America's Volcanic Past
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"Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Volcanic Highlights and Features:
[NOTE: This list is just a sample of various Idaho features or events and is by no means inclusive. All information presented here was gathered from other online websites and each excerpt is attributed back to the original source. Please use those sources in referencing any information on this webpage, and please visit those websites for more information on the Geology of Idaho.] |
Idaho, Oregon, and Washington:
Excerpt from: R.L. Whitehead, 1994, Ground Water Atlas of the United States: Idaho, Oregon, Washington: U.S. Geological Survey HA730-H |
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Basin and Range:9
Columbia Plateau:9 The Columbia Plateau province is enveloped by one of the worlds largest accumulations of lava. Over 500,000 square kilometers of the Earth's surface is covered by it. The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years. Over 170,000 cubic kilometers of basaltic lava, known as the Columbia River basalts, covers the western part of the province. These tremendous flows erupted between 17-6 million years ago. Most of the lava flooded out in the first 1.5 million years -- an extraordinarily short time for such an outpouring of molten rock. It is difficult to conceive of the enormity of these eruptions. Basaltic lava erupts at no less than about 1100 degrees C. Basalt is a very fluid lava; it is likely that tongues of lava advanced at an average of 5 kilometers/hour -- faster than most animals can run. Whatever topography was present prior to the Columbia River Basalt eruptions was buried and smoothed over by flow upon flow of lava. Over 300 high-volume individual lava flows have been identified, along with countless smaller flows. Numerous linear vents, some over 150 kilometers long, show where lava erupted near the eastern edge of the Columbia River Basalts, but older vents were probably buried by younger flows. Rocky Mountains:9 The Rockies form a majestic mountain barrier that stretches from Canada through central New Mexico. Although formidable, a look at the topography reveals a discontinuous series of mountain ranges with distinct geological origins. The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains. |
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Rocks of Idaho, Oregon, and Washington State:15
Unconsolidated Deposits:15
Volcanic Rocks:15
Pre-Miocene Undifferentiated Rocks:15
Miocene Volcanic Rocks:15
Miocene Basaltic-Rock Aquifers:15
Pliocene and Younger Volcanic Rocks:15
Silicic Volcanic Rocks:15
Idaho's Precambrian:13
Idaho's Paleozoic:13
Idaho's Mesozoic:13
Idaho's Eocene:13
Idaho's Miocene:13
Idaho's Pliocene:13
Idaho's Quaternary:13
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Big Southern Butte |
Big Southern Butte:16
The butte is composed of
light-colored silicic volcanic rocks
and stands nearly 760 meters
above the low relief surface of the
Eastern Snake River Plain. The site
is an ecological "island" supporting
vegetation such as lodgepole pine
(Pinus contorta), aspen (Populus
sp.), Douglas fir (Pseudotsuga
menziesii), and manzanita
(Arctostaphylos sp.) not common to
this region.
Big Southern Butte - National Natural Landmark:17
Butte County - 37 miles northwest of Blackfoot. The view from
this butte illustrates the scope and dimensions of Quaternary
volcanism in the western United States and the largest area of
volcanic rocks of young age in the United States. Owner:
Federal.
DESIGNATION DATE:
January 1976
Big Springs |
Big Springs:16
The springs emanate from rhyolite
lava flows of the Madison Plateau,
which comprise one of the largest
rhyolite lava fields in the United
States. Big Springs is the only first
magnitude spring in the United
States that issues forth from rhyolitic
lava flows.
Big Springs - National Natural Landmark:17
Fremont County - 54 miles northeast of Rexburg. The only
first-magnitude spring in the country which issues forth from
rhyolitic lava flows. It is the source of the South Fork of the
Henrys Fork River. Owner: Federal.
DESIGNATION DATE:
August 1980
City of Rocks National Preserve |
City of Rocks National Reserve:14
Nestled amidst the mountain
peaks of the Albion Mountain
Range lies City of Rocks
National Reserve. Embracing
a sagebrush steppe valley,
spectacular granite rock
formations, pinyon/juniper tree
stands and alpine-like
meadows, the reserve
presents an inspirational
landscape.
