Adel Mountains Volcanic Field
Encyclopedia
The Adel Mountains Volcanic Field (also known as the Adel Mountains, Adel Volcanics, and Adel Mountain Volcanics) is an ancient volcanic field
of heavily eroded 75 million year old igneous rock
s about 40 miles long and 20 miles wide (800 square miles, or 2,071 square kilometers) in west-central Montana
about 30 miles southwest of the city of Great Falls
. The area was named by geologist
John Bartholomew Lyons, who first described the general geology of the region in 1944. The Adel Mountains Volcanic Field is a significant and abundant source of shonkinite
, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario
, and Timor
. Because of its geologic structure, the Adel Mountains Volcanic Field has drawn the attention of geologists for more than 100 years.
on top of Cretaceous
sedimentary rock
of the Two Medicine Formation
on the edge of the Great Falls Tectonic Zone
. The extrusive rock is composed of breccia
and conglomerate
. The intrusive
rock has formed numerous dikes
, sills
, and plugs
. An "enormous number of dikes...emanate radially from its center", many of which are visually striking and vertical, and can be seen from the interstate highway
and other roads in the area. Many of the dikes clearly fed laccoliths in the area.
Two concentric rings of laccolith
s lie in the northwest section of the field 11 miles (18 kilometers) and 14 miles (23 kilometers) from the center of the area. Square Butte, Shaw Butte (near the town of Ft. Shaw
), and Cascade Butte (next to the town of Cascade
) are three laccoliths belonging to the Adel field which are famous local landmarks. More than 2,000 feet of rock which used to exist on top of Square Butte has subsequently eroded to reveal the igneous rock which forms the laccolith.
Adel Mountain is the highest peak in the volcanic field, at 7,093 feet (2,162 meters) above sea level, and an unnamed peak in the field is the second-highest at 6,845 feet (2,086 meters) above sea level.
, a period of mountain building in western North America which began about 80 million years ago and ended about 35 million years ago. The age of the volcanism and thrusting
overlap, with some intrusions being affected by the thrusting while others were intruded into already formed thrust horses.
Magma
welled up from below for several million years, creating the Adel volcanics as well as the nearby Highwood Mountains
and Judith Mountains
. The crustal weakness associated with the Great Falls Tectonic Zone may have provided a route for the magma to reach the surface. Magma which never made it to the surface nonetheless created the many dikes, sills, plugs, and laccoliths which can be seen in the Adel volcanic field.
Scientists have studied the Adel Mountains Volcanic Field for a number of reasons. Several teams of scientists have analyzed the polar wander
path to study continental drift
. Others have looked at the possibility of petroleum
or other hydrocarbon
s under the field, and concluded there is a strong possibility they exist there.
, several tribes camped in the Adel Mountains Volcanic Field on their way to and from bison
hunting rounds on the nearby plains (such as the pishkun
, or buffalo jump
, at nearby Ulm
—the largest in the world). The Lewis and Clark Expedition
passed up the Missouri River through the area in July 1805, becoming the first white Americans to see the formations.
Development of the area did not come until the 1930s. Cattle ranchers were the area's first white settlers. The primary route over the Rocky Mountains
to the Pacific Ocean
, the Mullan Road
, skirted the volcanic field to the north, although dirt stagecoach
roads were built into the Missouri River Canyon in the volcanic field in the 1860s. The road was upgraded and turned into a toll road
in 1866, and had become a major route from Helena through Great Falls to Fort Benton
by the 1870s. The Montana Central Railroad reached the region in 1887, which led to additional white settlement. A paved county road was laid down, and remained the sole access through the area for decades. In the 1920s, the Montana Highway Department
began planning a major highway through the area as part of their effort to build U.S. Highway 91
and provide a direct road connection between Helena
and Great Falls. Contracts were let in 1929, the majority of construction occurred from 1931 to 1934, and the highway was completed in 1935. The Hardy Bridge, built over the Missouri River in 1931, was one of two major bridges built for Highway 91. Interstate 15
was built largely on top of U.S. Highway 91 in 1967.
motion picture Thunderbolt and Lightfoot
was filmed on and next to Interstate 15 as it passed through the Adel volcanics. A portion of the 1987
movie The Untouchables was shot at the Hardy Bridge in the Adel Mountains Volcanic Field, which doubled for the U.S.-Canadian border.
