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Sunset Crater NM

GLY 3164, Spring 2006

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Arizona Parks Map

• Map shows the relationship of several park units

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Wupatki and Sunset NM’s

• Wupatki preserves Native American relicts

• Sunset is a large volcanic cone

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San Francisco Peaks

• San Francisco Peaks are the remnants of the only stratovolcano in the San Francisco volcanic field

• For decades, volcanologists suggested that the mountain now called San Francisco Peaks had simply worn away over time, eroded bit by bit to form its current bowl-shaped top

• Then, in 1980, the catastrophic explosion of Mount St. Helens forced us to rethink our ideas about volcanoes

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Reconstruction of the Volcano• Stratovolcanoes are known for

their powerful explosive eruptions, but they usually force their way upwards, producing a gaping crater at the top

• Mount St. Helens, as we know, blasted sideways, leaving a bowl-shaped amphitheater where a nearly symmetrical mountain top once stood

• San Francisco Peaks with a line showing the pre-1980 eruption outline of Mount St. Helens

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Name• Sunset Crater Volcano was originally named Sunset

Mountain by that intrepid explorer of the Colorado River, John Wesley Powell, for the bright sunset reds and yellows of its summit

• Of all the cinder cones of the San Francisco volcanic field, Sunset Crater Volcano is one of the most colorful

• “The contrast in the colors is so great that on viewing the mountain from a distance the red cinders seem to be on fire. From this circumstance the cone has been named Sunset Peak."

John Wesley Powell (Civil War soldier-turned-geologist), 1885

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Dating the Eruption

• How can we find out when Sunset Crater erupted?

• One way is to ask a tree• To find trees that grew at the time of the

Sunset Crater eruption, scientists looked at Wupatki, about 20 miles away

• Wupatki Pueblo was a thriving community after the eruption

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Dendrochronology• Roof beams used in some of

its rooms revealed several narrow, dark rings, indicating a period of stunted growth in the years 1064 and 1065

• Perhaps there was a severe drought or other disturbance during these years but, more likely, those narrow rings indicate the date of Sunset Crater's first eruption

Wupatki Ruin, a freestanding stone structure is built of blocks and bricks quarried from the surrounding bedrock, which is primarily a series of red-orange rock layers called the Moenkopi Formation

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Precursor to a Cinder Cone

• Lava with lots of gas is vital• Gas-rich lava expands as it travels toward the

surface• Shaking a can of soda simulates this effect• When the can is opened, or the volcano unplugged,

the pressure that holds the gas in is released • WHOOSH!- molten rock sprays into the air as a

fiery fountain

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Runny Lava

• Large cones are created when very fluid lava with lots of trapped gas is sprayed hundreds of feet into the air

• Basaltic lava has a low silica content, so it is very runny

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• Although lava erupted at 1200° centigrade, most airborne molten globs cooled and solidified to form cinders before they reached the ground

• Most cinders fell very near the central vent, building a small cone

Fiery Fountains

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Colored Volcanic

Cone

• While the base of Sunset Crater Volcano is mantled with dark gray cinders, the summit of the cinder cone is a striking rusty red

• Although you might speculate that the rocks at the top are of a different type, they are not!

• What makes these basalt cinders red?

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Volcanic Gas

• The cinder cone belched forth hot gasses as well as lava• The cinders on the rim of the cinder cone were bathed in these

vapors and chemically reacted with them to form iron oxide (rust), sulfur compounds, and gypsum

• The resulting red, yellow, purple, and green-colored basalt cinders decorate Sunset Crater Volcano’s summit

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Cinder Pictures

• Red cinders affected by volcanic gases

• Gray cinders unaffected by volcanic gas

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Vesicular Basalt

• The basaltic flow was very rich in gas, and the gas made the basalt extremely vesicular

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Mega Vesicle• When the surface of the

lava flow cools and hardens, a crust forms

• Lava is still-flowing beneath it

• Gas rises but is trapped beneath the crusty roof

• As more and more bubbles are trapped by the roof, they grow together to form a really big bubble like the mega vesicle seen here