Some of the granitic rock formations here are
2.5 billion years old. They
are some of the oldest formations
found in the continental United
States. The fantastic rocks are shaped
by weathering. Many people
see animals, faces, or buildings in
the rocks. What you see is limited
only by your imagination.
City of Rocks:4
Towering granite shapes resemble a silent city,
which is what James F. Wilkins named this area in
1849. A looming backdrop, Independence Mountain,
rises to 10,339 feet of elevation.
Cassia Silent City of Rocks - National Natural Landmark:17
Cassia County - Contains monolithic landforms created by
exfoliation processes on exposed massive granite plutons, and
the best example of bornhardts in the country. Owner: Federal,
State, Private.
DESIGNATION DATE:
May 1974
Twin Sisters of the City of Rocks:4
More like mother and daughter than
siblings, the Twin Sisters of City of
Rocks are made up of different rocks.
The difference helps explain how the
City of Rocks landscape came to be.
The darker sister is made of rock in a
formation that geologists call the Green
River Complex. It is 2.5 billion years
old and is some of the oldest rock in the
Lower 48 states. The lighter sister is
made of rock in a far younger formation
that geologists call the Almo Pluton. It
is 25 million years old. Both formations
began as molten matter in the Earth's
crust. Eventually the Almo Pluton was
thrust up though the Green River
Complex, while both formations still lay
beneath the Earth's surface and other
layers of rock. As time passed, the
overlying rocks and the formations
beneath them cracked. Along the
cracks and fissures erosion took place
more rapidly and exposed the rocks of
the Almo Pluton and Green River
Complex. The exposed rocks were
then shaped by the forces of erosion.
In weathering, the tops of rocks are dissolved by
rainwater; and minerals,
such as iron oxide, are redeposited to form crust-like caps.
These caps are more resistant to weathering than the
underlying rock, and this causes the
formation of spires and pinnacles.
When the caps erode, the inner rock can be
molded by erosion into many caves, arches, bath tubs,
and hollow boulders seen at City of Rocks. On the sides of
the spires water seeps into cracks and frost wedging occurs.
When the water freezes it expands and can crack great
slabs off the rock. This process already has removed
some of the layers of rock bearing 150-year-old signatures left
by the pioneers. Today, many people see animals,
faces, or buildings in the shapes of these rock formations. Only
your imagination limits what you can see in the City of Rocks.
Columbia Plateau |
Columbia Plateau8
Between 14 and 16 million years ago, "fissure" volcanic eruptions
in eastern Washington, eastern
Oregon, and western Idaho produced enormous
volumes of molten Columbia River
basalt that flowed
like water west into the Deschutes-Columbia
Plateau province in eastern Washington and
northeastern Oregon, with some lava continuing to
flow as far west as the Pacific Ocean via the
ancestral Columbia River valley. As the basalt
cooled and congealed, it formed the columnar cliffs
that dominate the landscape today. Erosions by the
Columbia River has exposed a particularly
spectacular sequence of these rocks in the
Columbia River Gorge on Oregon's northern
boundary.
Crater Rings |
Crater Rings - National Natural Landmark:17
Elmore County - Two adjacent and symmetrical pit craters that
are among the few examples of this type of crater in the
continental United States. The pit craters, which are volcanic
conduits in which the lava column rises and falls, were formed
by explosions followed by collapse. Owner: Federal.
DESIGNATION DATE:
April 1980
Craters of the Moon National Monument |
Craters of the Moon Lava Field:12
Craters of the Moon National Monument, Idaho,
U.S.A.
Craters of the Moon National Monument and surrounding lava
flows, are located along the northern border
of the Snake River Plain
This showcase of volcanic
formations, including lava flows, cinder cones,
and craters, was created by a fissure about
50 miles (80 kilometers) long in the Earth's crust.
Through this
zone of weakness, known as the Great Rift,
lava frequently welled up to the surface over
thousands of years. With each successive series of eruptions,
older volcanic formations were buried,
and new ones were superimposed on their surfaces.
The barren black lava flows were emitted during the most recent
eruptions, which occurred approximately 2,000 years ago.
