1. Geology 301
Oregon Caves National Park Service
U.S. Department of the Interior
Oregon Caves National Monument
Cave Junction, OR
An in depth, advanced look at the geology of the cave and the region.
Oregon Caves is located in the northern reaches
of the Klamath Mountain System, in a subset of
this range known as the Siskiyou Mountains.
Though Oregon is located within the “Ring of
Fire,” the Siskiyou Mountains are not the result of
recent volcanism, but rather of the addition
(accretion) of much older volcanic seafloors and
granites. Together these form the Klamath
superterrane. This is an assemblage of over half
a dozen terranes, (groups of rocks that share a
common history), which became a part of the
accretionary prism west of the North American
Plate; this process can be found in Geology 201.
The Cascade Mountains, found to the east, are
the mountains that were formed by volcanism
long after the Klamath Mountain System had
accreted to North America. The densest part of
the Farallon Plate, an oceanic plate, slid further
and deeper under the North American Plate until
it hit a melting point. The newly melted rock,
known as magma, was less dense than
surrounding rock which enabled it to rise and
erupt at the surface as lava through volcanoes.
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At the same time, an admirable effort is under
way to digitize the voluminous Jefferson Papers
and put them onto the Library’s American
Memory website. What is less well known to the
public is that almost two-thirds of Jefferson’s
original collection of books was destroyed in fire
on Christmas Eve of 1851.
Regional Geology
Volcanism at Oregon
Caves
Some magma never reaches the surface to
become lava. Batholiths are massive magma
chambers beneath the surface, (surface area >
100km2
), that never become a volcano.
Approximately 160 million years ago (think
Jurassic Park!) the Greyback batholith grew right
below the marble, and was exposed inthe “Ghost
Room” when the Caves formed much later Here,
we can see how the batholith shot through what
was a small crack in the marble and widened it,
creating an igneous dike composed of diorite.
With it, the intrusion brought along sulfide
minerals that, when mixed with water and certain
bacteria, formed sulfuric acid. Sulfuric acid is
quite a bit stronger than carbonic acid and thus
helped to make quick work chemically eroding
the “Ghost Room.”. In the course of just a few
hundred thousand years the sulfuric acid, along
with all the cracks, was able to dissolve away a
large portion of the room, making it the largest
chamber in our cave!
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Geology 301
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Farallon Plate
Siskiyou Mountains
Subduction
North
American
Plate

2. CO2 (g) + H2O (l) → H2CO3 (aqueous)
H2CO3 (aq)+ CaCO3 (s) → Ca(HCO3)2 (aqueous)
Ca(HCO3)2(aq) → CO2 (g) + H2O (l)+ CaCO3 (s)
Geology 101 explains and illustrates some of the
major formations that can be seen in the cave,
but the process was oversimplified. While
Geology 101 simplifies the building of formations
to evaporation, it is important to know that the
water, for the most part, is not actually
evaporating away. Instead, most of the process
can be attributed to different carbon dioxide
(CO2) levels in the air and water in the cave.
Carbonic acid (H2CO3) is a weak acid that is
created as carbon dioxide (CO2) in the soil
dissolves into rainwater (H2O) that has filtered
into the ground. Carbonic acid reacts with the
calcite (CaCO3) in the marble and creates an
aqueous solution called calcium bicarbonate,
Ca(HCO3)2. This solution is highly saturated with
CO2.
Calcium bicarbonate drips through the small
cracks and fractures in the rock until it finds a
cave where airflow has flushed out so much
carbon dioxide that the air can be 50 times less
saturated in CO2. than the water is. At this point,
the process is reversed as the CO2 reaches
equilibrium; if there is more CO2 in the water than
in the cave air, the gas will move into the air. CO2
is removed from the solution, resulting in
precipitation of the less soluble CaCO3 that was
once suspended in solution.
.
Cave Formations
Things you may not
have seen…
Calcite Crystals
Moonmilk
Vermiculations
Most calcite crystals in cave formations are so small
it is impossible to see them with the naked eye.
However, there are a few areas in the caves where
it is possible to see such big crystals. These larger
ones formed from a solution only slightly saturated
with calcite. Visible calcite crystals form when an
area has a fairly constant climate, one where calcite
only deposits on a few older crystals that then get
larger. Where oversaturation occurs too fast, small
crystals form on most surfaces. Once there is an
opening nearby, conditions change fast daily.
Formations with many tiny, invisible crystals such as
moonmilk or cave popcorn then begin to form.
Calcite moonmilk usually only grows in cold and wet
environments, and therefore can only accumulate
during the winter months. During this time, air
entering the cave must warm up, thus making the
air dry enough to evaporate cave water. It is
believed to form as the calcite-rich water moves
through the marble and reaches microbial life that
can use nutrients in the water. The microbes eat the
organic acids and then are wrapped around by the
now lesss soluble calcite. Strange shapes can be
produced when the moonmilk completely covers
tree roots.
Vermiculations typically look like convoluted pieces
of brown yarn a few inches long. They grow on
smooth surfaces of cave walls and ceilings that
were flooded with silt-rich water. The more intricate
vermiculations may show the effects of electric
charges and bacteria sticky enough to collect
suspended sediments from floodwaters. Now all
that remains is the particles of dirt clinging to the
roof.
E X P E R I E N CE Y O U R AM E R I C A
™