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 |
Location Map - Colorado National Parks and Monuments
[51K,InlineGIF]
Volcanic Highlights and Features:
[This list is just a sample of
various Colorado volcanic features or events and is by no means inclusive.]
The beginning of the Tertiary coincides with the birth of the Rocky Mountains. The event is known as the Laramide Orogeny (orogeny means "mountain building"). The cause of the Laramide Orogeny reaches back more than 200 million years. At the end of the Triassic period, the great supercontinent known as Pangea began to break apart, and North America began to separate from Europe. Far to the west, the North American crustal plate began colliding with and over-riding the Pacific-Farallon Plate. The collision between the two plates caused the crust to buckle and fold -- just like the fenders of two cars in a head-on collision! This folding started in California and gradually moved its way eastward, finally reaching Colorado about 60 million years ago. During the Tertiary, the stresses caused by the colliding plates to the west forced several Precambrian crustal "wedges" upwards, forming the Colorado Front Range and the Southern Rocky Mountains. In some areas, the mountain building was accompanied by volcanic eruptions and magma emplacement.
The uplift and volcanism of the early to
mid-Tertiary established the highland that
would serve as the headwaters for the
Gunnison River. Snowmelt from the Sawatch
Range to the east, the West Elk Mountains
to the north and the San Juans to the south
provided an ample supply of water to what
would eventually become the Gunnison
Basin. Geologists believe that the modern
Gunnison River became established in its
current course about 10 to 15 million years ago, just after the last eruptions in the
San Juans and West Elks. This coincides with the beginning of a period of rapid
uplift of the Great Basin and Colorado Plateau provinces that lie between the
Rockies and the Sierra Nevada Range in California. To date, geologists are at a
loss to explain the forces behind the uplifting of such an immense region.
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The Interior Plains:7
Colorado Plateau:7 The sculptured beauty and brilliant colors of the Colorado Plateau's sedimentary rock layers have captured the imaginations of countless geologists. This is a vast region of plateaus, mesas, and deep canyons whose walls expose rocks ranging in age from billions to just a few hundred years old. Ancient Precambrian rocks, exposed only in the deepest canyons, make up the basement of the Colorado Plateau. Most are metamorphic rocks formed deep within the Earth while continental collision on a grand scale produced the nucleus of the North American continent well over a billion years ago. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement. These deeply-formed rocks were uplifted, eroded, and exposed for eons. By 600 million years ago North America had been beveled off to a remarkably smooth surface. It is on this crystalline rock surface that the younger, more familiar layered rocks of the Colorado Plateau were deposited. Rocky Mountains:7 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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| Black Canyon of the Gunnison National Park |
Black Canyon of the Gunnison:2
In Black Canyon of the Gunnison National Park,
the Gunnison River cuts through
Precambrian rock nearly 2 billion years old!
Most of these rocks are
metamorphic and show evidence of exposure to extreme pressures and
temperatures.
Some of the rocks are igneous and formed from magma that
pushed its way up into cracks in the Earth's crust, where it cooled and
crystallized.
Formation of the Black Canyon of the Gunnison:2
The canyon's landscape was formed by the slow,
but continuous and unyielding process of erosion -- the effect of
one drop of water at a time or the scouring
by a seasonally flood-swollen river, the rush of mud-laden side
streams after heavy rains, occasional rockfalls
from high cliffs, and the relentless creep of landslides.
The river first
established its course over soft volcanic rock.
It then cut through this rock to the harder and older crystalline
rock of the present canyon that had been thrust
up in an earlier dome-shaped formation known as the Gunnison
Uplift. Once committed to its course,
the stream had no alternative but to continue
to cut through this once-buried
hard core, taking about two million years to carve the gorge.
The excavating process is still going on, but at a
slower pace because of the dams upstream.
Black Canyon Gneiss:2
The metamorphic rock that dominates the
walls of the Black Canyon is called gneiss
(pronounced "nice"), and is blended with
schist, another rock that normally has flat or
elongated crystals. You might spot the intense
folding of the alternating light and dark bands.
These rocks were once
buried deep below the Earth's surface where
they encountered extreme heat and pressure.
Painted Wall:2
The Painted Wall is the result of
molten material forced under great
pressure into the cracks and joints of the base rock.
Those sheer walls are an ideal home for the peregrine falcon.
