Igneous Rocks The Volcanic Stone of Missouri
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O F M I S S O U R I
VOLUME 3, ISSUE 1 Summer 2008
DNR photo by Jerry D. Vineyard and David Hoffman
Aerial view of Elephant Rocks State Park.
Igneous Rocks
The Volcanic Stone of Missouri
The St. Francois Mountains The igneous rocks are core and granite in the St. Francois
of southeast Missouri are known features in some of Missouri’s Mountains led to the develop-
for their exposures of 1.5 to 1.2 most spectacular scenery and ment of settlements and towns,
billion year old igneous rocks. popular state parks, including and was often the impetus behind
These exposures are remarkably Johnson’s Shut-Ins, Elephant the development of infrastructure
fresh and unaltered, and are Rocks and Taum Sauk Mountain. in the region. Today, this region is
unique within the mid-continent Each of these parks owes its still a major producer of mineral
region of the United States. The unique character to the igne- products.
area also gives a rare glimpse ous rocks and to the geologic
into rocks that are formed by the processes that have formed and
most violent volcanic eruptions shaped them.
known on the earth. Consequently, The rocks have also provided
geologists come from around the a livelihood for generations of MISSOURI
DEPARTMENT OF NATURAL RESOURCES
world to study them. Missourians. Early mining of iron Division of Geology and Land Survey
Missouri’s Volcanic Past Granites and
Approximately 1.4 billion years
ago, southeast Missouri was a land-
scape dominated by volcanic calderas,
the more viscous, or thick, will be the
lava. Viscous magmas tend to hold
more volcanic gas and therefore erupt
Rhyolites
some of which were up to 15 miles in explosively. Rhyolite calderas produce Granite is the most common
diameter. These calderas were different the most violent eruptions of any type of igneous rock type at depth in Mis-
from volcanoes active today in places volcano on earth. souri, but it makes up very little of
like Hawaii. Instead, they more closely the exposed rock in the St. Francois
resembled the Yellowstone caldera,
Formation of a Caldera Volcano Mountains. Most of the outcrops are
which today, is the largest dormant Volcanic Eruption Pre-existing rock is a volcanic rock called rhyolite. While
pushed upward and the two rocks look very different, they
volcano in the United States. Unlike Rhyolite fractured by magma
“typical” volcanoes, calderas do not have essentially the same chemical
in the chamber below.
erupt from one main vent or develop a composition and are made up of the
Rhyolite ash and lava
large cone-shaped mountain. Instead, MAGMA erupt to the surface. same minerals. Granite was formed
magma (molten rock) from an underlying from magma that never
Volcanic Collapse reached the surface.
chamber moves upward under pressure After eruptions, the
and erupts lava from multiple vents in a Because it cooled
overlying rocks collapse
series of smaller eruptions. Over time, into the mostly emptied slowly under-
the pressure in the magma chamber MAGMA magma chamber. ground, it had
time to de-
builds to the point that the overlying Erosion
rock cannot withstand the pressure and As the caldera becomes
velop large
a giant eruption occurs, forming a crater extinct, remaining magma crystals
miles in diameter. Layers of volcanic cools into granite. of
GRANITE Overlying igneous rocks
rock form thousands of feet thick. slowly begin to erode.
Close-up of Missouri red granite
Large calderas such as these gener-
ally erupt rhyolite (see Granites and Today the minerals quartz, feldspar and
Rhyolites), a volcanic rock that is very mica. Rhyolite, on the other hand,
The rhyolite has eroded, erupted from a volcano, cooling so
high in silica, a compound comprised exposing the large granite
of the elements silicon and oxygen. fast that the lava did not
GRANITE body at the surface. The
The higher the silica content in a magma, harder rhyolite forms knobs. have time to grow
large crystals.
Most of the
Lava vs. Ash crystals that
make up
When many people think of volcanic eruptions, they picture lava flows. rhyolite
While some of the volcanic rocks in Missouri are lava flows, most of them are a are
type called ash flow tuffs, similar to those formed by the Mt. St. Helens erup- so Close-up of Missouri rhyolite
tions in the early 1980s. Ash-flows occur when the lava explodes so forcibly that
it comes out in small droplets that immediately harden into fi ne, gritty pieces small that one needs a microscope
of rock, called volcanic ash. The ash is mixed with hot volcanic gases that are to see them.
also part of the eruption. This mixture of ash and hot gases flows like a liquid Because the mineral grains are
down the side of the volcano, and is called a pyroclastic flow. Pyroclastic flows larger, granite in the St. Francois
can move at speeds of up to 100 miles per hour, and may be near 1,800 degrees Mountains weathers and erodes
Fahrenheit. The ash is so hot that it welds together when it falls to the ground, faster than rhyolite. Therefore, areas
forming a solid rock. This is the type of eruption that destroyed the Roman with granite bedrock are often very
cities of Pompeii and Herculaneum, except that the eruptions from Missouri’s low knobs or lowlands. Rhyolite is
volcanoes are estimated to have been more than 100 times larger. much more resistant to weathering,
Other volcanic rocks in Missouri are called ash-fall tuffs. These igneous rocks and makes up most of the steep,
form when smaller eruptions occur. The volcanic ash is blown up into the air, but high knobs that form the St. Francois
does not contain as much volcanic gas as when a pyroclastic flow is unleashed. Mountains.
