America's Volcanic Past -
Montana

View the Geologic Time Scale

Volcanic Highlights and Features:
[This list is just a sample of various Montana volcanic features or events and is by no means inclusive.]

• Montana
• Montana Regions
• Absaroka Mountains
• Adel Mountains
• Bearpaw Mountains
• Beartooth Plateau
• Bitterroot Mountains
• Blacktail Mountains
• Boulder Batholith
• Bridger Range
• Gallatin Range
• Glacier National Park (USA) - Waterton Lakes National Park (Canada)
• Granite Peak
• Gravely Mountains
• Highwood Mountains
• Humbug Spires
• Idaho Batholith
• Judith Mountains
• Lost Creek State Park
• Madison Mountains
• Ruby Mountains
• Snowcrest Mountains
• Southwest Montana
• Square Butte
• Sweet Grass Hills
• Upper Missouri River Breaks National Monument

Absaroka Mountains:⁶ The Absarokas are the result of volcanic activity similar to that that created the Cascades. A chain of volcanoes erupted about 50 million years ago as a subducting ocean plate dived beneath the continent's western edge.

Adel Mountains

Adel Mountains:⁶
Volcanic activity also created mountains in west-central Montana -- the Adel, Highwood, Judith, and Bearpaw Ranges. Some of the magma never made it to the earth's surface but formed igneous intrusions.

Bearpaw Mountains:⁶
Volcanic activity also created mountains in west-central Montana -- the Adel, Highwood, Judith, and Bearpaw Ranges. Some of the magma never made it to the earth's surface but formed igneous intrusions.

Beartooth Plateau:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Bitterroot Mountains:⁶
The rugged, 470-mile long Bitterroot Range winds along the Montana-Idaho border. The Bitterroots are Montana's longest mountain range and include three groups of mountains: the Bitterroots, the Beaverheads, and the Centennials. They are also part of one of North America's best known batholiths, the Idaho Batholith of central Idaho and western Montana. A batholith is a large intrusion of solidified magma that does not reach the earth's surface as it cools. Later, with erosion, it can become exposed which is what has happened in the Bitterroots. So unlike the mountains of northwestern Montana, which are composed of sedimentary rock, this range is mostly granite. The rock is roughly 75 million years old.

Idaho Batholith:⁶
One theory is that the emplacement of Idaho Batholith raised the earth's crust like a blister. The bulging caused another large block of rock -- the Sapphire Block -- to gradually start sliding downslope in an easterly direction. According to the theory, this block of rock, like the mountains of northwestern Montana, slid about 50 miles. When it finished sliding, it had pushed up, bulldozer fashion, the semi-circular arc of isolated mountains we call the Garnet, Flint Creek, and Anaconda-Pintler Ranges.

Blacktail Mountains

Blacktail Mountains:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Boulder Batholith:⁶
The Boulder Batholith makes up the jumble of mountains along the Continental Divide between Helena and Butte, mountains well-known for their striking and strangely shaped rocks, some of which are the result of related volcanic activity. At the time of the batholith's doming, it is believed that this region of Montana was a geologic hot spot similar to today's Yellowstone Park. [Also See Idaho Batholith]

Boulder Batholith Mines:⁶
Forty-mile wide blister of granite that stretches from Helena to the Big Hole River. Rich in minerals. Mines in Helena, Boulder, Butte, and Silver Star.

Bridger Range - Intrusive Rocks:⁸
Dacite, diorite, diabase, and basalt dikes and sills. Diorite dike in northern part of Bridger Range is olive-gray, coarsely crystalline, pyroxene phenocrysts; composite dike in southern part of range may be similar in composition. Sills differentiated into upper syenite layer and lower layer of biotite-augite diorite. Thickness 80-200 feet. Eocene.

Bridger Range - Livingston Group:⁸
Livingston Group is volcaniclastic conglomerate, sandstone, and mudstone; and volcanic flows, sills, tuff, and breccia. Upper Cretaceous.

Bridger Range - Metamorphic Rocks:⁸
Primarily gneiss. Includes schist, granite, quartzite, pegmatite, amphibolite, and mafic intrusive rocks. Archean.