Black Canyon Pegmatite:2
Pink streamers of pegmatite animate the
towering cliffs of the canyon at Painted Wall
overlook.
This granite-like igneous rock formed
as hot magma forced its way into cracks.
It
cooled slowly, allowing large crystals to form.
The pegmatite is loaded with shiny muscovite
(mus'-ko-vite) mica and large crystals of a
pinkish mineral called potassium feldspar.
| Colorado National Monument |
Colorado National Monument and nearby Grand Mesa:3
The Uncompahgre Highland rises to
mountainous heights, then is worn down over
millions of years to a nearly level plain.
(These ancient crystalline rocks - granite, gneiss, schist -
which were at the core appear today in the canyon floors.)
Great
bodies of water follow, depositing layer upon layer of soft,
sedimentary rocks as distant mountains give themselves up, grain
by grain, to be reformed. Becoming entombed within the rock
are the remains of dinosaurs, fish and shellfish, early mammals,
and many other life forms that lived within
a span of more than 100 million years.
Finally the Rocky Mountains begin
their slow rise, and the surrounding land is raised as well.
Water falling as rain and snow invades rock crevices, expands
as ice, and begins prying pieces loose from solid material.
As erosion continues, the streams and rivers sort themselves out
into the ancestral Colorado River system.
Volcanic eruptions, the latest series of geological events,
spread molten lava over much of the land nearby.
The remnant lava today forms a resilient cap on Grand
Mesa to the northeast.
| Curecanti National Recreation Area |
Dillon Pinnacles and West Elk Mountains:2
Tertiary volcanism is
responsible for one of the
most notable geologic
features in Curecanti
National Recreation Area.
The Dillon Pinnacles
tower above the northern
shore of Blue Mesa's
Sapinero Basin. The rock
forming the pinnacles is called the
West Elk Breccia (pronounced bretch'-yuh or
bretch'-ee-yah). It formed from a
huge volcanic mud flow (called a lahar) of ash and
volcanic debris that spewed
from violent, pyroclastic eruptions in the West Elk
Mountains about
30 million years ago. You probably know that pyro refers to fire.
The term clastic is
used to describe bits and pieces of broken rock.
The West Elk Breccia contains a jumble of angular rock fragments that vary in
size and shape. These fragments are imbedded in a matrix of fine volcanic ash
and mud. Many of the larger clastic fragments are more resistant to the effects of
erosion and weathering than the soft, mud-ash matrix. These larger rock
fragments provide an "umbrella of protection" against the elements, sheltering the
rock immediately beneath it. The result is the mysterious spire-like form of the
pinnacles.
San Juan Mountains:2
The West Elk Mountains were not the only
volcanoes erupting during the Tertiary.
About 28 million years ago,
a series of volcanic ash flows that originated from the
San Juan Mountains blanketed much of southern Colorado.
The tremendous
caldera eruptions of the San Juans were
characterized by turbulent, flowing
clouds of hot incandescent ash, gasses and
tiny shards of volcanic glass. Such
plinian-type eruptions are sometimes
referred to as nuée ardentes or "glowing
avalanches". As the turbulent ash clouds settled out,
the burning-hot ash and
glass shards welded together to
form a dense, erosion-resistant rock called
welded tuff. The various layers of welded tuff
serve as cap rocks that protect the
softer rocks beneath them and give the
mesas of Curecanti their flat top (mesa
means "table" in Spanish).
| Dome Rock Natural Area |
Dome Rock:9
This scenic area is dominated by massive exfoliating outcrops
of Pikes Peak granite.
Dome Rock, among the most spectacular of the
formations, rises 800 feet from the canyon floor.
The site also contains good-quality examples of
ponderosa pine, limber pine, Douglas-fir
and montane grassland plant communities.
The area provides winter browse and lambing
grounds for a herd of bighorn sheep.
| Dotsero Crater |
Dotsero Crater:11
One of the youngest eruptions
in the continental U.S.
produced an explosion crater,
a lahar, and a 3-kilometer long lava
flow in north-central Colorado near the town of Dotsero.
The eruption is dated at
4,140 years before present,
based upon a carbon-14 date from wood under the scoria.
The Dotsero crater is 700 meters wide and 400 meters deep.
