America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
[This list is just a sample of
various South Dakota volcanic features or events and is by no means inclusive.]
In general, the oldest rocks in South Dakota formed more than 2 billion years ago,
during the Precambrian.
They consist of granites and metamorphic rocks
and are found in the core of the Black Hills and in two small areas in eastern South
Dakota.
In the Black Hills of western South Dakota, great sheets of granite
intruded the igneous
and metamorphic rocks. The
Harney Peak Granite was carved to form Mount Rushmore.
Many thousands of pegmatites were also intruded during
Precambrian time. In the southern Black Hills,
these pegmatites have been mined for their crystals of feldspar, quartz,
mica, beryl, and lithium minerals. Some have also been mined for tin and tungsten.
In northeastern South Dakota, the
Precambrian Milbank Granite is exposed and quarried at the surface. In southeastern
South Dakota, the Sioux Quartzite is exposed and quarried at the surface.
By the end of the Precambrian (570 million years ago),
South Dakota had been deeply eroded and worn to a nearly flat
plain interrupted by low knobs of granite and ridges of
resistant quartzite. The top of Precambrian rocks in South Dakota
slopes generally from east to west across all but the southwestern part of the state.
Excerpts from: South Dakota Department of Environment and Natural Resources Website, 2001 |
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The Interior Plains:5
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Precambrian:1
Paleozoic:1
Mesozoic:1
Tertiary:1
Pleistocene:1
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| Badlands National Park |
Badlands National Park:2
At Badlands National Park, weird shapes are etched into a plateau of soft sediments and volcanic ash, revealing colorful bands of flat-lying strata. The stratification adds immeasurably to the beauty of each scene, binding together all of its diverse parts. Viewed horizontally, individual
beds are traceable from pinnacle to pinnacle, mound to mound, ridge to ridge, across the
intervening ravines. Viewed from above, the bands curve in and out of the valley like contour
lines on a topographic map. A geologic story is written in the rocks of Badlands National Park, every bit as fascinating and colorful as their outward appearance. It is an account of 75 million years of accumulation with intermittent periods of erosion that began when the Rocky Mountains reared up in the West and spread
sediments over vast expanses of the plains. The sand, silt, and clay, mixed and interbedded
with volcanic ash, stacked up, layer upon flat-lying layer, until the pile was thousands of feet
deep. In a final phase of volcanism as the uplift ended, white ash rained from the sky to frost
the cake, completing the building stage.
White Ash Layer:2
A 65 million-year-old layer of black rock called the Pierre Shale
formed on the bottom of an ancient sea and is the oldest formation in the
Badlands. Next come the layers of the Eocene and Oligocene epochs. These
hold fossil bones of land mammals.
At the close of the Eocene (about 34 million years ago), this land was
a broad marshy plain crossed by sluggish streams
flowing from highlands to the
west. As the Oligocene Epoch drew to a close,
volcanoes to the west and
southwest ejected huge volumes of ash into the atmosphere.
Borne eastward by
the winds, the ash fell and became
the whitish layer near the top of the
Badlands formations.
Rockyford Ash:2
The climate began to dry and cool after the Eocene and the forests
gave way to open savanna. New mammals such as oreodonts -sheep-
like, her mammals - began to dominate. The Brule and
Sharps Formations were deposited by rivers during the Oligocene
Epoch from 26 to 32 million years ago. A thick layer with a very high
ash content, the Rockyford Ash, is the bottom layer of the Sharps
and serve as the boundary between the two formations.
Sharps Formation:8
The Sharps Formation reflects an overall change in geologic and environmental conditions that began 26 to
28 million years ago. The region became even drier, and mammoth ash fall deposits associated with
tremendous ignimbrite-style volcanic eruptions in the Great Basin region episodically blanketed the
Badland region. The Sharps consists mostly of tuffaceous sandstones, stream channel sand and floodplain
mud (paleosols) typical of steppe or even desert-like conditions. Harksen and others (1961) who first
described the formation thought its age was Miocene. The Sharps Formation is now recognized to be latest
Oligocene but is assigned to the Arikaree Group (a group dominated by Miocene-age rocks elsewhere in
the region).