The ash falls to the ground, or is blown by winds before being deposited. Volcanic Other igneous rocks found in
ash that is blown high into the atmosphere can go around the Earth several the St. Francois Mountains include
times before it settles to the ground. trachyte, diorite and syenite. These
look very much like rhyolite, except
Pyroclastic Flow Pyroclastic Flow they are black or near black in color.
They contain less quartz and feldspar
than rhyolite, and more minerals that
are dark in color that contain high
amounts of iron and magnesium. They
are exposed in only a few places, but
weather (and form knobs) like rhyolite.
2
A Moment in Time
Iron Mountain is located in St. Fran- the M.A. Hanna Company made the de- date, the company began processing
cois County and was originally given in cision to cease iron mining operations piles of unmineralized igneous rock,
1797 to James Pratt, a settler of great at the site. Several years before this called waste rock, which was left over
influence in the Upper Louisiana from the iron mining for use as
territory, as part of a land grant aggregate, or crushed stone.
by the Spanish government. Today, the site still produces
At the time, the mountain was igneous rock once considered
believed to be entirely iron ore. In waste from the mining opera-
1836, the property was conveyed tion as aggregate, making it
to the Missouri Iron Company, the oldest continually mined
DNR photo by Art Hebrank
which began mining the site. The property in the United States.
first mining was of ore exposed at The hoist for the underground
the surface, but operations soon workings and many of the origi-
changed to underground mining. nal buildings remain on the
Iron Mountain was mined almost site, preserved by the current
continuously by various compa- Headframe, buildings and water tanks from the Iron Mountain mine, which and past operators as a living
nies from 1836 until 1966, when began mining in 1836. piece of Missouri’s history.
St. Francois Mountains Young and Old
The history of the St. Francois Mountains began when When rocks are removed by erosion and the erosional
repeated rhyolite ash-flows, ash-falls and lava eruptions surface is then covered by younger rocks, unconformities
cooled and solidified on the surface. This process lasted for occur. Unconformities represent gaps in the geologic record
at least several hundred million years. When the magma similar to pages missing from the middle of a book.
chambers emptied after large eruptions, the overlying rock In the St. Francois Mountains, there are numerous places
would fracture and collapse. Large blocks of rhyolite would where a person’s hand can span rocks that bracket 900
tilt and slip against one another. Magma chambers refi lled million years of missing geology — a period of time that rep-
and the calderas erupted many times. At the same time, resents more than one-fifth of the geologic history of the Earth.
granites intruded into the pre-existing rock and solidified Fossils are often useful indicators of the age of rocks.
below the surface. The total thickness of the granite and Igneous rocks, however, do not contain fossils, because they
rhyolite sequence is not known, but is easily tens of thou- either formed deep
Sea with knobs forming islands
sands of feet. underground or
Eventually, the underlying magma cooled and the calde- were too hot for
ras became inactive. The igneous rocks eroded, forming a any life to survive
mountainous topography. Geologists estimate that as much their placement on
as 6,000 feet of igneous rock may have been eroded to form After eruptions stopped, igneous rocks and calderas the earth’s surface.
the range of knobs and hills that we call the St. Francois eroded into a series of knobs or mountains. Instead, radioactive
Mountains. The calderas themselves were also eroded, so Sedimentary rock isotopes are used
that today we see only the roots of the once giant calderas. to date igneous
Roughly 520 million years ago, the Ozark region was rocks. Radioactive
Around 530 million years ago
lowered due to continental movement. Seas inundated the elements, such as
area and began to deposit sedimentary rocks — fi rst beach- The seas inundated the area, depositing conglomer- uranium, naturally
like deposits of sandstone and then limestone and dolomite. ates and sandstone, then limestone and dolomite. decay. This decay
The gap in time between the igneous rocks forming
The seas advanced and retreated many times, burying the and the conglomerate is our great unconformity. occurs at a known
St. Francois Mountains under layers of sediment that lith- Sedimentary rock rate. Geologists
ified, or hardened, into sedimentary rocks. fi nd the percent-
Recently, in geologic terms, the Ozark region has been age of atoms that
uplifted due to pressure on the tectonic plate that carries the have experienced
North American continent. The uplift led to increased ero- decay and use this
sion, removing the sedimentary rocks from the core of the Sedimentary rocks continued to be deposited as the as a clock to tell
St. Francois Mountains. The same rugged topography that seas came and went. The knobs were likely, com- how long it has been
pletely covered by sedimentary rocks.
we see exposed in just a few counties in southeast Missouri since the rock formed.
lies buried under hundreds to thousands of feet of sedimentary Knobs This is how we know
rock throughout the Ozarks. As the area continues to be that the igneous
slowly uplifted, more of the buried mountain range will be rocks in the St.
exposed at the surface. Francois Mountains
The seas retreated for the last time and the sedimentary are 1.5 to 1.2 billion
rock eroded, exposing the knobs. The erosion occurred
because this area of Missouri was being uplifted. years old.
