Volcanic Ash

 

Mt. St. Helens ash collected from deposits a few kilometers from the mountain several years after the 1981 eruption.  Copper-top battery and Swiss army knife are in these photos so viewer can judge if his or her screen in properly reproducing colors.

 

Another perspective of the same ash shown above.  Note the range of particle sizes for ash that has not been sorted by the wind.

 

Mt. St. Helens ash collected several 10s of kilometers downwind, where about 3 cm of ash covered everything.  This ash is well sorted (narrow size range) and much finer than ash collected from near the volcano.

 

Another perspective of the previous ash.


 

Mt. Pinatubo ash collected from Clark Air Force Base in 1991.  This coarse ash fell about 10 km from the vent.  Check out http://vulcan.wr.usgs.gov/Volcanoes/Philippines/Pinatubo/images.html for photos of the eruption and some newly fallen ash deposits.


 

Another perspective of the Pinatubo ash.  Note similarities in color for all ash images so far.  This reflects a similar rhyolitic (high-silica) chemistry shared by highly explosive volcanoes.

 

Basaltic ash from Isla del Tigre, a small island cone in the Gulf of Fonseca on the south coast of Honduras.  Although basaltic volcanoes are not usually associated with explosive behavior, the nearby basaltic ConsigŘina volcano in Nicaragua erupted in 1835, sending ash into the atmosphere that reach Mexico City and Jamaica.


Another perspective of the Isla del Tigre ash.  This was collected from a quarry where cinder blocks were being made.  Ash was packed into a mold and soaked in water.  Minerals dissolved from the ash precipitated when the molded block dried, making a somewhat solid (hard to break with your hands), light-weight building block.

Ash from the crater of Costa Rica's Iraz˙ volcano, a shield (basaltic) volcano that last erupted in 1963 (President Kennedy was splattered with ash and rain during his visit to San JosÚ) through 1965, although there have been several fumarole - geyser-like or overly vigorous hot springs - eruptions several times since, not real eruptions but hazardous enough to convince authorities to close the national park.  The last time I was in Costa Rica (1997) the bus for the park left from near the national opera house at 8 AM sharp (not hora latina) Sundays.  It's a local, picking up and dropping off passengers all the way up the mountain.  Park admission for norteamericanos is about $10 (better check, you probably need the equivalent in Colones, the local currency).  This will be a good chance to practice your Spanish, as most bus passengers are reluctant to speak English.

Another perspective of Iraz˙ ash.  Some authors call Iraz˙ a stratovolcano, and indeed the summit crater exposes interlayered lava flows and phroclastics, but the overall shape of the mountain is clearly shield rather than stratovolcano.

 

Iraz˙ ash with magnets (under the paper).  Some (not all) of the ash sticks to the magnetic field because it contains the iron oxide mineral magnetite.  If you plan to travel to a beach on a tropical volcanic island, take along a strong little magnet and look for magnetite.  I put the magnet in a sandwich bag so it is easy to break it away from the magnet and collect in a second sandwich bag.  I've also introduced peasants in the mountains of Honduras to magnetic sand, giving away the magnets so they could show their children.

Video clip of moving ash and paper over rectangular magnet

Video clip of moving ash and paper over circular magnet

Ash from Mt. Spur, collected at Anchorage Airport by a former student whose flight was delayed by the eruption.  This is andesitic ash from a true stratovolcano.  Note the salt-and-pepper mix appearance, as well as the well sorted, wind-blown sample.  Some of the dark grains are magnetite.

 

Another perspective, Mt. Spur ash.

 

 

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