Bridger Range - Volcanic Ash Beds:⁸
Sandy and conglomeratic tuffaceous siltstone with interbedded volcanic ash, freshwater gastropod limestone, and fossiliferous lenses of conglomerate and gravel. Shown only in extreme northeast corner of Bridger Range. Exposures more than 100 feet thick. Lower Miocene.

Gallatin Range:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Lewis Overthrust:1 The Lewis Overthrust began 170 million years ago, when a collision of the Earth s crustal plates elevated numerous mountain chains and formed the ancestral Rocky Mountains. Ever-increasing stresses near the end of this great event shoved a huge rock wedge, several miles thick and several hundred miles wide, eastward more than 50 miles. Large masses of relatively stronger rocks were shoved over softer and more easily deformed rocks. Erosion stripped away the upper part of the original rock wedge and exposed the rocks and structures visible in the park today. Rarely have rocks of such ancient age been thrust over rocks that are so much younger. The overlying Proterozoic rocks are over 1,500 million years older than the underlying Cretaceous age rocks. MORE about the Lewis Overthrust Fault Exposed Rocks within the Park:1 Most of the rocks exposed in the park are sedimentary rocks of Proterozoic age, which were deposited from 1,600 to 800 million years ago. Rocks of that age in other parts of the world have been greatly altered by mountain building processes and no longer exhibit their original characteristics. These virtually unaltered Proterozoic rocks of Waterton/Glacier are unique in that they have preserved the subtle features of sedimentation such as ripple marks, mud cracks, salt-crystal casts, raindrop impressions, oolites, six species of fossil algae, mudchip breccias, and many other bedding characteristics. These Proterozoic sedimentary rocks, while outcropping over an area extending from southern Montana to southern British Columbia, are most impressively exposed in Waterton/Glacier. Due to the extreme relief and unexcelled exposures, over 2,100 meters of stratigraphic thickness is exposed to scientific examination. What are Sedimentary Rocks?

Glacier National Park/Waterton Park:²
It is thought that a little over 1.5 billion (1500 million) years ago, most of the continents came together at the equator to form a supercontinent called Rodinia. As the continents slowly rifted apart, a depression formed and filled with sea water. Pre-Alberta was next to this sea. The rocks now forming the mountains of Waterton/Glacier were once sediments along the coast of this inland sea, called the Belt Sea. For the hundreds of millions of years that the Belt Sea existed, its floor alternately sank with its sedimentary load and uplifted with plate movement. Its shores advanced and retreated many times. In the absence of land plants, pre-Alberta was a balmy, but barren, place. Changes in the area's climate, water depth and rate of deposition are all recorded in the rock. Around 1,300 million years ago, molten rock from the mantle squeezed up through cracks in the sea floor, injecting itself between sedimentary layers (intrusive magma). Some eventually flowed out at the surface as lava. This intrusive and extrusive magma now forms the only igneous rock in Waterton/Glacier - known as the Purcell Sill and Flow.

Purcell Sill:²
Although most of Waterton's rock is sedimentary, a striking band of igneous rock can be seen. This band, or sill, is a result of molten magma being injected between sedimentary layers. The heat of this intrusion forced out the dark organic matter from the surrounding limestone, changing it into white marble (metamorphic rock). The magma cooled very slowly, allowing large star-shaped crystals to form. These phenocrysts can be seen in the dark grey to pale violet gabbro rocks used in some of Waterton's buildings. A band of this igneous rock and marble, looking like striped toothpaste, can be seen stretched across the north face of Mount Cleveland and other park mountains, such as Mount Anderson and Mount Blakiston near Red Rock Canyon.

Purcell Flow:²
The intrusive magma also forced its way to the surface, where it oozed out as a lava flow. It cooled quickly, leaving many small gas-bubbles or holes in the basalt rock. Sometimes these holes were filled with calcium or quartz crystals which precipitated out of groundwater. These dark rocks with small specks of white crystals found in the park are called amygdaloidal basalts. Basaltic rocks without crystals and just holes can also be found throughout the park.

Granite Peak, Custer National Forest:⁶
At 12, 799 feet above sea level, Granite Peak is the state's highest mountain.

Gravely Mountains:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Highwood Mountains:⁶
Volcanic activity also created mountains in west-central Montana -- the Adel, Highwood, Judith, and Bearpaw Ranges. Some of the magma never made it to the earth's surface but formed igneous intrusions.

