America's Volcanic Past -
Utah

Location Map - Utah National Parks and Monuments
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Volcanic Highlights and Features:
[This list is just a sample of various Utah volcanic features or events and is by no means inclusive.]

• Utah
• Utah Regions
• Utah - Brief Geologic History (Volcanic)
• Utah's Volcanic Types
• Utah's Volcanic Rocks
• Bailey Ridge
• Big Cottonwood Canyon
• Black Rock Desert Volcanic Field
• Brighton Gap
• Capitol Reef National Park
• Clayton Peak
• Cove Fort
• Dugway Geode Beds
• Eightmile Hills
• Flagstaff Mountain
• Fumarole Butte
• Goshute Indian Reservation
• Grass Valley
• Green River Formation
• Gum Hill
• Harmony Mountains
• Honeycomb Hills
• Little Cottonwood Canyon
• Little Rockies
• Marysville Volcanic Field
• Mineral Mountains Volcanic Field
• Mountain Meadow Monument
• Mt. Belknap
• Mt. Millicent
• Mt. Wolverine
• Navajo Mountain, La Sal, Abajo, and Henry Mountains
• Pahvant Butte (Pavant Butte)
• Pine Valley Mountains
• Sheeprock Mountains
• Signal Peak
• Smelter Knolls
• Snow Canyon State Park
• St. George
• Sunstone Knoll
• Swett Hills
• Topaz Mountain
• Tushar Mountains
• Twin Peaks
• Vernon Hills
• Wildhorse Canyon
• Zion National Park

Composite or Stratovolcanoes:¹
Composite volcanoes (stratovolcanoes) develop from repeated explosive and nonexplosive eruptions of tephra (airborne lava fragments that can range in size from tiny particles of ash to house-size boulders) and lava that build up layer by layer. These volcanoes are the largest and form symmetrical cones with steep sides. Some composite volcanoes in Utah are in the Tushar Mountains (Mount Belknap, for example) in Piute County. Now extinct, they are too old (between 32 and 22 million years) to maintain the classic volcanic shape of their modern-day counterparts, such as Mount Hood and Mount St. Helens in the Cascade Range along the northwestern coast of the United States.

Shield Volcanoes:¹
Shield volcanoes form from "gentle" or nonexplosive eruptions of flowing lava. The lava spreads out and builds up volcanoes with broad, gently sloping sides. The low-profile shape resembles a warrior's shield. In Utah a good example is the one-million-year-old Fumarole Butte in Juab County. Currently active volcanoes of this type are found in the Hawaiian Islands.

Cinder Cones:¹
Cinder cones build from lava that is blown violently into the air and breaks into fragments. As the lava pieces fall back to the ground, they cool and harden into cinders (lava fragments about 1/2 inch in diameter) that pile up around the volcano's vent. Cinder cones are the smallest volcanoes and are cone-shaped. Cinder cones are found in many areas of Utah including Millard, Iron, Garfield, Kane, and Washington Counties, and they vary in age. The youngest, only about 600 years old, are in the Black Rock Desert in Millard County.

Dome Mountains:¹
Dome Mountains are formed from hot molten material (magma) rising from the Earth's mantle into the crust that pushes overlying sedimentary rock layers upward to form a "dome" shape. Unlike a volcano, the magma typically does not reach the Earth's surface. Instead, the magma cools underneath the surface and forms the core of the mountains. Dome mountains in Utah include Navajo Mountain and the La Sal, Abajo, and Henry Mountains in the southeastern part of the state.

Andesite:¹
Andesite has a higher quartz content than basalt and is usually lighter in color. Crystals of the minerals amphibole, biotite, and feldspar are sometimes visible without magnification. In Utah andesite can be seen at Signal Peak in the Tushar Mountains, Piute County.

Basalt:¹
Basalt is characteristically a dense, black, massive rock, high in calcium and iron-magnesium- bearing minerals and low in quartz content. Great examples of basaltic lava flows can be found in the Black Rock Desert, Millard County.

Diorite:¹
Diorite has the same texture as granite but has the mineral composition of an andesite, which is diorite's extrusive equivalent. Diorite forms the summits of Haystack Mountain, Mt. Tomasaki, Mt. Mellenthin, and Mt. Tuckuhnikivatz in the La Sal Mountains, Grand and San Juan Counties.

Granite:¹
Granite is the intrusive equivalent of rhyolite but has a coarser texture. A 12-square-mile outcrop of granite is visible on the southwestern flank of the Sheeprock Mountains, Tooele and Juab Counties.

Rhyolite:¹
Rhyolite is typically a fine-grained, white, pink, or gray rock, high in quartz and feldspar content with some amphibole and biotite. A well-known example is the Topaz Mountain rhyolite in the Thomas Range, Juab County.

Obsidian:¹
Approximately 2.5 million years ago (late Tertiary Period), volcanic eruptions in the Black Spring area of the Black Rock Desert in western Utah spewed out the volcanic rocks rhyolite, pumice, and obsidian. Obsidian is a dark-colored volcanic glass formed when molten lava cools quickly. It is usually black but colored varieties range from brown to red. Snowflake obsidian, a black obsidian with whitish-gray spots (spherulites) of radiating needle-shaped cristobalite (high-termperature quartz) crystals, is also found in the Black Rock Desert. Obsidian has been used for arrowheads and primitive cutting tools, and is presently used for jewelry.

