Welcome to beautiful Castle Rocks
State Park here in southern Idaho.
I'm Ranger Tara and today
I'm going to be taking you
on an adventure to see some of the unique
geology that Castle Rocks has to offer.
Castle Rocks Sate Park entrance sign
Geology Rocks
Castle Rock State Park and its sister
park City of Rocks National Reserve
have some really cool geology.
The landscape is scattered with giant
granite pillars that reach hundreds
of feet into the sky. The one behind me is
called Castle Rock.
towering granite formation
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There are two types of granite in
the parks. One is the Green Creek Complex
which is over 2.5 billion years old and
then there's the younger Almo Pluton
which is 28 million years old.
Primarily in Castle Rocks State Park
we find the Almo Pluton and that's
what we're going to be seeing today.
Green Creek Complex 2.5 billion years old
Almo Pluton 28 million years old
There are three types of rock. These are
sedimentary, metamorphic, and igneous.
Granite is an intrusive igneous rock.
Three rock types:
Metamorphic
Sedimentary
and Igneous
Intrusive igneous rock means that these
pillars were formed from cooling magma
under the Earth's crust. As the magma
cooled it created large masses of granite
also known as batholith under the ground.
Batholith:
large mass of intrusive igneous rock
under the earth's crust
For millions of years mountain ranges
around the parks started to rise and
wide valleys were formed. As these
valleys formed the sediments around the
granite started to erode leaving just
the pillars standing in the landscape.
scenic view of granite spires
Uncovered on the Earth's surface,
the granite pillars were suddenly exposed
to mechanical and chemical weathering.
Mechanical Weathering:
Physical process that breaks down
the structure of rocks without
chemical change, like wind abrasion
and freezing.
Chemical Weathering:
The disintegration of rocks
by a chemical reaction, like acid rain
or acidic standing water on the rock.
This weathering has created bizarre and
fantastic shapes and other features in
the granite. Let's go see some.
view of Castle Rock
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Weathering Examples
Welcome to Three Pools Boulder. It got its
name from the fact that it has panholes
on the top, which are unique weathering
effect. We're going to take a look now.
Three Pools Boulder
water in a panhole
We've climbed up onto Three Pools Boulder
and you're looking at one of the
larger panholes that occurs up here.
Panholes begin as tiny depressions or
cracks where water sits for long periods
of time. The longer water stays pooled on
the granite, the more acidic the water
becomes. The water becomes acidic by
either mixing with decaying vegetation or
mixing with minerals that are leaching
out of the granite. But once the sitting
water becomes acidic enough it starts to
eat through the granite causing a panhole.
This would be an example of
chemical weathering.
Panhole:
is an example of chemical weathering.
The water pooling on the granite becomes
acidic and slowly eats away at the
rock underneath.
I'm sitting in another one of the panholes
up here on Three Pools Boulder.
You can see this one doesn't contain any
water at the moment. A neat feature about
these panholes is that they are home to
a really cool little species called
Fairy Shrimp. You'll see them hatching
here in either late spring
or early summer.
Fairy Shrimp in a panhole at Castle Rocks
Close-up view of fairy shrimp
This rock behind me is right next to
Three Pools Boulder and it exhibits
a different type of weathering.
This weathering is called tafoni.
Tafoni is formed when salt is transported
by rain and water from the Great Salt
Lake Basin eats through the granite
causing small little caverns to form.
Tafoni: rounded, cavernous openings in
rock caused by chemical weathering.
Tafoni is also known as honeycomb
weathering and I'm sure you can see why.
Another principle associated with tafoni
called case hardening. As the salt eats
away at the interior portions of the
granite the outside erodes much more
slowly causing a hardened crust to form.
Let's take a closer look.
Here we are with a closer look at the
tafoni weathering. You can see the
caverns on the inside that have been
completely eaten away by the salt and
then the outside is the harder areas that
are slowly eroding
and this is the case hardening.
Advanced stages of tafoni
can cause windows and arches in the rocks!
Window caused by tafoni weathering
Thank you for joining me on this brief
tour of the geology of Castle Rocks
State Park. We look forward to seeing you
in the parks in the future. Remember to
like and subscribe to our videos and if
you have any questions on geology please
leave them in the comment section below.
City of Rocks National Reserve
Castle Rocks State Park