Flood Basalts |
Flood Basalts and Lava Flows:3
In some shield-volcano eruptions, basaltic lava
pours out quietly from long fissures
instead of central vents and
floods the surrounding countryside with
lava flow upon lava flow, forming broad plateaus.
Lava plateaus of this type can be seen in Iceland,
southeastern
Washington, eastern Oregon, and southern Idaho.
Along the Snake River in Idaho,
and the Columbia River in Washington and
Oregon, these lava flows are beautifully exposed and
measure more than a mile in total thickness.
[See Columbia Plateau above and Snake River Plain below]
Great Rift |
Great Rift:7
The Crater's of the Moon National Monument's
central focus is the Great Rift,
a 62-mile long crack in the earth's crust.
The Great Rift is the source of a remarkably
preserved volcanic landscape with an
array of exceptional features.
Craters, cinder cones,
lava tubes, deep cracks, and
vast lava fields form a strangely beautiful
volcanic sea on central Idaho's Snake River Plain.
MORE Great Rift:5
The Great Rift volcanic rift zone is a zone of cracks
running approximately northwest to southeast across
almost the entire eastern part of the
Snake River Plain.
The entire Great Rift is 62 miles long.
The Great Rift is an example of basaltic
fissure eruption. This type of volcanic
activity is characterized by extrusion of
lavas from fissures or vents that is relatively
quiet in comparison with highly explosive
eruptions such as the 1980 Mount Saint
Helens eruption.
Where the Great Rift intersects the earth's
surface, there is an array of cinder cones,
lava cones, eruptive fissures, fresh-appearing
lava flows, noneruptive fissures, and
shield volcanoes.
Great Rift System - National Natural Landmark:17
Blaine County and extends into Minidoka and Power Counties -
43 miles northwest of Pocatello. As a tensional fracture in the
Earth's crust that may extend to the crust-mantle interface, the
Great Rift System is unique in North America and has few
counterparts in the world. It also illustrates primary vegetation
succession on very young lava flows. Owner: Federal.
DESIGNATION DATE:
April 1968
Hagerman Fossil Beds National Monument |
Hagerman Fossil Beds:6
No other fossil beds preserve such varied land and aquatic species from the time period called the Pliocene Epoch. More than 140 animal species of both vertebrates and invertebrates have been found in hundreds of individual fossil sites. Eight species are found nowhere else, and 44 were found here first. The Hagerman Horse, Equus simplicidens, exemplifies the quality of fossils. From these fossil beds have come both complete and partial skeletons of this zebra-like ancestor of today's horse. The 600-foot-high bluffs rising above the Snake River and comprising the Hagerman Fossil Beds reveal the environment at the end of the Pliocene Epoch. Grassy plains dotted with ponds and forest stands then received over twice today's ten inches of yearly precipitation. Mastodons, sabre-tooth cats, beavers, muskrats, otters, camels, antelope, deer, ground sloths, hyena-like dogs, and fish, frogs, snakes, and waterfowl lived here. The sediment layers from river level to bluff tops span some 550,000 years; from 3.7 million years old at river level to 3.15 million years old atop the bluff. These layers were deposited when rivers flowing into ancient Lake Idaho flooded the countryside. The much later Bonneville Flood (15,000 years ago) carved the high bluffs, exposing the layers and fossils. |
Banbury Basalt:6
The Banbury Basalt
forms floor of what is now the Hagerman Valley (8-11 million years ago, Pliocene)
Bruneau-Jarbridge Eruption South of Hagerman:6
Deposits ash as far east as
Nebraska
(11 million years ago, Pliocene)
Challis Area:6
Volcanism in the Challis area
began
51 million years ago (Oligocene).
McKinney Butte Basalt:6
Damming of Snake River by McKinney Butte Basalt occurred 50,000 years ago (Holocene).
Melon Gravel:6
The Bonneville Flood (15,000 years ago) deposited fields of so-called melon gravel
(lava boulders ranging in size
from a compact car to watermelons) from today's river level to gravel bars 225 feet higher.