Bailey Ridge:²
The Mineral Mountains-Cove Fort volcanic field is a Quaternary bimodal, basalt-rhyolite association with some intermediate composition units. Silicic volcanism began at 800,000 years ago with eruption of two fluid, aphyric, rhyolite flows (3 kilometers long, 80 meters thick) along Bailey Ridge and Wildhorse Canyon. [See Mineral Mountains-Cove Fort below]

Big Cottonwood Canyon:¹
Between 33 and 32 million years ago, molten intrusive igneous rock invaded the surrounding rocks in the Big Cottonwood Canyon area. As the magma cooled and solidified, it created an environment that encouraged growth and concentration of metallic minerals in the surrounding rocks. Millions of years later, after the rocks of the Wasatch Range were uplifted and erosion began, rocks of the Alta stock were exposed at the earth's surface. Pleistocene glaciers caved two magnificent peaks at the head of this canyon. Mt. Wolverine (10,795 feet in elevation) and Mt. Millicent (10.452 feet in elevation) are both composed of igneous rocks of the Alta stock. In the 1860s, lead, zinc, silver, and copper were found and the Big Cottonwood mining district was officially formed.

Black Rock Desert Volcanic Field:²
The Black Rock Desert volcanic field consists of a rhyolite dome and many mafic cones and flows erupted from about two dozen vents within the past 1.5 million years. Early flows are covered extensively by sediments from Lake Bonneville. The Black Rock Desert is approximately 250 kilometers south of Salt Lake City in west-central Utah, immediately west of Interstate 15 between the communities of Kanosh and Holden; several lava flows and cinder cones can be recognized from the freeway.

Ice Springs Volcano - Recent Activity:²
The most recent activity (around 800 years ago) at Black Rock Desert volcanic field produced the five nested cinder cones and lavas of the Ice Springs volcano. Lava flows have a maximum length of 8 kilometers. Both flows and tephra contain abundant xenoliths of partially fused granite.

Pavant Butte:²
Pavant Butte, a large (275-meter-high, 3-kilometer-diameter) tuff cone, was built 16,000 years ago on top of pahoehoe and aa lavas of the Pavant field, and erupted through a water depth of 85 meters in Lake Bonneville. A prominent shoreline terrace is developed on the tuff cone. The then subaerial portion has been entirely converted to palagonite. [See Pavant Butte below]

Tabernacle Hill:²
Eruptions during the waning stages of Lake Bonneville at Tabernacle Hill produced two tuff cones and lavas emanating from a central vent which collapsed to form a crater 20 meters deep and 0.4 kilometers in diameter. Lava tubes are common, and pit craters dot the surface of the volcano, which is roughly circular and 4 kilometers in diameter. Several features suggest that lavas from the Tabernacle were erupted into Lake Bonneville. Rounded and irregular pillows with glassy rinds and course grained interiors are common on the edge of the flow, and tufa encrustations and wave-rounded cobbles occur as high as 9 meters above the base of the flow. Tephra deposits from these two eruptions form distinctive stratigraphic markers in Quaternary Lake deposits of the Sevier and Black Rock Deserts.

White Mountain:²
White Mountain rhyolite dome (0.1 cubic kilometer) is the youngest rhyolite yet dated in Utah. Its pumiceous carapace has been stripped away by Lake Bonneville and deposited as pumice cobbles in a bar southeast of the dome.

Black Springs Obsidian:¹
Approximately 2.5 million years ago (late Tertiary Period), volcanic eruptions in the Black Spring area of the Black Rock Desert in western Utah spewed out the volcanic rocks rhyolite, pumice, and obsidian. Obsidian is a dark-colored volcanic glass formed when molten lava cools quickly. It is usually black but colored varieties range from brown to red. Snowflake obsidian, a black obsidian with whitish-gray spots (spherulites) of radiating needle-shaped cristobalite (high-temperature quartz) crystals, is also found in the Black Rock Desert. Obsidian has been used for arrowheads and primitive cutting tools, and is presently used for jewelry.

Brighton Gap:¹
A you approach Brighton Gap from the east, you drive past a road-cut with rocks of multiple colors. These rocks alternate between sedimentary rocks and igneous dikes. Between 245 and 240 million years ago, an inland sea deposited limestones of the Triassic Thaynes Formation. At Brighton Gap, these fossil-rich limestones have been baked by "fingers" of igneous dikes from the Clayton Peak stock. Similar to cooking food in an oven, heat and pressures associated with metamorphism cause physical and chemical properties to change in rocks causing new, interesting minerals to form. Careful examination of these rocks will yield a shiny, silver-colored, flaky, iron oxide mineral called specular hematite.

Non-Volcanic Background:4 The Waterpocket Fold defines Capitol Reef National Park. A nearly 100-mile long warp in the Earth's crust, the Waterpocket Fold is a classic monocline: a regional fold with one very steep side in an area of otherwise nearly horizontal layers. The rock layers on the west side of the Waterpocket Fold have been lifted more than 7,000 feet higher than the layers on the east. The Waterpocket Fold formed between 50 and 70 million years ago. The most scenic portion of the Waterpocket Fold, found near the Fremont River, is know as Capitol Reef. Nearly 10,000 feet of sedimentary strata are found in the Capitol Reef area. These rocks range in age from Permian (as old as 270 million years old) to Cretaceous (as young as 80 million years old). The Waterpocket Fold has tilted this geologic layercake down to the east. The older rocks are found in the western part of the park, and the younger rocks are found near the east boundary. This layer upon layer sequence of sedimentary rock records nearly 200 million years of geologic history. Rock layers in Capitol Reef reveal ancient climates as varied as rivers and swamps, Sahara-like deserts, and shallow ocean. The tilt of the Waterpocket Fold dies out at Thousand Lake Mountain near the northwestern boundary of the park.