Humbug Spires near Melrose:⁶
White granite spires rise 600 feet above surrounding trees.

Bitterroot Mountains:⁶
The rugged, 470-mile long Bitterroot Range winds along the Montana-Idaho border. The Bitterroots are Montana's longest mountain range and include three groups of mountains: the Bitterroots, the Beaverheads, and the Centennials. They are also part of one of North America's best known batholiths, the Idaho Batholith of central Idaho and western Montana. A batholith is a large intrusion of solidified magma that does not reach the earth's surface as it cools. Later, with erosion, it can become exposed which is what has happened in the Bitterroots. So unlike the mountains of northwestern Montana, which are composed of sedimentary rock, this range is mostly granite. The rock is roughly 75 million years old. One theory is that the emplacement of Idaho Batholith raised the earth's crust like a blister. The bulging caused another large block of rock -- the Sapphire Block -- to gradually start sliding downslope in an easterly direction. According to the theory, this block of rock, like the mountains of northwestern Montana, slid about 50 miles. When it finished sliding, it had pushed up, bulldozer fashion, the semi-circular arc of isolated mountains we call the Garnet, Flint Creek, and Anaconda-Pintler Ranges.

Judith Mountains:⁶
Volcanic activity also created mountains in west-central Montana -- the Adel, Highwood, Judith, and Bearpaw Ranges. Some of the magma never made it to the earth's surface but formed igneous intrusions.

Limestone Cliffs and Granite Formations:⁶
These spectacular gray limestone cliffs and pink and white granite formations rise 1,200 feet above the canyon floor a short way outside industrial Anaconda. You're likely to see wildlife in this park and you will enjoy a short hiking trail to Lost Creek Falls cascading over a 50-foot drop to provide one of the most popular spots in the park. Wildlife, especially mountain goats and bighorn sheep, are frequently seen on the cliffs above. How to get there: 1.5 miles east of Anaconda on Montana 1, then 2 miles north on Secondary 273, then 6 miles west.

Madison Mountains:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Ruby Mountains:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Snowcrest Range:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Southwest Montana:⁶
A different sort of geologic activity built the mountains of southwestern Montana: the Ruby, Snowcrest, Gravely, Blacktail, Madison, and Gallatin Ranges, and the Beartooth Plateau. These ranges are the result of block faulting -- blocks of the earth's crust pushed up along faults. The mountains contain sedimentary, metamorphic, and igneous rock, the latter of which -- pink granites and streaky gneisses and schists -- are exposed across large areas. Over three billion years old, these ancient crystalline rocks are part of the body of rock that makes up most of the continent's crust.

Square Butte:⁶
A National Natural Landmark visible for 100 miles, this granite butte rises 2,500 feet above the valley floor.

Square Butte - National Natural Landmark:⁹
Chouteau County - An igneous rock intrusion between sedimentary beds (a laccolith) which provides one of the best examples of banded magmatic rock in the United States, with a clear distinction between dark and light fractions. The butte's flat crest supports relatively natural grassland communities. Owner: Federal, Private. DESIGNATION DATE: August 1980

Sweet Grass Hills:⁷
The Sweet Grass Hills are prominent land marks, rising nearly 3,000 feet above the surrounding plains and are visible for more than 50 miles. The Sweet Grass Hills consist of three separate butte complexes, East, Middle and West Buttes, and two smaller features, Grassy and Haystack Buttes. The Sweet Grass Hills consist of igneous intrusive rocks that are considered Eocene in age (Ross, 1950). These igneous rocks range in composition from shonkinite to syenite to intrusive trachyte with a minor amount of lamprophyre. The surrounding sedimentary rocks domed up by the intrusives range in age from Mississippian to Cretaceous. These are primarily limestones and shales with minor amounts of sandstone. The older Mississippian Madison Formation occurs near the central portions of East and West Buttes, and the younger Jurassic-Cretaceous formations are found adjacent to Middle Butte, which is of smaller size. The sedimentary rocks, particularly limestone, have been chemically and physically altered due to contact metamorphism and hydrothermal fluids associated with the igneous activity.