Dotsero Crater, Triangle Peak, Willow Peak:1
Igneous activity has occurred as
recently as 4,150 years ago at Dotsero volcano, the
youngest known volcano in Colorado. Volcanic
activity has also occurred during the
Quaternary at other locations in this area, including Triangle
Peak
and Willow Peak. However, much
and perhaps all the demonstrable
Tertiary deformation within the area, where evaporitic rocks
are at shallow depths and in the
adjoining Grand Hogback Monocline, appears to be directly due
to salt tectonism and salt dissolution.
| Elephant Rocks Natural Area |
Elephant Rocks:9
Elephant Rocks are a weathered remnant of the Fish Canyon tuff
(late Oligocene, about 28 million years old). The rocks themselves
support an occurrence of the rare parsley Neoparrya lithophila,
(rock-loving neoparrya). An interesting mixture of shortgrass prairie
and foothill shrubland vegetation occurs on the site.
| Eleven Mile State Park |
Eleven Mile State Park:9
Eleven Mile State Park features a 3,405
surface-acre reservoir lying on the South Platte River at
the southern edge of South Park. Formed behind
Elevenmile Canyon Dam, other drainages emptying into
the reservoir include Cross, Prudence, Union,
Balm-of-Gilead, Simms and Spring creeks. The South
Platte River Valley is mantled with Wisconsin-aged
glacial outwash material, Pleistocene alluvium, which covers older
formations. The southeastern portion of the park has been covered by
Como-age surface deposits.
Most of the park is underlain by Precambrian
rocks:
Silver Plume granite
east and south of Howbert Point and
Pikes Peak granite
along the river west of Howbert.
The western and southern edges
consist of Thirtynine Mile
andesite deposits; igneous
and metamorphic rocks
of Tertiary origin.
Cross Creek cuts through
another Tertiary deposit,
Trachyte (also igneous
and metamorphic rock)
Along the southwestern
reservoir margin, Wall Mountain tuff,
part of a south-dripping sequence of
welded tuffs
that incorporated
volcanic ash and pumice
is exposed as steep
cliffs. Thirtynine Mile andesite
has been deposited over both the
granite and tuff formations
on the southern reservoir edge.
| Florissant Fossil Beds National Monument |
Florissant Fossil Beds:4
Today, Florissant Fossil Beds National Monument
is a wonderland of meadows,
forests, and wildflowers. Yet 34 to 35 million years ago, Lake
Florissant, stretching 15 miles through an
ancient forest valley, dominates
the scene. Nearby a volcano rumbles. In the past, volcanic mudflows
blanketed parts of the forest surrounding Lake Florissant, killing entire
trees. Now as the volcano again erupts violently, the devastation is
widespread.
The exploding volcano showers the countryside with million of
tons of ash, dust, and pumice.
Caught in this deadly cloud, insects, leaves,
fish (anything that cannot escape)
die, and many fall to the lake bottom,
where they are buried.
These eruptions occur again and again for perhaps as
long as 700,000 years. Each time,
fragments of life are trapped in a layer of
volcanic sediments at the bottom of the lake.
Eventually these sediments
become a finely layered shale and
transform the buried plan and animal life
into fossils.
Florissant Fossil Beds National Monument preserves this site.
Lake Florissant:5
During the Oligocene (35-34 million
years ago) a complex of composite volcanoes,
18 miles to the southwest, began to erupt.
Early eruptions sent volcanic mudflows into the
Florissant area and buried forest of giant
redwoods and other trees. A later mudflow
formed a dam across the ancient drainage,
resulting in the formation of a large lake. The
fine grained muds and volcanic ash deposited in
this ancient Lake Florissant eventually became
the shales that contain the exquisitely preserved
fossils of insects, leaves and flowers.
Sedimentation in the lake varied from slowly
deposited, thin layers of organic-rich muds
enriched in diatoms to rapidly deposited, thick
layers of coarse ash and pumice.
Pikes Peak Granite:5
The Precambrian Pikes Peak Granite,
1.02 billion years old, is the oldest unit at
Florissant. This pink granite
forms the rounded rocky hills in the monument. The
Pikes Peak Granite formed as a large intrusive
batholith that covers more than 1,150 square miles
and is composed of pink to reddish-tan, medium-to
coarsely crystalline, biotite and hornblende-biotite
granite. These Precambrian rocks were
uplifted during the Laramide orogeny and
exposed at the surface by Eocene time (37
million years ago) after erosion of the overlying
Paleozoic and Mesozoic units.