Motion picture star Gary Cooper
was raised on the Seven Bar Nine Ranch, which was located in the Adel Mountains.
Volcanic field
A volcanic field is an area of the Earth's crust that is prone to localized volcanic activity. They usually contain 10 to 100 volcanoes, such as cinder cones and are usually in clusters. Lava flows may also occur...
of heavily eroded 75 million year old igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s about 40 miles long and 20 miles wide (800 square miles, or 2,071 square kilometers) in west-central Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
about 30 miles southwest of the city of Great Falls
Great Falls, Montana
Great Falls is a city in and the county seat of Cascade County, Montana, United States. The population was 58,505 at the 2010 census. It is the principal city of the Great Falls, Montana Metropolitan Statistical Area, which encompasses all of Cascade County...
. The area was named by geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
John Bartholomew Lyons, who first described the general geology of the region in 1944. The Adel Mountains Volcanic Field is a significant and abundant source of shonkinite
Shonkinite
Shonkinite is a mafic foidal syenite, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar in the form of sanidine, nepheline, augite, biotite, and olivine. Shonkinite is also used for mafic nepheline syenite with aegerine-augite as the pyroxene, and...
, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, and Timor
Timor
Timor is an island at the southern end of Maritime Southeast Asia, north of the Timor Sea. It is divided between the independent state of East Timor, and West Timor, belonging to the Indonesian province of East Nusa Tenggara. The island's surface is 30,777 square kilometres...
. Because of its geologic structure, the Adel Mountains Volcanic Field has drawn the attention of geologists for more than 100 years.
Description
The Adel Mountains Volcanic Field is a 3,280 feet thick (1,000 meters) unit which lies unconformablyUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
on top of Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
of the Two Medicine Formation
Two Medicine Formation
The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma , during Campanian time, and is located in northwestern Montana...
on the edge of the Great Falls Tectonic Zone
Great Falls Tectonic Zone
The Great Falls Tectonic Zone is a major intracontinental shear zone between the Hearne craton and Wyoming craton, basement rock of the Archean Eon which form part of the North American continent. The zone is an area about 100 miles wide extending from the southwestern Idaho-Montana border across...
. The extrusive rock is composed of breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
and conglomerate
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
. The intrusive
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
rock has formed numerous dikes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
, sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
, and plugs
Volcanic plug
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an...
. An "enormous number of dikes...emanate radially from its center", many of which are visually striking and vertical, and can be seen from the interstate highway
Interstate Highway System
The Dwight D. Eisenhower National System of Interstate and Defense Highways, , is a network of limited-access roads including freeways, highways, and expressways forming part of the National Highway System of the United States of America...
and other roads in the area. Many of the dikes clearly fed laccoliths in the area.
Two concentric rings of laccolith
Laccolith
A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively...
s lie in the northwest section of the field 11 miles (18 kilometers) and 14 miles (23 kilometers) from the center of the area. Square Butte, Shaw Butte (near the town of Ft. Shaw
Fort Shaw, Montana
Fort Shaw is a census-designated place in Cascade County, Montana, United States. The population was 274 at the 2000 census. It is part of the 'Great Falls, Montana Metropolitan Statistical Area'.-Geography:...
), and Cascade Butte (next to the town of Cascade
Cascade, Montana
Cascade is a town in Cascade County, Montana, United States. The population was 819 at the 2000 census. It is part of the 'Great Falls, Montana Metropolitan Statistical Area'.-Geography:...
) are three laccoliths belonging to the Adel field which are famous local landmarks. More than 2,000 feet of rock which used to exist on top of Square Butte has subsequently eroded to reveal the igneous rock which forms the laccolith.
Adel Mountain is the highest peak in the volcanic field, at 7,093 feet (2,162 meters) above sea level, and an unnamed peak in the field is the second-highest at 6,845 feet (2,086 meters) above sea level.