Peters Gulch Ash:6
Volcanic eruption at Yellowstone
deposits Peters Gulch Ash at Hagerman (3.7 million years ago, Pleistocene)
Sediment Layers include Ash and Basalt:6
The sediments in the bluffs include river sands,
thin shale layers deposited in ponds, clay flood deposits, and
occasional volcanic deposits such as ash and basalt.
It is the radioactive elements such as potassium 40 in the
volcanic ashes that allowed scientists to determine
the age of the fossils by measuring the rate at which one
radioactive element breaks down into another.
Hell's Half Acre Lava Field |
Hell's Half Acre Lava Field:16
The lava field is an outstanding
example of a single event, single
process geologic feature that is fully
preserved and fully exposed.
Fractures, depressions, and small
lava caves are common features on
the flow, which otherwise has a fairly
smooth surface.
Hell's Half Acre Lava Field - National Natural Landmark:17
Bingham County and extends into Bonneville County - The
center of the site is 20 miles west of Idaho Falls. A complete,
young, unweathered, fully exposed pahoehoe lava flow and an
outstanding example of pioneer vegetation establishing itself on
a lava flow. Owner: Federal, State.
DESIGNATION DATE:
January 1976
King's Bowl |
King's Bowl:2
Composite flow field; Basalt composition; 1,500 meter elevation.
The King's Bowl Field is small and cover less than 2.6 square kilometers.
It is situated on the King's Bowl Rift Set, one of several such sets which
collectively make up Idaho's Great Rift, a series of tension fractures
that cross cut the eastern Snake River Plain. The King's Bowl field is a
composite feature made up of flows from several point sources along the
Rift as well as a larger, apparently dike-fed sheet flow, which for a time
was held in a lava lake. These flows locally overlap, indicating that the
eruptive sequence was complex and issued from different vents at different
times.
Part of the Monument:5
Established in 1924, Craters of the Moon National
Monument celebrated its 75th birthday in 1999. On
November 9, 2000, President Clinton issued a
proclamation greatly expanding the size of the
monument. The
monument encompasses three major lava fields
and the surrounding sagebrush steppe
grasslands. The Craters of the Moon lava field
covers 618 square miles and is the largest young
basaltic lava field in the conterminous United
States. The Kings Bowl and Wapi lava
fields, which are about 2,200 years old, are now
part of the monument. All three lava fields lie along
the Great Rift, with some of the best examples of
open rift cracks in the world.
Lava Hot Springs |
Lava Hot Springs:18
For centuries, many Indian tribes gathered at these
natural hot water springs, calling them the healing waters. Geologists
theorize that the water has been a consistent 110 degrees
for at least 50 million years. Today, the State of Idaho
maintains this world-famous resort complex year-round.
The facility offers hot mineral baths at 110 degrees Fahrenheit
that are sulfur and odor free.
Malad Gorge State Park |
Malad Gorge State Park:14
Most of the history at this park is on the geologic scale.
The cracks and folds of rock along the canyon
cliffs record the movements of earth, lava and water.
This 652-acre park is located
just off Interstate 84 at the Tuttle exit.
The Malad River crashes down stairstep falls
and into the Devils Washbowl, then cuts through a
beautiful 250-foot gorge on its way to the Snake River, 2-1/2 miles
downstream. Views of the gorge are best from
the sturdy bridge that crosses the
canyon. You can take a short hike to discover nearby fingers of the
gorge where crystal-clear springs produce ponds and streams.
Massacre Rocks State Park |
Massacre Rocks State Park:14
The park is rich in geological history.
Volcanic evidence is everywhere.
The Devil's Gate Pass is all that
remains of an extinct volcano.
Gate of Death and Devil's
Gate were names given to this
area during the Oregon Trail
period. These names referred
to a narrow break in the rocks
through which the trail
passed. Emigrants apparently
feared that Indians might be
waiting in ambush.
The prehistoric Bonneville Flood shaped the landscape of the area,
rolling and polishing the huge boulders found throughout the park. The
flood was caused when eroding waters broke through Red Rock Pass
near the Idaho/Utah border. Lake Bonneville, which covered much of
what is today the state of Utah, surged through the pass and along the
channel of the Snake River in a few short months. For a time, the flow
was four times that of the Amazon River. It was the second largest
flood in the geologic history of the world.