Thirty-Nine Mile Volcanic Field and Guffy Volcano:4
Thirty-four million years ago (Eocene)
volcanism begins at Thirty-Nine Mile Volcanic Field.
Products include: ashflows, lava flows,
agglomerates from lahars, breccias, ashfalls,
lake formations at Florissant and Antero in South Park.
Guffey Volcano and others in the field
are formed as composite volcanoes.
This indicates a violent type of eruption similar to Mount St. Helens type.
Wall Mountain Tuff:4
Thirty-six million years ago (Eocene)
massive ashflows emanating from the
Mount Aetna caldera in the
Sawatch Range resulted in the formation
of the Wall Mountain Tuff.
Stream conglomerates containing
clasts of the tuff were deposited over
the tuff and are found today as the
Tallahassee Creek and Castle Rock conglomerates.
| Front Range |
Front Range Foothills:1
For simplicity, we can think of the
geology of the Front Range Foothills areas as falling into
three basic categories of rock and soil deposits:
"really old," "old," and "young."
The "really old" deposits consist of the
igneous and metamorphic rocks that make up the
mountains in the Front Range itself.
These rocks date back to the Precambrian era. Most of
them are over 1 billion years old.
The "old" deposits consist of
sedimentary rock formations: mostly sandstone and shale with
some limestone and coal layers.
These rocks date back to the Paleozoic, Mesozoic, and
Cenozoic eras. They range in age from about
300 million years to 63 million years. These
formations have been folded and uplifted by
the Rocky Mountains along the foothills, whereas
they are flat-lying beneath Denver.
The "young" deposits consist of unconsolidated
(i.e., soil) sediment deposits that lay atop the
older rock deposits. These sediments are
alluvial (deposited by streams), colluvial (slope-wash),
and eolian (wind). Many of these deposits
date back to 1 million to 25 thousand years, when
Colorado's mountains experienced several
periods of glaciation and melt-off.
North Table Mountain:1
Looking east from Golden, Colorado,
we see the broad slopes of North Table Mountain.
These slopes are covered with
landslide deposits, which are underlain by
the Denver Formation of Cretaceous-Tertiary age (the
K-T boundary is hidden up there, somewhere).
The rimrock cliffs at the top of the mountain
are the remnants of a series of 63-64 million-year-old
lava flows.
The lava originally flowed down a valley.
After cooling, it became very hard
and resistant to erosion.
The surrounding highlands later eroded away leaving the former valley
as today's high point.
This type of geomorphic feature is known as a topographic inversion.
South Table Mountain:1
Tertiary Volcanic Rocks
| Glenwood Springs, Garfield County |
Glenwood Springs Basalt:1
The rock in and surrounding the
sinkhole at Colorado Mountain College is basalt.
Basalt is an extrusive igneous rock. Basalt is dark colored,
fine grained and may appear dull, almost velvety
on fresh surfaces. Numerous, smooth cavities or
vesicles, which represent entrapped gas bubbles
released from the rock as it cooled, are common
in the upper part of basalt flows. slightly larger
crystals within the fine-grained basalt are called phenocrysts.
Isotopic dating of basalt flows in the
Glenwood Springs area suggest widespread volcanic
activity from about 20 to 25 million years ago
and 8 to 11 million years ago (Larson and others,
1975). Isolated volcanoes have been active about
4 million, 3 million, 1.5 million, and 4,000
years ago.
Colorado Mountain College Campus:1
Turn left at the Colorado Mountain College Campus.
Drive an additional 0.3 miles and park in the dirt parking
lot next to the soccer field.
Walk 200 feet to rock tower ruin and
view large sinkhole in basalt.
Flat Tops and Basalt Mountain Shield Volcano:1
Basalt, olivine basalt, and andesitic basalt flows,
and interbedded siltstone,
conglomerate, sandstone, and claystone --
over a dozen flows form the caprock for the Flat
Tops or can be seen on south flank of the
Basalt Mountain shield volcano (thickness
approximately 250 feet).
Spring Valley:1
Spring Valley is a salt-collapse
half graben into which a large part of the hills to the east are
collapsing. The 22 million years old basalt
exposed in the roadcut has been downdropped 3,000
to 4,000 feet into the Carbondale collapse center.
