Hey folks, Dr. Chapman here. Today we're
going to talk about Yosemite, the Sierra
Nevada batholith, and igneous processes.
The massive cliff faces, canyons, and
domes of Yosemite, Kings Canyon, and
Sequoia National Parks in the Sierra
Nevada Mountains in California are
mostly made of granitic rocks. "Granitic"
is a generic term that just means like
granite. 
"Granite" is also a catch-all term for
non-specialists as applied to all sorts
of igneous and metamorphic rocks, but
true granite refers to a rock with a
specific composition. The next time
someone shows you their granite
countertops in their home there's a
very high chance that those countertops
are not true granite. Not to say that
there's anything inherently better about
true granite countertops compared to
other rock types. Some of the rocks
in Kings Canyon, Yosemite, and Sequoia
National Park are true granite, but there
are many other intrusive igneous rock
types as well like diorite, granodiorite, and tonalite that are equally or
even more abundant. You can classify
intrusive igneous rocks by looking at
the percentage of the minerals quartz,
plagioclase feldspar, and potassium
feldspar - three of the most abundant
minerals in granitic rocks.
These rocks are called
intrusive because they intruded into the
earth as magmas and then cooled down
and crystallized and hardened very
slowly over thousands to millions of
years. Most of the intrusive rocks in
Yosemite were originally emplaced
as magma seven tp ten kilometers
beneath the surface of the Earth. The
surrounding country rocks helped to
insulate the magma and keep them from
cooling down too fast.  Because these
magmas cooled slowly, the minerals or
crystals had time to grow large enough
to be seen with the naked eye. Igneous
rocks with relatively large interlocking
minerals are called phaneritic and this
texture is what tells us that they are
intrusive rocks.
Conversely, magma extruded or erupted
at the surface of the Earth into air or
water are quenched and cool down very
rapidly so the minerals in those rocks
do not have time to grow
and a microscope is usually needed in
order to see the individual minerals or
crystals. This texture is called a
aphanitic and it helps us recognize
extrusive or volcanic rocks. There's also
an intermediate form or texture of
igneous rocks called porphyritic where
some crystals were able to grow and
others were not. These rocks generally
formed at shallow depths in Earth's
crust, partially crystallized at depth, 
and then were erupted before the entire
rock could fully crystallize. For example
consider an aphanitic andesite and a
porphyritic
andesite. The white crystals in this rock
are mainly plagioclase and the black
minerals are mainly amphibole. The granitic
rocks in Yosemite, Kings Canyon, and
Sequoia National Parks are all part of
the Sierra Nevada batholith
A batholith is a huge mass of intrusive
igneous rock and itself made up of many
smaller intrusive bodies including
plutons and dikes and sills. Most of the
Sierra Nevada batholith formed around
100 million years ago during the
Cretaceous period. The batholith
represents an amalgamation of magma
chambers that once fed a chain of large
explosive volcanoes called a continental arc
because the volcanoes form on the edge
of a continent - in this case the North
American continent. The volcanoes and
volcanic rocks that were once present
above the Sierra Nevada have been eroded
away. If we could take a peek several
kilometers into the Earth beneath
active volcanoes like Mt. St. Helens
or Mt. Rainier, we would find rocks
with similar compositions to those in
the Sierra Nevada batholith. I mentioned
earlier that the intrusive rocks in the
Sierra Nevada span a range of
compositions from more mafic rocks like
gabbro and diorite that have lots of
magnesium- and iron-bearing minerals to
more felsic rocks like granodiorite
and granite that are mostly composed of
feldspar in quartz. To understand why the
composition of these igneous rocks
varies so much let's take a look at the
Tuolumne Intrusive Suite in Yosemite
National Park. Tuolumne Intrusive Suite is
a zoned plutonic complex with older more
mafic phases on the margins and younger
more felsic phases in the center. This would
suggest that the source the Tuolumne
magma evolved over time from a more
mafic composition to a more felsic
composition. The three main processes
that have been hypothesized to produce
this range of magma compositions are
magma mixing, assimilation, and fractional
crystallization. Magma mixing or mingling
is the easiest to understand. An
intermediate composition, like granodiorite,
could be produced by mixing of a
mafic gabbroic magma with a felsic
granitic magma - just like mixing black
and white paint.
Irregular shaped enclaves of mafic rocks within
larger felsic intrusion units have been
associated with magma mixing. The second
process, assimilation, also involves
mixing, but in this case the magma mixes
in and melts part of the host rock or
the country rock. A mafic gabbroic magma
could melt and incorporate a sandstone
country rock and the result would be a
more quartz rich or felsic final
composition. Pieces of country rock are
commonly found inside igneous bodies and are evidence for assimilation. These
pieces of country rock are called
xenoliths. The prefix "xeno" comes from
ancient Greek and means stranger or
foreigner. So xenolith is a rock foreign to
the original magma. The third process,
fractional crystallization, is a little
harder to understand, but it may be the
most important for producing the
spectrum of igneous rock compositions. As a magma body cools, mafic minerals tend
to crystallize first and these may
settle to the to the bottom or to the
sides of a magma chamber. The melt left
over is now more felsic than it
previously was because the early
forming crystals have pulled out
elements like magnesium. This process can
continue through time so that when the
last bit of magma crystallizes the
resulting rock coming from that melt is
very felsic like a granite. Intrusive
suites like the Tuolumne Suite that show
a complete spectrum of rock compositions
from old, early crystallizing mafic rocks
to young, late crystallizing felsic rocks
support the model of fractional
crystallization. The average composition
of the continental crust on Earth is
intermediate, like an andesite or granodiorite. And many geoscientists have
suggested that fractional
crystallization processes in continental arcs
like the one that created the Sierra Nevada Batholith
are what originally created continental
crust. Once created, continental crust
tends to stick around and the oldest
rocks on Earth are pieces of continental
crusts that are three to four billion
years old. So unless you live in Hawaii
or some other island in the ocean, you
may have fractional crystallization to
thank for the ground beneath your feet.
Perhaps millions or billions of years
ago there was a place like Yosemite and
the Sierra Nevada batholith where you
are now. Hey, thanks for watching - check
out some more videos and share them with
friends and family.
Take care.
