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 |
MORE America's Volcanic Past - Wyoming
Visit A Volcano - Yellowstone National Park
Volcanic Highlights and Features:
| [NOTE: This list is just a sample of various Yellowstone 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 Yellowstone.] |
Yellowstone's vast collection of thermal features provides a constant reminder of the park's recent volcanic past. Indeed, the caldera provides the setting that allows such features as Old Faithful to exist and to exist in such great concentrations. At the heart of Yellowstone's past, present, and future lies volcanism. Catastrophic eruptions occurred here about 2 million years ago, then 1.2 million years ago, and then 600,000 years ago. The latest eruption spewed out nearly 240 cubic miles of debris. What is now the park's central portion then collapsed, forming a 28- by 47- mile caldera (or basin). The magmatic heat powering those eruptions still powers the park's famous geysers, hot springs, fumaroles, and mud pots.
In the last decade, geological research has determined that the two
volcanic vents, now known as "resurgent domes", are rising again. From
year to year, they either rise or fall, with an average net uplift of about
one inch per year. During the period between 1923 and 1985, the Sour
Creek Dome was rising. In the years since 1986, it has either declined or
remained the same. The resurgence of the Sour Creek dome, just north of
Fishing Bridge is causing Yellowstone Lake to "tilt" southward. Larger
sandy beaches can now be found on the north shore of the lake, and
flooded areas can be found in the southern arms.
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| Yellowstone Caldera |
Yellowstone Caldera:1
About 600,000 years ago, huge volcanic eruptions occurred in
Yellowstone, emptying a large underground magma chamber. Volcanic
debris spread for thousands of square miles in a matter of minutes. The
roof of this chamber collapsed, forming a giant smoldering pit. This
caldera was 30 miles (45 km) across, 45 miles (75 km) long, and several
thousand feet deep. Eventually the caldera was filled with lava.
| Yellowstone "Hot Spot" |
Yellowstone Hot Spot:3
Yellowstone National Park in northwest Wyoming currently
lies above a source of magma called a hot spot or a plume.
The giant volcanic depression that contains the park and its
geysers and hot springs (for example, Old Faithful) formed
about 100,000 to 5 million years ago. Over the last 16
million years, the North American Plate has moved
westward over this plume, creating the Snake River Plain.
The Yellowstone caldera (indicated by the red outline) is a
large crater-like feature covering more than 1300 square
miles. It formed when an underground magma chamber
collapsed 630,000 years ago.
| Geysers, Mudpots, etc. |
Geysers and Hot Springs:1
Today's geothermal activity is a link to past volcanism. A partially molten
magma chamber, remnant of a cataclysmic volcanic explosion 600,000
years ago in central Yellowstone, supplies one of the ingredients, heat.
In the high mountains surrounding the Yellowstone Plateau, water falls as snow or rain and slowly percolates through layers of porous rock, finding its way through cracks and fissures in the earth's crust created by the ring fracturing and collapse of the caldera. Sinking to a depth of nearly 10,000 feet, this cold water comes into contact with the hot rocks associated with the shallow magma chamber beneath the surface. As the water is heated, its temperatures rise well above the boiling point to become superheated. This superheated water, however, remains in a liquid state due to the great pressure and weight pushing down on it from overlying rock and water. The result is something akin to a giant pressure cooker, with water temperatures in excess of 400 degrees F.
The highly energized water is less dense than the colder, heavier water sinking around it. This creates convection currents that allow the lighter, more buoyant, superheated water to begin its slow, arduous journey back toward the surface through rhyolitic lava flows, following the cracks, fissures, and weak areas of the earth's crust. Rhyolite is essential to geysers because it contains an abundance of silica, the mineral from which glass is made. As the hot water travels through this "natural plumbing system," the high temperatures dissolve some of the silica in the rhyolite, yielding a solution of silica within the water.
At the surface, these silica-laden waters form a rock called geyserite, or sinter, creating the massive geyser cones; the scalloped edges of hot springs; and the expansive, light-colored, barren landscape characteristic of geyser basins. While in solution underground, some of this silica deposits as geyserite on the walls of the plumbing system forming a pressure-tight seal, locking in the hot water and creating a system that can withstand the great pressure needed to produce a geyser.
With the rise of superheated water through this complex plumbing system, the immense pressure exerted over the water drops as it nears the surface. The heat energy, if released in a slow steady manner, gives rise to a hot spring, the most abundant and colorful thermal feature in the park. Hot springs with names like Morning Glory, Grand Prismatic, Abyss, Emerald, and Sapphire, glisten like jewels in a host of colors across the park's harsh volcanic plain.