The floor of Spring Valley was occupied by a lake
until being drained by homesteaders prior to 1900.
Tunnel Eastbound I-70:1
This tunnel passes through a unique igneous rock with
large crystals called porphyroblastic biotite granodiorite.
Samples may be collected along the bike path from
Glenwood Springs.
| Great Wall Dike |
The Great Wall Dike:13
The Great Wall dike north of Spanish Peaks
in Colorado, is a spectacular
example of a volcanic wall. The well-developed
dike pattern in this area has been largely the result of a
pre-existing system of joints that developed
during the early stages of late Cretaceous orogeny. It is thought
that in the formation of dikes, particularly
at shallow depths, the magma does not force the fissures apart
prior to entry. Instead the fissures were
pulled apart by dilation or stretching,
and then the magma flowed into the cracks.
| Green River Formation |
Green River Formation:6
Three ancient great lakes existed in the region of Wyoming,
Utah, and Colorado 50 million years ago: Lake Gosiute, 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.
| Jefferson County |
Crushed Stone:1
Crushed stone is a construction material
used mainly as an aggregate for the manufacture of
concrete, and for road base material.
Several large quarries are present in the foothills west of
the Denver metro area. The hard, dense
metamorphic and igneous rocks of Precambrian age
(about 1.7 billion years old in this area)
that form the core of the Front Range make excellent
sources of for aggregate. Hard volcanic rock
of Tertiary-age (about 60 million years old) also
makes good aggregate, and is being quarried
at the Ralston Quarry (Asphalt Paving Co.)
Morrison Quarry:1
This quarry produces crushed stone
from Precambrian metamorphic and igneous rock.
The quarry site was chosen partially
because the hills screen most of the operation
from view.
Ralston Quarry:1
Hard volcanic rock
of Tertiary-age (about 60 million years old) also
makes good aggregate, and is being quarried
at the Ralston Quarry (Asphalt Paving Co.)
| Mini-Wheeler Natural Area |
Mini-Wheeler Natural Area:9
The unusual "Devil's Dunce Cap" formations are the
eroded remnants of a 1,200-square-mile volcanic ash flow.
This Gribbles Park Tuff was
deposited 29 million years ago during the Oligocene.
Although located within the
Thirtynine Mountain Volcanic Field, the tuff's source is
thought to be outside of the field to the west.
The exact source is unknown, and research
on this may be useful in determining the timing of
the opening of the upper Arkansas valley.
The more well-known Wheeler Geologic site
in Mineral County has higher formations (and is
from a different volcanic source),
but this site covers a larger area than Wheeler.
Wheeler Geologic Natural Area:9
Wheeler Geologic Natural Area is a mass of pinnacles and domes,
some more than several hundred feet high. Erosive forces have sculpted
these forms in
white, beige and lavender layers of lava and ash.
The geologic features occur within virgin forests of
Engelmann spruce and
subalpine fir.
| Mount Antero and White Mountain |
Colorado Gemstone:1
The aquamarine was adopted as the
official state gemstone on April 30, 1971,
by an act of the General Assembly. The mountain
peaks of Mount Antero and White Mountain in Colorado
are among the finest quality localities known for gem aquamarine. They
are also among the highest in elevation, located at 14,000 feet.
The granite rock of these peaks contains pegmatite bodies that are
characterized by large miarolitic cavities containing the gem
quality aquamarine crystals.
The cavities are found through a vertical
area of a mere 500 feet. The crystals in these cavities range
in color from light blue to pale and deep aquamarine green, and in
size from very small to 6 centimeters in length.
| Needle Rock Natural Area |
Needle Rock:9
Needle Rock towers 800 feet above the floor of the
Smith Fork of the Gunnison River valley.
It originated as the throat of a large volcano
about 28 million years ago (Miocene epoch)
when molten rock intruded between existing sedimentary formations.
As the surrounding
country rocks eroded over millions of years,
the resistant igneous core was exposed.
| Ouray Hot Springs |
Ouray Hot Springs:1
The cliffs around Ouray are composed
of Paleozoic and lower Cretaceous sedimentary
rocks overlain by Tertiary San Juan Formation volcanic rocks.
