These rocks from the northern shore of Georgian
Bay once were deeply buried. Having been thrust upon the North American
craton during the Grenville Orogen, they now are proof of this event
which took place prior to 1,000 Ma, that is one billion years ago!
This is an example of using geological field mapping for tracing
events which took place in the distant past and at various depths
within the crust of the lithosphere. This includes the use of geochemical,
geochronological, and petrological methods to determine the constituents
(source), history (time of formation), and depth (temperature and
pressure at formation) of the exposed rock units and the minerals
contained in them.
Other surface observations which can be made in outcrops include
the orientation of magnetic minerals, which aligned themselves with
the magnetic field at the time of their solidification. (We already
touched upon this earlier.) Their preserved orientation depends
on their geographic location when their alignment occurred, that
is their geographic latitude. Thus, the very concept of plate tectonics
can be tested here, since these rock units were transported as their
host plates moved.
Thirdly, their juxtaposition in relation to other rock units has
to be taken into account since tectonic forces moved some rock units
into new positions, as was the case with the thrust rock unit shown
in the slide. All of the above observations come into play when
one undertakes to unravel the present position of observed rock
Of course, we want to know what is going on below the surface,
deep inside the lithosphere on which we live.
Remember the seismic cross section from the Atlantic coast? The
sound waves we send from the surface to great depths in the lithosphere
return as echoes (reflections) to the surface, where we measure
them with geophones. More about the seismic reflection technique
later; it is quite fascinating!