Sprinkled amid the hot springs are the rarest fountains of all, the geysers. What makes them rare and distinguishes them from hot springs is that somewhere, usually near the surface in the plumbing system of a geyser, there are one or more constrictions. Expanding steam bubbles generated from the rising hot water build up behind these constrictions, ultimately squeezing through the narrow passageways and forcing the water above to overflow from the geyser. The release of water at the surface prompts a sudden decline in pressure of the hotter waters at great depth, triggering a violent chain reaction of tremendous steam explosions in which the volume of rising, now boiling, water expands 1,500 times or more. This expanding body of boiling superheated water bursts into the sky as one of Yellowstone's many famous geysers.
Fumaroles:1
Fumaroles, or steam vents, are hot springs with a lot of heat,
but so little
water that it all boils away before reaching the surface. At places like
Roaring Mountain, the result is a loud hissing vent of steam and gases.
| Grand Canyon of the Yellowstone River |
Grand Canyon of the Yellowstone River:1
The spectacular Grand Canyon of the Yellowstone River
provides a glimpse of
Earth's interior: its waterfalls highlight
the boundaries of lava flows and thermal areas.
Rugged mountains flank the park's volcanic plateau,
rewarding both eye and spirit.
The specifics of the geology of the canyon are not well understood, except that it is an erosional feature rather than the result of glaciation. After the caldera eruption of about 600,000 years ago, the area was covered by a series of lava flows. The area was also faulted by the doming action of the caldera before the eruption. The site of the present canyon, as well as any previous canyons, was probably the result of this faulting, which allowed erosion to proceed at an accelerated rate. The area was also covered by the glaciers that followed the volcanic activity. Glacial deposits probably filled the canyon at one time, but have since been eroded away, leaving little or no evidence of their presence.
Formation:1
About 600,000 years ago, huge volcanic eruptions occurred in
Yellowstone, emptying a large underground magma chamber. Volcanic
debris spread for thousands of square miles in a matter of minutes. The
roof of this chamber collapsed, forming a giant smoldering pit. This
caldera was 30 miles (45 km) across, 45 miles (75 km) long, and several
thousand feet deep. Eventually the caldera was filled with lava.
One of these lava flows was the Canyon Rhyolite flow, approximately 590,000 years ago which came from the east and ended just west of the present canyon. A thermal basin developed in this lava flow, altering and weakening the rhyolite lava by action of the hot steam and gases. Steam rises from vents in the canyon today and the multi-hued rocks of the canyon walls are also evidence of hydrothermally altered rhyolite.
Other lava flows blocked rivers and streams forming lakes that overflowed and cut through the various hard and soft rhyolites, creating the canyon. Later the canyon was blocked three different times by glaciers. Each time these glaciers formed lakes, which filled with sand and gravel. Floods from the melting glaciers at the end of each glacial period recarved the canyon, deepened it, and removed most of the sand and gravel.
The present appearance of the canyon
dates from about 10,000 years
ago when the last glaciers melted.
Since that time, erosional forces (water,
wind, earthquakes, and other natural forces)
have continued to sculpt the
canyon.
| Volcanic Features and Places to Visit |
Absaroka Range, Mount Washburn, Tower Falls:1
Mount Washburn (10,243 feet) and the Absaroka Range are both remnants of ancient
volcanic events that formed the highest peaks in the Tower District.
Ancient eruptions, perhaps 45 to 50 million years ago, buried the forests of
Specimen Ridge in ash and debris flows. The columnar basalt formations
near Tower Falls, the volcanic breccias of the "towers" themselves, and
numerous igneous outcrops all reflect the district's volcanic history.
Firehole Canyon Drive:1
Firehole Canyon Drive, a side road, follows the Firehole River upstream
from Madison Junction to just above Firehole Falls. The drive takes
sightseers past 800-foot thick lava flows. Firehole Falls is a 40-foot
waterfall.
Gibbon Falls:1
This 84-foot (26-meter) waterfall tumbles over remnants of the
Yellowstone Caldera rim. The rock wall on the opposite side of the road
from the waterfall is the inner rim of the caldera.
Glacial Boulder:1
Along the road to Inspiration Point there is a house-sized granite boulder
sitting in the pine forest alongside the road. It was plucked from the
Beartooth Mountains by an early Pinedale Glacier and dropped on the
north rim of the Grand Canyon of the Yellowstone nearly 80,000 years
ago. Continued glacial advances and retreats led to the present-day
appearance of the canyon and surrounding area.
Lava Creek:4
Ash, cinders, bombs, and blocks, though less well known
than lava, are the most common volcanic eruption
products. Ash flows (pyroclastic flows) result from
expansion of gas bubbles at the vent, which fragment the lava.