Formation
The age of the Adel Mountain Volcanic Field has been re-estimated several times since 1944. Based on fossil evidence, Lyons estimated in 1944 that the formation was late Cretaceous in origin, making it about 65 million years old. This estimate was reinforced in the early 1970s. But a 1987 estimate based on new data put the age at early Tertiary, or about 60 million years old. But separate analyses made in 1991 and 2004 re-estimated the rock as having formed in the late Cretaceous, about 81 to 71 million years ago. Most estimates today place the age at 75 million years. The Adel volcanics are part of the Laramide orogenyLaramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
, a period of mountain building in western North America which began about 80 million years ago and ended about 35 million years ago. The age of the volcanism and thrusting
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
overlap, with some intrusions being affected by the thrusting while others were intruded into already formed thrust horses.
Magma
Magma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
welled up from below for several million years, creating the Adel volcanics as well as the nearby Highwood Mountains
Highwood Mountains
The Highwood Mountains cover approximately 4,659 km² in north central Montana in the U.S., east of Great Falls and Benton Lake National Wildlife Refuge, at the northern end of the Lewis and Clark National Forest...
and Judith Mountains
Judith Mountains
The Judith Mountains are located in the north central area in the U.S. State of MontanaJudith Peak is the highest summit of this range, with an elevation of 6,428 feet....
. The crustal weakness associated with the Great Falls Tectonic Zone may have provided a route for the magma to reach the surface. Magma which never made it to the surface nonetheless created the many dikes, sills, plugs, and laccoliths which can be seen in the Adel volcanic field.
Scientists have studied the Adel Mountains Volcanic Field for a number of reasons. Several teams of scientists have analyzed the polar wander
Polar wander
Polar wander is the movement of the North and South Poles of the Earth with respect to the continents. This motion can be divided into two components: that due to continental drift , and true polar wander, in which the mantle and the crust rotate together into new orientations....
path to study continental drift
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
. Others have looked at the possibility of petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
or other hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s under the field, and concluded there is a strong possibility they exist there.
Development of the area
Although not permanently inhabited by Native AmericansNative Americans in the United States
Native Americans in the United States are the indigenous peoples in North America within the boundaries of the present-day continental United States, parts of Alaska, and the island state of Hawaii. They are composed of numerous, distinct tribes, states, and ethnic groups, many of which survive as...
, several tribes camped in the Adel Mountains Volcanic Field on their way to and from bison
American Bison
The American bison , also commonly known as the American buffalo, is a North American species of bison that once roamed the grasslands of North America in massive herds...
hunting rounds on the nearby plains (such as the pishkun
First Peoples Buffalo Jump State Park
First Peoples Buffalo Jump State Park is a Montana state park in Cascade County, Montana in the United States. The park is and sits at an elevation of .It is located about northwest of the small town of Ulm, which is near the city of Great Falls...
, or buffalo jump
Buffalo jump
A buffalo jump is a cliff formation which North American Indians historically used in mass killings of plains bison. Hunters herded the bison and drove them over the cliff, breaking their legs and rendering them immobile. Tribe members waiting below closed in with spears and bows to finish the kills...
, at nearby Ulm
Ulm, Montana
Ulm is a census-designated place in Cascade County, Montana, United States. It was originally a large ranch owned by Indiana-born cattleman William Ulm. The population was 750 at the 2000 census...
—the largest in the world). The Lewis and Clark Expedition
Lewis and Clark Expedition
The Lewis and Clark Expedition, or ″Corps of Discovery Expedition" was the first transcontinental expedition to the Pacific Coast by the United States. Commissioned by President Thomas Jefferson and led by two Virginia-born veterans of Indian wars in the Ohio Valley, Meriwether Lewis and William...
passed up the Missouri River through the area in July 1805, becoming the first white Americans to see the formations.