Black Boulders:⁴
The black bounders, found scattered throughout the Fremont River valley and along other drainages, are recent geologic arrivals to Capitol Reef. These volcanic rocks came from the 20 to 30 million years old lava flows which cap Boulder and Thousand Lake Mountains. The boulders made their way to Capitol Reef during the Ice Ages when the High Plateaus supported small mountain glaciers. Landslides, debris flows, and possibly heavy stream outwash from these glaciers carried the boulders to lower elevations in the park.

Cathedral Valley:⁴
Dikes and sills, which are thin bodies of igneous rock and small volcanic plugs, are found in Upper Cathedral Valley. Theses features formed during volcanic activity 3 to 6 million years ago.

Clayton Peak Stock:¹
The intrusive igneous rocks of the Clayton Peak stock are granodiorites, a slightly different chemical composition than true granites. They have played an important role in the mineralization in the area. Geologists believe that this intrusion occurred between 41 and 37 million years ago.

Mineral Mountains-Cove Fort Volcanic Field:²
The Mineral Mountains-Cove Fort volcanic field is a Quaternary bimodal, basalt-rhyolite association with some intermediate composition units. Silicic volcanism began at 800,000 years ago with eruption of two fluid, aphyric, rhyolite flows (3 kilometers long, 80 meters thick) along Bailey Ridge and Wildhorse Canyon. Subsequent activity from 700,000 to 500,000 years consisted of pyroclastic eruptions and extrusion of at least 11 domes distributed over 10 kilometers along the crest and western flank of the Tertiary Mineral Mountains pluton. Tephra from these eruptions are abundant in the lacustrine deposits of the Beaver Basin. East of the Mineral Mountains are lavas of basalt, basaltic andesite, and latite which erupted before the silicic episode of the Mineral Mountains and persisted afterwards. Activity began with outpourings of the tholeiitic basalt of the Black Rock field from vents on the eastern margin of the Mineral Mountains, followed by basaltic andesite of the Maderfield and Crater Knoll fields. Latite lavas were then erupted from Red Knoll cinder cone, followed by quartz-bearing basaltic andesite from the topographically dominant Cove Fort cinder cone. The youngest lavas are latite of the Cedar Grove field, erupted from a cinder cone on the southwest margin of the Cove Fort field. In these two younger units surface features are readily apparent, including pressure ridges, squeezeups, and pahoehoe textures. The Mineral Mountains-Cove Fort volcanic field is approximately 300 kilometers south of Salt Lake City and approximately 100 kilometers north of Cedar City, Utah. Access to the silicic volcanoes of the Mineral Mountains is from the west via Milford, Utah. Interstate 15 crosses the Cove Fort flows immediately north of the interchange with Interstate 70.

Dugway Geode Beds:¹
Approximately 6 to 8 million years ago (Miocene epoch), volcanic activity occurred in western Utah and deposited an extrusive igneous rock called rhyolite. Trapped gasses formed cavities within the rhyolite, and millions of years of ground-water circulation allowed minerals to precipitate into the cavities. The result is geodes with spherical shapes and crystal-lined cavities. Roughly 32,000 to 14,000 thousand years ago, a large body of water known as Lake Bonneville covered most of western Utah. The lake's wave activity eroded the geode-bearing rhyolite and redeposited the geodes several miles away in the Dugway geode bed area as lake sediments. Most geodes are typically hollow whereas others are completely filled with massive, banded quartz. The most common mineral found within the geodes is quartz in various colors: clear (rock crystal), purple (amethyst), and pink (rose).

Eightmile Hills:¹⁴
Eightmile Hills - made up of Tertiary ash-flow tuffs dipping southeast off the flank of Granite Mountain intrusion, which is partially visible above the hills. Large abandoned open-pit iron mines surround Granite Mountain.

Flagstaff Mountain Stock:¹
The gray to green rocks of the Flagstaff Mountain stock contain light-colored minerals (up to 1/2 inch long) floating in a fine-grained host. This "spotted" igneous texture is called a porphyry. The rock, named for the type of minerals it contains and how it forms, is a diorite. The Flagstaff intrusion is thought to have contributed to the emplacement of the Park City district ores.

Fumarole Butte:²
Fumarole Butte is a Quaternary basaltic andesite shield volcano. It overlies minor outcrops of Tertiary basalt and rhyolite erupted 6.1 million years ago, contemporaneously with rhyolite of the Keg and Thomas range 10-30 kilometers to the north and west. Fumarole Butte has normal magnetic polarity indicating eruption during the Jaramillor event. The volcanic neck which provides the name Fumarole Butte is at the center of the volcano and rises approximately 30 meters above the gentle slope of the shield. The volcano was inundated briefly by Lake Bonneville, and there are scattered remnants of lacustrine deposits. Benches developed at the Provo level (1,463 meters). Crater Springs (also known as Baker Hot Springs and Abraham Hot Springs), on the eastern margin, produced thermal water (87-90 degrees C) at an estimated discharge rate of about 17 liters per second in the summer of 1967. Fumarole Butte is in west-central Utah approximately 30 kilometers north of Delta, Utah.

