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Volcano Calendar 2020
: We're proud to present our 2020 volcano calendar: 13 different and attractive images of volcanoes, volcanic landscapes and phenomena taken during volcano tours over the past two years.
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The Volcano Adventure Guide
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Illustrated Volcano Glossary
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite. Because obsidian is metastable at the earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian has a low water content when fresh, typically less than 1% water by weight, but becomes progressively hydrated when exposed to groundwater, forming perlite. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis.
Obsidian arrowhead.Obsidian was valued in Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It was also polished to create early mirrors.
Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated, including carved and worked obsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon.
Native American people traded obsidian throughout North America. Each volcano and in some cases each volcanic eruption produces a distinguishable type of obsidian, making it possible for archaeologists to trace the origins of a particular artifact. Similar tracing techniques have allowed obsidian to be identified in Greece also as coming from either Milos, Nisyros or Yiali, islands in the Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.
Obsidian was also used on Rapa Nui (Easter island) for edged tools such as Mataia and the pupils of the eyes of their Moai (statues).
Modern archaeologists have developed a dating system Obsidian hydration dating to calculate the age of Obsidian artifacts.
More on VolcanoDiscovery:
Volcanoes & Earthquakes
: Our popular app is now also available for iPhone and iPad! Get the latest volcano news and earthquakes world-wide - it's free!
17 days volcano trekking
: This is one of our most exciting volcano tours! Halmahera, North Sulawesi and the Sangihe Islands are not only very scenic and interesting as well as nearly unspoiled by tourism, but also home to some of the country's most active (and often erupting) volcanoes, inclduing Dukono, Ibu, Lokon, Soputan and Karangetang.
Top 20 quakes in 2016
: Several magnitude 7 quakes occurred in 2016 (but none above M8): Papua New Guinea, NZ, Ecuador, Solomon Islands, Sumatra, Chile, Alaska and others - nearly all major earthquakes were located near active subduction zones.
See the top 20 list of largest quakes in 2016 with this map.