hi there so you may wonder if we talk
about these rocks in the northern East
Humboldt Range here we're on clover
Hill coming from depths as great as 35
kilometers and you may be wondering how
could we possibly know something what
could be somehow in the rock that could
tell you the depth or the temperature
that it was at and the secret is in the
minerals minerals are really a wondrous
little kind of a zip drives of the
geologic history of the rocks and in
this case the the story of the pressures
and the temperatures that they formed at
is captured by the chemical composition
of some minerals so really we
wouldn't know what pressure and
temperature these rocks were at and
therefore wouldn't know what depth they
formed in the crust unless you had the
right rock so a lot of these rocks
around here the wrong kind of rock
they're made entirely of quartz than
whatever pressure and temperature
they're at that the rock is going to be
made out of quartz but I found one here
this dark it might look ugly to you but
it's beautiful to me rock with these
nice shiny Micas in it and not only
that but there's a pretty blue mineral
when you look closely called kyanite
when you slice there's tiny little red
minerals called garnets the locals
around here sometimes they get large and
old-timers used to call them Ruby
garnets and hence the name Ruby range
the in addition to the garnets there are
black flaky minerals biotite there's a
mineral sometimes the kyanite the blue
mineral will have these little fibers of
sillimanite on it and all those minerals
are sensitive to the pressure and
temperature at which they formed so some
the the kyanite and sillimanite
are actually exactly the same
position kind of like diamond and and
graphite are both made out of carbon but
the kyanite and the sillimanite  are the
kyanite is a higher pressure version
that you get that the the same elements
rearrange themselves into a denser form
at higher pressure and that's the
kyanite and so you know if you have
kyanite in a rock you're above a certain
pressure at any given temperature some
minerals however have a kind of a more
complex story it's not so much the
individual mineral as the geochemistry
the chemical components exchanging
between that mineral on its neighbors so
for example the amount of iron that goes
into Garnet versus biotite when they're
right next to each other changes as a
function of the temperature and this has
been shown experimentally that same
garnet also has calcium in it and the
partitioning or the exchange of calcium
between the garnet and the that is
neighboring plagioclase crystals can
give you important controls on the
pressure at which the rocks were so we
have a number of things going on in this
kind of complex rock that ultimately
record its pressure and so several years
ago a scientist Kip Hodges is was his
name collected these and studied them
and he got pressures as high as 10 Kilo
bars for these rocks and not only that
but as the minerals grew they changed in
composition and so he was able to trace
out a path a a sequence of changes and
pressure and temperature that showed
that these rocks went from a kyanite
grade metamorphism into sillimanite so
you have these little feathers of  sillimanite
on the kyanite
the temperature increased and at the
same time the pressure was going down
what could that mean
the rocks even as they were heating were
somehow being brought closer to the
surface and so that's the kind of really
amazing stories that you can get out of
looking at something an incredible
detail and that's a lot of the story of
geology is that these little clues these
little hints can lead to really much
kind of grandeur stories and you know a
kind of amazing vision of how the earth
has evolved through time and how
ultimately Clover Hill formed and how it
got uplifted to the Earth's surface
