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More information from NOAA on volcanoes...
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Volcanoes
The word volcano comes from the island of Vulcano in the Mediterranean Sea. Long ago people thought this island mountain was the chimney of the blacksmith forge of the Roman God Vulcan. The steam and ash that came out of the vent was a sign that Vulcan was working at his forge making weapons for Jupiter and Mars. In Hawaii and other Polynesian islands, local people once attributed volcanic eruptions to the Goddess Pele. They believed Pele was moving from island to island as she sought to escape her evil sister, Na Maka O Kaha'i, the goddess of the sea. Today, scientists understand that volcanic eruptions are surface reminders of Earth's still hot interior.

The ash cloud formed by the May 18, 1980, eruption at Mount Saint Helens, WA. More images...
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Sixty percent of all active volcanoes are found at crustal plate boundaries such as the Pacific Plate. Earth's crust, like the cracked shell of a hard-boiled egg, is broken into a number of "plates". These floating pieces of crust are moving about very slowly on the hotter interior. Where the plates are moving apart or colliding with one another, volcanoes may form. Volcanoes also form oceanic islands in the Pacific Ocean or Mediterranean Sea where "hot spots" occur in the crust and mantle.
Many kinds of volcanic activity can endanger the lives of people and property. Most of the activity involves the explosive ejection or flowage of rock fragments and molten rock. Volcanoes that erupt explosively can send particles as far as 20 miles high and many miles away from the volcano. The volcanic ash from these types of eruptions is a significant hazard to aviation. If an airplane flies into a volcanic cloud, it can lead to engine damage and malfunction and to many other kinds of aircraft damage. Volcanic eruptions near coastlines can generate damaging tsunami waves that can cause death and destruction among coastal communities.
To ensure safe navigation and monitor possible climatic impact, NOAA records global historic volcanic eruptions, tracks volcanic ash eruptions affecting the United States, issues volcanic ash advisories and provides ash cloud forecasts. For more information on volcanoes, visit NOAA's Volcano website.
Recent Volcano Observatory Activity Reports from USGS
Pagan Weekly Update issued May 24, 2013 08:00 ChST Volcano Alert Level ADVISORY - Aviation Color Code YELLOW A constant steam and gas plume from Pagan was visible in satellite images over the past week. This is typical of recent months of observation of Pagan. A pilot report from a commercial aircraft flying over Pagan on Tuesday reported a vigorous plume although no ash was identifiable in satellite images and there were no reports of eruptive activity by residents on the island. The gas/vapor plume may just have been more vigorous on this day.
Because of this ongoing emission of volcanic gas from Pagan, it remains possible that residents of the CNMI may notice sulfurous odors under certain ... Details...
Kilauea Daily Update issued May 23, 2013 08:40 HST Volcano Alert Level WATCH - Aviation Color Code ORANGE Activity Summary: At the summit, tiltmeters recorded slowing DI inflation since yesterday morning. The lava lake level rose slightly, mimicking the tilt change. At the middle east rift zone, there was no lava activity within Pu`u `O`o crater, but the Kahauale`a II lava flow remained active north of Pu`u `O`o. To the southeast, the Peace Day flow had active breakouts on the pali and coastal plain, but most of the flow stayed within its lava tube until reaching the ocean on both the east and west sides of the Hawai`i Volcanoes National Park boundary. Gas emissions from the volcano ... Details...
Cleveland Daily Update issued May 23, 2013 10:39 AKDT Volcano Alert Level WATCH - Aviation Color Code ORANGE Elevated surface temperatures were observed by satellite over the past 24 hours indicative of ongoing low-level activity. AVO has received no reports of ash emission or other indications of eruptive activity over the past 24 hours.
Sudden explosions of blocks and ash are possible with little or no warning. Ash clouds, if produced, could exceed 20,000 feet above sea level. If a large ash-producing event occurs, nearby seismic, infrasound, or volcanic lightning networks should alert AVO staff quickly. However, for some events, a delay of several hours is possible. Cleveland Volcano does not ... Details...
Pavlof Daily Update issued May 23, 2013 10:39 AKDT Volcano Alert Level WATCH - Aviation Color Code ORANGE The eruption of Pavlof continues at low levels today. Small discrete events, likely indicative of small explosions continue to be detected on seismic and pressure sensor networks over the past 24 hours. Seismic tremor declined markedly on Tuesday morning and has not resumed. Cloudy conditions obscured satellite observations of the volcano over the past 24 hours but a pilot report and photos from yesterday afternoon showed a very weak steam and gas plume with little to no ash issuing from the vent. Elevated surface temperatures were observed in satellite images, even through cloud cover, ... Details...
Ugashik-Peulik Information Statement issued May 22, 2013 13:55 AKDT Volcano Alert Level NORMAL - Aviation Color Code GREEN Ukinrek Maars Volcano
57°50'2" N 156°30'50" W, Summit Elevation 299 ft (91 m)
Current Volcano Alert Level: NORMAL
Current Aviation Color Code: GREEN
Over the past several weeks seismic instruments at Ugashik-Peulik and Ukinrek volcanoes have slowly returned to operation. These instruments are solar powered and have likely returned to operation as a result of increased solar input with the changing season. AVO is now able to seismically monitor these volcanoes and we have changed the Aviation Color Code from UNASSIGNED to GREEN and the Volcano Alert level to NORMAL. Details...
Cleveland Daily Update issued May 22, 2013 12:20 AKDT Volcano Alert Level WATCH - Aviation Color Code ORANGE Clouds obscured views of Cleveland by satellite over the past 24 hours. AVO has received no reports of ash emission or other indications of eruptive activity over the past 24 hours.
Sudden explosions of blocks and ash are possible with little or no warning. Ash clouds, if produced, could exceed 20,000 feet above sea level. If a large ash-producing event occurs, nearby seismic, infrasound, or volcanic lightning networks should alert AVO staff quickly. However, for some events, a delay of several hours is possible. Cleveland Volcano does not have a local seismic network and is monitored ... Details...
Pavlof Daily Update issued May 22, 2013 12:20 AKDT Volcano Alert Level WATCH - Aviation Color Code ORANGE The eruption of Pavlof continues. Seismic tremor declined markedly yesterday around 11:00 AM AKDT and has been replaced by small discrete events likely indicative of small explosions. These explosion signals were also recorded on pressure sensors. A meteorological cloud deck has moved in over the volcano with a top of approximately 15,000 feet (4.5 km) above sea level thus obscuring satellite observations. If a volcanic ash plume is being generated, it is below this cloud deck. The last observations of the volcanic ash plume were on Monday afternoon. Trace ash fall with rain on Nelson ... Details...
Kilauea Daily Update issued May 22, 2013 09:06 HST Volcano Alert Level WATCH - Aviation Color Code ORANGE Activity Summary: At the summit, tiltmeters recorded the onset of DI deflation at about 11 AM yesterday and a return to DI inflation at about 7 AM this morning. The lava lake level mimicked the tilt change. At the middle east rift zone, there was no lava activity within Pu`u `O`o crater, but the Kahauale`a II lava flow remained active north of Pu`u `O`o. To the southeast, the Peace Day flow had active breakouts on the pali and coastal plain, but most of the flow stayed in its lava tube until reaching the ocean on both the east and west sides of the Hawai`i Volcanoes National Park ... Details...
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