LAVA FLOW TRAIL
STOP 5
How to build a
cinder cone |
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| What
did the eruption of Sunset Crater Volcano look like? This photo of
Pu`u
`O`o cone erupting in Hawaii might help you imagine it.
|
Start with the right ingredients
 To make a cinder cone, you need to have lava with lots
of gas. Gas-rich lava expands as it travels toward the surface. If
you shake up a soda, then open it, you can simulate this effect. Open
the can, or unplug the volcano, and you release the pressure that holds
the gas in and -WHOOSH!- the molten rock sprays into the air as a fiery
fountain.
That brings us to the second critical cinder cone ingredient,
runny lava. Only very fluid lava with lots of gas can
be sprayed hundreds of feet into the air as lava globs to form a nice
cone.
Basaltic lava is exactly the right stuff! It has a low silica
content, so it is very runny (the amount of silica in molten rock determines
how runny or thick the lava will be). |
 |
| This
view shows the of curtains of fire that erupted prior to the
growth
of Pu`u `O`o cone in Hawaii. The initial eruption at Sunset
Crater may have looked very much like this.
|
Curtains, please!
The basalt lava that erupted to make the Sunset Crater
cinder cone made its way to the surface through a deep fracture that tapped
a magma-filled
pocket (magma
chamber) beneath the sedimentary layers you've seen at the Grand Canyon.
Rather than spewing from a single vent, lava gushed onto the scene
where this deep fracture cut the surface. A glowing 'curtain of fire'
may have extended over several miles before eruptions focused their attention
on the vents that built Sunset Crater Volcano. |
Fiery fountains
As the
outpouring of lava became focused beneath what is now Sunset Crater, lava
fountains threw blobs of molten basalt hundreds of feet into the air.
Although lava erupted at 1200° centigrade (2200° Fahrenheit), 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.
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