The Ouray hot springs are
heated by geothermal waters emanating from faults
beneath the alluvium of the valley
bottom and are a faint reminder of past volcanic activity.
| Pikes Peak |
Pikes Peak Granite:4,10
Pikes Peak Granite is to the north of Florissant.
Samples collected on top of Pikes Peak are
1,030,000,000 years old, dated by various atomic clock methods.
| Raton Mesa |
Raton Mesa - National Natural Landmark:14
Las Animas County - Illustration of a mesa preserved by a thick
lava cap which has resisted destruction from weathering and
erosion. Only significant reference available illustrating the
magnitude of erosion involved in developing the land surface of
the Great Plains adjacent to the lower foothills of the Rocky
Mountains. Owner: Private.
DESIGNATION DATE:
April 1967
| Red and White Mountain |
Red and White Mountain Laccolith:13
Less fluid magma may form a sill variation
known as a laccolith. These shallow intrusive bodies are actually
domes that did not quite break through to
the Earth's surface but instead heaved up the overlying beds of
older rocks. Both laccoliths and sills may
be fed by either dikes or from the central conduit. The rocks
overlying large laccoliths are often relative
light sedimentary types. This is probably because the lighter
sedimentary rock layers are easier for the
magma to lift. A more dense rock would instead be broken by the
magma in its push toward the surface. ... (One example)
Red and White Mountain, Colorado, is of Tertiary
age. Overlying layers of rock have been eroded.
| Rocky Mountain National Park |
| Saddle Mountain Research Natural Area |
Saddle Mountain:9
Saddle Mountain Research Natural Area
supports a variety of pristine plant communities.
Included in the natural area are dense stands of
Engelmann spruce and subalpine fir, aspen,
and bristlecone pine. Extensive meadows of
Danthonia parryi (Parry oatgrass) occur on
shallower soils derived from basalt.
| Saint Mary's Falls |
Saint Mary's Falls:8
From Colorado Springs, take either 26th Street or
Cheyenne Road to North Cheyenne Canyon Road to Gold Camp
Road. About one mile past the intersection of North Cheyenne
Canyon Road/Gold Camp Road and High Drive is the trailhead.
Foot and horse trail.
Motorized vehicles are prohibited.
St. Mary's Falls cascades
250-300 feet down a solid granite wall
and is about 2.0 miles
from Gold Camp Road. The falls elevation is 8,880 feet.
| San Juan Mountains |
San Juan Mountains:2
About 28 million years ago, a series
of volcanic ash flows that originated from the
San Juan Mountains blanketed much of
southern Colorado. The tremendous
caldera eruptions of the San Juans were
characterized by turbulent, flowing
clouds of hot incandescent ash, gasses
and tiny shards of volcanic glass. Such
plinian-type eruptions are sometimes
referred to as nuée ardentes or "glowing
avalanches". As the turbulent ash clouds
settled out, the burning-hot ash and
glass shards welded together to form a dense,
erosion-resistant rock called
welded tuff. The various layers of welded tuff
serve as cap rocks that protect the
softer rocks beneath them and give the
mesas of Curecanti their flat top (mesa
means "table" in Spanish).
San Juan Mountains:1
The mountains that you see while
driving along Red Mountain Pass were shaped over
billions of years through multiple
episodes of mountain building and uplift, ancient seas,
volcanic upheavals, and icy glaciers.
In the San Juan Mountains we see evidence of
rock formations that span a vast amount of
geologic time. From recent landslide features
and relatively young volcanic events to
billion-year-old basement rocks, we see a
large part of the geologic time scale
represented in these rocks.
The San Juan Mountains are mostly composed
of rocks that erupted from Tertiary
volcanoes beginning about 40 million years ago.
The volcanic activity continued
sporadically for another 30 million years.
Lava flows covered vast areas and mixed with
older rocks to form conglomerates and breccias.
Underlying the volcanic rocks, the older
geologic sequence is visible in the Ouray area
and south along the Uncompahgre River Canyon.
Small outcrops are present in Ironton
Park.
Precambrian rocks (>600 million years before present)
in much of Colorado are igneous
or extensively metamorphosed rocks, but the
Precambrian Uncompahgre Formation near
Ouray comprises former sedimentary rocks that
have been only moderately
metamorphosed, and retain much of their
original character. Sandstones have become
quartzites; shales and mudstones have become slate.