Fragments may flow downhill at speeds up to 200 kilometers/hour
and come to rest on the lower slopes of the volcano. if
volcanic debris is so charged with fragments that it is too
heavy to rise, it becomes a nuee ardente or glowing
cloud. Where the flowing ashes are overlain by airborne ash,
an ignimbrite or tuff is formed. These ignimbrites
may cool slowly over a period of years, promoting
welding in the interior and partial crystallization of the glassy
fragments. ... (One example) is a thick layer of
welded tuff at Lava Creek in Yellowstone, Wyoming.
Mammoth Hot Springs:1
Mammoth Hot Springs are a surficial expression of the deep volcanic
forces at work in Yellowstone. Although these springs lie outside the
caldera boundary, their energy is attributed to the same magmatic system
that fuels other Yellowstone thermal areas.
At Mammoth Hot Springs, a rarer kind of spring is born when the hot
water ascends through the ancient limestone deposits of the area instead
of the silica-rich lava flows of the hot springs common elsewhere in the
park.
As ground water seeps slowly downward and laterally, it comes in
contact with hot gases charged with carbon dioxide rising from the magma
chamber. Some carbon dioxide is readily dissolved in the hot water to
form a weak carbonic acid solution. This hot, acidic solution dissolves
great quantities of limestone as it works up through the rock layers to the
surface hot springs. Once exposed to the open air, some of the carbon
dioxide escapes from solution. As this happens, limestone can no longer
remain in solution. A solid mineral reforms and is deposited as the
travertine that forms the terraces.
National Park Mountain:1
The mountain is actually part of the lava flows that encircle the Madison
Junction area.
Obsidian Cliff:2
Obsidian Cliff exposes the interior of a thick rhyolite lava flow
erupted about 180,000 years ago. The vertical columns are
cooling fractures that formed as the thick lava flow cooled and
crystallized. The flow consists of obsidian, a dark volcanic glass.
Old Faithful and the Upper Geyser Basin:1
Evidence of the geological forces that have shaped Yellowstone are found
in abundance in this district. The hills surrounding Old Faithful and the
Upper Geyser Basin are reminders of Quaternary rhyolitic lava flows.
These flows, occurring long after the catastrophic eruption of 600,000
years ago, flowed across the landscape like stiff mounds of bread dough
due to their high silica content.
Red Mountains and Mount Sheridan:1
This small range of mountains, located just west of Heart Lake, is
completely contained within the boundaries of Yellowstone National Park.
In 1871, F.V. Hayden named present-day Mount Sheridan "Red
Mountain." In 1872, members of the second Hayden Survey transferred
that name to the entire range. The name was "derived from the prevailing
color of the volcanic rocks which compose them" (Hayden, Twelfth
Annual Report, p. 470). In 1878, Henry Gannett reported that there were
12 peaks in the range, with 10,308-foot-high Mount Sheridan being the
highest.
West Thumb Geyser Basin:1
The West Thumb Geyser Basin, including Potts Basin to the north, is
unique in that it is the largest geyser basin on the shores of Yellowstone
Lake. The heat source of the thermal features in this location is thought to
be relatively close to the surface, only 10,000 feet down! The West
Thumb of Yellowstone Lake was formed by a large volcanic explosion
that occurred approximately 150,000 years ago (125,000-200,000). The
resulting collapsed volcano, called a caldera ("boiling pot" or caldron), later
filled with water forming an extension of Yellowstone Lake. The West
Thumb is about the same size as another famous volcanic caldera,
Crater Lake in Oregon, but much smaller than the great Yellowstone caldera
which formed 600,000 years ago. It is interesting to note that West Thumb
is a caldera within a caldera.
Ring fractures formed as the magma chamber bulged up under the
surface of the earth and subsequently cracked, releasing the enclosed
magma. This created the source of heat for the West Thumb Geyser
Basin today.
Yellowstone Lake:1
Geologists indicate that large volcanic eruptions have occurred in
Yellowstone on an approximate interval of 600,000 years. The most
recent of these (600,000 years ago) erupted from two large vents, one
near Old Faithful, the Mallard Lake Dome, and one just north of Fishing
Bridge, the Sour Creek Dome. Ash from this huge explosion, 1,000 times
the size of Mount St. Helens, has been found all across the continent. The
magma chamber then collapsed, forming a large caldera filled partially by
subsequent lava flows. Part of this caldera is the 136-square mile basin of
Yellowstone Lake. The original lake was 200 feet higher than the
present-day lake, extending northward across Hayden Valley to the base
of Mount Washburn.
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