Menan Buttes |
Menan Buttes:16
The site is an outstanding example
of a glassy tuff cone, which is found
in only a few places in the world. The
buttes are composed of small
fragments of basaltic glass formed
by sudden chilling of magma.
Menan Buttes:2
Mid- to late-Pleistocene tuff cones, olivine tholeiitic basalt composition.
North and South Menan Buttes are the tow most prominent phreatomagmatic
cones of the Menan Complex, a group of six cones roughly aligned along a
north-northwest trend, 16 kilometers west-southwest of Rexburg, Idaho.
North Menan Butte is the larger, standing 250 meters above the surrounding
Snake River Plain. It is elliptical in plan with axes 3.5 and 2.5
kilometers in length. South Menan Butte measure 3 kilometers by 2
kilometers and has 145 meters of relief. The buttes are asymmetrical with
a greater accumulation of material on their northeast flanks, presumably
due to strong southwest winds during eruption.
Menan Buttes - National Natural Landmark:17
Jefferson County and extends into Madison County)--Contains
outstanding examples of glass tuff cones, which are found in only
a few places in the world. Their large size and unusual
composition make them particularly illustrative of an unusual
aspect of basaltic volcanism. Owner: Federal, Private.
DESIGNATION DATE:
April 1980
Nez Perce County |
Nez Perce County:19
Nez Perce County is on the eastern margin of the
Columbia River Plateau, a large
area that was covered by lava
(basalt) flows in Miocene times. The terrain covered
by the flows was very irregular;
therefore, the depth of the lava varies widely from
place to place. Elevated portions
of the old pre-lava surfaces are exposed in places,
like islands surrounded by basalt.
There were several flows laid down
over an epoch of thousands of years. Between
some of the lava flows there were
long periods of inactivity in which weathered
surfaces developed. Sediment washed
from the weathered surfaces and was
deposited in lakes, streambeds, or
depressions along the eastern margins of the
basaltic plain. Later, lava flows
covered these weathered slopes and deposits of
sediment, forming a layer of sand,
gravel, silt, and volcanic ash known as the "Latah
beds", but they are exposed on
canyon walls below the elevation of the present
plateaus. Deposits of sand, gravel,
and clay, of economic value, are found in some
of these Latah beds.
Along the Snake and Salmon Rivers
on the southwestern border of Nez Perce County
erosion has uncovered metamorphosed
rhyolite and pyroclastics related to the
Seven Devils volcano. The color of
these rocks is predominately green and they are
commonly designated greenstone.
Recent stream action has produced
sand and gravel bars along the present course of
the rivers. Deposits of sand,
gravel, silt, and boulders in former stream channels are
found in the Lewiston area. These
deposits are worked to some extent for the sand
and gravel. Other deposits are on
the floor of the Lapwai Valley and in the Cow
Creek area south of Genesee.
Rocky Mountains |
Rocky Mountains:9
The Late Cretaceous and Early Tertiary periods were geologically eventful in the West.
The Rocky Mountains, which were uplifted about 50 to 100 million years ago,
extend from southern Colorado
northwest to the Canadian border.
Their rocks and topography are diverse and highly complex.
Many of the individual ranges that make up the Rocky Mountains appear on maps as variously
shaped bull's-eyes surrounding a center. Each crudely ringed pattern was created by the Tertiary
erosion of Paleozoic and Mesozoic sedimentary rocks that once overlay and now surround a core
of uplifted Precambrian granite.
Seven Devils Volcano |
Seven Devils Volcano:19
Along the Snake and Salmon Rivers
on the southwestern border of Nez Perce County
erosion has uncovered metamorphosed
rhyolite and pyroclastics related to the
Seven Devils volcano. The color of
these rocks is predominately green and they are
commonly designated greenstone.
