Transcript Geology of Australia and New Zealand, HWS/UC 2007 2. Plate

2. Plate Tectonics
Geology of Australia and New
Zealand, HWS/UC 2007
Continental drift, sea floor spreading, evidence for Plate Tectonics
The magnetic field of the earth reverses from
time to time, in a random pattern. Magmas that
cool during times when the magnetic field is
“normal” or “reversed” become magnetized by the
prevailing field of the time. These magnetized
rocks retain their magnetization and either add
to or subtract from the present day field
producing positive or negative magnetic anomalies.
The pattern is like a tape recording of the earth’s
field through time, spread out in space,
symmetrically on either side of ocean ridges. The
best explanation is spreading of the sea floor, a
key element in plate tectonics (and continental
drift)
Earth structure, Plate features
Note the difference in thickness between oceanic and continental crust (and lithosphere). This
produces the characteristic topography of ocean basins and continental platforms, as well the
high elevations of the mountain ranges. Also note how relatively thin is the rigid outer part of
the earth, the lithosphere. Most of the earth is relatively plastic, i.e. is capable of flowing over
geologic periods of time.
Divergent Margins
(the plates are moving away from each other)
Iceland is on a divergent plate
boundary. The rocks that make up
most of Iceland are basalts, which
are the result of melting of upper
mantle rocks, followed by rising and
freezing of that magma along the
divergent plate boundary, where it
forms new oceanic (basaltic) crust.
The crust typically doesn’t
accumulate to thicknesses of more
than about 10km because of the
balance between magma generation
and sea-floor spreading. Iceland is
unusual because it overlies a “hot
spot” in the mantle where magma
generation rates are unusually high,
supplying extra magma to make a
thicker ocean crust. This thicker
than average ocean crust is why
Iceland sticks up above sea level.
Convergent margins
(the plates are moving toward
each other)
The top diagram shows subduction of
oceanic lithosphere (with ocean crust on
top) beneath oceanic lithosphere. The gray
beneath the upper plate of ocean crust on
the left side is cold upper mantle that is
part of the lithosphere. You can imagine a
similar thickness of upper mantle being part
of the lithosphere of the subducting plate
(on the right hand side). Release of water
and melting of ocean crust from the
subducting slab rises to the surface and is
deposited within and on top of the upper
slab to make thick crust and an island arc,
like Japan. Unlike a spreading center, the
magma is emplaced again and again in the
same location, building up thick (and
eventually continental) crust.
The bottom diagram shows subduction at a
continental margin (as in the Andes
Mountains). The lithosphere beneath the
continent and the continental crust is
actually thicker than shown here. Note that
the magma is also emplaced at the same
location for an extended period of time,
producing thick continental crust.
Transform Margins
Plates slide past each other at
transform faults. These often
produce shallow, and
destructive, earthquakes.
Often there is an element of
convergence or divergence
(i.e. the plates do not slide
perfectly parallel to one
another) at a transform
boundary, which produces
differential uplift across the
fault.
Hot Spots-what are they and how
are they important?
Plate movement across hot spots is a classic demonstration of relative movement of the plate
and some part of the underlying mantle (where the magma is produced). The sinking of older
parts of the chains of islands associated with hot spots gives rise to atolls and guyots (see
lecture on carbonate reefs).
The convective origin of plate motions
Think about the “stew-pot model” of plate tectonics while looking at this diagram for plate
tectonic motions. The cold lithosphere at the top (which includes the crust) is the product of
magmatism. Continental crust differs from oceanic crust in important ways (see Lecture 4),
but both begin as a result of magmatic activity.