EPSC201 - Lecture 9 Notes
Earthquakes pick out the boundaries of
the tectonic plates, zones of spreading
and subduction quite accurately.
An upward push of the mantle causes the crust to
crack. The oceanic crust is quite thin; about 6-
10 km thick. This causes earthquakes.
The dots on the figure below indicate earthquakes.
Pushing the brittle crust downwards causes it to
fracture and break. Anywhere these fractures oc-
cur, there is a potential for deep focus earthquakes.
Subduction = deep and shallow focus earthquakes
Here is a map of the tectonic plates. There is and
equal amount of subduction and spreading, which
keeps the size of the earth constant.
The ocean floors are very young. The age of the
rock can be deter- mined by the mag-
netic striping. The rock nearest the mid ocean ridge is the
youngest, and it gets older moving out-
wards. Obviously, it would be very
difficult to obtain rock samples
from under the oceans; however, it
is easy to drag a magnetometer
across the ocean. Comparing the
magnetic patterns to rock samples
gathered on the surface with the
same magnetic pattern, it is possi-
ble to date the sea floor rocks. The
surface rocks can be dated.
Notice the rate of movement is extremly vari-
able. In someplaces the expansion is rapid,
and other areas, for the same color, it didn’t
expand as quickly.
Zig-zag pattern is called a transform fault.
Only observed in areas of spreading. It is very easy to recognize our conti-
nents from Pangea. However, looking
at Rodinia, our continents are not
obvious at all. Sometimes conti-
nents try to separate. They are not
always successful. Quebec tried to
separate from North America and
failed. There is a mid continent valley
going through Africa, but current calcula-
tions suggest it will also fail to separate.
Passive margin – bit of continental crust
stuck to the oceanic crust. If the crust had
been subducting, it would have been an ac-
tive margin. On the East coast of N.A. we have a passive margin, but on the West
coast of N.A. we have an active margin.
Note the oceanic crust is thinner, and
more dense then the continental crust.
The idea of the continental crust floating
on the mantle is comparable to an ice-
berg, which is mostly below water.
At an ocean trench, the depth of water
could be up to 6-7 km deep, which is
much deeper then the rest of the ocean.
The rest of ocean is about 1-3 km deep.
The subducting plate generates a
trench. The subducting slab will even-
tually break up. As it moves deeper,
the subducting plate becomes more
ductile, and will assimilate with the
Key Point – the mantle is heteroge-
nous Transform Faults: (Professor