The highest peaks in the badlands consist of the eroded remnants of Sharp Formation. More
complete exposures of the Sharps Formation can be seen at Agate Fossil Beds National Park in
northwestern Nebraska (about 200 miles southwest of Badlands National Park). The Sharps Formation
marks the end of floodplain-style deposition preserved in the park region. If younger Tertiary deposits
existed in the region they were stripped away by erosion. At the close of the Tertiary Period erosion was
the dominating force in the region. Down-cutting streams began to exhume the Oligocene strata. On some
of the highest tableland in the park area are gravel deposits consisting of materials carried by fast-moving
streams flowing away from the Black Hills. These deposits, called the Medicine Root Gravels, are about 2
million years old and were deposited prior to or near the beginning of the ice ages of the Quaternary period.
Much of the Badlands landscape is a reflection of the erosional history of the region over the past two
million years.
The Rockyford Ash defines the base of the Sharps Formation.
The ash bed forms a white, massive resistant, ledge-forming cap rock.
| Bear Butte State Park |
Bear Butte State Park:6
Mato Paha or "Bear Mountain" is the Lakota name given to the
unique formation at Bear Butte State Park. This formation is a lone
mountain, not a flat-topped "butte" as its name implies. It is one of
several intrusions of igneous rock that formed millions of years
ago along the northern edge of the Black Hills.
Bear Butte - National Natural Landmark:9
Five miles north of Fort Meade in Meade County. A
cone-shaped mass of igneous rock standing alone, 1,300 feet
above the surrounding plains, which illustrates the geological
processes of igneous intrusion, folding and faulting, and
exposure by differential erosion. Owner: State.
DESIGNATION DATE:
April 1965
| Black Hills |
The Black Hills:7
The Black Hills is a huge, elliptically domed area
in northwestern South Dakota and northeastern Wyoming,
about 125 miles long
and 65 miles wide. Rapid City, South Dakota,
is on the Missouri Plateau at the east edge of the Black Hills.
Uplift caused
erosion to remove the overlying cover
of marine sedimentary rocks and expose the
granite and metamorphic rocks that form the
core of the dome.
The peaks of the central part of the Black Hills
presently are 3,000 to 4,000 feet above the surrounding plains.
Harney Peak,
with an altitude of 7,242 feet, is the highest point in South Dakota.
These central spires and peaks all are carved from
granite and other igneous and
metamorphic rocks that form the core of the uplift.
The heads of four of our great Presidents are
sculpted from this granite at
Mount Rushmore National Memorial.
Joints in the rocks have controlled weathering processes that
influenced the final shaping of many
of these landforms. Closely spaced
joints have produced the spires
of the Needles area, and widely spaced joints
have produced the rounded forms of
granite that are seen near Sylvan Lake.
Black Hills Geologic History:1
During most of Cretaceous time,
North America contained a central
north-south seaway that accumulated thick deposits
of marine sediments. The end of the
Cretaceous marks the final retreat
of the continental seas from South Dakota. It also
marks the birth of the Black Hills in
western South Dakota. As the Black Hills
area was domed upwards, erosion actively
attacked the soft shales and finally
the harder and older sediments.
Erosion removed more than 6,000 feet of
sedimentary rock layers from the central part of the
Black Hills. During the later stages of
uplift, great volumes of molten
rock were forced upwards, forming many of today's
valuable mineral deposits (i.e., gold) in the northern Black Hills.
Erosion continued to be the dominant
force as Tertiary rock history began.
By Early Oligocene time, stream gradients
were so reduced that the streams could no
longer carry away their erosion products,
and deposition started on the plains
adjacent to the Black Hills. Gradually,
the lower two-thirds of the Black Hills
became buried by light-colored clays and
sands, derived not only locally,
but from mountain areas to the west.
Volcanic activity, probably near Yellowstone Park,
contributed large volumes of windblown
volcanic ash to the sediments.
By the end of Oligocene time, it is possible that
the Black Hills projected less than
2,000 feet above this apron of sediments.
Uplift, or a change in climate, or both,
caused a renewal of erosion.
The soft unconsolidated sediments were attacked
and gradually, the lower part of the Black Hills were exhumed.
The Black Hills today probably looks very much like they
did 40 million years ago.
The sediments, eroded and carved
into a very distinctive type of topography, can be viewed in
the Badlands in southwestern South Dakota.
| Cathedral Spires |
Cathedral Spires - National Natural Landmark:9
Custer State Park, 23 miles southwest of Rapid City in Custer
County. An excellent, rare example of joint-controlled
weathering of granite. Owner: State.
DESIGNATION DATE:
May 1976
| Custer State Park: |
Custer State Park:6
Custer State Park in the Black Hills encompasses 73,000 acres
of spectacular terrain and an abundance of wildlife.