3Elephant Rocks Minerals and Mining
The granite boulders in Elephant The unique geology in the St. souri until 2001, when Pea Ridge,
Rocks State Park are not only beautiful Francois Mountains led to the forma- the last operating underground iron
and enjoyable, but are also a textbook tion of numerous mineral deposits. mine in the United States, ceased
example of weathering granite. The The location of the deposits, in turn, production. The mine is located ap-
granite formed thousands of feet below affected settlement proximately 16 miles
the surface, but has been uplifted and in the area and the Granite has a long history of use as southeast of the town
exposed by erosion. Cracks formed in area’s history. Names a building stone, and is found in houses of Sullivan. Iron from
the granite when it cooled, and stresses like Ironton, Iron Pea Ridge was used
caused by the uplift led to greater frac- around the state and in buildings from
Mountain and for steel production,
turing of the rock. When the granite was Graniteville reflect the California to Massachusetts. coal desulfurization,
exposed at the surface, water and ice mineral products around which high-density concrete
began to widen the fractures and erode those communities were founded. and iron pigments. Large reserves of
the granite. The granite at the surface Metals like iron, copper and manganese high-grade iron ore still remain under-
became a series of oblong blocks. The and stone products from rhyolite and ground in Missouri’s igneous rocks,
corners and edges of the blocks became granite have been produced from this giving the state the potential to return
rounded due to weathering and chemi- region for centuries. to a position as a major iron producer.
cal reactions of the rock with water Iron was the
and air. Eventually, the rounded blocks primary mineral
became the huge isolated boulders, commodity
or “elephants,” that are sitting on the produced from
granite outcrop. igneous rocks.
The basins, often called “birdbaths,” The exposed St.
that are near the “elephants” were origi-
Francois Moun-
nally shallow depressions that could
tains, as well as the
accumulate
igneous rocks still
water. The
buried beneath
combined
the sedimentary
chemical
rocks, host a
reaction
world class iron
DNR photo
between the
granite and ore district. Iron
DNR photo by Bill Duley
the water, metal was fi rst
as well as produced in 1815, An example of a granite quarry, near Graniteville.
continual when surface
freezing and veins of magnetite (iron oxide) were Granites and rhyolites are also
thawing, mined on Shepherd Mountain, near mineral commodities produced in
continues Numerous birdbaths in the granite at Ironton. The surface iron mines at Missouri. Missouri granite has a long
Elephant Rocks State Park. Iron Mountain and Pilot Knob were history of use as a building stone, and
to cause
the granite to fragment and weather, also developed in the fi rst half of the is found in houses around the state
gradually deepening and 19th century. The fi rst iron furnace and in buildings from California to
widening the basins. west of the Mississippi River was con- Massachusetts. Graniteville granite,
structed near Ironton at this time as found at and near Elephant Rocks
well. The iron was shipped by wagon State Park, was used extensively in St.
to St. Louis and other river ports. This Louis for cobblestone streets, build-
Rock Types eventually proved too costly, and led
to construction of the St. Louis and
Iron Mountain Railroad, completed
ings at Washington University and
for Eads Bridge. Graniteville granite
is also in the Illinois and Iowa state-
All rocks found on and in the
in 1858. It later became the Missouri houses and the Missouri Governor’s
earth can be divided into three
Pacific Railroad, one of the major rail Mansion. Rhyolite is also quarried, and
main types. Igneous rocks are
systems in the nation. today, is used to make the granules
rocks that form by the cooling and
Iron production continued in Mis- found on roofi ng shingles.
solidification of molten rock. Sedi-
mentary rocks like limestone and
sandstone form either by precipita-
tion from water or by the erosion of
other rocks and redeposition and
cementing of the eroded grains.
