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Outcrop at Georgian Bay.

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 units.

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!

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