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Xenoliths• In addition to lava and gas, strange

pieces of rock, not created by this eruption, were also erupted

• These pieces of rock can be seen within the lava flows and are called xenoliths, xeno meaning strange and lith referring to rock

• These pieces of rock were ripped up from the sides of the eruption conduit

• This xenolith is a piece of Kaibab limestone, a rock unit found as far as 1 kilometer under the present ground surface

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Subsequent Eruptions

• Perhaps as spectacular as the original pyrotechnics were two subsequent lava flows: Kana-A flow in 1064 Bonito flow in 1180

• They destroyed all living things in their paths

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Bonito Lava Flow

• The Bonito Lava Flow is one of several flows that streamed out from the base of Sunset Crater Volcano about 825 years ago

• The 1200°C liquid formed a river of black lava that inundated over four and a half square kilometers of the landscape before it cooled and solidified

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Squeeze Up Formation

• The top of the lava flow was exposed to cold air, so it began to cool and solidify first

• The core of the lava river continued to flow while the top of the flow formed a rocky basalt roof

• Like many other materials, rock shrinks as it cools

• As the solid basalt cap began to shrink, gaping fractures formed, pulling the sides apart

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Squeeze Up• In some places, hot, semi-

solid basalt lava was able to squeeze up through the fracture

• The semi-solid lava is very plastic, like modeling clay or ®silly putty

• As the lava oozed up, it scraped the jagged, solid sides, creating grooves in the plastic-like mass and, a squeeze-up was born

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Spatter Cone• One to fifteen meter (3-50

feet) high spatter cones form a string of lumpy beads along a once-active vent system near the base of Sunset Crater Volcano

• Like bubbling spaghetti sauce, they form when gasses escape from molten lava beneath the crusty, solid surface of a flow

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Hornito

• Picture a kettle of hot, bubbling spaghetti sauce• In your imagination, can you see the bubbles rise to the

surface, then spatter blobs of red sauce in a ring around the bubbles?

• That’s the process that formed these spatter cones or hornitos ('little ovens' in Spanish) on the surface of Bonito Lava Flow

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Lava Tubes• Beneath the thick lava crust lie caves called lava tubes,

which are remnants of the Bonito Lava Flow’s plumbing system

• These pipes first form while the lava is actively streaming downhill

• The scorching, runny basalt lava cools and hardens quickest on the surface of the flow where it contacts air

• The first solid rock forms plate-like sheets that are swept along like rafts on this swiftly-flowing stream of lava

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Insulation

• As the surface continues to cool the plates will pile up and fuse together to form a kind of crusty roof over the gushing lava river below

• The solid roof insulates the still-molten lava below from the cooling effects of the air

• The long, straight tubes may extend miles from the vent where the lava emerges, emptying their molten contents far downstream

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Exhaustion of Lava• Eventually the vent exhausts its lava supply or simply

becomes plugged up• Lava already in the tube drains out at the down stream

end, leaving an empty lava tube behind• Sometimes part of the thin, crusty roof collapses, and an

entrance to the tube opens up as it did here at Sunset Crater

• If the lava is not able to drain completely, lava within the tube solidifies to form a flat floor

• The last drips of molten rock often form lava stalactities to decorate the drained tube

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Ice

• A lava tube at Sunset Crater is unusual because ice is found inside it year-round

• Ice is rarely found year round in large lava tubes because large tubes generally have good air circulation that eventually melts any ice that accumulates there during the winter

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Ice Cave

• The lava tube you see here has a very narrow interior compared to other lava tubes

• During the winter months, ice forms from water seeping through the porous, fractured basalt

• Cold, heavy air settles into the lava tube, allowing the ice to form thick deposits along the walls and floor Although much of the ice melts each year, basalt is such an effective insulator that some ice remains in the cave even through the summer months

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Unconformity

• Photo shows two lava flows

• The lighter colored flow underneath is younger than the overlying flow

• It pushed up under the older flow