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 Long Valley 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 Long Valley.] |
About 760,000 years ago a cataclysmic volcanic eruption in the Long Valley,
California area blew out 150 cubic miles of
magma (molten rock) from
a depth of about 4 miles beneath the Earth's surface.
Rapidly moving flows of glowing hot ash
covered much of east-central California, and
airborne ash fell as far east as Nebraska.
The Earth's surface sank more than 1 mile into the
space once occupied by the erupted magma, forming a large
volcanic depression that geologists call a caldera.
The massive Long Valley eruption was followed by hundreds of
smaller eruptions over the next few
hundred thousand years. These eruptions of lava flows, domes, and pyroclastic flows
were concentrated in the central and western parts of the caldera.
Volcanic activity then moved northward to the Mono Lake area about
35,000 years ago to build the Mono Craters. The most recent eruptions in the area
occurred from the Mono and Inyo Craters about 600 years ago, and from Negit Island
in Mono Lake about 250 years ago.
Excerpts from: Hill, et.al., 1996, Living With a Restless Caldera -- Long Valley, California: USGS Fact Sheet 196-96 |
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| Bishop Tuff |
Bishop Tuff:4
Fallout from the Long Valley eruption blanketed most of the western states
with ash as far east as Nebraska, and covered the Mono Basin, Owens Valley,
and parts of the Sierras in 600-3,000 feet of 1300 degrees F.
burning ash. This ash solidified into a
pinkish igneous rock
known as the Bishop Tuff, a layer of which covers 580 square miles of
California and Nevada and is especially exposed along the
highways of the Mono Basin and Owens River Valley region.
| Black Point |
Black Point:4
Black Point is the low, black, mesa-like hill that lies on
Mono Lake's northwestern shore. Black Point is a basaltic volcano
that erupted approximately 13,000 years ago, in the middle of the last ice age.
At the time, glacially-inundated Mono Lake was 900 feet
deeper than it is today, and completely submerged Black Point's eruption.
The weight of hundreds of feet of water is responsible for its flat
top, and for the intriguing fissures on its southwestern face.
These fissures -- some as deep as 80 feet and as narrow as four or five
across -- are presumed to be cooling fissures, although geologic debate
over their origins continues. Now exposed by Mono's receded
lakeline, Black Point makes an excellent specimen of an exposed
underwater-volcano for scientific study.
| Devil's Punchbowl |
Devil's Punchbowl:4
In the Mono Basin, a great example of a smaller explosion pit is the Devil's Punchbowl,
located at the south end of the Mono Craters.
| Inyo Craters and Domes |
Mono-Inyo Craters Volcanic Chain:1
The Long Valley Caldera is only one part of a large volcanic system
in eastern California that also includes the
Mono-Inyo Craters volcanic chain. This chain extends from
Mammoth Mountain at the southwest rim of the
caldera northward 25 miles to Mono Lake.
Eruptions along this chain began 400,000 years ago, and
Mammoth Mountain itself was formed by a series of
eruptions ending 50,000 years ago. The volcanic system
is still active. Scientists have determined that
eruptions occurred in both the Inyo Craters and Mono Craters
parts of the volcanic chain as recently as 600 years ago
and that small eruptions occurred in Mono Lake
sometime between the mid-1700's and mid-1800's.
| Long Valley Caldera |
Long Valley Caldera:2
Long Valley, from the headwaters of Owens River to Lake Crowley,
is a giant 10-mile-wide, 20-mile-long volcanic caldera.
Long Valley occupies the eastern half of this caldera.
Magma still underlies the
caldera and heats underground water. The heated water feeds local
hot springs and natural steam vents and drives three
geothermal power plants,
producing a combined 40 megawatts of electricity.
Resurgent Dome:2
The resurgent dome is a broad area of the central caldera floor that was pushed upward within
100,000 years or less of the caldera-forming eruption 730,000 years ago. This uplift was caused
by upward pressure related to the intrusion of molten rock into the magma reservoir beneath the
caldera. The resurgent dome is made of layers of lava flows, tephra, and pyroclastic flows that
were erupted onto the caldera floor soon after the caldera formed. The uplift arched and faulted
these volcanic rocks to form a central highland area about 10 kilometers in diameter and as
high as 500 meters above the surrounding caldera floor.
| Mammoth Mountain |
Mammoth Mountain:2,3
Mammoth Mountain - a massive volcanic dome -
has grown on the Long Valley caldera margin.
Mammoth Mountain was built by
eruptions between about 200,000 and 50,000 years ago.