Metamorphic rocks, which are rare
in Missouri, form when the minerals
and textures of a rock are changed
by extreme heat or pressure. Igneous rock Sedimentary rock Metamorphic rock
4Fascinating Geology Revealed
In December 2005, a hydropower geologists, geology students and to New geologic drill hole information was
reservoir facility located on top of Prof- anyone interested in the geologic collected and used in conjunction with
fit Mountain in rural Reynolds County history of this unique region. archived data. Support was also provided
failed. The event affected Johnson’s The division has supported efforts for assessment and repair of the environ-
Shut-Ins State Park and water quality to understand the failure mechanism mental damage related to the event. The
and aquatic habitat in the East Fork for the reservoir and the geologic and division has also provided geologic ex-
of the Black River as well as in down- geophysical limitations for the safe pertise for interpretative displays within
stream sections of the Black River. rebuild of a new pumped storage facility. Johnson’s Shut-Ins State Park.
The flood scar revealed a dramatic
exposure of the igneous and sedimen-
tary bedrock that make up the west
flank of Proffit Mountain. The hillside
provides a unique geologic workshop
for current and future generations of
The flood scar is slightly more than 1.5 miles in length
from the reservoir to the former bed of the East Fork of
the Black River. The flood scar left behind after approximately 1.3 billion gallons of water rushed down the side of the Proffit
Mountain, exposing igneous bedrock.
Archiving Geologic Data
A mission of the Div ision of vary from surficial ronmental studies,
Geology and Land Survey is to archive materials collected and academic
geologic data of different types and to for earthquake haz- purposes. Core
make this information available for ard and glacial mate- samples are an im-
inspection by geologic professionals rial studies to deep drill portant scientific and
and the public. The division archives cores that enhance our un- economic resource that
include everything from paper derstanding of the igneous rocks would be very expensive or
information to rock samples. The that are buried under thick layers of impossible to duplicate. Maintaining a
McCracken Core Library contains sedimentary rock. The drill cores are core library greatly increases the de-
more than 2.5 million feet of drill core used to explore for ore deposits, for partment’s ability to provide technical
from around the state. The drill cores geologic mapping, hydrologic and envi- assistance and geologic information
to those who develop, market, manage
or regulate the state’s energy, land,
and water resources.
The Division also archives infor-
mation about past mining activity in
DNR photo by Mark Gordon
Missouri. Part of this archive contains
historic mine maps. The maps show
surface and underground mines that
produced commodities including:
iron, lead, zinc, limestone and
sandstone, among others.
5Web Sites
Taum Sauk State Park Elephant Rocks State Park
www.mostateparks.com/taumsauk.htm www.mostateparks.com/elephantrock.htm
Published by the
Johnson’s Shut-Ins State Park Elephant Rocks Fact Sheet
www.mostateparks.com/jshutins.htm www.dnr.mo.gov/pubs/pub683.pdf Missouri Department
of Natural Resources
Division of Geology
Publications and Land Sur vey
RI-64. Geology of the Precambrian St. Francois Terrane, Southeastern Missouri, (Contribu-
tion to Precambrian Geology No. 8), by Eva B. Kisvarsanyi, 58 p., 9 figs., 10 tbls., 1:250,000-scale map,
Joe Gillman
Director and State Geologist
1981. Division of Geology and Land Survey
RI-67. Guidebook to the Geology and Ore Deposits of the St. Francois Mountains,
Missouri, (Contribution to Precambrian Geology No. 9), by Eva B. Kisvarsanyi, Arthur W. Hebrank, Bill Duley
and Richard F. Ryan, 119 p., 75 figs., 8 tbls., 1981. Deputy Director and
Assistant State Geologist
OFM-83-160-MR. Geologic Map of Exposed Precambrian Rocks in the Iron Mountain Division of Geology and Land Survey
Lake Quadrangle, Missouri, www.dnr.mo.gov/geology/sitemap/rolla/ro8301.htm
OFM-83-161-MR. Geologic Map of Exposed Precambrian Rocks in the Wachita Mountain
Cher yl Seeger
Contributing Author
(Fredericktown NW¼) Quadrangle, Missouri, (Contribution to Precambrian Geology No. 11), by
M.E. Bickford and J.R. Sides, 1:24,000
Hylan Beydler
OFM-83-162-MR. Geologic Map of Exposed Precambrian Rocks in the Lake Killarney Division Information Officer
Quadrangle, Missouri, (Contribution to Precambrian Geology No. 12), by J.R. Sides, 1:24,000.
Mark Gordon
These and other publications may be purchased from the Missouri Department of Natural Resources’ Layout and Graphics
Division of Geology and Land Survey. To order, contact the publications desk at: 573-368-2125 or
1-800-361-4827, or use our online form at: www.dnr.mo.gov/geology/adm/publications/MapsOrder.
htm. For additional information visit our Web site: www.dnr.mo.gov/geology/.
MISSOURI DEPARTMENT OF NATURAL RESOURCES PRSRT. STD.
Division of Geology and Land Survey U.S. POSTAGE
P.O. Box 250, Rolla, MO 65402-0250 PAID
PERMIT #215
ROLLA, MO
O F MISSOURI
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