In Ouray and south to the vicinity of Bear Creek,
sedimentary rocks from the Paleozoic
era outcrop on the canyon walls.
The rocks unconformably overlie the Uncompahgre
Formation. Paleozoic rocks include the
Ouray Limestone of Devonian age, the Leadville
Limestone of Mississippian age, the
Pennsylvanian Molas and Hermosa Formations, and
the Permian Cutler Formation. The horizontally
bedded, tan rocks to the east of the hot
springs are part of the Ouray Limestone.
Unconformably overlying the Paleozoic
sequence are the Mesozoic rocks of the Triassic
Dolores Formation, Jurassic Wanakah and
Morrison Formations, and the Cretaceous
Dakota Formation. These rocks are exposed
north and west of Ouray, but are not visible
south along the Uncompahgre River Canyon.
After the Mesozoic sediments were deposited,
a time of uplift and erosion ensued and the
Tertiary Eocene Telluride Conglomerate was deposited.
Another period of erosion
removed most of the Telluride Conglomerate
in this area. Subsequent volcanic activity
began in the area of the San Juan Mountains.
The San Juan Formation volcanic material
was erupted from stratovolcanoes (like Mount St. Helens)
and was deposited
unconformably above the older formations
exposed at the surface. The San Juan
Formation volcanics are predominately andesites.
Subsequent to deposition of the San
Juan Formation, the Silverton Volcanic Group
was deposited. The early members of this
volcanic sequence are contemporaneous with
caldera development in this area. Its
members, from oldest to youngest, are the
Eureka Member (rhyolite), Burns Member
(andesite, rhyodacite), Henson Formation (andesite),
and a pyroxene andesite member.
The San Juan, Uncompahgre, Silverton, and
Lake City Calderas and their associated
volcanic deposits record a 15-20 million-year
history of volcanism in the Ouray-Silverton
area.
| Silverton Caldera |
Silverton Caldera:1
Volcanic activity, related faulting, and
subsequent erosion formed the landscape and
geology of this area. From a stop
on the northwestern side of the roughly circular
Silverton caldera, to the southeast
Red Mountains No.2 and 3 can be seen. They are
within the subsided or down-dropped
central portion of the caldera.
The upper part of
these mountains consists of the Henson Formation.
The lower part consists of the older
Burns Formation. Both units are Tertiary Oligocene in age
(approximately 27-28 million years), a time
when intense volcanic activity in the
San Juan Mountains area caused deposition of thick
beds of volcanic rock. Looking west to the outside of the
Silverton caldera, the Burns
formation is at the surface and the
Henson Formation is not present due to erosion.
The stunning red, maroon, orange, and yellow colors of the Red Mountains are primarily due to past hydrothermal alteration of these rocks. Hydrothermal alteration is literally the process of "hot water" circulation through the strata that changes the mineral composition of the original rocks. Hydrothermal fluids commonly contain significant concentrations of metals and sulfur. Hydrothermally altered rocks can contain significant amounts of disseminated sulfide minerals, primarily pyrite (iron sulfide). This process happened within and surrounding the Silverton caldera. Subsequent erosion of the strata exposes the pyrite to oxidation, and as the iron combines with oxygen it turns the rock various colors evident in the Red Mountains.
Stop just south of Red Mountain Pass,
in the upper reaches of the Mineral Creek
drainage basin, a tributary of the Animas River.
We are now standing on the northwest
structural boundary or rim of the Silverton
caldera.
The caldera center is to the east-southeast.
Mineral Creek, which State Highway
550 follows to the south, traces the
western rim of the caldera. The Silverton caldera
formed after the last major eruption cycle of the
older San Juan caldera in the middle
Tertiary period (approximately 27 million years ago).
As the caldera collapsed on itself, vertical faults
and fractures formed pathways for the upward
movement of gases, ore bearing fluids, and
igneous intrusive plugs. The rim faults also
became pathways for circulating
hydrothermal fluids, which caused extensive
alteration and emplacement of sulfide
minerals.
| Slumgullion Earthflow Natural Area |
Slumgullion Earthflow:9
Two major earthflows included in this natural area illustrate
a striking example of mass wasting
(the movement of large masses of earth
material). Slumgullion earthflow is over four miles long and
2,000 feet wide.
A huge mass of volcanic rock slumped down the valley about
700 years ago, damming Lake San Cristobal.