Goshute Indian Reservation:⁷
The Goshute Indian Reservation includes 109,013 acres located on the border between Utah and Nevada. It encompasses parts of Deep Creek Valley, Antelope Valley, and the Deep Creek Range. Reservation land includes Proterozoic schist and quartzite; Paleozoic quartzite, carbonate, and shale; and Tertiary volcanic, intrusive, and sedimentary rocks. Complex structural control results from Jurassic-Cretaceous thrusting associated with the Sevier Orogeny, which is overprinted by Tertiary Basin and Range extension. Numerous splays on a range-bounding fault, which extends along the western flanks of the northern and central Deep Creek Range, are probably related to Miocene uplift. Structure and stratigraphy appear to control mineralization, as carbonate breccias below silty rocks host gold, silver, and tungsten. The occurrence of faulting appears less prominent south of the reservation.

Grass Valley:¹⁴
From North of Grass Valley on Grass Valley Road -- Pull off at crest of hill to view geologic features on east side of Grass Valley. Grass Valley is another eroded anticline produced by an unexposed intrusion that is interpreted to be an extension of the Pine Valley intrusion exposed in the hills to the right. The white cliffs just above the valley floor are exposures of the ash-flow tuff member of the rocks of Paradise overlain by the Big Mountain slide mass. The Rencher Formation was not deposited this far east. The Big Mountain slide is overlain by fanglomerates of the Page Ranch Formation and the Pine Valley slide mass. Overlying the Pine Valley slide is the Timber Mountain flow member of the Pine Valley Latite that extruded northward from the Pine Valley laccolith following the collapse of its flank by gravity sliding. Rencher Peak (source area for the slightly older Rencher Peak flow member of the Pine Valley Latite) is the high peak visible on the north side of Grass Valley. Continue south on Grass Valley road for one mile. Cinder cone of quartz-bearing basalt on right. Lava from this and other vents dammed Grass Valley, which then filled in with fluvial sediments and minor lacustrine deposits to form the relatively broad valley floor. Many fertile valleys in this area formed in this manner, including Pine Valley, Grassy Flat, and Diamond Valley.

Green River Formation:⁶
Three ancient great lakes existed in the region of Wyoming, Utah, and Colorado 50 million years ago: Lake Goshute, Lake Uinta, and Fossil Lake, the smallest. All are gone today, but they left behind a wealth of fossils in lake sediments that turned into the rock layer known as the Green River Formation, made up of laminated limestone, mudstone, and volcanic ash. The fossils are among the most nearly perfectly preserved remains of ancient plant and animal life in the world.

Gum Hill:¹⁴
Gum Hill - consists of basin-fill deposits capped by basalt of Gum Hill of Blank (1993), which has recently been dated with a minimum age of 5.7 Million Years Ago (Cornell and others, 2001). The basalt dips to the east into the southern extension of the Antelope Range fault (range-front fault) of Siders and others (1990).

Harmony Mountains:¹⁴
Harmony Mountains consist mostly of faulted Tertiary ash-flow tuffs.

Harmony Hills Tuff:¹⁴
Above the Bauers Tuff is the brown and tan, crystal-rich Harmony Hills Tuff (22.5 Million Years Ago). These unfractured ash-flow tuffs represent autochthonous rocks tilted eastward by the Iron Mountain Intrusion to the west.

Honeycomb Hills:²
The small rhyolite dome of the Honeycomb Hills is of interest because it displays chemical and mineralogical features characteristic of a pegmatite magma. Twelve meters of pyroclastic deposits are overlain by passively emplaced domal lavas. Erosion has exposed the internal structure of the dome, including the central conduit with its concentric, vertical flow banding and an increase in size and abundance of phenocrysts. The Honeycomb Hills are in westernmost central Utah, approximately 180 kilometers southwest of Salt Lake City.

Little Cottonwood Canyon:¹
Quartz monzonite, a very close relative of, and locally known as "granite", is a gray, "salt and pepper" igneous rock exposed in the lower reaches of Little Cottonwood Canyon. It is exposed on the Temple Quarry Nature Trail on the south side of Little Cottonwood Road (SR209) near the mouth of Little Cottonwood Canyon. The quartz monzonite intruded into the Wasatch Range between 24 and 31 million years ago.

Little Rockies - National Natural Landmark:¹⁵
43 miles south-southeast of Hanksville in Garfield County. An igneous structure. Owner: Federal, State. DESIGNATION DATE: May 1975

Marysvale Volcanic Field

Marysvale Area:¹
The Marysvale area is dominated by Tertiary igneous rocks ranging from intermediate to silicic compositions with both volcanic and plutonic representatives. The especially thick volcanic section is thought to be the result of a large stratovolcano complex and associated calderas that existed during mid-Tertiary time. One of the common units found near Marysvale is a white to dark gray rhyolite with extremely well-developed flow banding. Crystals, which are found in cavities in the rhyolite, formed in gas pockets that were trapped in the lava flow as it cooled.

Marysvale Volcanic Field:¹³
Marysvale volcanic field, southwestern Utah, one of the largest Cenozoic volcanic fields in the Western United States. The Marysvale field occurs mostly in the High Plateaus, a subprovince of the Colorado Plateau and structurally a transition zone between the complexly deformed Great Basin to the west and the stable, little-deformed main part of the Colorado Plateau to the east. The western part of the field is in the Great Basin proper. The volcanic rocks and their source intrusions in the volcanic field range in age from about 31 million years (Oligocene) to about 500,000 years (Pleistocene). These rocks overlie sedimentary rocks that range in age from Ordovician to early Cenozoic. The area has been deformed by thrust faults and folds formed during the late Mesozoic to early Cenozoic Sevier deformational event, and later by mostly normal faults and folds of the Miocene to Quaternary basin-range episode.