Sheep Rock |
Sheep Rocks:16
The site provides perhaps the best
view of the horizontally layered lavas
that represent successive flows of
the Columbia River Basalt. The
Columbia River Basalt covers an
area of some 518,000 square
kilometers in Idaho, Washington,
and Oregon. It represents one of the
great lava extrusions of geologic
time.
Sheep Rock - National Natural Landmark:17
Adams County - In Payette National Forest, 35 miles northwest
of Council and two miles east of the Snake River. Provides the
best view of the horizontally layered lavas that represent
successive flows on the Columbia River Basalt Plateau, and an
unobstructed view of two contrasting series of volcanic rocks
separated by a major unconformity--an important geologic
phenomenon. Owner: Federal.
DESIGNATION DATE:
December 1976
Snake River Plain |
Snake River Plain:5
Craters of the Moon lies at the north
edge of the eastern Snake River
Plain, a broad flat arc, concave to the
north, which covers nearly 10,000
square miles of southern Idaho. It
extends from the Yellowstone Plateau
and the Teton Mountains on the east
to the Oregon state line on the west.
The Snake River borders the southern
edge of the plain, which occupies
almost a quarter of the surface of Idaho and
contrasts markedly with the mountainous
terrain that dominates the northern, central,
and far southern parts of the state.
The eastern Snake River Plain is essentially flat in
this vicinity: vertical relief is a few
hundred feet at Craters of the Moon and less than
that elsewhere.
Snake River Plains Volcanism:1
The Snake River Plain forms a broad arch across the southern part of Idaho extending
600 kilometers eastward from the Oregon
border to the Yellowstone Plateau. Its width ranges from 65 to 100 kilometers.
Typical of plains volcanism, most flows on the Snake River Plain accumulate as
1) small, low shields,
2) fissure flows, and
3) or
large tube-fed flows.
All were probably emplaced relatively slowly, often advancing
only a few meters per hour, forming "toey" lava
flows with hummocky surfaces of several meters relief.
Pressure ridges and collapse craters are common.
MORE Snake River Plain:9
The Snake River Plain stretches across Oregon,
through northern Nevada,
southern Idaho, and ends at the
Yellowstone Plateau in Wyoming.
Looking like a great spoon scooped out the Earth surface,
the smooth topography of this province
forms a striking contrast with the
strong mountainous fabric around it.
The Snake River Plain lies in a
distinct depression. At the western end,
the base has dropped down along
normal faults, forming a graben structure.
Although there is extensive
faulting at the eastern end, the structure
is not as clear. Like the
Columbia River region, volcanic eruptions
dominate the story of the
Snake River Plain in the eastern part of
the Columbia Plateau Province.
The earliest Snake River Plain eruptions
began about 15 million years ago,
just as the tremendous early eruptions of
Columbia River Basalt were ending.
But most of the Snake River Plain volcanic
rock is less than a few million years old,
Pliocene age (5 - 1.6 million years ago) and younger.
In the west, the Columbia River Basalts are
just that - almost exclusively
black basalt.
Not so in the Snake River Plain,
where relatively quiet eruptions of
soupy black basalt lava flows
alternated with tremendous explosive
eruptions of rhyolite,
a light-colored volcanic rock.
Cinder cones dot the landscape of the Snake River Plain.
Some are aligned along vents,
the fissures that fed flows and cone-building eruptions.
Calderas, great pits formed by explosive volcanism,
and low shield volcanoes, and rhyolite hills are also part of
the landscape here, but many are obscured by later lava flows.
Snake River Plain Volcanoes:10
Large volcanoes appear as
bumps on the otherwise smooth
surface of Snake River Plain, a
kidney-shaped expanse in
southern Idaho. These volcanoes
are made of a lava known as
rhyolite, which produces very
explosive eruptions and cone-shaped volcanoes.
The oldest of these volcanoes,
about 17 million years old, are in the western
and southern parts of the Plain.
The age of the rhyolite volcanoes in the Snake
River Plain decreases from the southwest
to the northeast.
As recently as 2,000 years ago,
a different type of lava known as basalt
flowed onto the surface and
covered the rhyolitic flows.
Basalt is a very fluid type of
lava which produces low, smooth volcanoes.