This flow is gradually being covered by a
younger flow which began 300 years ago. Trees on
the earthflow are being pushed over by
the forward advance of the younger flow.
Slumgullion Earthflow - National Natural Landmark:14
Hinsdale County - Seven hundred year-old, 1,000-acre landflow
composed of volcanic rock which formed a dam that created
Lake San Cristobal. A younger landflow, currently active, is
moving as much as 20 feet per year along the path of the earlier
flow. Owner: Federal, Private.
DESIGNATION DATE:
October 1965
| Spanish Peaks |
Spanish Peaks:12
The mountainous sections of the Great Plains
were formed long before the remaining areas
were outlined by erosion. Uplift of the Black Hills and the
Central Texas Uplift began as the
continental interior was raised and the
last Cretaceous sea was displaced, 65 to 70 million years ago.
They stood well
above the surrounding plains long before
any sediments from the distant
Rocky Mountains began to accumulate at their bases.
In southern Colorado and
northern New Mexico, molten rock invaded
the sedimentary layers between 22 and 26 million years ago.
The Spanish Peaks were formed at this time
from hot magma that domed up the surface layers
but did not break through; the magma has since cooled
and solidified and has been exposed by
erosion. Elsewhere the magma reached the surface,
forming volcanoes, fissures, and basalt flows.
A great thickness of basalt flows accumulated at
Raton Mesa and Mesa de Maya between 8 and 2 million years ago.
Volcanism has continued intermittently,
and the huge cinder cone of Capulin
Mountain was created by explosive eruption
only 10,000 to 4,000 years ago. Most of these
volcanic masses were formed before major downcutting by
the streams began. Other igneous intrusions
and volcanic areas in the northern Great Plains
similarly were formed before the streams were incised.
Spanish Peaks - National Natural Landmark:14
Huerfano County and extends into Las Animas County - One of
the best exposed examples of igneous dikes known; dikes are
formed when molten igneous material is forced into a fracture
or fault before becoming solidified. There may be over 500
such dikes in the area. Owner: Federal, Private.
DESIGNATION DATE:
January 1976
| Specimen Mountain Research Natural Area |
Specimen Mountain and Shipler Mountains:9
Specimen Mountain Research Natural Area
straddles the Continental Divide
in Rocky Mountain National Park and includes Specimen and
Shipler Mountains.
This natural area is capped with volcanic material
from a nearby volcano that erupted approximately 28 million years
ago and now supports virgin stands of Engelmann spruce-subalpine fir
and pristine alpine meadows. The area is important calving grounds for
Rocky Mountain bighorn sheep. Natural salt licks in the "crater"
attract elk, deer, and bighorn sheep.
| Wheeler Geologic Natural Area |
Wheeler Geologic Natural Area:9
Wheeler Geologic Natural Area is a mass of pinnacles and domes,
some more than several hundred feet high.
Erosive forces have sculpted
these forms in white, beige and lavender layers of lava and ash.
The geologic features occur within virgin forests of Engelmann spruce and
subalpine fir.
Excerpts from:
1) Colorado Geological Survey, Division of Minerals and Geology,
Department of Natural Resources Website, 2002
2) U.S. National Park Service Website,
Black Canyon of the Gunnison National Park, 2001
3) U.S. National Park Service Website,
Colorado National Monument, 2000;
4) U.S. National Park Service Website,
Florissant Fossil Beds National Monument, 2000
5) U.S. National Park Service Website,
Florissant Fossil Beds National Monument, 2002,
Geology Field Trip Guides by Anabelle Foos
6) U.S. National Park Service Website,
Fossil Butte National Monument, 2001
7) USGS/NPS Geology of the Parks Website, 2001
8) U.S. Forest Service Website, Pike's Peak Ranger District, 2002
9) Colorado State Parks Website, 2002
10) William L. Newman, Geologic Time:
USGS Online Information Publication, 1997
11) Wood, 1990, IN:
Wood and Kienle, 1990, Volcanoes of North America:
Cambridge University Press
12) Trimble, D.E., 1980, The Geologic Story of the Great Plains:
U.S. Geological Survey Bulletin 1493
13) NOAA National Data Centers, NGDC Website, 2003
14) U.S. National Park Service, National Natural Landmarks Website, 2003
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