General Information on Obsidian &
Dacite
Obsidian
Obsidian is mineral-like, but not a true mineral because as a glass it is not
crystalline. Pure obsidian is usually dark in appearance, though the color
varies depending on the presence of impurities. Iron and magnesium typically
give the obsidian a dark green to brown to black color. Very few samples are
nearly colorless. In some stones, the inclusion of small, white, radially
clustered crystals of cristobalite in the black glass produce a blotchy or
snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles
remaining from the lava flow, aligned along layers created as the molten rock
was flowing before being cooled. These bubbles can produce interesting effects
such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen
(rainbow obsidian).
Obsidian can be found in locations which have experienced rhyolitic eruptions.
It can be found in Argentina, Armenia, Azerbaijan, Canada, Chile, Greece, El
Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, Peru,
Scotland and United States. Obsidian flows which may be hiked on are found
within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade
Range of western North America, and at Inyo Craters east of the Sierra Nevada in
California. Yellowstone National Park has a mountainside containing obsidian
located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits
can be found in many other western U.S. states including Arizona, Colorado, New
Mexico, Texas, Utah, Washington, Oregon and Idaho. Obsidian can also be found in
the eastern U.S. state of Virginia.
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. Modern archaeologists
have developed a relative dating system, obsidian hydration dating, to calculate
the age of obsidian artifacts.
Lithic analysis can be instrumental in understanding prehispanic groups in
Mesoamerica. A careful analysis of obsidian in a culture or place can be of
considerable use to reconstruct commerce, production, distribution and thereby
understand economic, social and political aspects of a civilization. This is the
case in Yaxchilán, a Maya city where even warfare implications have been studied
linked with obsidian use and its debris. Another example is the archeological
recovery at coastal Chumash sites in California indicating considerable trade
with the distant site of Casa Diablo, California in the Sierra Nevada Mountains.
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 the Americas. 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 Melos, Nisyros or Yiali, islands in the
Aegean Sea. Obsidian cores and blades were traded great distances inland from
the coast.
In Chile obsidian tools from Chaitén Volcano have been found
as far away as in Chan-Chan 400 km north of the volcano and also in sites 400 km
south of it.
Dacite
Dacite is an igneous, volcanic rock. The word dacite comes from Dacia, a
province of the Roman Empire which lay between the Danube River and Carpathian
Mountains (now modern Romania) where the rock was first described.
Dacite usually forms as an intrusive rock such as a dike or sill. Examples of
this type of dacite outcrop are found in northwestern Montana and northeastern
Bulgaria. Nevertheless, because of the moderate silica content, dacitic magma is
quite viscous and therefore prone to explosive eruption. A notorious example of
this is Mount St. Helens in which dacite domes formed from previous
eruptions.
Dacitic magma is formed by the subduction of young oceanic crust under a thick
felsic continental plate. Oceanic crust is hydrothermally altered causing
addition of quartz and sodium. As the young, hot oceanic plate is subducted
under continental crust, the subducted slab partially melts and interacts with
the upper mantle through convection and dehydration reactions. Once at the cold
surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende.
Accessory elements like pyroxenes provide insight to the history of the
magma.
The formation of dacite provides a great deal of information
about the connection between oceanic crust and continental crust. It provides a
model for the generation of felsic, buoyant, perennial rock from a mafic, dense,
short-lived one.
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