Mammoth Mountain was built by the eruption of
at least 12 different steep domes and thick lava
flows. These eruptions occurred between about
200,000 and 50,000 years ago. Volcanic activity
then moved northward to the Mono Lake area
about 35,000 years ago to build the Mono
Craters, a collection of more than 30 overlapping
lava flows, domes, cones, and craters. The most
recent eruptions in the area occurred from the
Mono Craters and Inyo Craters about 600 years
ago, and from Negit Island in Mono Lake about
250 years ago.
| Mono Basin and Craters |
Mono-Inyo Craters Volcanic Chain:1
The Long Valley Caldera is only one part of a large volcanic system
in eastern California that also includes the
Mono-Inyo Craters volcanic chain. This chain extends from
Mammoth Mountain at the southwest rim of the
caldera northward 25 miles to Mono Lake.
Eruptions along this chain began 400,000 years ago, and
Mammoth Mountain itself was formed by a series of
eruptions ending 50,000 years ago. The volcanic system
is still active. Scientists have determined that
eruptions occurred in both the Inyo Craters and Mono Craters
parts of the volcanic chain as recently as 600 years ago
and that small eruptions occurred in Mono Lake
sometime between the mid-1700's and mid-1800's.
Mono Basin:4
The Mono Basin lies in one of the most geologically active areas on the planet.
Its eventful history of volcanic activity is evident in both the
extinct volcanic ranges of the Bodie and Anchorite Hills
to the north and east and the dormant Mono Craters to the south.
While the rolling
Bodie and Anchorite hills were last active over hundreds of millions of years ago,
the Mono Craters are the youngest mountain range in
North America--the oldest of its 9,000' peaks is only 40,000 years old.
Panum Crater, the northernmost and youngest of the Craters, erupted
only 640 years ago. Panum, a textbook example of a rhyolitic plug-dome volcano,
is easily reached from Hwy. 120, three miles east of Hwy. 395.
Mono Craters:2
The Mono Craters is a 17-kilometer-long chain of rhyolite domes and flows that were erupted from
35,000 to about 600 years ago. All but four of the 24 exposed domes and flows of the Mono
Craters are less than 10,000 years old. The most recent eruptive episode occurred between A.D.
1325 and 1365, during which time there were several explosive eruptions and five separate lava
flows that oozed onto the surface, including Panun Dome and North Coulee flow.
| Negit Island |
Negit Island:4
In addition to the volcanoes surrounding Mono Lake,
the two islands that rise above Mono's surface are also volcanic.
Negit, a classic black
cinder cone poking out of the water, first erupted some 1,600 years ago and
most recently flowed 270 years ago.
Paoha Island, the large
white island in the center, is less than 300 years old.
| Obsidian Dome |
Obsidian Dome:4
Obsidian Dome, of the nearby Inyo Craters, is an
example of a dome that has completely overrun its debris ring.
| Panum Crater |
Panum Crater:4
The twenty-one volcanic cones to the south of Mono Lake form the youngest
mountain range in North America, the Mono Craters.
While the earliest eruptions occurred about 35,000 years ago,
the most recent activity dates back merely 600 years. The most accessible
cone is Panum Crater, located three miles east of Highway 395 on Highway 120.
This 600 year-old volcano exhibits all of the
characteristics of the textbook rhyolitic, plug-dome volcano.
| Paoha Island |
Paoha Island:4
The youngest member of the Mono Basin volcanic family is Paoha,
the white island in the middle of the lake, which formed only 300 years
ago. An unusual volcanic formation, Paoha looks very different
from the classic cinder cone of Negit Island. White and rolling, it resembles
more the sediment that covers that bottom of Mono Lake.
And with good reason! Paoha is, indeed, exposed lake-bottom sediment,
pushed up over the surface of the lake in a volcanic upheaval
that lacked the momentum to fully erupt. Although there have been a few
small lava flows on the north end of the island,
Paoha has yet to let go in a full-scale eruption.
Still, steam vents and fumaroles on the island
indicate its potential for future activity!
| Red Cones |
Red Cones:2
Several small cinder cones are located on the floor and rim of Long Valley caldera.
The youngest cones in the area include Red Cones 5 kilometers south of
Mammoth Mountain and Black Point on the
north shore of Mono Lake. Geologists estimate that
Red Cones may have erupted as recently as
about 5,000 years ago and Black Point about 13,000 years ago.
The other cones on the caldera
floor and along the rim range in age from 228,000 to 64,000 years.
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