would look like today (t=0) had motion
started 20 Ma ago (i.e. to = 20 Ma)
• draw isochrones every 4 Ma in the manner of magnetic reversals
• indicate in each column the position of the anchor, starfish or ammonite which
has been brutally nailed and superglued to the plates!
There is a worked example for your guidance.
Further note: these exercises only crudely approximate plate motion and
geometry in planar two dimensions. The Earth is, of course, a sphere and plates
move with angular motion around a pole.
Diagram for Question 1
Hypothetical Experiment to Distinguish Between Inactive Fracture
Zones and Transforms
1. Plates A and B begin spreading apart 50Ma . As usual, a fracture zone develops
along an offset in the ridge.
2. Astrogeophysicists arrive 30Ma and set up a double row of survey markers on
the seafloor as shown and go home to Krypton, never to return.
3. Seafloor spreading grinds on.
4. While scuba diving, you discover the markers on the seafloor. What do you
make of the pattern? Note that when the circles and squares were attached to the
seafloor at 30Ma, plate B had already moved eastward and both plates had grown
symmetrically, as may be seen from the symmetry of the 50 and 40my isochrones.
Note also that the transform fault (double arrows depicting shear) is active only
between the two ridges, and it is only here that the fracture zone is a transform.
To the east of the transform, the rocks on both sides of the fracture zone labelled
FZ are part of plate B and both sides are moving together. To the west the rocks
on both sides of the fracture zone (FZ) are part of plate A.
History of a Plate Named B
70Ma A former plate B is born and begins
to move toward the east. Luckily a
double row of submarine volcanoes
(labelled a-h) is strategically placed to
help us follow the action. Because of the
polarity of the trench to the east, we
know that plate B is doomed to die.
60Ma In 10 my the ridge has migrated
500km eastward from plate A and plate A
has grown to the same amount. Plate B
has moved 1000km east of plate A. To
the east, 1000km of plate B has been
subducted into the trench. To the west
plate B has grown by 500km, like plate A,
so the net length of plate B has
decreased by 500km.
53Ma Most of the original lithosphere of
plate B (t>70Ma) has now gone down the
trench and some of the new (t<70Ma)
lithosphere is starting down in the
northeast.
33Ma Because the ridge is offset by a
transform, when the ridge meets the
trench, plate B is split into two separate
plates (B and C). Plate C goes under at
26Ma and subduction stops along the
southern part of the trench. The last
piece of plate B goes under at 19Ma. At
this time plate B has been destroyed, all
subduction stops, and plate A is declared
the winner.
Question 2
Definitions and Symbols:
Figures for Question 2 on following pages