Development of the area did not come until the 1930s. Cattle ranchers were the area's first white settlers. The primary route over the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
to the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
, the Mullan Road
Mullan Road
Mullan Road was the first wagon road to cross the Rocky Mountains to the Inland of the Pacific Northwest. It was built by US Army Lieut. John Mullan between the spring of 1859 and summer 1860. It led from Fort Benton, Montana, the navigational head of the Missouri River to Fort Walla Walla,...
, skirted the volcanic field to the north, although dirt stagecoach
Stagecoach
A stagecoach is a type of covered wagon for passengers and goods, strongly sprung and drawn by four horses, usually four-in-hand. Widely used before the introduction of railway transport, it made regular trips between stages or stations, which were places of rest provided for stagecoach travelers...
roads were built into the Missouri River Canyon in the volcanic field in the 1860s. The road was upgraded and turned into a toll road
Toll road
A toll road is a privately or publicly built road for which a driver pays a toll for use. Structures for which tolls are charged include toll bridges and toll tunnels. Non-toll roads are financed using other sources of revenue, most typically fuel tax or general tax funds...
in 1866, and had become a major route from Helena through Great Falls to Fort Benton
Fort Benton, Montana
Fort Benton is a city in and the county seat of Chouteau County, Montana, United States. A portion of the city was designated as a National Historic Landmark District in 1961. Established a full generation beforethe U.S...
by the 1870s. The Montana Central Railroad reached the region in 1887, which led to additional white settlement. A paved county road was laid down, and remained the sole access through the area for decades. In the 1920s, the Montana Highway Department
Montana Department of Transportation
The Montana Department of Transportation is a governmental agency in the U.S. state of Montana, responsible for numerous programs related to the construction, maintenance, and monitoring of Montana's transportation infrastructure and operations...
began planning a major highway through the area as part of their effort to build U.S. Highway 91
U.S. Route 91
U.S. Route 91 is a north–south United States highway. The highway currently serves as a connection between the Cache Valley area of Utah and Idaho to the Salt Lake and Idaho Falls population centers. Prior the mid-1970s, U.S. 91 was an international commerce route from Long Beach, California...
and provide a direct road connection between Helena
Helena, Montana
Helena is the capital city of the U.S. state of Montana and the county seat of Lewis and Clark County. The 2010 census put the population at 28,180. The local daily newspaper is the Independent Record. The Helena Brewers minor league baseball and Helena Bighorns minor league hockey team call the...
and Great Falls. Contracts were let in 1929, the majority of construction occurred from 1931 to 1934, and the highway was completed in 1935. The Hardy Bridge, built over the Missouri River in 1931, was one of two major bridges built for Highway 91. Interstate 15
Interstate 15
Interstate 15 is the fourth-longest north–south Interstate Highway in the United States, traveling through the states of California, Nevada, Arizona, Utah, Idaho, and Montana from San Diego to the Canadian border...
was built largely on top of U.S. Highway 91 in 1967.
In popular culture
The final scenes of the 19741974 in film
The year 1974 in film involved some significant events.-Events:*February 7 - Blazing Saddles is released in the USA.*August 7 - Peter Wolf, lead singer of The J...
motion picture Thunderbolt and Lightfoot
Thunderbolt and Lightfoot
Thunderbolt and Lightfoot is a 1974 American crime film written and directed by Michael Cimino and starring Clint Eastwood, Jeff Bridges, George Kennedy, and Geoffrey Lewis.-Plot:...
was filmed on and next to Interstate 15 as it passed through the Adel volcanics. A portion of the 1987
1987 in film
-Events:*January 31 - The Cure for Insomnia premieres at The School of the Art Institute in Chicago, Illinois, to officially become the world's longest film according to Guinness World Records....
movie The Untouchables was shot at the Hardy Bridge in the Adel Mountains Volcanic Field, which doubled for the U.S.-Canadian border.
Motion picture star Gary Cooper
Gary Cooper
Frank James Cooper, known professionally as Gary Cooper, was an American film actor. He was renowned for his quiet, understated acting style and his stoic, but at times intense screen persona, which was particularly well suited to the many Westerns he made...
was raised on the Seven Bar Nine Ranch, which was located in the Adel Mountains.