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 Arkansas volcanic features or events and is by no means inclusive.]
Most surficial rocks in Arkansas are sedimentary,
but there are some igneous rocks
(with adjacent contact metamorphic rocks)
and very low grade regional metamorphic
rocks in Arkansas also.
The most recent igneous activity in Arkansas
occurred some 90 to 100 million years ago.
Excerpt from: Arkansas Geological Survey Website, 2001, Geology of Arkansas, and U.S. National Park Service, Hot Springs National Park Website, 2003 |
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Arkansas Regions:1
Ouachita-Ozark Interior Highlands:3
Arkansas's Ozark/Ouachita:1
The Atlantic Plain:3 The Atlantic Plain is the flattest of the provinces. It stretches over 2,200 miles in length from Cape Cod to the Mexican border and southward another 1000 miles to the Yucatan Peninsula. The Atlantic plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf. This region was born during the breakup of the supercontinent Pangea in the early Mesozoic Era.
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Precambrian (570 to 4,600 million years ago):1
Ouachita region: Erratic boulders of late Precambrian
igneous and metamorphic rocks and sparse tectonically
emplaced bodies of metamorphosed igneous rocks
(approximately 1,000 million years ago)
are present in Paleozoic rocks.
In these areas, there is no evidence of the
succession of events during Precambrian time.
Although not exposed elsewhere, Precambrian rocks are
assumed to underlie the entire state.
Paleozoic Era (245 to 570 million years ago):1
Triassic (208 to 245 million years ago):1
Jurassic (144 to 208 million years ago):1
Jurassic or Triassic:1
Cretaceous (66 to 144 million years ago):1
Paleocene (47.8 to 66 million years):1
Eocene (36.6 to 57.8 million years ago):1
Pliocene (1.6 to 36.6 million years ago):1
Quaternary (Present to 1.6 million years
ago):1
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Arkansas's Igneous Rocks |
Arkansas's Igneous Rocks:1
The exposures of igneous rocks
in Arkansas are less than 0.1
percent of the entire area of the state.
Most are exposed over 15 square miles,
principally in Pulaski, Saline,
Hot Spring, Garland, and Pike Counties.
A few small igneous dikes and sills are
present outside the Ouachita region,
mostly in the Arkansas Valley, and in at least
one case, in the Boston Mountains.
Except for some localized contact metamorphism adjacent to the
larger igneous intrusions,
only very low grade metamorphic rocks are present in the state.
Arkansas's Rocks and Minerals and related Volcanic Activity |
Arkansas's Bauxite and Aluminum:1
The Arkansas bauxite region covers about 275 square
miles in the northern part of the West Gulf Coastal
Plain and is divided into two
mining districts. One area is in Pulaski County
south and east of Little Rock and the other is in
nearby Saline County, northeast and east
of Benton.
The bauxite is present mostly as sheet or
blanket deposits
in very close proximity to outcrops
of the intrusive igneous rock, nepheline syenite.
The deposits formed in early
Tertiary time, developing as soils along the western
edge of a shallow marine basin that
occupied the Mississippi River Embayment.
During that time, hills and knobs of syenite as
islands were exposed to intense chemical
weathering in a tropical or near-tropical
environment (lateritic weathering). In the weathering
process, leaching by rain, ground water, and
perhaps by salt spray, decomposed the original
igneous rock minerals (feldspar and nepheline),
removed much of the silica, and
concentrated the newly formed oxides and
hydroxides of aluminum as the rock we term bauxite.
These are residual deposits because they
formed essentially in place (in situ paleo-soils).
Many other deposits, generally smaller, consist of
bauxite removed by erosion from its
site of origin and redeposited nearby
(transported deposits).
The principal ore of aluminum is bauxite,
a complex mixture of a number of aluminum
hydroxides and hydrous aluminum oxides.
Bauxite ranges in color from off-white to deep
reddish brown, and structurally from a soft
earthy material to a well-cemented rock.
Commercial bauxite usually has a
minimum content of 50 to 55 percent alumina.
Arkansas's Bentonite:1
In the Gulf Coastal Plain of Arkansas,
beds of bentonite
--
a type of clay formed from altered volcanic ash --
are present in the Midway Group (Paleocene).
Bentonite has not
been mined in Arkansas, although small
deposits of certain clays that have the desired bentonitic
properties after treatment have been mined.
Arkansas's Diamonds:2
Crater of Diamonds offers
you a one-of-a-kind adventure-the opportunity to
hunt for real diamonds
and keep any you find! You'll search over a 37-acre
plowed field-the
eroded surface of an
ancient, gem-bearing
volcanic pipe.
[See Crater of Diamonds State Park below]
Arkansas's Fuller's Earth:1
Fuller's earth, or natural bleaching clay,
is any natural or treated clay which, when used as a filter,
effectively removes color and clarifies
various mineral and organic oils.
Mineralogically, fuller's earth is
predominantly smectite (calcium montmorillonite),
but typically includes some kaolinite and attapulgite.
The importance of fuller's earth to industry is
due to its bleaching properties. Deposits of fuller's earth
were mined by underground methods near Olsen Switch,
7 miles south of Benton in Saline County,
between 1901 and 1922.