Mineral Mountains Batholith:¹⁴
Mineral Mountains batholith -- the largest exposed batholith in Utah. Despite its size and prominent expression, most of it is younger than 22 Million Years.

Mineral Mountains-Cove Fort Volcanic Field:²
The Mineral Mountains-Cove Fort volcanic field is a Quaternary bimodal, basalt-rhyolite association with some intermediate composition units. Silicic volcanism began at 800,000 years ago with eruption of two fluid, aphyric, rhyolite flows (3 kilometers long, 80 meters thick) along Bailey Ridge and Wildhorse Canyon. Subsequent activity from 700,000 to 500,000 years consisted of pyroclastic eruptions and extrusion of at least 11 domes distributed over 10 kilometers along the crest and western flank of the Tertiary Mineral Mountains pluton. Tephra from these eruptions are abundant in the lacustrine deposits of the Beaver Basin. East of the Mineral Mountains are lavas of basalt, basaltic andesite, and latite which erupted before the silicic episode of the Mineral Mountains and persisted afterwards. Activity began with outpourings of the tholeiitic basalt of the Black Rock field from vents on the eastern margin of the Mineral Mountains, followed by basaltic andesite of the Maderfield and Crater Knoll fields. Latite lavas were then erupted from Red Knoll cinder cone, followed by quartz-bearing basaltic andesite from the topographically dominant Cove Fort cinder cone. The youngest lavas are latite of the Cedar Grove field, erupted from a cinder cone on the southwest margin of the Cove Fort field. In these two younger units surface features are readily apparent, including pressure ridges, squeezeups, and pahoehoe textures. The Mineral Mountains-Cove Fort volcanic field is approximately 300 kilometers south of Salt Lake City and approximately 100 kilometers north of Cedar City, Utah. Access to the silicic volcanoes of the Mineral Mountains is from the west via Milford, Utah. Interstate 15 crosses the Cove Fort flows immediately north of the interchange with Interstate 70.

Mountain Meadow Monument and Overlook:¹⁴
From the entrance road to Mountain Meadow monument -- proceed to the overlook parking area on Dan Sill Hill. You will pass a gravel road on the left that leads to the gravesite in Mountain Meadow. This is the site of the infamous 1857 massacre of about 120 emigrants while they were traveling the Old Spanish Trail that traverses Mountain Meadow. Park in monument parking lot and take short paved trail to monument overlook on Dan Sill Hill. The hill is made of the upper ash-flow tuff member of the Rencher Formation overlying allochthonous Claron rocks of the Big Mountain slide. Northwest of Mountain Meadow is Big Mountain (with radio towers on top) at the northern end of the Bull Valley-Big Mountain arch. The Big Mountain dome consists of Iron Springs rocks and locally some Carmel limestone and intrusive quartz monzonite. In this denuded area of the arch, the Iron Springs and/or Claron are overlain by the upper ash-flow tuff of the Rencher Formation. The hills on the east side of Mountain Meadow (east of SR-18 behind you) consist of the Big Mountain slide that originated on Big Mountain. These hills contain a thick section of the conspicuous allochthonous white lower Rencher, which has slid eastward from the crest or flank of the Bull Valley – Big Mountain arch prior to the eruption of the upper Rencher.

Mt. Belknap:¹
Composite volcanoes (stratovolcanoes) develop from repeated explosive and nonexplosive eruptions of tephra (airborne lava fragments that can range in size from tiny particles of ash to house-size boulders) and lava that build up layer by layer. These volcanoes are the largest and form symmetrical cones with steep sides. Some composite volcanoes in Utah are in the Tushar Mountains (Mt. Belknap, for example) in Piute County. Now extinct, they are too old (between 32 and 22 million years) to maintain the classic volcanic shape of their modern-day counterparts, such as Mount Hood and Mount St. Helens in the Cascade Range along the northwestern coast of the United States.

Big Cottonwood Canyon:¹
Between 33 and 32 million years ago, molten intrusive igneous rock invaded the surrounding rocks in the Big Cottonwood Canyon area. As the magma cooled and solidified, it created an environment that encouraged growth and concentration of metallic minerals in the surrounding rocks. Millions of years later, after the rocks of the Wasatch Range were uplifted and erosion began, rocks of the Alta stock were exposed at the earth's surface. Pleistocene glaciers caved two magnificent peaks at the head of this canyon. Mt. Wolverine (10,795 feet in elevation) and Mt. Millicent (10.452 feet in elevation) are both composed of igneous rocks of the Alta stock. In the 1860s, lead, zinc, silver, and copper were found and the Big Cottonwood mining district was officially formed.

Big Cottonwood Canyon:¹
Between 33 and 32 million years ago, molten intrusive igneous rock invaded the surrounding rocks in the Big Cottonwood Canyon area. As the magma cooled and solidified, it created an environment that encouraged growth and concentration of metallic minerals in the surrounding rocks. Millions of years later, after the rocks of the Wasatch Range were uplifted and erosion began, rocks of the Alta stock were exposed at the earth's surface. Pleistocene glaciers caved two magnificent peaks at the head of this canyon. Mt. Wolverine (10,795 feet in elevation) and Mt. Millicent (10.452 feet in elevation) are both composed of igneous rocks of the Alta stock. In the 1860s, lead, zinc, silver, and copper were found and the Big Cottonwood mining district was officially formed.