Within the park, you’ll discover a world of adventure!
Favorite outdoor activities include hiking 7,242-foot Harney Peak,
mountain biking, horseback riding, rock climbing, fishing,
chuckwagon suppers and jeep rides to see the bison.
The park boasts scenic drives such as the Needles Highway (SD 87),
which twists and turns its way past towering rock
formations and through narrow tunnels.
At the end of one tunnel stands the Needles Eye,
a granite spire with a slit only 3 to 4 feet
wide but reaching 30 to 40 feet in the air.
Cathedral Spires - National Natural Landmark:9
Custer State Park, 23 miles southwest of Rapid City in Custer
County. An excellent, rare example of joint-controlled
weathering of granite. Owner: State.
DESIGNATION DATE:
May 1976
| Mount Rushmore National Memorial |
Mount Rushmore National Memorial:3
Mount Rushmore National Memorial is located along the
northeast edge of what is known as the Harney Peak
Granite Batholith
in the Black Hills of South Dakota.
A batholith is a geologic feature that formed by the
cooling of a large igneous body of magma
below the earth's surface;
if a similar igneous body reaches the earth's
surface, it would form a volcanic feature such as a lava flow.
The Black Hills magma was emplaced into the older "host"
mica schist rocks during
Precambrian time, approximately 1.7 billion years ago!
Harney Peak Granite:3
The Harney Peak Granite
(of which Mount Rushmore is carved) consists of
fine-grained minerals including quartz, feldspar, muscovite and biotite. It is believed that
these minerals formed approximately 8 miles below the earth's
surface from molten magma. Some cracks developed as a result of the cooling of the magma and were
later "patched" with molten magma. The result was the emplacement
of pegmatite dikes that filled the fractures and zones of weakness in the granite. Today these
pegmatite dikes are expressed as white streaks on the foreheads of
Presidents Washington and Lincoln. Elsewhere in the Black Hills,
economically significant mineral
deposits are found associated with these pegmatite bodies.
The Harney Peak granite was likely exposed at the surface prior to
Cambrian time, but was covered by sandy sediment when the Cambrian seas invaded the Black
Hills some 550 million years ago. Today,
these sands are part of the Deadwood Formation sandstones that contain grains derived from the ancient Harney Peak
granite and the exposed
Precambrian surface.
The granite core of the Black Hills continued to be further
buried during the rest of the Paleozoic and Mesozoic eras of
geologic time and wasn't exposed to surface processes again
until some 50 million years ago when today's Black Hills began to take on their present form.
| Wind Cave National Park |
Wind Cave National Park Pegmatite:4
Wind Cave formed in the Pahasapa Limestone.
This limestone was deposited in a warm
shallow sea about 350 million years ago and is composed mostly of fragments of
calcium carbonate sea shells. Coinciding with the accumulation of limestone, bodies of
gypsum (calcium sulfate) crystallized from the sea water, when arid conditions caused
evaporation. The gypsum formed irregular shaped masses within the limestone.
The oldest rocks are exposed in the northwest part of the park. These
are schists and pegmatites. The schists are
metamorphic rocks which
formed under heat and intense pressure during an early episode of
mountain building, about 2 billion years ago. They have almost
parallel bands, or foliation, caused by the growth of mica crystals
under pressure.
Pegmatites are made of large crystals of glassy-gray quartz, pink
feldspar, silvery micas, and shiny black tourmaline. Pegmatite is an
igneous rock, similar to granite.
It hardened from magma and hot
fluids. In places, the pegmatite intruded into the schists. This proves
the pegmatite is younger than the schists, but still very old at 1.7 billion years.
The emplacement of the pegmatite probably
occurred during another mountain building event.
Schists and pegmatites are visible along State Route 87.
Excerpts from:
1) South Dakota Department of Environment and Natural Resources Website, 2001
2) U.S. National Park Service Website - Badlands National Park, 2000, 2001
3) U.S. National Park Service Website, Mount Rushmore National Memorial, 2001
4) U.S. National Park Service Website, Wind Cave National Park, 2001
5) USGS/NPS Geology in the Parks Website, 2001
6) South Dakota State Parks Website, 2002
7) Trimble, D.E., 1980, The Geologic Story of the Great Plains:
U.S. Geological Survey Bulletin 1493
8) Stoffer, P.W., 2003, Geology of Badlands National Park: A Preliminary Report:
U.S. Geological Survey Open-File Report 03-35
9) U.S. National Park Service, National Natural Landmarks Website, 2003
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