The deposits were formed by
the weathering of igneous dikes of Cretaceous age.
The original igneous rock was altered to a depth of
200 feet, probably in early Tertiary time, while bauxite
was forming nearby. Several deposits of bentonite,
which upon treatment with weak acid have good
bleaching or clarifying properties, are reported in
Pulaski and Saline Counties. These bedded deposits are
in the Midway and Wilcox Groups (Paleocene and Eocene, respectively).
Arkansas's Lamproite:1
Magmatic lamproite is a
dark-colored igneous rock
with a
texture that has two distinct grain sizes (porphyritic).
Some of it was
broken explosively as it neared the earth's surface.
This broken rock material is lamproite breccia tuff.
Rarely have diamonds been reported in
the soils formed by the weathering of magmatic lamproite; most
have been recovered in the lamproite
breccia tuff or in the thin residual soils overlying this rock.
Microdiamonds, however, have been
recovered from the magmatic rock by special techniques.
Arkansas's Lithium and Spodumene:1
Lithium is the lightest of all the metals,
having an atomic weight of 6.939 and a specific gravity of 0.534.
The mineral spodumene, which is often present in extremely
coarse-grained igneous rocks called pegmatites,
is the most important commercial ore mineral of lithium.
In the 1960s, with the discovery of lithium in brines
and in arid evaporative lakes and lake deposits (evaporites), new
commercial sources of lithium became available.
Lithium occurs in significant amounts in geothermal waters,
oil-well brines, and as a trace element in a variety of rocks.
The three lithium minerals that exist in Arkansas
occur in the Ouachita Mountains.
Cookeite is in small hydrothermal quartz veins, most
commonly filling fractures in the Jackfork Sandstone (Pennsylvanian)from Pulaski County westward through Saline and into Perry County.
Taeniolite is present in smoky quartz veins in the recrystallized
novaculite adjacent to the Magnet Cove intrusion (Cretaceous), Hot
Spring County, and in a chalcedony-fluorite-pyrite vein in the "V"
intrusive (Cretaceous age igneous dikes), Garland County.
Lithiophorite has been reported in the manganese deposits in Polk
and Montgomery Counties, and as a late secondary mineral in quartz
veins from many localities in the Ouachita Mountains.
Arkansas's Syenite:1
Nepheline syenite is a medium to coarse-grained, light- to medium-gray, igneous rock that is composed predominantly of a silicate mineral called orthoclase and has a granite-like appearance.
It may be distinguished from granite by little or no quartz content.
In Arkansas, nepheline syenite has been intruded into the
sequence of Paleozoic sedimentary rocks. Locally, quarrymen recognize "blue granite" and "gray granite" varieties.
Syenite is Arkansas' most abundant igneous rock.
Much of Arkansas' nepheline syenites have high strength and
weather-resistant properties and are crushed for use as roofing granules,
road materials, riprap, and asphalt and concrete aggregate.
Fines are used as a colorizing and fluxing agent in the manufacture of brick
and as compaction fill. Historically, this rock has been used as a
building, monument, and paving stone and for railroad culvert and bed
construction. Syenite was also used extensively as riprap for
the protection of river banks and road embankments.
Nepheline syenite and its associated igneous rocks are exposed
in four areas of the state: Pulaski County, Saline County,
Garland County, and Hot Spring County. The total surface exposure of syenite in Arkansas is about 13 square miles.
Arkansas's Titanium:1
Titanium-bearing minerals in Arkansas are in Pulaski, Saline,
Hot Spring, Garland, Pike, Howard,
Sevier, and Little River Counties, and in
alluvial sands of the Arkansas River.
Titanium-bearing minerals from Pulaski and Saline Counties
are in intrusive bodies of nepheline
syenite and deposits of bauxite.
Ilmenite is also present in bauxite in both counties.
Arkansas's Uranium:1
In Arkansas, several uranium anomalies were discovered during the 1950s.
Several localities yielded samples with 0.1 percent or more
uranium oxide. At most localities the radioactive mineralization is
secondary or related to organic matter. In most instances, the
uranium-bearing minerals have not been identified.
The Potash Sulphur Springs igneous intrusion in Garland County is
probably the best known and perhaps the first site where uranium
was discovered in Arkansas.
The mineralization is at the contact
of the Cretaceous syenite complex with folded Paleozoic novaculite and
shale beds. The U.S. Geological Survey identified the uranium-bearing
mineral as pyrochlore, a primary mineral.
Soil samples assaying up to 0.4 percent
uranium were collected from this site.
Although samples from Potash Sulphur Springs and the other
prospects contain uranium-bearing minerals, no economically viable
deposits have been discovered.
Arkansas's Vanadium:1
Vanadium deposits at Potash Sulphur Springs
consist of highly altered sedimentary rocks,
mainly the Arkansas Novaculite
(Mississippian-Devonian) and
altered igneous rocks
in and adjacent to the contact metamorphic zone.
