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Vanuatu's Volcano

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Hidden : 11/4/2008
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Visits should be undertaken only with a local tour operator who can keep you safe. Sturdy shoes and long pants should be worn because of the sharp volcanic rocks and a flashlight should be brought if you are viewing after dusk.

Vanuatu is located in the south west Pacific. It is part of the Pacific Ring of Fire. The 83 islands were formed by volcanic activity over millions of years. Flights to Vanuatu operate from Australia, New Zealand, Fiji, New Caledonia, and Solomon Islands.

Yasur is one of the world's most active volcanoes. It is known as the Lighthouse of the Pacific because of its regular eruptions for hundreds of years. The volcano is located on Tanna Island in southern Vanuatu. It lies on a subduction zone at the boundary of the Indo-Australia and Pacific tectonic plates.

Yasur is a potentially hazardous volcano. Approaching the crater is dangerous at any time. Often quoted activity levels are for general scientific classification only, and do not relate to visitor safety. During periods of low activity, there can still be a large eruption without warning.
A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot, molten rock, ash, and gases to escape from below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.
Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by "divergent tectonic plates" pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earth's crust (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America and the European Rhine Graben with its Eifel volcanoes.
Volcanoes can be caused by "mantle plumes". These so-called "hotspots" , for example at Hawaii, can occur far from plate boundaries. Hotspot volcanoes are also found elsewhere in the solar system, especially on rocky planets and moons.
Stratovolcanoes (composite volcano)
Stratovolcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, then lava flows on top and dries and then the process begins again. Classic examples include Mt. Fuji in Japan, Mount Mayon in the Philippines, and Mount Vesuvius and Stromboli in Italy. In recorded history, explosive eruptions by stratovolcanoes have posed the greatest hazard to civilizations. Mount Yasur is a Stratovolcano.
An aspect of stratovolcanic eruptions are the creation of Volcanic Bombs
A volcanic bomb is a globe of molten rock (tephra) larger than 65 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Lava bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of Asama in Japan expelled bombs measuring 5-6 m in diameter up to distances of 600 m from the vent.
Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone. One such incident occurred at Galeras volcano in Colombia in 1993; six people near the summit were killed and several seriously injured by lava bombs when the volcano erupted unexpectedly.
Types of Bombs
Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed.
1. Ribbon or cylindrical bombs
These bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name- ribbon bombs. These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.
2. Spherical bombs
These bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.
3. Spindle, fusiform, or almond/rotational bombs
These bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shape, the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.
4. Cow-dung bombs
These are formed when highly fluid magma falls from moderate height (so the bomb does not solidify before impact) which are still liquid when they strike the ground. They consequently flatten or splash and form irregular roundish disks which resemble cow-dung.
5. Bread-crust bombs
If the outside of a lava bomb solidifies during its flight, it may develop a cracked outer surface as the interior continues to expand. This type of lava bomb is known as a bread-crust bomb.
6. Cored bombs
Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or in rare cases bits of lava formed earlier during the same eruption.

Yasur, the best-known and most frequently visited of the Vanuatu volcanoes, has been in more-or-less continuous strombolian and vulcanian activity since Captain Cook observed ash eruptions in 1774. This style of activity may have continued for the past 800 years. Yasur is a mostly unvegetated 361-m-high pyroclastic cone with a nearly circular, 400-m-wide summit crater. Yasur is largely contained within the small Yenkahe caldera and is the youngest of a group of Holocene volcanic centers constructed over the down-dropped NE flank of the Pleistocene Tukosmeru volcano. The Yenkahe horst is located within the Siwi ring fracture, a 4-km-wide, horseshoe-shaped caldera associated with eruption of the andesitic Siwi pyroclastic sequence. Active tectonism along the Yenkahe horst accompanying eruptions of Yasur has raised Port Resolution harbor more than 20 m during the past century.

To gain credit for this earth cache, you must measure the average height of the Pyroclastic eruptions during your visit, record the types of the Pyroclastic rocks (bombs) found on the crater rim and record the duration between events and email the results to the cache owner for verification. Pictures should be posted showing yourself at the volcano with a pyroclastic eruption in the background or with examples of volcanic bombs. Please do not post the answers in your log. Any answers/spoilers will be deleted.

Additional Hints (Decrypt)

Gur ibypnab pengre vf nccebkvzngryl sbhe uhaqerq zrgref npebff nf n ersrerapr

Decryption Key


(letter above equals below, and vice versa)



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