Hi! This is Bob with the Pinal Geology
and Mineral Museum in Coolidge, Arizona.
Today I'm going to talk about Pinal
Schist. Pinal Schist is a metamorphic rock,
made up mostly of quartz and mica. The
rock has been metamorphosed from
sandstone and slate by burial and
heating. In this Boulder near Casa Grande
you can see the layered structure of the
Schist. The rock here has also been
weathered and broken, almost splintered.
This has happened to much of the
Pinal Schist now at the surface. You can
see it at a smaller scale in the rock
behind me which is close-up of the one
pictured in the window. A closer look at
a piece of Schist, this one I'm holding,
shows the glittering pieces of grayish
mica. This gives the rock a special shine when
moved in the light, at least close up. The
Pinal Schist was named in 1903 from
the Pinal mountains between Globe and Superior. It's the basement rock in southern Arizona.
This means that it forms the bulk of the continental crust below our feet.
It's the rock floating on the mantle supporting this part of the North
American continent. This means also that
it's the oldest rock in southern Arizona,
including our own Pinal County.
Pinal Schist was formed about
1.6 billion years ago. At the time, what is
now Arizona was located
at the South Pole. It had been underwater for most of the previous billion or so years.
The nearest continent was land that is now in Wyoming. The erosion off that continent
built up the sediments under the ocean
was compressed into sandstone and slate
and then further buried and heated to be
metamorphosed into the Pinal Schist.
That rock was not completely melted,
which is why we can see the quartz,
and mica, and the layers in the Pinal Schist. Today the Pinal Schist can be found
near the surface over a wide area of Southern Arizona.
There are outcrops on many of the uplands from Phoenix, south to Bisbee.
If you go looking you should be able to find some Pinal Schist for your photo album.
or your collection. Thanks for watching our video. If you're watching on YouTube,
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Thanks again until next time.
This is Bob, with the Pinal Geology and Mineral Museum, signing off.