Arkansas's Vermiculite:1
Vermiculite is a mica-like silicate mineral
that rapidly expands upon heating,
resulting in a low-density material. The expanded
material is used as a lightweight aggregate and
insulation in the construction industry, a
carrier for fertilizers and a soil conditioner in
agriculture, a fragrance carrier, and a filler
and texturizer for plastics and rubber.
Vermiculite were observed during mining of iron ore
in the 1940s and 1950s at Magnet Cove,
Hot Spring County.
Residual flake
vermiculite originated from the iron-rich mineral
biotite in igneous rock,
then processes of alteration and weathering created vermiculite.
These deposits have unknown commercial potential.
Exploration for vermiculite deposits is
straightforward since they are the products of
surface and near-surface weathering processes.
Although Arkansas does not have any vermiculite
mining operations, companies in Arkansas
expand imported vermiculite, placing
Arkansas among the principal expanded-vermiculite
producing states in the United States.
Arkansas's Volcanic Tuff:1
Tuff is a pyroclastic rock composed
mostly of angular fragments of
volcanic material deposited from the air.
If deposited on land while hot, the particles weld
together as a welded tuff;
otherwise, normal lithification occurs.
In Arkansas, two tuff units are present.
The Hatton tuff lentil of the Stanley Shale
(Mississippian) is exposed in Polk County.
Southwest of the community of
Hatton, the tuff has a maximum
thickness of 300 to 400 feet, but 90 feet is more common.
The tuff is massive, homogeneous, and jointed so that determination of
bedding is difficult.
The unweathered fine-grained rock
is dark gray and may appear spotted
due to light-colored feldspar crystals. Under the microscope,
numerous broken volcanic glass
fragments (shards) compose much of the rock.
The unweathered rock is tough, compact,
and may contain Late Pennsylvanian
milky quartz veins.
The Hatton tuff is not used in Arkansas for aggregate,
although it does pass concrete-aggregate
specifications of the Texas Highway Department. It is quarried
near Hatton and exported
to east Texas for use in concrete.
The Hatton have potential as a cementing agent.
Tuff beds are
also present in southwest Arkansas in the
Woodbine Formation (Cretaceous),
but have no resource potential for aggregate.
Arkansas's Gulf Coastal Plain |
Arkansas's Gulf Coastal Plain:1
In the Gulf Coastal Plain of Arkansas,
beds of bentonite are present in
the Midway Group (Paleocene).
The Arkansas bauxite region covers about 275 square
miles in the northern part of the West Gulf Coastal
Plain.
Arkansas's Oldest Surface Rock |
Arkansas's Oldest Surface Rock:1
Arkansas's oldest known surface rock,
about 1,025,000,000 years old, is an
altered igneous body
in Saline County.
Crater of Diamonds State Park |
Crater of Diamonds State Park:2
Crater of Diamonds offers
you a one-of-a-kind adventure-the opportunity to
hunt for real diamonds
and keep any you find! You'll search over a 37-acre
plowed field-the eroded
surface of an ancient, gem-bearing
volcanic pipe.
Prospectors access
the field through the visitor center. Exhibits and an A/V
program explain the
area's geology and offer tips on recognizing diamonds in
the rough.
Diamonds were first discovered here in 1906 and over 75,000 diamonds have
been unearthed at this site.
Notable diamonds include the 40.23-carat -Uncle
Sam,- [the largest diamond ever unearthed in the United States], the
16.37-carat -Amarillo Starlight,- the 15.33-carat -Star of
Arkansas- and the
3.03-carat -Strawn-Wagner Diamond,- cut to a 1.09-carat
gem graded -D-
flawless O/O/O (the highest grade a diamond can achieve)
by the American
Gem Society.
Crater of Diamonds became a state park in 1972,
and since then over 22,000
diamonds have been carried home by visitors.
Amethyst, garnet, jasper, agate,
quartz and more can also be found here.
Digging tools are available for rent,
and the park staff provide free identification
and certification of diamonds.
Crater of Diamonds is two miles southeast
of Murfreesboro on Arkansas 301.
Prairie Creek Volcanic Pipe:1
One of the few places in North America where diamonds
are present in their host rock and the only place
tourists may hunt for diamonds is the
Prairie Creek pipe in Arkansas. It is roughly triangular in surface
outcrop, exposed over 73 acres,
and is situated 2.5 miles southeast of Murfreesboro in Pike County. The
site has been known to geologists since 1842.
It is a
breccia-filled volcanic pipe
of Cretaceous age, formed
by a series of gaseous explosions as are several other pipes nearby.
Various rock types are present in the
Arkansas diamondiferous pipes.
Magmatic lamproite is a
dark-colored igneous rock with a
texture that has two distinct grain sizes (porphyritic).
Some of it was
broken explosively as it neared the earth's surface.
This broken rock material is lamproite breccia tuff.
Rarely have diamonds been reported in
the soils formed by the weathering of magmatic lamproite; most
have been recovered in the lamproite
breccia tuff or in the thin residual soils overlying this rock.
Microdiamonds, however, have been
recovered from the magmatic rock by special techniques. Epiclastic
rock, which is a rock formed by the
mechanical mixing of tuffaceous volcanic material and local Cretaceous
sediments, was recently recognized by geologists.
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