Sweet Grass Hills - East Butte:⁷
East Butte consists of alkalic igneous rocks intruding Paleozoic and Mesozoic sedimentary rocks as plugs, laccoliths, stocks, dikes, and sills. The igneous rocks consist of monzonitic and syenitic trachytes and latite porphyries. The domed sedimentary rocks are silicified, altered and highly fractured. Limestone alteration includes recrystallization as marble and some skarn mineralization. Quartz with pyrite, magnetite, and fluorite mineralization also occur primarily as fracture filling in places (Gavin, 1991). The Jurassic and Cretaceous rocks composed primarily of shales and fine siltstones show signs of contact metamorphism ranging from recrystallization to silicification and intense fracturing. East Butte is the largest of the 3 Buttes with a 9 square mile area of uplifted terrain. The combination of igneous intrusive rock and altered sedimentary strata has not been mapped in enough detail to differentiate between the separate sedimentary units. Much of the valley bottoms and the lower slopes of ridges are covered with Quaternary alluvium and colluvium. This makes interpretation through surface mapping more difficult. through surface mapping more difficult.

Sweet Grass Hills - Middle Butte:⁷
Middle Butte lies 3 miles west-southwest of Whitlash and covers a hilly area about 5 miles wide and 5 miles long. The ghost town of Gold Butte, formerly a gold mining camp, rests on the northwest flank of the tallest hill in the Middle Butte complex, which bears the same name as the former town. Gold Butte is an exposed laccolith consisting of diorite porphyry. It is separated from two other buttes, of similar composition, by carbonaceous shale which is cut by numerous dikes and sills. The sedimentary rocks are less resistant and form swales between the higher ridges composed of igneous rock.

Sweet Grass Hills - West Butte:⁷
West Butte lies 13 miles west-northwest of Whitlash, the nearest town. The central core is an exposure of diorite porphyry and monzonite, a rock similar to common variety granite, but with less quartz.

Upper Missouri River Breaks National Monument

Baker Monument:³
This is an erosional remnant of a volcanic vent rising about 180 feet above the surrounding terrain on state lands. Geologically it is one of about 35 features described as diatremes that occur between the Highwood Mountains to the west and the Little Rocky Mountains to the east. The mineral composition of the igneous centers are similar to diamond bearing kimberlites that produce diamonds in other parts of the world. Although diamonds have yet to be found, these features constitute the emphasis of mining exploration in the Missouri Breaks area.

Birdtail Butte, Chimney Rock, Dark Butte, Citadel Rock, Eagle Buttes, LaBarge Rock, Pilot Rock, Steamboat Rock:³
These are fine-grained igneous rocks, dominated by dark-colored minerals occur as dikes, sills and stocks injected into fractures in the cretaceous age sandstones and shales. They range in age from Tertiary to late Cretaceous. They are more resistant to weathering than the enclosing sedimentary rocks causing them to from promontory features in the surrounding terrain. Some of these that have been named along the river are Dark Butte, LaBarge Rock, Citadel Rock, Pilot Rock and Steamboat Rock. North of the river, some of the natural features are Eagle Buttes, Birdtail Butte and Chimney Rock.

Saskatchewan Butte:³
This is an erosional remnant of a volcanic vent rising about 200 feet above the surrounding terrain located on federal land. The Butte is about 10 acres in size and has potential for gemstone occurrence. It is typical of other features described as the Missouri Breaks Diatremes in numerous professional papers and mineral reports prepared by the U. S. Geological Survey and Bureau of Mines.

Excerpts from:
1) U.S. National Park Service Website, Glacier National Park, 2001
2) Water Lakes National Park Information Website, Alberta, Canada, 2001
3) U.S. Department of the Interior, Bureau of Land Management, Montana/Dakotas Website, 2001
4) USGS/NPS Geology in the Parks Website, 2001
5) State of Montana Website, 2002
6) Montana Fish, Wildlife, & Parks Website, 2002
7) U.S. Department of the Interior, Bureau of Land Management, Sweet Grass Hills Final Amendment and Environmental Impact Statement, April 1996
8) Wilson and Elliott, 1997, Geologic Maps of Western and Northern Parts of Gallatin National Forest, South-Central Montana: Geologic Investigations Series I-2584,
9) U.S. National Park Service, National Natural Landmarks Website, 2003

[Return to America's Volcanic Past - States and Regions]
[Return to America's Volcanic Past - National Parks and Monuments]
[Return to Visit A Volcano Menu]