The apparent movement of the volcanoes from west to east
over the last 17 million years marks the
movement of the North American Plate westward across a source of magma.
Such hot spots or plumes
rise into the Earth's crust from the underlying mantle.
Yellowstone National Park in northwest Wyoming
lies above the current location of the hot spot.
A similar hot spot formed the Hawaiian Islands.
Spencer Opal Mine |
Opal Deposits within Volcanic Rocks:11
In Idaho, opal is the second largest contributor
to the total value of gem material produced.
The varieties produced include precious (white
and pink), yellow, blue, pink, and common.
The Spencer opal mine, the largest privately
owned gem stone producer in the State, is the major
producer of opal. At Spencer the precious
opal occurs as one or more thin layers within
common opal partially filling gas cavities within a
rhyolite-obsidian flow.
About 10% of the material is thick enough
to cut into solid gems; the remainder
is fashioned into doublets and triplets. The
Spencer Mine is the source of pink common
opal and pink precious opal.
Split Butte |
Split Butte:1
Split Butte on the south-central Snake River Plain overlies basalt flows
of the Snake River Group and was encroached from the southeast by a lobe
of the Wapi lava flow, which has been dated at 2,270 years ago. The butte
consists of vitric ash forming a ring 600 meters across. The ring is
asymmetrical, having a greater accumulation on the east, the result of
prevailing westerly winds during the eruption. Although the eroded ring
stands 50 meters above the surrounding plain, an original ash thickness of
80 meters on the east is estimated from the dip of the beds and the ring
diameter. A topographic notch or erosional "split", approximately 150
meters wide occurs in the thick eastern ash accumulation.
Wapi Volcanic Field |
Wapi:1
The Wapi lava field is one of several Holocene to Pleistocene volcanic
fields on the Snake River Plain. In many respects, it is typical of the
older fields of low shields that make up the present surface of the plain.
It covers a large (300 square kilometer) area that is elongate in the
north-south direction and has three prominent lobes extending east, west,
and northwest from the main mass of the field.
Part of the Monument:5
Established in 1924, Craters of the Moon National
Monument celebrated its 75th birthday in 1999. On
November 9, 2000, President Clinton issued a
proclamation greatly expanding the size of the
monument. The
monument encompasses three major lava fields
and the surrounding sagebrush steppe
grasslands. The Craters of the Moon lava field
covers 618 square miles and is the largest young
basaltic lava field in the conterminous United
States. The Kings Bowl and Wapi lava
fields, which are about 2,200 years old, are now
part of the monument. All three lava fields lie along
the Great Rift, with some of the best examples of
open rift cracks in the world.
Excerpts from:
1) Greeley, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p
2) King, 1990, IN: Wood and Kienle, 1990,
Volcanoes of North America: United States and Canada:
Cambridge University Press, 354p
3)
Tilling, 1985, Volcanoes: USGS General Interest Publication
4) U.S. National Park Service Website,
City of Rocks National Preserve, 2001
5) U.S. National Park Service Website,
Craters of the Moon National Monument, 2000, 2002
6) U.S. National Park Service Website,
Hagerman Fossil Beds National Monument, 2000
7) U.S. Bureau of Land Management, Idaho Office, Website, 2001
8) Geologic Sightseeing:
Oregon Department of Geology and Mineral Resources Website, 2001
9) USGS/NPS Geology of the National Parks Website, 2001
10) USGS A Tapestry of Time and Terrain Website, 2001
11) Gemstones, An Overview of Production of Specific U.S. Gemstones:
U.S. Bureau of Mines Special Publication 14-95
12) NASA "Earth From Space" Website, 2002
13) Idaho Geological Survey Website, 2002
14) Idaho State Parks and Recreation Website, 2002
15) R.L. Whitehead, 1994, Ground Water Atlas of the United States:
Idaho, Oregon, Washington:
U.S. Geological Survey HA730-H
16) U.S. National Park Service, Columbia Cascades Cluster Website, 2002
17) U.S. National Park Service, National Natural Landmarks Website, 2003
18) Visit Idaho Website, 2002
19) Nez Perce County Website, 2002
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