Navajo Mountain, La Sal, Abajo, and Henry Mountains:¹
Dome Mountains are formed from hot molten material (magma) rising from the Earth's mantle into the crust that pushes overlying sedimentary rock layers upward to form a "dome" shape. Unlike a volcano, the magma typically does not reach the Earth's surface. Instead, the magma cools underneath the surface and forms the core of the mountains. Dome mountains in Utah include Navajo Mountain, and the La Sal, Abajo, and Henry Mountains in the southeastern part of the state.

Pavant Butte:²
Pavant Butte, a large (275-meter-high, 3-kilometer-diameter) tuff cone, was built 16,000 years ago on top of pahoehoe and aa lavas of the Pavant field, and erupted through a water depth of 85 meters in Lake Bonneville. A prominent shoreline terrace is developed on the tuff cone. The then subaerial portion has been entirely converted to palagonite.

Pahvant Butte:¹
About 15,500 years ago, in a place that is now known as the Black Rock Desert in west-central Utah, a volcano explosively erupted from the bottom of the rising Ice Age Lake Bonneville. Pahvant Butte (also known as Pavant Butte of Sugarloaf) ejected shreds of basalt lava high into the air that quickly cooled into glassy particles the size of sand (volcanic ash) and gravel (volcanic cinders) collectively known as tuff. The explosion produced a crater on the south face of Pahvant Butte. During the eruption the wind must have been blowing to the northeast; black volcanic ash from the eruption is found in sand dunes northeast of Pahvant Butte. When the eruption ceased, a volcanic cone called a tuff cone was left to the mercy of erosion by Lake Bonneville. The rising Lake Bonneville was only 50 feet below its highest level when the eruption occurred. The highest point on Pahvant Butte was at least 435 feet above the water at the time. Waves carved a shelf around most of the volcano except for the north face where intense storm waves cut a vertical cliff into the cone. The cut exposes an intricate lacey pattern caused by the partial cementing of the tuff by minerals in ground water. The cliff is known as the "Lace Curtain" because of its white color and mysterious lacey pattern. Today the highest point on Pahvant Butte is 5,486 feet above sea level. The entire volcano is about 740 feet above the ground surface and 2 miles in diameter.

Pine Valley Mountains:¹⁴
Rocks of the Pine Valley Mountains consist mostly of volcanic and intrusive rocks that range in age from Oligocene to Quaternary that were erupted upon or intruded into Mesozoic and Tertiary sedimentary rocks. The laccolithic bodies belong to a group of more than a dozen closely related, early Miocene intrusions that constitute a magmatic province trending northeasterly across the structural transition zone between the Basin and Range and Colorado Plateau in this region, generally along the trend of the Sevier orogenic front. Because laccoliths of the Iron Springs district and eastern Bull Valley Mountains are well aligned within the belt and have produced sizable iron deposits, the belt has known informally as the “Iron Axis” (Toby, 1976; Blank and others, 1992; Rowley and others, 1995; Hacker, 1998). Intrusions of Iron Axis affinity were forcibly emplaced within 3.0 to 0.25 kilometers of the surface as bulbous laccoliths, sills, and other partly concordant bodies, and were emplaced within the axial zones of some of the older, southeast-vergent Sevier thrusts and folds (Mackin, 1960). The largest Iron Axis intrusion forms the gigantic (>200 square kilometers) igneous mass capping the Pine Valley Mountains.

Sheeprock Mountains:¹
Granite is the intrusive equivalent of rhyolite but has a coarser texture. A 12-square-mile outcrop of granite is visible on the southwestern flank of the Sheeprock Mountains, Tooele and Juab Counties.

Signal Peak:¹
In Utah andesite can be seen at Signal Peak in the Tushar Mountains, Piute County. Andesite has a higher quartz content than basalt and is usually lighter in color. Crystals of the minerals amphibole, biotite, and feldspar are sometimes visible without magnification.

Smelter Knolls:²
The Smelter Knolls consist of several coalescing rhyolite domes and flows with an estimated total erupted volume of 2.2 cubic kilometers. No pyroclastic deposits are exposed, although flow breccias and basal vitrophyres are present. The lava is topaz rhyolite. The two main bodies of the Smelter Knolls are separated by a north-south-trending normal fault, consistent with the orientation of normal faults in Fumarole Butte 15 kilometers to the north. Nearby mafic volcanism dated at 6.1 and 0.3 million years bracket the silicic episode. Smelter Knolls is 24 kilometers west-northwest of Delta, Utah.

Snow Canyon State Park:⁹
Red Navajo sandstone, capped by an overlay of black lava rock, makes photography, hiking, biking, and camping in Snow Canyon a double treat. Early spring and fall use of the park is especially appealing due to southern Utah's moderate winter climate. Two recent volcanic cones are found near the head of the canyon.

