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America's Volcanic Past

"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

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Map, Location of Arkansas

Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Arkansas features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Arkansas.]

  • Arkansas
  • Arkansas Regions
  • Arkansas - Brief Geologic History
  • Arkansas's Igneous Rocks
  • Arkansas's Rocks and Minerals and related Volcanic Activity
  • Arkansas's Gulf Coastal Plain
  • Arkansas's Oldest Surface Rock
  • Crater of Diamonds State Park


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 Commission Website, 2001, Geology of Arkansas, and U.S. National Park Service, Hot Springs National Park Website, 2003
Arkansas Regions

Arkansas Regions:1
Arkansas is divided into a highland area in the northwest and a lowland region in the south and east. The rocks in the highland area are dominated by well-lithified sandstones, shales, limestones, and dolostones of Paleozoic age. A thin drape of younger unconsolidated clays, sands, and gravel, termed alluvium, is often found in valley floors and associated with the streams and rivers. The sedimentary deposits of the lowlands are mainly unconsolidated clay, sand, and gravel of Quaternary age, poorly consolidated deposits of clay, sand, silt, limestone, and lignite of Tertiary age, and consolidated (to a limited extent) deposits of Cretaceous marl, chalk, limestone, sand, and gravel.

Ouachita-Ozark Interior Highlands:3
The ancient, eroded mountains of the Ouachita-Ozark Highlands stand surrounded by the nearly flat-lying sedimentary rocks and deposits of the Interior and Atlantic Plains provinces. Unlike the relatively young rocks that characterize neighboring provinces, the rocky outcrops that make up the core of the Ouachita-Ozark Highlands are Paleozoic age carbonate and other sedimentary rocks that were originally deposited on the sea floor. In the Ouachita Mountains these ancient marine rocks are now contorted by folds and faults. These rocks closely match deformed strata found today in the Marathon Mountains of Texas and the southern Appalachians -- strong evidence that the Ouachita-Ozark Highlands were once part of a mighty folded, uplifted mountain range that stretched from the Appalachians Highlands to the northeast through Texas to the southwest.

Arkansas's Ozark/Ouachita:1
When most of the sediments that compose the rocks in the highland region of Arkansas were being deposited, north Arkansas was a shallow south-sloping sea floor (continental shelf), the Arkansas River Valley was near the edge of the shelf, and the Ouachita area was a deep abyssal plain. An abyssal plain is the relatively smooth and deep (more than 3,000 feet below sea level) parts of the ocean floor where accumulating sediments have buried the pre-existing topography. In the late Paleozoic Era, a broad uplift domed the Ozark strata with little structural disruption. Simultaneously, a collision of two of the earth's mobile continental plates compressed the sediments of the abyssal plain into the Ouachita Mountains. This multimillion-year-long process folded and faulted the Ouachita strata into a structurally complex mountain chain. The Arkansas River Valley area is the transition zone between the structurally simple Ozarks and the structurally complex Ouachitas with subdued characteristics in each region.

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.

Arkansas - Brief Geologic History

Precambrian (570 to 4,600 million years ago):1
Ozark region and northern part of Mississippi Embayment: Granitic-type igneous rocks (some approximately 1,400 million years ago) have been encountered in some wells.

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
(Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian) Seas; Sedimentation and Erosion

Triassic (208 to 245 million years ago):1
Accumulation of predominantly red clay and silt, sand and gravel, and minor beds of anhydrite in a non-marine environment. Present only in southern Arkansas (in the subsurface). Age is indefinite. Erosion occurs in the Interior Highlands.

Jurassic (144 to 208 million years ago):1
Deposition in a shallow marine environment. Present only in the subsurface in southern Arkansas. Erosion occurs in the Interior Highlands.

Jurassic or Triassic:1
Intrusion of small localized bodies of magma in southern Arkansas, which crystallized to form igneous rocks.

Cretaceous (66 to 144 million years ago):1
Deposition of water-laid volcanic debris preceded the accumulation of sand, marl, and chalk in shallow marine seas during the Late Cretaceous. About 100 million years ago, downwarping of the Mississippi Embayment resulted in the invasion of the region by a shallow sea. Igneous activity in central, southwestern, and eastern Arkansas; nearshore deposition of clastic and carbonate debris in the Early Cretaceous, along with deposition of gypsum and anhydrite in highly saline waters in southwestern Arkansas. Erosion occurs in the Interior Highlands.

Paleocene (47.8 to 66 million years):1
Shallow marine sea in the Mississippi Embayment; development of bauxite on islands of exposed Cretaceous igneous rocks. Erosion occurs in the Interior Highlands.

Eocene (36.6 to 57.8 million years ago):1
Marine and fluvial clastic sediments. Erosion occurs in the Interior Highlands.

Pliocene (1.6 to 36.6 million years ago):1
Erosion over most of the state.

Quaternary (Present to 1.6 million years ago):1
Sedimentation and Erosion


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.

Excerpts from:   [links checked 06/06]
1) Arkansas Geological Commission Website, 2001, 2002
2) Arkansas State Parks Website, 2002
3) USGS/NPS Geology in the Parks Website, September 2001
4) U.S. National Parks System, Hot Springs National Park Website, 2003

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06/19/06, Lyn Topinka