Snow Canyon Inverted Topography:¹
About 3 million years ago, after erosion coupled with regional uplift profoundly denuded the area of overlying rocks, volcanic eruptions began spitting scalding, pungetn, black seas of basalt onto the land. Fiery channels of hot, molten rocks snaked their way over the earth and down into stream beds, valleys, and canyons; enveloping all that stood in their paths. These rivers quickly hardened into rocks, forming resistant, thick sheets of basalt that invaded and obstructed paths of rivers and steams. Seeking avenues of least resistance, drainages continued along their courses by shifting to the edges of the basalt flows (that now filled the earlier channels) and slicing new routes through the softer sedimentary rocks of the Navajo Sandstone. Erosion continued along the new channels until the water routes grew in size from stream beds, to ravines, to deep canyons. The sheets of basaltic rocks that initially filled low areas, cooled into resistant masses and eventually stood in relief as high ridges and plateaus. New volcanic eruptions occurred with lavas again invading flat lands, furrows, gullies, and depressions. This second blanket of basalt covered an area topographically lower than the first. Three distinct phases of the "inverted topography" are evident in Snow Canyon State Park. The oldest layer forms the plateau to the east of State Highway 18 (the road from St. George to Vejo). The next forms the pateau on which the highway is built, and the third forms the floor of Snow Canyon itself.

Snow Canyon Cinder Cones:¹
Sometime between 1,000 to 10,000 years ago the youngest series of volcanic eruptions began emanating from cinder cones and vents in the north section of the park. Following drainage channels etched in soft sandstones along the sides of solidified volcanic flows, the new, scalding flows crept south onto the floor of Snow Canyon and nearby areas. Today, these flows line the canyon floor stretching south to the Santa Clara River.

Flows in the Park:¹
Numerous features and textures characteristic of volcanic flows are well preserved in the park. For example, visitors to West Canyon can see motionless black cascades of basalt, and areas where the flows encircled mounds of Navajo Sandstone and cascaded down steep embankments onto the canyon floor. Hikers will notice that these black falls appear to have ended abruptly when they touched the floor of West Canyon. In truth, however, they extended across the canyon and, in places, may have touched the walls on the opposite side. Shifting desert sands and flash-flood debris of more recent times have obscured floor basalts in most areas.

St. George³
A line of Quaternary lava flows and cinder cones stretches from St. George, Utah, nearly 200 kilometers northeastward to the village of Loa. Additional young vents and flows extend approximately 50 kilometers north of St. George. Volcanism near St. George is best known and most spectacular; lava flows erupted from vents in the Pine Mountains flowed downslope into river valleys. Four different episodes of flow emplacement have been recognized, each preserving underlying Mesozoic rock from further erosion.

Oldest Flows:³
The oldest flows, formed 3-6 million years ago, are up to 300 meters thick above their surroundings, and the younger flows occur at approximately 120 meters (1-2 million years ago), on the present drainage (500,000 years ago), and fill stream valleys (a few thousand years).

Santa Clara Flow:³
The most recent flows came from two cinder cones in Diamond Valley, 16 kilometers north of St. George. The cones, approximately 400 meters wide and 60 meters high, are the sources for the Santa Clara flow which traveled 16 kilometers to the south. Geomorphological features to be seen along the flow include inverted valleys, lava dammed lakes, displaced drainages, and 120-meter-high lava cascades. The Santa Clara flow can be viewed along State Highway 18, west of St. George in and around Snow Canyon State Park.

Sunstone Knoll:¹
Sunstone Knoll is formed of volcanic vents that erupted during the early Pleistocene (1.6 million years to about 750,000 years ago). These eruptions left deposits of basaltic lava and volcanic breccia (angular, broken rock fragments held together in a matrix of finer grained material). Sunstone is a transparent, yellowish labradorite (a plagioclase feldspar mineral) found as crystals in these volcanic rocks and on the flats surrounding the knoll.

Swett Hills:¹⁴
Swett Hills consist of east-dipping Tertiary ash-flow tuffs as found in Eightmile Hills. [See Eightmile Hills above]

Topaz Mountain:¹
Topaz, Utah's state gem, is a semiprecious gemstone that occurs as very hard, transparent crystals in a variety of colors. The topaz crystals at Topaz Mountain are naturally amber colored, but become colorless after exposure to sunlight. The crystals formed within cavities of the Topaz Mountain Rhyolite, a volcanic rock which erupted approximately six to seven million years ago (Tertiary Period) from volcanic vents along faults in the area.

Tushar Mountains:¹⁴
Interstate-15 Exit 95, intersection with State Road 20 - Virtually all rocks you see to the west, north, and east consist of volcanic mudflow breccia of the Mount Dutton Formation. The high Tushar Mountains ahead are The high Tushar Mountains are mostly rhyolites of the Mount Belknap Volcanics, within the Mount Belknap caldera.

Mt. Belknap:¹
Composite volcanoes (stratovolcanoes) develop from repeated explosive and nonexplosive eruptions of tephra (airborne lava fragments that can range in size from tiny particles of ash to house-size boulders) and lava that build up layer by layer. These volcanoes are the largest and form symmetrical cones with steep sides. Some composite volcanoes in Utah are in the Tushar Mountains (Mt. Belknap, for example) in Piute County. Now extinct, they are too old (between 32 and 22 million years) to maintain the classic volcanic shape of their modern-day counterparts, such as Mount Hood and Mount St. Helens in the Cascade Range along the northwestern coast of the United States.

Signal Peak:¹
In Utah andesite can be seen at Signal Peak in the Tushar Mountains, Piute County. Andesite has a higher quartz content than basalt and is usually lighter in color. Crystals of the minerals amphibole, biotite, and feldspar are sometimes visible without magnification.

Twin Peaks:²
Twin Peaks is a late Tertiary, bimodal volcanic field of tholeiite, basaltic andesite, and low- to high-silica rhyolite. Silicic volcanism spanned the period from 2.7 to 2.3 million years ago, with two evolutionary cycles beginning with rhyodacite and culminating in high-silica rhyolite. The total volume of silicic eruptives is estimated to be approximately 12 cubic kilometers, much of which is now buried under younger sediments. Silicic volcanics comprise North Twin Peak rhyolite dome, South Twin Peak rhyolite dome (the largest of the domes, being 400 meters high and 1.4 kilometers in diameter), and coalescing domes and flows of the Coyote Hills, which define a semi-circular pattern around the 10-kilometer-diameter complex. Mafic lavas are confined primarily to the south. Local subsidence at the end of the first silicic cycle resulted in deposition of 300 meters of calcareous lacustrine sediments with interbedded silicic tuffs and mafic lava flows. Resurgent doming in the south produced a doubly plunging anticline with 400 meters of relief. Basalt from Burnt Mountain erupted on the crest of the resurgent dome and flowed down the eroded flanks of the anticline, indicating that resurgence closely followed cessation of silicic volcanism. Twin Peaks is in west-central Utah, approximately 275 kilometers south of Salt Lake City.

Vernon Hills Wonderstone:¹
The Vernon Hills wonderstone is a welded-vitric tuff (vitric means glassy) of rhyolitic composition. It is a volcanic rock composed predominantly of volcanic glass particles which have been welded or stuck together by heat and compacted by the weight of overlying material. Alteration of the rock by circulating ground water produced the colorful banding. The maroon and yellow-brown colors are due to the presence of iron oxides.

Wildhorse Canyon:²
The Mineral Mountains-Cove Fort volcanic field is a Quaternary bimodal, basalt-rhyolite association with some intermediate composition units. Silicic volcanism began at 800,000 years ago with eruption of two fluid, aphyric, rhyolite flows (3 kilometers long, 80 meters thick) along Bailey Ridge and Wildhorse Canyon. [See Mineral Mountains-Cove Fort above]

Wildhorse Canyon Obsidian:¹²
In Utah's Mineral Mountains, overlooking the west desert, is the entrance to Wildhorse Canyon. Here the terrain is rough, blanketed with pinyon and juniper forest underlain by sagebrush, cacti, and grasses. The canyon has no year-round stream. Above its mouth, Wildhorse forks into a small side channel and the main canyon. Lying between the tines is a short ridge. On this ridge, mixed with tuff, perlite and basalt stone, are three obsidian flows with a total thickness of 65 feet. Archaeological and scientific studies suggest this site has served as a major obsidian quarry and manufacturing station for this part of the Great Basin for not hundreds but thousands of years.

Three types of Obsidian:¹²
Archaeological exploration of ... villages (nearby Fremont Indian villages (between A.D. 900 and 1300)) ... has uncovered artifacts made of three types of obsidian: True obsidian, which is transparent with closely-spaced, horizontal black bands; pitchstone, actually a dense, opaque, black volcanic glass; brown obsidian, streaked with flowing bands of brown, red, and black. All three obsidians have been found at the Wildhorse Canyon quarry.

Wildhorse Canyon Obsidian Quarry:¹⁰
Added to National Register of Historical Places in 1976. Historic Significance: Information Potential. Area of Significance: Prehistoric. Cultural Affiliation: Fremont. Period of Significance: 1000-500 A.D., 1499-1000 A.D.

Zion National Park

Geologic Location:5 Zion is located along the edge of a region called the Colorado Plateau. The rock layers have been uplifted, tilted, and eroded, forming a feature called the Grand Staircase, a series of colorful cliffs stretching between Bryce Canyon and the Grand Canyon. The bottom layer of rock at Bryce Canyon is the top layer at Zion, and the bottom layer at Zion is the top layer at the Grand Canyon.

Volcanic Rocks:⁵
Sedimentary rock, mostly sandstone. Some limestone, shale, mudstone, and conglomerate. Mostly Triassic through Jurassic (250 million to 150 million years ago). Some recent volcanic activity in the form of cinder cones and lava flows.

Excerpts from:
1) Utah Geological Survey Website, 2001, 2002
2) Nash, 1990, IN: Wood and Kienle, 1990, Volcanoes of the North America: Cambridge University Press
3) Wood, 1990, IN: Wood and Kienle, 1990, Volcanoes of the North America: Cambridge University Press
4) U.S. National Park Service Website - Capitol Reef National Park, 2000;
5) U.S. National Park Service Website - Zion National Park, 2000, 2001;
6) U.S. National Park Service Website - Fossil Butte National Monument, 2001
7) Bureau of Indian Affairs, Division of Energy and Minerals Website, 2001
8) USGS/NPS Geology in the Parks Website, 2001
9) Utah State Parks Website, 2002
10) National Register of Historic Places Website, 2002
11) Utah.gov Website, 2002, "A Brief History of Utah"
12) Utah.gov Website, 2002, "Utah History To Go"
13) Rowley, et.al., 2002 Geologic Map of the Central Marysvale Volcanic Field, Southwestern Utah: USGS Geologic Investigations Series I-2645-A
14) Lund (ed.), 2002, Field Guide to Geologic Excursions in Southwestern Utah and Adjacent Areas of Arizona and Nevada: U.S. Geological Survey Open-File Report 02-172.
15) U.S. National Park Service, National Natural Landmarks Website, 2003

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